PhD Theses

Alleyne, D. N. ‘The non-destructive testing of plates using ultrasonic Lamb waves’, Mechanical Engineering Department, Imperial College London, 1991

Alleyne D. N. (pdf)

The major objective of this thesis is the development of quantitative methods of applying Lamb waves in industrial non-destructive testing (NDT). The key problem associated with the measurement of the characteristics of Lamb waves is that invariably more than one mode is excited at any given frequency. This has led to problems when interpreting the typically complicated Lamb wave signals which are commonly found in NDT applications.

The first two chapters of this thesis provide an introduction to the field of NDT using Lamb waves, reviewing past work and relevant theory. The review has shown that quantitative or qualitative time domain methods may be adopted in low frequency-thickness product regions where only two propagating modes are possibly as they may be easily decoupled from one another by the orientation of the transducers. However, in higher frequency-thickness regions the multi-mode dispersive nature of Lamb waves makes this approach unreliable for most NDT applications. In chapter 3 a new method is presented for measuring the amplitudes and velocities of Lamb waves. The method involves a two-dimensional Fourier transformation (2-D FFT) of the time history of the signals received at a series of equally spaced positions along the propagation path. The output of this transform is a three-dimensional plot of the amplitude versus frequency and wavenumber, from which the amplitudes of the different propagating Lamb modes may be obtained.

In chapters 4 and 5 the 2-D FFT method is used to measure the characteristics of propatgating Lamb waves in finite element modelling studies, where single Lamb modes have been launched. Numerical predictions of Lamb wave reflection from boundaries and interaction with straight sided notches are presented. In chapters 6 and 7 the numerical model is validated by experimental investigations carried out on a variety of plates with straight sided notches. The correlation between the experimental results and the numerical predictions is excellent and the results are presented in terms of Lamb wave amplitudes as a function of frequency-thickness product and Lamb wave amplitudes as a function of notch depth at particular frequency-thicknesses, this being the more useful format in NDT applications. The final two chapters discuss the practical implementation of quantitative and qualistative Lamb wave techniques in the NDT of plate-like st ructures and present the major conclusions of the thesis. Here, the emphasis is on practical problems such as signal-to-noise considerations, coupling requirements, excitation methods, and on methods of distinguishing the signals from defects from those produced by boundaries or other impedance changes.

The main conclusion of the thesis is that Lamb waves may be used very successfully for the quantitative NDT of plates. In localised, detailed NDT applications the detectability of a defect may be optimised by choosing the most suitable mode at the appropriate frequency-thickness product. Since stresses are produced throughout the thickness of the plate by Lamb waves (in some cases there may be stress nodes which must be carefully considered), the entire thickness of the plate is interrogated, which means it is possible to find defects that are initiated at either surface or internal locations. Lamb waves may be propagated over considerable distances, so they are ideally suited to the long range NDT of plates and plate-like structures where a fast, coarse inspection may be carried out. The finite element predictions and experimental results have shown that Lamb wave techniques maybe used to find defects when the wavelength to critical defect dimension is as high as 40. The computational requirements of the 2-D FFT method are fairly modest and can be handled by most IBM compatible micro-computers interfaced to a data capture system. The multi-element transducers which are now available make the implementation of the 2-D FFT method feasible in industrial NDT.

Allin, J. M., 'Disbond detection in adhesive joints using low frequency ultrasound', Mechanical Engineering, Imperial College London, 2002

Allin, J. M. (pdf)

Adhesive use in the automotive industry is limited by a lack of suitable non-destructive testing methods. Ultrasonic methods have been used successfully in some applications. However, current techniques cannot be used in the automotive industry due to large variations in the thickness of the attenuative adhesive and the need for couplant-free testing.
This thesis details the development of a novel ultrasonic technique for the detection of disbonds in the automotive industry, based on the fundamental through-thickness resonance (mode 1) frequency of the joint. For a specified range of adhesive thickness in a bonded joint, a corresponding range of mode 1 frequencies can be predicted. Where the joint is disbonded these frequencies are much higher. During testing, the mode 1 resonance is excited in the joint and the received signal is windowed, leaving the ringing of the first mode. If the frequency of this resonance falls into the range known for bonded joints, then the bond integrity is confirmed. Further investigation has shown that narrow beads of adhesive and tapered adhesive layers, which commonly occur in practice, do not affect the reliability of this technique.

In order to make spot measurements of the bond condition, a novel dry-contact dabber probe was developed. This comprises a low loss rubber delay line with a highly attenuative rubber bonded to the side walls to eliminate side wall reflections. This allowed results to be successfully collected in the factory.

Testing a wide range of adhesive thicknesses requires a very wide band, well damped, low frequency transducer. Such devices are not commercially available, which led to the development of a novel non-resonant transducer. The device is constructed from small undamped piezo-electric elements bonded to a thin membrane. It operates below the first resonance mode and provides an operating frequency range of 200-500kHz

Andreas Zimmermann, ‘Shear-horizontal guided wave tomography’, Mechanical Engineering Department, Imperial College London, 2022

Andreas Zimmermann

The petrochemical industry’s pipe-network is increasingly suffering from corrosion-induced shutdowns and thus a quantitative measurement approach is vital for making adequate service-life predictions and to guarantee safety. At pipe-supports, access is limited and traditional point-inspection methods such as ultrasonic thickness (UT) testing are not feasible. Guided wave tomography has been proposed as a solution to this problem. In this technique ultrasonic guided waves are excited from an array around the defect and received signals capture the interaction of the defect with these waves. From these signals, velocity maps are reconstructed, and the dispersive nature of the waves utilised to invert to thickness maps. Recently, it was found, that although reconstruction algorithms such as the hybrid algorithm for robust breast ultrasound tomography (HARBUT) allow for very high-resolution reconstructions, in practice, the mismatch of traditional guided wave scattering of Lamb waves used in guided wave tomography with the acoustic scattering models on which algorithms are based lead to poor resolution. In this thesis, another type of guided wave mode is investigated instead, shear-horizontal (SH) guided waves. Traditionally, these have been harder to excite but recent advances in electromagnetic acoustic transducer (EMAT) excitation made them a practical alternative. Since the fundamental SH0 mode is non-dispersive, the aim of this thesis is to assess whether the first higher-order SH mode, SH1, is applicable to guided wave tomography. SH1 exists at a higher frequency than those used in guided wave tomography with fundamental Lamb wave modes, leading to a smaller wavelength and it offers no out-of-plane displacement, which should avoid leakage into any pipe-loading. The thesis presents a thorough numerical investigation of the SH1 mode, examining its scattering behaviour and reconstruction of various defect types using three different SH excitation sources; (i) pure SH1 point source (ii) SH surface point source and (iii) directional SH surface source. These numerical results were validated experimentally, confirming the robustness of SH1 based guided wave tomography. In addition, the thesis investigates a technique to separate out individual modes via the application of the nonuniform fast Fourier transform (NUFFT) algorithm to non-uniformly sampled guided wave data. The benefits of SH1 for guided wave tomography are shown throughout the thesis and have been demonstrated by accurate and reliable thickness mapping of the defects. The SH1 wave was able to achieve a 2.4 times better resolution than was previously achieved using Lamb waves; yielding the best numerical and experimental guided wave tomography reconstructions to date. The application of the NUFFT algorithm showed how filtering in the frequency-wavenumber domain could be used to separate modal components even without the application of time-based windowing approaches. This technique can also be used to robustly determine bulk shear velocity even from directional data. Finally, this thesis demonstrated proof of concept for SH1 guided wave tomography, suggesting that this technique should be further investigated for employment in the accurate sizing and quantification of corrosion in pipelines.

Attarian, V.A. 'Long-term structural health monitoring of plate-like structures using distributed guided wave sensors', Mechanical Engineering, Imperial College London, 2013

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Aircraft, containers, and storage tanks contain plate-like structures that are safety critical. The structures often undergo non-destructive inspections. The inspection frequency tends to be over-conservatively high, and it may be possible to reduce the intervals between inspections to realize cost savings. This goal can possibly be realized by automated structural health monitoring (SHM) of structures using sparse active guided wave sensor arrays. Guided waves are sensitive to small defects and can propagate long distances across feature dense plates. Thus, a guided wave SHM system that enables reliable detection of critical defects or monitoring of their growth can potentially be used to reduce the frequency of inspections for real structures.

Industrial guided wave SHM systems must be reliable throughout prolonged exposure to temperature, humidity, and loading changes encountered in operation. Research at Imperial College shows that temperature compensation and subtraction between monitored guided wave signals and baselines acquired from healthy plates enables detection of 1.5% reflection change over areas ∼1 m2 in the presence of thermal swings and uniform liquid layers. These results and findings from scattering studies indicate it may be possible to detect reflections from hole type defects and notches affecting structures during their operation. An issue is that demonstrations of SHM system capabilities have only been shown in controlled laboratory tests within short periods following baseline acquisition. There is concern whether sustained exposure to service conditions will subject transducer elements to irreversible changes and introduce variability in baseline subtraction results that would mask signals due to slowly growing damage.

This thesis studies the reliability of guided wave SHM for monitoring plate-like structures over longer time periods. The theoretical characteristics of the fundamental Lamb waves and their use to monitor and detect damage are reviewed. Strategies for sensing and signal processing are described alongside experimental validation of their performance. The effectiveness of the SHM system is tested in experiments where damage-free plates are exposed to British weather as well as thermal variations in an environmental chamber. The monitoring capabilities of bonded piezoelectric sensors are quantified and compared to the performance achieved using electromagnetic acoustic transducers. Experimental results and findings from simulations of bonded piezoelectric transduction establish that performances achieved with bonded sensors degrade due to variations in the properties of adhesives used to attach sensors to plates. EMATs are relatively stable and capable of enabling detection of 1.5% reflection change at points away from the edges of plates after sustained exposure to thermal cycling loads.

Baly, S. ‘Interaction d’un faisceau ultrasonore avec un matériau multicouche anisotrope: generation, propagation, rayonnement d’ondes de Lamb’, Mechanical Engineering Department, l’Université de Technologie de Compiègne, 2002

The object of the work presented in this thesis concerns the study of the interaction of an ultrasonic beam with an anisotropic multilayered structure. We have developed a model which allows one to simulate an ultrasonic non-destructive experiments conducted on anisotropic multilayered materials. The study reveals the physics of the interaction of the acoustic beams with the plate and the nature of the propagation of the modal waves. 

Consideration of the ultrasonic bounded beam is made by decomposition into monochromatic plane waves. We consider a multilayered anisotropic plate immersed in a fluid media as well as two transducers, one emitter, the other receiver, at arbitrary angle and arranged freely in the space. The goal of this model is to simulate the signal detected by the receiver, following a pressure variation on the front face, and an arbitrary excitation of the emitter.

Our software allows one to deal with either two or three-dimensional geometry. Hence, the various phenomena which require the three-dimensional geometry to be represented can be studied, such as the deviation of the energy of Lamb waves with regard to the direction of propagation.

We show that the direction of propagation of the modal wave beam, normal to the Lamb wave slowness curve, does not belong any more to the sagittal plane, due to the anisotropy. The deviation of Lamb wave beams is predicted by using two different methods: an analytical method which appeals to Lamb slowness curves, and a numerical method using the software described above. This phenomenon of deviation is illustrated in a numerical and experimental way in the cases of a Carbon-Epoxy unidirectional and multilayered (only numerical results are shown) plates, where the fibres are not contained in the sagittal plane.

Then, we present the determination of the Lamb wave energy velocity, when the plate is immersed in water, by using an analytical formulation of the energy velocity vector. Energy vectors are thus studied for miscellaneous structures and notably, the phenomenon of focalization is presented. Following from that we examine the relation between the attenuation of the waves and the fluid/structure coupling, according to the modes which are excited.

Finally, a study of Lamb wav e propagation is presented for a Carbon-Epoxy unidirectional plate immersed in a fluid medium and subjected to transient excitation. We show the Lamb waves time-space dispersion phenomena according to the mode excited.

Beard, M. ‘Guided wave inspection of embedded cylindrical structures’, Mechanical Engineering Department, Imperial College London, 2002

Beard, M (pdf)

This thesis investigates the use of ultrasound for two specific non-destructive testing applications, made possible by recent developments in the understanding of guided wave propagation in embedded cylindrical structures. Guided waves offer an opportunity to inspect the rock bolts used to support coal mine roadways, where there is a need for an effective non-destructive test. Rock bolts are secured into pre-drilled holes in the mine roof with an epoxy resin, and provide roof support by resisting the movement and expansion of rock strata. As a result they are prone to failure from tensile overload, and a test is proposed that would identify defects and the residual length of the bolt using a pulse-echo technique. The use of rock bolts in the mining industry is increasing throughout the world, and the industrial use of such a test would have significant safety and economic benefits. In addition, this thesis continues the work of previous authors on the use of guided waves to inspect concrete post-tensioning tendons, and identifies the limitations of such a technique.

The behaviour of guided waves in the two systems is predicted through modeling, and the effect of material and geometry changes on modes that have the potential for long range inspection is investigated. These predictions are compared with experimental results from laboratory and site specimens. Further experimental work investigates the optimum excitation signal and the reflection of waves from selected features and effects, contributing to the general understanding of guided waves. The effect of specimen curvature has been found to be highly significant, and has been explained by comparing mode shapes in flat and curved plates. The previously unreported dispersion curves for a curved bar have also been calculated using a finite element technique, thus laying the foundations for further analytical work on guided wave propagation in curved bars.

Belanger, P. 'Feasibility of Thickness Mapping Using Ultrasonic Guided Waves', Mechanical Engineering Department, Imperial College London, 2009

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Detection and sizing of corrosion in pipelines and pressure vessels over large, partially accessible areas is of growing interest in the petrochemical and nuclear industries.
Traditionally, conventional ultrasonic thickness gauging and eddy current techniques have been used to precisely measure the thickness in structures. These techniques
only allow the measurement of the local thickness under the probe. Consequently obtaining the remnant thickness of a specimen over a large area requires the probe to
be scanned, which is a long and tedious process. Moreover, with these techniques, the scanning may become impossible when the area of inspection is inaccessible.
There is therefore a need for a rapid, accurate, long range inspection technique to measure the remaining thickness in corrosion patches.

Low frequency guided waves are now routinely used to screen large area of pipes and other structures for cracks and corrosion. Their detection and location capability is
very good, but the standard screening technique only gives a rough estimate of the remaining wall thickness. Guided waves have multiple properties which can be used
for thickness mapping over large partially accessible areas e.g. dispersion and cutoff frequency thickness product of the high order modes.

The present work aims to demonstrate the potential of guided waves for thickness mapping over large partially accessible areas. It starts with a general introduction
on ultrasonic guided waves and a literature review of the different techniques for the evaluation of thickness with guided waves. The severity of the errors introduced in
time-of-flight tomography for thickness reconstruction by breaking the assumption of the ray theory are investigated. As these errors are significant, the possibility of
using the cutoff property of the high order modes is investigated in a frequency range where the ray theory is valid. It is found that the attenuation due to the scattering
of the waves in corrosion is too large for this technique to work. Finally the use of low frequency guided wave for diffraction tomography is examined. Finite element
simulations of a 64 element circular array on a plate show that when the scattering mechanism of the object to be reconstructed satisfies the Born approximation the 
reconstruction of the thickness is accurate. However the practical implementation is more challenging when the incident field is not known. Experimental results demonstrate
that ultimately the scattering from the array of transducer is a major source of error in the tomographic reconstruction, but when there is no scattering from the array of transducers the reconstructions are very similar to the finite element simulations.

Benstock, D. J. ‘Extreme value analysis of ultrasonic thickness measurements’, Mechanical Engineering Department, Imperial College London, 2016

Benstock-D-2016-PhD-Thesis

Modern infrastructure and industrial plants have a finite design life. Their effective and profitable operation depends on well organized maintenance and condition assessments. Non-destructive evaluation and inspection is a key tool for condition assessment. However, despite best efforts, full inspection coverage of a plant is not always possible because of access problems, time constraints and limited budgets.

Many inspection companies are beginning to use partial coverage inspection (PCI) techniques to solve this problem. PCI describes the use of inspection data collected from a small area of the component to extrapolate to the condition and the rest of the component. Extreme value analysis (EVA) is a technique of particular interest for this application as it allows an inspector to construct a statistical model of the smallest thickness measurements across the component. This model can then be used to extrapolate to the most likely minimum thickness.

In this thesis, an analysis of the uncertainties that can arise when using EVA for extrapolation is performed. A clear outline of the uncertainties expected to result from EVA extrapolation is presented and it is made usable for inspectors. In addition, a simple test algorithm to analyse when EVA is suitable for a set of inspection data is described. It is hoped that the work described in this thesis will enhance confidence in the practical use of the technique in the field.

Furthermore, the effect of surface roughness on ultrasonic thickness measurements is investigated with joint experimental and computational studies. It is shown that the thickness measurement distribution can differ significantly from the actual thickness distribution, particularly for the smallest values of thickness and with rougher surface conditions. Consequently, extrapolations from extreme value models using ultrasonic thickness data are shown to be conservative compared to the true condition of the component.

Bernard, A. 'Ondes de plaques guidees : approche temporelle et spatiale', Mechanical Engineering Department, Bristol University, 1978.

Bernard, A (pdf)

Brierley, N. 'The Computational Enhancement of Automated Non-Destructive Inspection', Mechanical Engineering Department, Imperial College London, 2014

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In industrial NDE it is increasingly common for data acquisition to be automated, driving a recent substantial increase in the availability of data. The collected data need to be analysed and currently this is largely done manually by a skilled operator - a rather painstaking task given how rarely defects occur. Moreover, in automated NDE a region of an inspected component is typically interrogated several times, be it within a single data channel due to multiple probe passes, across several channels acquired simultaneously or over the course of repeated inspections. The systematic combination of these diverse readings is recognised to offer an opportunity to improve the reliability of the inspection, for example by enabling noise suppression, but is not achievable in a manual analysis. Hence there is scope for the inspection reliability to be improved whilst reducing the time taken for the data analysis by computational means. This thesis describes the development of a software framework providing a partial automation capability, aligning then fusing the available experimental data to declare regions of the component defect-free to a very high probability whilst readily identifying indications, thereby optimising the use of the operator's time. The framework is designed to be applicable to a wide range of automated NDE scenarios, but the focus in development has been on two distinct, industrial inspections: the ultrasonic inspection of power station turbine rotor bores and the ultrasonic immersion inspection of aerospace turbine disks. Results obtained for industrial datasets from these two applications convincingly demonstrate the benefits of using the developed software system.

Carandente, R. 'Interaction Between The Fundamental Torsional Guided Wave Mode And Complex Defects In Pipes', Mechanical Engineering Department, Imperial College London, 2011

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The presence of defects in pipelines is a concern especially in petrochemical applications where the service integrity of pipes is a fundamental requirement to avoid process interruptions and to full safety standards. Guided wave inspection is now routinely used in industry for screening long lengths of pipe for corrosion, any suspect areas then being followed up with conventional ultrasonic thickness gauging. However, this is difficult in cases where the suspect area is inaccessible (e.g. buried pipelines or pipes passing though walls), so it would be very useful to apply guided wave techniques for sizing as well as the detection and location of defects. This target is challenging due to the complexity of the profiles encountered in practice.

The present work aims to improve the understanding of the scattering of the fundamental torsional mode T(0, 1) from complex shaped discontinuities and to determine the controlling parameters of this phenomenon. The overall analysis starts with a study of the reflection from axi-symmetric tapered steps and notches in pipes. After that the scattering from three dimensional (3D) defects with different shapes has been studied. Firstly, at-bottomed defects with different surface profiles have been analyzed, and then the study of the reflection behavior from 3D defects with varying depth profile has been carried out. All of the work presented here uses the T(0,1) mode for inspection.

It is revealed that the reflection coefficient maxima from axi-symmetric tapered defects decrease with increasing frequency as the slope of the taper becomes more gradual, this effect being more pronounced when the ratio of the average defect length to the wavelength increases. Tapered defects are therefore expected to be more difficult to detect at higher inspection frequencies; this effect is more evident for shallower tapers. It is also found that at a given maximum depth of a finite discontinuity, the peak of the reflection coefficient from a defect is linearly dependent on the circumferential extent of the defect, and is independent of its shape. The results from these analyses have been used to propose a practical approach to determine the maximum depth of a complex discontinuity from the reflection coefficient behavior, provided that the external circumferential extent of the defect is known. This method has been applied to real corrosion patches and the results validated with experiments. Its main limitation is on defects with a gradual corrosion section profile, but with a sudden change of the depth over a small circumferential region. It is shown then that a possible way to diagnose sharp circumferential profile changes is to measure the reflection coefficient spectrum at frequency higher than usually used in long range guided wave inspection.

Cawley, P. 'Defect location in structures by vibration technique', Mechanical Engineering Department, Bristol University, 1978

Cawley, P (pdf)

A vibration technique for non-destructively assessing the integrity of structures has been described. The method uses measurements of changes in the lower structural natural frequencies, which can be made at a single point in the stucture, in conjunction with a dynamic analysis of the system to detect, locate and roughly to quantify damage. It has been shown that the mathematical model of the stucture need not be sophisticated and only one full analysis is required for each type of structure to be tested. The dynamic analysis was carried out using the finite element method as this is applicable to all structures. The dynamic finite element program written for this work has been described and the natural frequency and nodal pattern predictions made using this program have been compared with experimental results from an aluminium plate and from carbon fibre reinforced plastic plates with a variety of ply orientations. Excellent agreement was shown between the theoretical and experimental results. A program had been developed which enables the location of the damage site and the estiamtion of the severity of the damage from the results of the dynamic analysis and the measured changes in the structural natural frequencies. The computational requirements of the location routine are small and the program could readily be adapted to run on a micro-computer. The measurement of natural frequencies from the Fourier transform of the structural response to an impulse has been investigated and a method for improving the frequency resolution obtained from this type of test developed. Preliminary tests have shown that it is possible to obtain frequency resolution of one-tenth of the spacing between the frequency points produced by the Fourier transform at a low cost in terms of computer time and store. The results indicate that this technique would be the most suitable method of frequency measurement for the proposed non-destructive test since it combines accuracy with a short test time. However, because of the unreliability of the available transient recording equipment, the main test programme was carried out using steady-state frequency measurement. Results have been presented from tests of the non-destructive testing technique on an aluminium plate and a variety of carbon fibre reinforced plastic structures, including two honeycomb panels obtained from the aero-space industry. Five different forms of damage have been used and it has been shown that the method can successfully be used to detect and locate each type of damage.

Cegla, F. 'Ultrasonic Waveguide Sensors For Fluid Characterisation And Remote Sensing', Mechanical Engineering Department, Imperial College London, 2006

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This thesis addresses two physical problems which both benefit from a new approach using guided ultrasonic waves.

The first application relates to fluid characterisation. Conventional equipment for fluid characterization has drawbacks due to the need of a straight, unobstructed path across the fluid specimen, a perfectly parallel reflector, diffraction effects and penetration problems in highly attenuating fluids. The use of ultrasonic waveguides can alleviate these problems by separating the transducer from the measurement area and by guiding the ultrasonic energy along a flexible waveguide of fixed geometry. The theoretical modelling, design and construction of a wave guide sensor for fluid characterization of hot or radioactive fluids and liquids in general is presented. The sensor makes use of a guided interface wave. This wave was named the quasi-Scholte wave because of its similarity to the Scholte wave that is widely known in geophysics. It is a non-leaky guided wave that travels in a plate immersed in a fluid. A substantial fraction of its energy travels in the fluid and is trapped at the interface. It thus does not radiate energy away from the waveguide. This makes this mode very sensitive to the fluid properties. It is shown that the fluid bulk velocity and attenuation can be retrieved accurately using this method. Furthermore it is shown that the use of other guided wave modes can be used to extract further fluid properties so as to completely characterize the fluid acoustically.

The second application relates to non-destructive testing in harsh environments. Conventional ultrasonic non-destructive testing uses a piezoelectric transducer close to the area to be inspected. This becomes impossible above temperatures of about 300-400 C when conventional piezo-electric materials reach their Curie point and become depolarized, which removes their ability to send or receive ultrasonic signals. A remedy to this problem was found in using waveguides for remotely monitoring thickness and defects within a structure under extreme conditions. The waveguide separates the hot structure from the transducer which is located in a cool and safe place. Essentially, this represents an acoustic cable along which ultrasound is sent. The two main issues that had to be investigated are the wave propagation along waveguides of different candidate geometries and the geometry and method of attachment of the waveguide to the sample that is to be tested. The problems are that the acoustic pulse has to remain strong and as undistorted as possible while propagating along the waveguide, and when transmitting from the waveguide into the sample. A system was designed and tested successfully at temperatures over 550 C.

Chan, C. W. ‘The ultrasonic non-destructive evaluation of welds in plastic pipes’, Mechanical Engineering Department, Imperial College London, 1996

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The use of plastic as a pipe material for the transportation of potable water has increased over the last few years due to the improvements made to the material properties which have resulted in light, tough materials. In order to join the pipes together, two of the main welding technologies used are butt-fusion welding and electrofusion welding. At present, very limited quality assurance is carried out on such welded joints. Given that failures can occur due to the welding not being performed adequately, a nondestructive inspection technique is highly desirable.
Ultrasonics has been chosen as the tool to be used for the inspection. The approach of the studies is to assess the feasibility of several ultrasonic techniques with the aim of choosing the most appropriate for further development.

The likely failure in plastic water pipes is due to the slow brittle growth of a crack to a size which leads to fracture. The critical crack sizes required to cause failure in 50 years have been calculated using the theory of fracture mechanics.

For the butt-fusion weld, a technique using Lamb waves is investigated. In order to decide which mode(s) to use, dispersion curves were predicted and were verified experimentally. Studies were also carried out experimentally and using finite element analysis to assess the feasibility of using a low attenuation Lamb mode for defect detection. The results obtained were promising, indicating that a Lamb wave technique could be used for defect detection in plastic pipes. The practical implementation of the Lamb wave technique is discussed. Possible dry coupled transducers for exciting Lamb waves in pipes have been investigated and are described.

For the electrofusion weld, two techniques are assessed. The first is based on normal incidence inspection and was found to work extremely well although inspection times were long. The second technique uses Lamb waves and a finite element analysis was used to assess its feasibility. Although such a technique has potential in low damping materials such as steel, its use is very limited in plastics due to the high attenuation and low reflectivity from defects. Therefore, the normal incidence technique is the recommended method for electrofusion welds. The implementation of such a technique can be carried out using commercial systems currently available and by tailoring the system to use the appropriate transducers.

Chua, C.A. 'Crack growth monitoring using shear guided waves', Mechanical Engineering Department, Imperial College London, 2019

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Crack monitoring in critical sections of steel structures is a subject of growing interest. High frequency ultrasonic techniques have good detection sensitivities but poor inspection coverage per transducer that makes it impractical to monitor large areas. Corrosion detection and monitoring of pipelines can be performed by using low frequency guided waves, but are insufficiently sensitive for the detection of small cracks. This study evaluates the crack monitoring performance of a fundamental shear horizontal mode (SH0) system at frequencies just below the high-order mode cut-off. The scattering solutions valid at low frequencies were developed for both 2D and 3D cracks; most importantly the 3D solution showed that the SH0 reflection ratio is proportional to frequency to the power 1.5, to the effective crack size cubed, and is inversely proportional to the plate thickness and to the square root of the distance from the crack to the receiving sensor. Finite element (FE) analysis was used to validate these power coefficients and to calculate the proportionality constant. The predicted 3D solution was validated using experimental data generated by a ring of transducers on a 6-inch diameter pipe with a progressively grown notch that simulates crack growth; baseline subtraction with temperature compensation was applied to compensate for changes in the environmental and operational conditions. It was found that the residual signals after baseline subtraction can be assumed to be normally distributed so the random fluctuations could be reduced by coherent averaging; it was thereby possible to reliably detect a 2 mm wide and 1 mm deep notch simulating a crack located one pipe diameter along the pipe from the transducer ring. The damage detection performance at different locations along the pipe was assessed by analysing receiver operating characteristic (ROC) curves generated by adding simulated defects to multiple experimental measurements without damage. At a fixed standoff distance, the damage detection performance increases with the square root of the number of averaged signals, and is also improved by averaging the signals received by transducers covering the main lobe of the reflection from the defect; the common source method can be applied to reduce the effects of phase cancellation when using signals from multiple receivers. When the defect is located more than about one pipe circumference from the transducer ring, the optimal performance is obtained by averaging across all the transducers in the ring, corresponding to monitoring the T(0,1) pipe mode. The effects of temperature cycling and the presence of a large reflector, in this case a weld, near a simulated crack were investigated experimentally by data generated from electromagnetic acoustic transducers (EMATs) on a 6-inch diameter pipe with a weld. The results show that the measured reflection ratios after temperature compensation can still be assumed to be normally distributed, similar to the findings from the previous experiment, but there were uncompensated changes which result in larger variations towards the second half of the temperature cycling experiment. A notch was progressively grown near the weld of this same pipe and it was shown that a 1 mm wide and 0.5 mm deep notch located one pipe diameter along the pipe from the EMAT receiver can be detected; a 95% probability of detection with a 1% false alarm rate can be achieved after a sufficiently large number of measurements. In conclusion, an SH0 mode monitoring system with a short transducer standoff distance from the inspected area has good potential for crack monitoring applications, for example for girth welds in pipes, given that transducers designed for stable and long term use are utilised.

Cicero, T. 'Signal processing for guided wave structural health monitoring', Mechanical Engineering Department, Imperial College London, 2009.

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The importance of Structural Health Monitoring (SHM) in several industrial fields has been continuously growing in the last few years with the increasing need for
the development of systems able to monitor continuously the integrity of complex structures. In order to be competitive with conventional non destructive evaluation
techniques, SHM must be able to effectively detect the occurrence of damage in the structure, giving information regarding the damage location. Ultrasonic guided
waves offer the possibility of inspecting large areas of structures from a small number of sensor positions. However, inspection of complex structures is difficult as the
reflections from different features overlap. Therefore damage detection becomes an extremely challenging problem and robust signal processing is required in order to
resolve strongly overlapping echoes.

In our work we have considered at first the possibility of employing a deconvolution approach for enhancing the resolution of ultrasonic time traces and the potential
and the limitations of this approach for reliable SHM applications have been shown. The effects of noise on the bandwidth of the typical signals in SHM and the effects
of frequency dependent phase shifts are the main detrimental issues that strongly reduce the performance of deconvolution in SHM applications.
The second part of this thesis is concerned with the evaluation of a subtraction approach for SHM when changes of environmental conditions are taken into account.
Temperature changes result in imperfect subtraction even for an undamaged structure, since temperature changes modify the mechanical properties of the material
and therefore the velocity of propagation of ultrasonic guided waves. Compensation techniques have previously been used effectively to overcome temperature effects, in
order to reduce the residual in the subtraction. In this work the performance of temperature compensation techniques has been evaluated also in the presence of other
detrimental effects, such as liquid loading and different temperature responses of materials in adhesive joints. Numerical simulations and experiments have been conducted
and it has been shown that temperature compensation techniques can cope in principle with non temperature effects. It is concluded that subtraction approach
represents a promising method for reliable Structural Health Monitoring. Nonetheless the feasibility of a subtraction approach for SHM depends on environmental
conditions.

Clarke, T. ‘Guided wave health monitoring of complex structures', Mechanical Engineering Department, Imperial College London, 2009.

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Structural Health Monitoring (SHM) systems are widely regarded as capable of significantly reducing inspection costs of safety-critical structures in industries such as aerospace, nuclear, and oil and gas, among others. Successful SHM systems can be considered those which combine good sensitivity to defects, preferably with the capability of localization and identification, with a low sensor density. Techniques based on sparse arrays of sensors which generate and receive guided waves are among the most promising candidates. Guided waves propagate over large distances and certain modes have the ability to transmit through a variety of structural features leading to a relatively small number of distributed sensors being able to cover the structure.

In complex structures, which contain high densities of structural elements, the timetraces obtained are often too complex to be directly interpreted due to the large number of overlapping reflections. In this case, the Baseline Subtraction technique becomes attractive. In this method a current signal from the structure is subtracted from a signal which has been acquired during the initial stages of operation of the structure. This eliminates the need for interpretation of the complex raw time signal and any defects will be clearly seen provided the amplitude of the residual signal obtained after subtraction of the baseline signal is sufficiently low when the structure is undamaged. However, it is well known that environmental effects such as stress, ambient temperature variations and liquid loading affect the velocity of guided waves; this modifies the time-traces and leads to high levels of residual signal if a single baseline, taken under different conditions, is used. Of these effects, temperature variations are the most commonly encountered and are critical since they affect not only the wave propagation but also the response of transducers.

The present work aims to demonstrate the potential of guided wave health monitoring of large area complex structures. It starts with a general literature review on inspection and monitoring of large area structures, in which the advantages and disadvantages of this technique compared to other well-established SHM techniques are presented. The design and behaviour of two different temperature-stable transducers generating high A0 or S0 mode purity in the sub-200kHz frequency region are described. The efficiency of different signal processing techniques aimed at reducing or eliminating the influence of temperature on wave propagation is evaluated and a temperature compensation signal processing strategy is proposed. Finally, a large metallic structure is used to demonstrate a sparse-array SHM system based on this signal processing strategy, and imaging algorithms are used to combine the information from a large number of sensor combinations, ultimately leading to the localization of defects artificially introduced in the structure.

Connolly, G. D. 'Modelling of the propagation of ultrasound through austenitic steel welds', Mechanical Engineering Department, Imperial College London, 2009.

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In the nuclear power and chemical industries, austenitic steel is often used in the construction of pipework and pressure vessels due to resistance to corrosion and high
fracture toughness. A completed weld may host a variety of defects including porosity, slag and cracks. Under the stress of operation, defects may propagate and
mechanical failure may have severe consequences. Thus detection either during manufacture or service is of critical importance.

Currently, inspection and evaluation of austenitic materials using ultrasonic methods is difficult due to material inhomogeneity and anisotropy, causing significant
scattering and beam-steering. Radiography is used instead. A reliable ultrasonic inspection method would potentially replace radiography and reduce inspection time
and costs, improving plant availability.

The aim of this thesis is to develop a forward model to simulate the propagation of ultrasonic waves through V-welds whose orientations of elastic constants are
determined using definitions from a previously published and well-established model. The behaviour of bulk wave propagation in free space is presented and a ray-tracing
model is constructed. The predicted interaction of bulk waves at an interface is validated against the results of finite element simulations.
Synthetically focused imaging algorithms are presented and used to build reconstructions of the weld interior in order to locate and size defects. These images
are formed using data from both ray-tracing models and finite element simulations. It is shown that knowledge of the ray paths, via the simulation model, can enable
significant improvement of the array images of defects. 

Additionally, a study investigating the transformation of space via a novel process  known as "Fermat mapping" is presented. In this approach, geometry of the real space
is mapped to a Fermat space such that the material becomes uniformly isotropic and homogeneous, unique to a specified point source or receiver. An application of the
transformation is discussed.

Corcoran, J. 'Creep Monitoring Using Permanently Installed Potential Drop Sensors', Mechanical Engineering Department, Imperial College London, 2015

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Creep is the primary life limiting mechanism of static high temperature, high pressure power station components. Creep state evaluation is currently achieved by surface inspection of microstructure during infrequent outages; a methodology which is laborious, time consuming and considered inadequate. The objective of this work is to develop a monitoring technique that is capable of on-load creep damage monitoring. A continuous update of component integrity will enable better informed, targeted inspections and outage maintenance providing increased power generation availability.A low-frequency, permanently installed potential drop system has been previously developed and will be the focus of this thesis. The use of a quasi-DC inspection frequency suppresses the influence of the electromagnetic skin effect that would otherwise undermine the stability of the measurement in the ferromagnetic materials of interest; the use of even low frequency measurements allows phase sensitive detection and greatly enhanced noise performance.By permanently installing the electrodes to the surface of the component the resistance measurement is sensitive to strain. A resistance - strain inversion is derived and validated experimentally; the use of the potential drop sensor as a robust, high temperature strain gauge is therefore demonstrated.The strain rate of a component is known to be an expression of the creep state of the component. This concept was adopted to develop an interpretive framework for inferring the creep state of a component. It is possible to monitor the accumulation of creep damage through the symptomatic relative increase in strain rate. By taking the ratio of two orthogonal strain measurements, instability and drift common to both measurements can be effectively eliminated; an important attribute considering the necessity to monitor very low strain rates over decades in time in a harsh environment.A preliminary study of using the potential drop technique for monitoring creep damage at a weld has been conducted. Welds provide a site for preferential creep damage accumulation and therefore will frequently be the life limiting feature of power station components. The potential drop technique will be sensitive to both the localised strain that is understood to act as precursor to creep damage at a weld and also the initiation and growth of a crack.Through the course of this project, two site trials have been conducted in power stations. A measurement system and high temperature hardware that is suitable for the power station environment has been developed. The focus of this thesis is the effective transfer of the technique to industry; the realisation of this is detailed in the final chapter.

Dalton, R., ‘Propagation of LAMB waves in metallic aircraft fuselage structure’, Mechanical Engineering Department, Imperial College London, 2000

Dalton, R (pdf)

Owing to their unique potential for long-range, in-plane propagation through thin plates, guided waves seen to offer an obvious solution in the development of an onboard structural health-monitoring (SHM) system, to provide assurance of structural integrity for ageing metallic aircraft. This thesis evaluates the potential of guided waves for this application, by focusing on their propagation through the fuselage structure.

The fuselage structure of all semi-monocoque aircraft is characterised by a number of simplified structural features and the acoustic properties of constituent materials are measured, enabling dispersion curves of the associated waveguide systems to be plotted. Dispersion predictions, supported by experiments, are used to identify the most promising modes in each of the structural features. For joints where dispersion curves cannot adequately describe the mode of interaction with the discontinuous geometry, dynamic finite element modelling is employed and model predictions are also validated by experiment.

The investigation found that the simple, painted and tapering skin presents little problem for long-range propagation, providing dispersion is avoided. The application of sealant layers, however, causes severe damping of virtually all modes, except at very low frequencies. The transmission efficiency of modes across joints was found to be critically dependent upon the behaviour of 'carrier modes' in the overlap region. For narrow joints, including aircraft stringers, the sensitivity of carrier-mode interference to joint parameters effectively prevented propagation across a succession of joints, though excellent transmission across a single joint was demonstrated. Active SHM systems, requiring long-range propagation, are therefore not considered viable, owing to the high density of structural features. A brief study of the modal characteristics of acoustic emission employing numerical predictions and experiments, utilising simulated AE signals, found that AE signals are not impeded by twinned carrier-mode interference, owing to their low frequency. As a result of this work possible improvement of current AE defect location methods is suggested.

Davies, J. 'Inspection Of Pipes Using Low Frequency Focused Guided Waves', Mechanical Engineering Department, Imperial College London, 2008

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In recent years there have been several examples of the successful commercial exploitation of guided acoustic waves for long range inspection of large structures. One such successful application of guided wave has been the screening of long sections of pipework. This application employs guided waves essentially as a screening tool, and hence research has been driven by the need to maximize coverage which has necessarily been achieved at the expense of sensitivity. However, there is a clear need for a high sensitivity guided wave technique that can perform accurate defect sizing while still being deployed some distance away from the inspected region. Such a system will be utilized for inspecting critical regions of a structure to which direct access, and hence inspection by conventional local NDE techniques, is not possible.

The aim of the work presented here is to develop a pipe inspection tool that is capable of detecting, locating and then sizing defects that may be present in the pipe section under test. The work is primarily directed towards quantifying any improvements that can be made to the current commercially available system by using synthetically focused imaging algorithms. All of the work presented here uses torsional type wave modes for inspection.

It is found that a version of the Common Source Method of imaging which has been modified to deal with cylindrical pipe geometry works well for imaging the reflectors in the pipe. The system has been rigorously tested using data from 3D finite element model predictions. The performance of the system is established in terms of detection sensitivity to circumferential cracks, resolution and robustness towards set up errors. It is found that cracks of circumferential extent larger than around 1.5λ_SH can be directly sized from the image. This result is valid for any inspection frequency, axial defect location and pipe size. Laboratory validation experiments give results which show excellent agreement with the finite element predictions. Amplitude gains of around 18 dB over an unfocused system have been observed experimentally in 8 inch pipe.

Demma, A., 'The interaction of guided waves with discontinuities in structures', Mechanical Engineering Department, Imperial College London, 2003

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The thesis investigates the effect of geometrical discontinuities in plates and pipes on the propagation of guided waves. The discontinuities studied are both defects in the structure and features of the structure.

Firstly the scattering of the SH0 mode from discontinuities in the geometry of a plate is presented. Both Finite Element and modal decomposition methods have been used to study the reflection and transmission characteristics from a thickness step in a plate, very good agreement being obtained. A method to approximate the reflection from rectangular notches by superimposing the reflection from a step down (start of the notch) and a step up (end of the notch) is proposed. The limits of this method in approximating crack-like defects are discussed.
The second part of this thesis reports an experimental and numerical (Finite Element method) study of the reflection of the T(0,1) mode from defects in pipes. Both crack-like defects with zero axial extent and notches with varying axial extents are considered in this study. An interpretation of the crack-like reflection coefficients in terms of the wavenumber-defect size product is proposed.

The third part focuses on the reflection from notches in pipes. A systematic numerical analysis (Finite Element) of the effect of pipe size, defect size, guided wave mode and frequency on the reflection from notches is presented. A generalization of the results obtained for different test configurations is proposed. As a result, maps of reflection coefficient depending on the circumferential extent and depth of the defect are shown for a particular pipe size and an approximate formula for extrapolation to other pipe sizes is proposed. This study addresses problems encountered in practical testing and offers guidance for the interpretation of measurements.

The last part of the thesis studies guided wave propagation in pipes with bends. The dispersion curves for toroidal structures are derived using a Finite Element modal solution and the main characteristics of the modes of a curved pipe are described. A series of pipes with different bend radii were investigated experimentally and with numerical simulation (Finite Element). The influence of both bend radius and bend length on the transmission of the incident wave is shown. The modes travelling after the interaction with a bend are identified.

Dewen, P. N. ‘The non-destructive evaluation of the cohesive properties of adhesively bonded joints’, Mechanical Engineering Department, Imperial College London, 1991

The use of adhesive bonding in structural applications has been limited by the lack of robust, quantitative, nondestructive evaluation (NDE) methods for the strength of the joint. This thesis describes the progress made towards developing an NDE technique for the determination of the cohesive properties (those of the adhesive layer) of bonded joints.

The techniques considered in this thesis are all ultrasonic, as the work of other authors has shown such techniques to have the greatest potential for solving this problem. The dynamic properties of the joint (through-thickness resonances and leaky Lamb modes) are considered and are found to be heavily influenced by the elastic properties of the adherends, which in aerospace applications are typically at least one order of magnitude thicker than the bondline. In contrast, the measurement of the reflection coefficient from the adherend/adhesive boundary and the transit time of an ultrasonic pulse through the adhesive layer are found not to be dominated by the adherends, and from these measurements the cohesive properties of longitudinal bulk wave velocity and thickness of the adhesive layer can be calculated. Attention is focussed on joints with aluminium adherends, but consideration is also given to joints between fibre composite substrates. It is found that for these joints, reflection coefficient measurement is not possible, but the transit time can be used to calculate the bulk wave velocity if the bondline thickness is known.

A scanning procedure based on the reflection coefficient/transit time technique and implementible in a standard C-scan tank is developed and tested on a number of aluminium-epoxy-aluminium specimens in which the properties of the adhesive and adherends are varied. The results show that this technique can determine the adhesive bulk wave velocity to within 6% of the nominal value, and its thickness to within micrometer accuracy. Such a technique represents a significant advance in NDE technology, and could easily be developed for the quality control of bonded joints.

Diligent, O. (pdf)Diligent, O. (pdf)‌

Diligent, O., 'Interaction between fundamental lamb modes and defects in plates', Mechanical Engineering Department, Imperial College London, 2003

The thesis is motivated by the goal of developing techniques for inspecting plate structures using ultrasonic Lamb waves. Many engineering structures are composed of large areas of flat or curved plates, including, for example, oil and chemical storage tanks. The inspection of a plate by Lamb waves requires the propagation of multiple signals in order to achieve full coverage. This can be achieved using a plate tester device which has been developed separately. The work presented in this thesis investigates the interaction of the fundamental Lamb modes with a free edge of a plate and with different types of defects with very simple geometries. This is studied in order to bring understanding of the detection capabilities of the inspection system, as well as to aid the signal processing procedures used by the system. Finite Element, analytical and experimental studies are compared.

The reflection of Lamb waves when the fundamental antisymmetric Lamb mode (A0) or the fundamental symmetric Lamb mode (S0) is incident at the free end of a plate is studied, in order to identify the extent to which the generation of non-propagating modes influences the field local to the end of the plate. The results of this work are important if the pate tester device is placed close to a defect, or close to the edge of a plate, because the non-propagating modes could then interfere with propagating modes and pollute the signal processing. Two frequencies are investigated. A simple case is a frequency below the second anti-symmetric mode cut off frequency, where there is only one anti-symmetric mode (A0 mode). A second and more complex case is above this cut-off frequency when there are more than one anti-symmetric mode. These two cases show that there is some additional motion due to the non-propagating modes. It is also shown, in contrast, that no such additional motion happens in the case when the fundamental symmetric mode S0 is incident at the end of the plate.

The interaction of the S0 Lamb mode with circular defects is investigated. The low frequency mode S0 is the most attractive of the two fundamental modes for Non Destructive Evaluation (NDE) because it has low dispersion (the velocity is approximately constant for low frequencies), it has a high group velocity, it is equally sensitive to defects at any depth in the plate and, if the plate is immersed in a fluid medium, the attenuation due to leakage is very small. Two types of generation and two types of defect are studied. First the S0 Lamb mode is excited by a plane wave and interacts with a circular hole through the full-thickness of the plate. Second, the S0 mode is still excited by a plane wave but interacts with a part-depth circular hole. These two studies give the first understanding of the reflection behavior, showing the mode conversion at the defect, the beam spreading of the reflected wave and the creation of circumferential waves that propagate around the hole. Finally the S0 mode is excited by a small circular source and interacts with the same defects. This is of particular interest because it gives information which is directly applicable to what takes place with the plate tester device.

Drinkwater, B. ‘The use of dry coupling in ultrasonic non-destructive testing’, Mechanical Engineering Department, Imperial College London, 1995

Standard ultrasonic non-destructive testing techniques are used in a wide variety of applications, but require a liquid of gel to couple the transducer to the test structure. The need for a liquid couplant is often inconvenient and precludes the use of ultrasonic testing in cases where the couplant would cause unacceptable contamination of the test structure.

IN order to overcome these problems an alternative approach has been taken in which the transducer is coupled to the test structure via a soft solid (i.e. a soft polymer or rubber). In this system the soft rubber conforms to the surface of the test structure allowing the transmission of ultrasonic energy without the need for coupling liquid. The soft rubber can be made in the form of a tyre to allow fast scanning of large areas. This thesis describes the design of dry coupled probes and discusses the scientific principles on which it is based.

A model of the acoustic pressure field generated by a transducer in such a probe and the interaction of this field with the test structure is described to allow the best choice of transducer to be made. Good agreement between experiment and theory is shown. Also an experimental and theoretical study of the transmission of ultrasonic energy across solid-solid interfaces is described. Reflection coefficients from solid-rubber interfaces have been measured experimentally. A numerical model of the solid-solid contact has been used to predict the contact geometry and a spring model has then been used to predict the reflection coefficient. In this way the reflection coefficient from an imperfect solid-solid interface can be predicted if the topography of the surfaces and material properties are known. Good agreement was found between measured and predicted reflection coefficients.

The design of both static probes for point measurements and wheel probes for scanning is discussed and results obtained using these devices on a variety of test structures are presented. These results show that dry coupled probes can produce results similar to immersion testing on many samples making them generally applicable NDT tools.

Drozdz, M. 'Efficient Finite Element Modelling Of Ultrasound Waves in Elastic Media', Mechanical Engineering Department, Imperial College London, 2008

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The aim of the work presented in this thesis is to provide tools to extend modelling capacities and improve quality and reliability of bulk and guided wave propagation models using commercially available finite element (FE) packages.

During the development process of NDT inspection techniques, the knowledge of the interaction of waves with defects is key to the achievement of robust and efficient techniques as well as identifying potential weaknesses. The reflection of ultrasound from cracks and notches of simple geometry and orientation is already well understood, but there are few results for more complex cases. A discrete approach is needed to model how the waves interact with discontinuities, including structural features, cracks, corrosion or other forms of defects. FE methods have been used to model a wide range of bulk and guided waves problems and have successfully provided important information about wave interaction with discontinuities. In these studies, defects were strongly simplified. One reason for this is that initial work is bound to focus on the simplest cases, but many modellers are ready to go on to more complex problems. The reason that so little of that is happening is that, despite rapid growth in computer power, many of the more complex realistic problems are still beyond the capacity of the models. The more complex problems require much larger models than the simplified ones, and so have remained out of reach.

This can be changed by using innovative techniques and improving the quality and reliability of modelling by taking the right decisions during the modelling process.

Perfectly matched layers (PML) and absorbing layers using increasing damping (ALID) enabling a reduction in the model geometric size are implemented in commercially available FE packages. Analytical models are developed in order to facilitate the achievement of high computational efficiency. Demonstrator cases highlight the gains achieved by the use of these techniques.

As the choice of mesh density is crucial in defining the resources necessary to solve a model, a study of the influence of meshing parameters for various element types and numerical schemes on the propagation velocity is performed. This provides information helping modellers to reach the right modelling compromises thanks to an improved understanding of the consequences of the decisions made. The accuracy of defect modelling is investigated for a range of situations and modelling strategy. The weight of the choice of the right strategies is demonstrated.

The potential implementation of local mesh refinement in commercially available FE packages is considered and discussed in the context of the choices open to the modellers.

The outcome of the use of the techniques and information presented in this thesis is a significant improvement in FE modelling of waves in elastic media.

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Duxbury, D., 'Calibration and control of advanced ultrasonic array technology', Mechanical Engineering, Imperial College London, 2012

Ultrasonic inspection is the primary method of Non-Destructive Evaluation (NDE) for the detection of planar flaws in engineering components. In recent years phased array technology has been adopted for use in NDE following success in related fields, such as medical and sonar applications. Phased array technology provides increased flexibility relative to single element monolithic transducers and the development of controlling hardware with large numbers of parallel channels has allowed the use of large phased arrays able to focus at long range, and offer increased performance. Full Matrix Capture (FMC) is a method of recording data using a phased array transducer that allows image reconstruction to be performed for any inspection technique than could be deployed using delay laws applied to the transmit voltage pulses applied to the array and receiving amplifiers. FMC technology provides a step change in inspection flexibility, and also provides the opportunity to take advantage of imaging techniques that are not practical to implement using phased arrays in the conventional way. However, existing inspection calibration procedures defined in standards do not allow these benefits to be fully realised. This thesis reports the development of a calibration framework designed for FMC based inspection for both rigid and conformable wedge mounted arrays. A large part of this work has been the development of acceptance limits on transducer performance variations. The developments of these limits have required a significant amount of modelling work, often using a Monte Carlo approach. To accommodate this, modelling tools have been developed to investigate the effect of array element directivity, sensitivity, and relative phase on system performance. For conformable phased arrays the effect of surface profile measurement accuracies has also been assessed. The developed calibration framework includes the tools necessary to monitor transducer performance throughout an inspection, with minimum impact on inspection duration. A means of calibrating imaging tools against known reflectors, in accordance with established industrial practice, has also been produced.

Egerton S. J., ‘Ultrasonic inspection of highly attenuating media’, Mechanical Engineering Department, Imperial College London, 2018

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The aim of the project was to improve the ultrasonic array inspection of highdensity polyethylene (HDPE) heat-fused pipe joints of cooling water pipework that is installed in EDF Energy’s nuclear power stations. Whereas ultrasound array inspection is now established for safety-critical metal components, HDPE poses a hugely challenging problem, that the ultrasound waves are heavily attenuated by the material. This impacts multiple aspects of the inspection and of the modelling that is needed to design and qualify inspection. The thesis reports a range of research that was needed to overcome this challenge.

The work of the thesis has:

• obtained accurate acoustic properties of HDPE that are necessary for improved simulated or real ultrasonic array imaging of HDPE pipe joints

• developed a simulation technique for representing ultrasound in such inspections that has both high accuracy and efficiency

• extended analytical analysis of ultrasound scattering from cylindrical voids from elastic media to general, attenuative media

• used the Huygens-Fresnel principle to represent ultrasound scattering from volumetric and planar voids, to image sub-wavelength features of these defects in an ideal circular array setup, and to image angled ultrasonic array nondestructive evaluation (NDE) of potential defects occurring in HDPE pipe joints

• devised an automated and antidispersive system for reducing coherent and incoherent noise in waveforms with an isolated wave reflection signal

• produced an imaging and analysis method for ultrasonic array NDE that can represent defects in a refractive, reflective, and scattering environment in attenuating media, which is applied to data from the above developed simulation technique

• applied much of the above imaging and analysis method to defects machined into HDPE pipe material, with an experimental ultrasonic array controller, yet with an array of limited suitability

• specified parameters for ultrasonic arrays and a water-filled wedge, which are optimum for HDPE pipe joint inspection, and have been designed and built by Imasonic SAS, France, for research use at Imperial College London

Evans, M. J. ‘The use of diffuse field measurements for acoustic emission’, Mechanical Engineering Department, Imperial College London, 1997

Evans, M. J (pdf)

An understanding of the behaviour of diffuse fields in solid structures would greatly enhance the accuracy and applicability of Acoustic Emission (AE) measurements. The diffuse field approximation is a method whereby a complex wave field can be represented statistically in a very simple manner. However, there are certain conditions which must be satisfied before these approximations are valid. The aims of this thesis are to determine whether the necessary conditions are met by the wave fields generated by acoustic emission sources in real structures, to develop signal processing techniques to take advantage of diffuse field approximations, and to demonstrate the benefits which can be gained by treating AE signals in this manner. In practical applications the ultrasonic source would be an AE event, such as an extending crack, but the for the purpose of this project a simulated source was used instead to generate a pseudo-AE signal which was both repeatable and controllable. A measurement system consisting of conical piezoelectric transducers has been developed and calibrated.

Variable transducer coupling has been highlighted as a major shortcoming of standard contact transduction methods. The unpredictable nature of the effect of coupling on the transducer response causes uncertainties as to the amplitude and frequency content of the incident signal. A novel method has been developed to measure the transducer coupling independently which shows great promise for improving the repeatability of AE measurements using contact transducers.

Experiments were carried out using this equipment on aluminium plate structures to determine whether the field generated was diffuse. The size, geometry and damping were systematically varied, thus demonstrating required conditions for a diffuse field to be sustained. Results have shown that diffuse fields are readily sustained in aluminium plate structures in the absence of additional damping; however, bolted and adhesively bonded structures are unlikely to behave diffusely due to the damping introduced by the joint.

Fan, Z. 'Applications Of Guided Wave Propagation On Waveguides With Irregular Cross-Section',Mechanical Engineering Department, Imperial College London, 2010

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Guided waves are interesting for Non-destructive Testing (NDT) since they offer the potential for rapid inspections of a large variety of structures. Analytical methods are well known for predicting properties of guided waves such as mode shapes and dispersion curves on regular geometries, e.g. plain plates or cylindrical structures. However these methods cannot be used to study guided wave propagation in waveguides having irregular cross-sectional geometries, such as railway lines, T-shape beams or stiffened plates. This thesis applies and develops a Semi-Analytical Finite Element (SAFE) method, which uses finite elements to represent the cross-section of the waveguide and a harmonic description along the propagation direction, to investigate the modal properties of structures with irregular cross-section. Two attractive applications have been investigated with the SAFE method, and the results are encouraging.

The first application relates to fluid characterization. Guided torsional waves in a bar with a non-circular cross-section have been exploited by previous researchers to measure the density of fluids. However, due to the complexity of the wave behavior in the non-circular cross-sectional shape, the previous theory can only provide an approximate prediction; thus the accuracy of the measurement has been compromised. The SAFE method is developed to model accurately the propagation velocity and leakage of guided waves along an immersed waveguide with arbitrary non-circular cross-section. An accurate inverse model is then provided to measure the density of the fluid by measuring the change of the torsional wave speed. The model also enables the optimization of the dipstick sensor by changing the material of the dipstick and the geometry of the cross-section. Experimental results obtained with a rectangular bar in a range of fluids show very good agreement with the theoretical predictions.

The second application relates to the inspection of large areas of complex structures. An experimental observation on a large welded plate found that the weld can concentrate and guide the energy of a guided wave traveling along the direction of the weld. This is attractive for NDE since it offers the potential to quickly inspect for defects such as cracking or corrosion along long lengths of welds. The SAFE method is applied to provide a modal study of the elastic waves which are guided by the welded joint in a plate. This brings understanding to the compression wave which was previously observed in the experiment. However, during the study, a shear weld-guided mode, which is non-leaky and almost non-dispersive has also been discovered. Its characteristics are particularly attractive for NDT, so this is a significant new finding. The properties for both the compression and the shear mode are discussed and compared, and the physical reason for the energy trapping phenomena is explained. Experiments have been undertaken to validate the existence of the shear weld-guided mode and the accuracy of the FE model, showing very good results. The sensitivity of compression and shear weld-guided modes to different types of defects close to the weld is investigated, by both finite element simulations and experiments. Due to similar reasons for energy trapping, the feature guiding phenomena also exists in a wide range of geometries. This thesis finally discusses feature guided waves on lap joints, stiffened plates and interconnected heat exchanger tube plates, and their potential applications.

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Fan, S. 'Elastic Wave Scattering From Randomly Rough Surfaces', Mechanical Enigeering Department, Imperial College London, 2016

Elastic wave scattering from randomly rough surfaces and a smooth surface are essentially different. For ultrasonic nondestructive evaluation (NDE) the scattering from defects with smooth surfaces has been extensively studied, providing fundamental building blocks for the current inspection techniques. However, all realistic surfaces are rough and the roughness exists in two dimensions. It is thus very important to understand the rough surface scattering mechanism, which would give insight for practical inspections. Knowledge of the stochastics of scattering for different rough surfaces would also allow the detectability of candidate rough defects to be anticipated. Hence the main motivation of this thesis is to model and study the effect of surface roughness on the scattering field, with focus on elastic waves. The main content of this thesis can be divided into three contributions.

First of all, an accurate numerical method with high efficiency is developed in the time domain, for computing the scattered waves from obstacles with arbitrary shapes. It offers an exact solution which covers scenarios where approximation based algorithms fail. The method is based on the hybrid idea to combine the finite element (FE) and boundary integral (BI) methods. The new method efficiently couples the FE equations and the boundary integral formulae for solving the transient scattering problems in both near and far fields, which is implemented completely in the time domain. Several numerical examples are demonstrated and sufficiently high accuracy is achieved with different defects. It enables the  possibility for Monte Carlo simulations of the elastic wave scattering from randomly rough surfaces in both 2D and 3D.

The second contribution relates to applying the developed numerical method to evaluate the widely used Kirchhoff approximation (KA) for rough surface scattering. KA is a high-frequency approximation which limits the use of the theory for certain ranges of roughness and incidence/scattering angles. The region of validity for elastic KA is carefully examined for both 1D and 2D random surfaces with Gaussian spectra. Monte Carlo simulations are run and the expected scattering intensity is compared with that calculated by the accurate numerical method. An empirical rule regarding surface parameters and angles is summarized to establish the valid region of both 2D and 3D KA. In addition, it is found that for 3D scattering problems, the rule of validity becomes stricter than that in 2D.

After knowing the region of validity, KA is applied to investigate how the surface roughness affects the statistical properties of scattered waves. An elastodynamic Kirchhoff theory particularly for the statistics of the diffused field is developed with slope approximations for the first time. It provides an analytical expression to rapidly predict the expected angular distribution of the scattering intensity, or the scattering pattern, for different combinations of the incidence/scattering wave  modes. The developed theory is verified by comparison with numerical Monte Carlo simulations, and further validated by the experiment with phased arrays. In particular the derived formulae are utilized to study the effects of the surface roughness on the mode conversion and the 2D roughness caused depolarization, which lead to unique scattering patterns for different wave modes.

Fleming, M. J. 'Far-Field Super Resolution Imaging', Mechanical Engineering Department, Imperial College London, 2008.

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Wave based imaging methods aim to build an accurate reconstruction of the physical properties of an object by recording the scattered field caused by illumination from multiple directions. Classically the minimum distance between the characteristics of the object that can be resolved by an imaging method is limited by the wavelength, λ, of the interrogating field. In order to improve the resolution shorter wavelengths can be propagated; however, due to material absorption, this limits the penetration depth of the wave which consequently reduces the potential imaging range. Any imaging technique which can overcome the resolution limit is of great practical and academic interest and represents the subject of this thesis.

Subwavelength characterisation has become well established in the field of Nearfield Scanning Optical Microscopy which requires part of the probing system to be within λ of the object being illuminated (near field), in order to detect the non-propagating evanescent waves. The super oscillatory properties of the evanescent waves are subsequently used to achieve subwavelength resolution. However, access to the near field of an object is not always feasible and since evanescent waves decay exponentially they cannot be directly detected in the far field (greater than λ from the object).

The aim of this thesis is to define and investigate an imaging strategy that will allow super resolution to be achieved from the far field. Conventional imaging techniques, which are constrained by the resolution limit, neglect the distortion of the scattered field caused by the internal structure of the object. This thesis will show that a more accurate description of the interaction of the incident field with the object, which includes the multiple scattering of evanescent waves, can lead to subwavelength resolution from the far field.

Fletcher-2011-PhD-Thesis

Fletcher, S., 'Guided waves for power plant applications', Department of Mechanical Engineering, Imperial College London 

This study explores the possibility of using the guided wave non-destructive testing technique for power plant applications. Guided waves are already used extensively in the petrochemical industry, however the nature of pipework in a power station has meant that guided waves have not been studied for use in this environment. Power station pipework is more challenging to inspect than petrochemical pipework using guided waves because the pipelines tend to be shorter, and the feature density is much higher, with welds, hangers, supports and bends all contributing to make analysis of results more difficult.

A particular focus of the study was detecting axially aligned defects in pipes, a problem that emerged in the UK coal power station fleet in 2006. Guided waves provided a desirable inspection technique because large volumes of pipework can be screened quickly, this being particularly advantageous due to the high volume of pipework that requires inspection.

Two guided wave approaches to detecting axial cracks in pipes were pursued. Long-range guided waves were initially examined as they are able to examine large quantities of pipework in a short amount of time. Unfortunately, long-range guided waves are sensitive to the change in cross-sectional-area of a pipe, and axially aligned defects produce only a very small change in cross-section. Therefore guided waves were not sensitive enough to detect a critically sized axial crack. The sensitivity of long-range guided waves was improved using a synthetic focusing algorithm, although this was still insufficient to detect a critically sized defect.

The second guided wave approach was to utilise circumferential guided waves to detect axial cracks in pipes. Although many of the advantages of long-range guided waves are lost, using circumferential guided waves is much faster than an alternative manual ultrasonic inspection. The results of circumferential guided wave experiments suggest that they would be capable of detecting a critically sized axial crack in a pipe. 

Besides attempting to detect axial cracks guided waves have been tested on a small number of other power station pipework systems. These systems were tested as a way to examine the viability of using guided waves as a general inspection tool at a power station. Although guided waves are not suitable for every application, there are a good number of potential applications due to the wide variety of pipework systems at a power station.

Fong, K. L. J. ‘A Study Of Curvature Effects On Guided Elastic Waves’, Mechanical Engineering Department, Imperial College London, 2005

Fong, K. L. J (pdf)

Long range guided wave inspection of large engineering structures has been proven to be very effective. However, there are still many aspects of the guided wave behaviour which remain unknown. One of these aspects is the curvature effect which can substantially change the physical properties of the guided wave mode, especially in a leaky system where limiting the extent of energy radiation into the surrounding medium is critical for successful inspection.

This thesis examines the curvature effect on the guided wave properties using a 2-D curved plate system. Both unloaded and loaded cases are investigated systematically. Model studies comprise exact and asymptotic analyses, including investigations of their limits. The curvature effect in an unloaded case is examined by comparing the phase velocity and the displacement mode shapes of fundamental modes between a straight case and curved cases of various curvature radii, at all frequencies. The percentage difference of these properties due to the curvature effect is found to increase exponentially with an increase in radius, and is frequency dependent. This provides a graphical tool to pick the best frequency at which the properties are least affected by the curvature. Results of Finite Element (FE) modelling and experiment prove the validity of the analytical predictions. For the loaded case (leaky case), the analytical solution is substantially more complicated, partly due to the fact that the numerical calculations of the Bessel functions with a complex order are hard to implement. The solutions produce the dispersion relation of phase velocity and attenuation of an embedded curved plate system. The distribution of energy, determining the amount of coupling between the guiding layer and the surrounding medium, can be obtained, and can also be related to the changes of attenuation in a particular mode when the plate is curved. Experimental and FE validations are provided.

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Gajdacsi A., 'High Accuracy Ultrasonic Degradation Monitoring', Mechanical Engineering Department, Imperial College London, 2016

This thesis is concerned with maximising the precision of permanently installed ultrasonic time of flight sensors. Numerous sources of uncertainty affecting the measurement precision were considered and a measurement protocol was suggested to minimise variability. The repeatability that can be achieved with the described measurement protocol was verified in simulations and in laboratory corrosion experiments as well as various other experiments. One of the most significant and complex problems affecting the precision, inner wall surface roughness, was also investigated and a signal processing method was proposed to improve the accuracy of estimated wall thickness loss rates by an order of magnitude compared to standard methods.

It was found that the error associated with temperature effects is the most significant among typical experimental sources of uncertainty (e.g. coherent noise and coupling stability). By implementing temperature compensation, it was shown in laboratory experiments that wall thickness can be estimated with a standard deviation of less than 20 nm when temperature is stable (within 0.1 oC) using the signal processing protocol described in this thesis. In more realistic corrosion experiments, where temperature changes were of the order of 4 oC), it was shown that a wall thickness loss of 1 µm can be detected reliably by applying the same measurement protocol.

Another major issue affecting both accuracy and precision is changing inner wall surface morphology. Ultrasonic wave reflections from rough inner surfaces result in distorted signals. These distortions significantly affect the accuracy of wall thickness estimates. A new signal processing method, Adaptive Cross-Correlation (AXC), was described to mitigate the effects of such distortions. It was shown that AXC reduces measurement errors of wall thickness loss rates by an order of magnitude compared to standard signal processing methods so that mean wall loss can be accurately determined. When wall thickness loss is random and spatially uniform, 90% of wall thickness rates measured using AXC lie within 7:5 _ 18% of the actual slope. This means that with mean corrosion rates of 1 mm/year, the wall thickness estimate with AXC would be of the order of 0.75-1.1 mm/year.

In addition, the feasibility of increasing the accuracy of wall thickness loss rate measurements even further was demonstrated using multiple sensors for measuring a single wall thickness loss rate. It was shown that measurement errors can be decreased to 30% of the variability of a single sensor.

The main findings of this thesis have led to 1) a solid understanding of the numerous factors that affect accuracy and precision of wall thickness loss monitoring, 2) a robust signal acquisition protocol as well as 3) AXC, a post processing technique that improves the monitoring accuracy by an order of magnitude. This will benefit corrosion mitigation around the world, which is estimated to cost a developed nation in excess of 2-5% of its GDP. The presented techniques help to reduce response times to detect industrially actionable corrosion rates of 0.1 mm/year to a few days. They therefore help to minimise the risk of process fluid leakage and increase overall confidence in asset management.

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Galvagni, Andrea. 'Pipeline health monitoring',  Mechanical Engineering Department, Imperial College London, 2014

Worldwide, BP operates many thousand kilometres of pipelines carrying valuable yet toxic and corrosive fluids. The structural integrity of these pipelines is crucial, as any failure may result in environmental damage, economic losses and injuries to personnel. Convention- ally, pipeline integrity is assessed on a time basis. This inherently limits the amount of infor- mation available about its structural health, as any damage which develops in unexpected circumstances or while the pipeline is not being inspected may remain undetected. Such lack of information hinders the reliability of any prognosis and of Risk-Based Inspection and Maintenance strategies, increases the risk of unexpected critical damage development and pipeline failure, and forces the use of costly time-based maintenance, following the safe-life design approach. Conversely, if sufficient information about pipeline integrity were avail- able to produce reliable prognoses, then it would become possible to dramatically reduce the risk of unexpected failures and to utilise cost-efficient condition-based maintenance, which prescribes the replacement of a pipeline only when it is about to suffer critical dam- age and has therefore reached the actual end of its operational life. In this way, pipeline networks would become safer and more reliable while at the same time more productive and less costly. This thesis introduces and demonstrates a Structural Health Monitoring ap- proach that has the potential to fill the integrity information gap and ultimately enable the use of condition-based pipeline maintenance. This approach, embodied by a practical au- tomated pipeline damage detection procedure, complements permanently installed guided wave sensors to create a complete pipeline health monitoring solution. Utilising experimen- tal data from a permanently installed guided wave sensor installed on a purpose-built NPS 8 Schedule 40 pipe loop facility at BP’s Naperville Campus, it is shown that the procedure is very effective at detecting and quantifying actual damage, thereby achieving the intended aim of this thesis.

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Guo, N. Q. ‘The vibration characteristics of piezoelectric discs’, Mechanical Engineering Department, Imperial College London, 1990

Most of the techniques to analyse the vibration characteristics of piezoelectric discs are one dimensional, which assumes that the piezoelectric disc vibrates in the thickness direction only (piston-like motion) and is applicable to discs with either very large diameter to thickness ratio (D/T ratio) or very small D/T ratio. However, it cannot predict other modes of vibration of the piezoelectric disc, which may affect the transducer behaviour in the frequency range of interest, especially for those discs with finite D/T ratios.

Finite element method and modal analysis techniques have been used to predict the vibration characteristics of piezoelectric discs. The modal constant has been employed to evaluate the strength of excitation of the modes which can be excited by applying voltages across the disc.

The finite element study of piezoelectric discs shows that many modes including radial, edge, thickness shear, thickness extensional, and high frequency radial modes are predicted in the frequency range of interest. However, no mode has been predicted having piston-like motion assumed by the one dimensional model. The most strongly excited modes of the discs are the thickness extensional modes, which are in the frequency range of the first through thickness mode predicted by the one dimensional model, and have non-zero mean value of the axial displacement over the surface of the disc, and the number of thickness extensional modes have much larger modal constants than the other modes especially in discs with D/T ratio larger than 5. When the D/T ratio is very large, one single thickness extensional mode which has a very large modal constant occurs and dominates the response, this is analogous to the one dimensional assumption. The finite element model has been validated by the excellent agreement between the predicted and measured electrical impedance responses and by the qualitative agreement between the predicted and measured mode shapes.

Guyott, C.C.H. ‘The Non-destructive testing of adhesively bonded structures’, Mechanical Engineering Department, Imperial College London, 1987

Guyott, C.C.H (pdf)

In spite of the potential advantages, the use of adhesive bonding in primary structure has been limited by the lack of non-destructive testing procedures to guarantee the reliability of the joint. The three main types of defect that are commonly found in adhesive joints have been identified, the first type being complete voids, porosity and disbonds in the adhesive layer, the second type of defect being poor cohesive strength i.e. a weak adhesive layer, whilst the final type is low adhesion strength or a weak bond between the adhesive and the adherends. At present there is only one commercially available instrument, the Fokker Bond Tester Mk II, that attempts to predict the cohesive strength of a joint. However, an investigation into the sensitivity of the instrument has shown that it is not able to detect changes in adhesive modulus and thickness, and hence cohesive strength, unless the adhesive layer has either much lower modulus or a significantly higher thickness than is commonly employed in high strength applications. There is therefore a need for the development of improved testing techniques. The technique of ultrasonic spectroscopy has been thoroughly investigated and used to measure the resonant frequencies of plain plates and joints. A model was developed to predict the resonant frequencies and mode shapes of plain plates and joints, the predicted values showing excellent agreement with the measured resonant frequencies over a wide range of adhesive properties. The tests reported here also show that measurements of the resonant frequencies of adhesive joints obtained using ultrasonic spectroscopy can be used to detect changes in adhesive thickness and modulus, accuracies of approximately 10% in thickness and 20% in modulus being obtained in joints typical of those used in primary structure. Consequently, it has been demonstrated that ultrasonic spectroscopy can be used to monitor the cohesive properties of a joint, a change from the normal values indicating a fault in the process control and a likely reduction in the cohesive strength of the joint. Since there is no method suitable for the non-destructive detection of poor adhesion strength this problem is currently overcome by careful control of the surface preparation procedures. Further work is now required to develop a satisfactory method of testing for poor adhesion strength once the joint has been manufactured.

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Haith, M. 'Radiographic Imaging of Subsea Pipelines', Mechanical Engineering Department, Imperial College London, 2016

Subsea pipelines are increasingly being used both around offshore drilling facilities and for long distance oil and gas transport. Accidents can have devastating environmental and economic impact, amplifying the need for accurate, reliable detection and characterisation of pipeline defects. Inspection of these pipelines for corrosion and other defects is crucial for safe operation. Radiography holds a significant advantage over many other inspection methods in that it does not require surface preparation or insulation removal. 
Subsea pipeline radiography is a relatively new technique, and underwater conditions are not covered by radiographic standards. Water can have a significant impact on a radiographic image and access is very difficult, meaning standardised above-water methods may not be applicable. This is particularly the case for defect characterisation; standard methods often call for  calibration objects to be included in the setup, which can be a very complex operation in subsea conditions. There is also a lack of experimental data for research, due to the difficulty  and high costs associated with subsea radiography. Simulation is one of the key ways of assessing inspection problems, however radiographic simulation models have not been  alidated for subsea inspections.
This thesis addresses the two problems of accurate subsea simulation and alternative defect characterisation methods. Firstly the accuracy of a radiographic simulation model applied to subsea pipeline inspections is investigated. Experimental measurements of a sample in a water tank are used to adjust the simulation, with the aim of matching image quality  parameters - such as signal-to-noise ratio and contrast. The simulation has been partially matched to experiment, with some differences found in contrast to-noise ratio. Possible c causes of the differences are analysed, with the most likely cause found to be detector backscatter and additional scatter from out-of-setup objects within the experimental exposure bay.
The simulation model is then used to provide data for development and testing of a defect characterisation method. The method relies on knowledge of the setup geometry and use of multiple images, and does not require calibration objects to be included in the setup. It is specifically aimed at use in situations where access is difficult such as in subsea pipeline  inspections. The method is tested on simulated and experimental flat bottomed hole defects and simulated corrosion patch defects. Results demonstrate a good, consistent ability to calculate lateral and axial defect dimensions. Defect thickness calculations are more difficult and as such errors are more significant. However, errors in thickness are due to overestimation, meaning the calculation could be used to place a maximum limit on potential defect size rather than as an actual estimate of the thickness.

Heinlein, S. ‘Structural health monitoring of pipes using permanently installed guided wave sensors’, Mechanical Engineering Department, Imperial College London, 2019

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Low frequency ultrasonic torsional T(0,1) guided waves allow for the full volume of tens of meters of a pipe to be inspected from a single measurement position. Currently, these inspections are primarily carried out with detachable transducer rings and data collected is evaluated manually by the operator. Permanently installed guided wave sensors have been introduced by Guided Ultrasonics Ltd. in 2005 and since have been deployed on thousands of pipes. Measurements collected with these sensors are, in the current framework, evaluated in the same manual fashion as those obtained from detachable transducer rings. The contents of this thesis focus on the development and evaluation of a novel SHM procedure for guided waves in pipes utilising existing permanently attached transducer ring technology. This work can be subdivided into four main fields of investigation. Firstly, a blind trial was conducted in order to test the sensitivity of a structural health monitoring (SHM) algorithm based on independent component analysis. It was found that compared to standard one-off inspections an improvement of approximately a factor of 5 could be obtained. Defects introduced on straight sections of a test pipe either before or after a 1.5D bend were identified with cross-sectional area losses between 0.5% and 1.5%. The sensitivity of the SHM algorithm to a defect located on a pipe bend was found to be significantly lower with a 3.5% cross-sectional area loss at the time of first detection. The reduced sensitivity to the defect located on the pipe bend was investigated in the second part of this thesis utilising finite element (FE) simulations. It was shown that the sensitivity of guided wave inspections using the T(0,1) mode is highly dependent on the location of a defect both in circumferential and axial position, as well as the size of the bend itself. This sensitivity is proportional to the distribution of the squared von Mises stress across the structure. The results of this study allowed for the conclusion to be made that the reduced sensitivity during the blind trial was caused by the geometry of the inspected structure and the propagation of the T(0,1) wave mode rather than a shortcoming of the SHM algorithm. The third main part of this thesis investigates the potential of using defect reflections obtained from FE simulations in combination with real measurements of a structure in a constant state in order to estimate the sensitivity of an installed SHM system. The validity of this method was verified by the use of measurements obtained from the blind trial setup. The growth patterns of the defects introduced into the pipe setup were replicated using FE simulations and superimposed onto measurements of the trial pipe in its baseline state. The applied SHM algorithm was found to have the same sensitivity to these synthetic datasets as to those containing the real defect growth. The final part of this thesis discusses a novel temperature compensation approach developed for application to results obtained from the SHM procedure. It was found that, even if a stretch based temperature compensation algorithm is applied to guided wave measurements, temperature related variations in the coherent noise floor of the inspected measurements as well as frequency response changes in the signals transmitted by the transducer ring will continue to be present. A newly developed compensation procedure allows for these variations to be significantly reduced enabling lower rates of false calls as well as an improved probability of detection for small defects.

Herdovics, B. 'Structural Health Monitoring with Torsional Guided wave EMATs', Mechanical Engineering Department, 2018

Non-destructive tests are performed regularly to asses a component's fitness for service. Guided wave ultrasound inspection is a well-established method for non-destructive testing of structures. As the propagation of guided waves can cover larger areas, fewer measurements are required to inspect large structures. While the sensitivity is lower, this enables rapid screening of structures for large defects. Piezoelectric guided wave transducers are widely available, but they require surface preparation and bonding. There is a concern, that adhesive property changes, especially when exposed to repeated temperature changes, can result in output signal variability. The highest possible stability is required for improved sensitivity, therefore the aim of this project is to investigate EMATs, which are non-contact and might be more stable. The EMAT's excitation mechanism is contactless, therefore it can potentially offer a good alternative for the bonded piezoelectric transducers. The thesis presents the design, simulation and testing of a torsional guided wave EMAT, which excites the fundamental torsional guided wave mode. A high temperature version of the transducer was designed, and signals were recorded at temperatures above 200°C. The benefit of the permanently installed transducers is that they can deliver more repeatable measurement data, as uncertainties associated with the installation process are more likely to remain constant. However, temperature variations affect the recorded signal, and they need to be compensated for. Previous work has focused on the compensation of propagation speed in ultrasonic signals. This thesis investigates the benefit of additionally compensating the transducer induced waveform changes. By adding the extra compensation term, significant improvement is reached at the location of pipe features. Structural Health Monitoring (SHM) systems rely on stable transducers to guarantee the required sensitivity for damage detection. The stability of the EMAT prototype was evaluated during a 1.5-year long test where the transducer and the pipe was exposed to repeated temperature variations. The results show good performance (the changes at the coherent noise level are less, than 0.8% of the signal amplitude), further improvements to transducer design that potentially increase the stability near reflectors are suggested. These can form the basis for future work.

Hesse, D. 'Rail Inspection Using Ultrasonic Surface Waves', Mechanical Engineering Department, Imperial College London, 2007

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The detection of critical surface cracks in the railhead is a major challenge for the railway industry. Conventional inspection methods have proven not to be reliable enough in this context, therefore the aim of this work was to develop an alternative or complementary screening method. The approach was to scan a pulse-echo probe along the rail which deploys low frequency surface waves.

The results of an initial study on plates with about the thickness of a railhead were encouraging, even though the interference of multiple guided wave modes complicated the signal interpretation.

The properties of the dominant surface wave modes of rails were determined and a mode suitable for inspection purposes was identified. However, it was found that there was a number of similar unwanted modes which would be easily excitable from the railhead surface as well.

In order to ensure correct and reliable signal interpretation it was necessary to suppress such unwanted modes. Two signal processing methods were developed, one involving focussing of a phased array across the railhead, the second mimicking an increased probe length along the rail by a spatial averaging method. The latter was found to be highly effective and robust, rendering the phased array obsolete and thus reducing both system complexity and data acquisition time.

The performance of this method was studied on rail specimens containing artificial and real defects. Areas with defects were reliably distinguished from areas without defects or with tolerable surface damage. Furthermore, deep defects were detected even with multiple smaller ones in front. However, due the complex geometry of real cracks and the interference of reflections from multiple defects, accurate sizing appeared to be very difficult. Nevertheless, the inspection method developed appears suitable for defect detection and could be used to complement existing methods and thus enhance their reliability.

Huthwaite, P. 'Quantitative Imaging With Mechanical Waves', Mechanical Engineering Department, Imperial College London, 2012

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Quantitative imaging complements structural imaging by providing quantitative estimations of subsurface material properties as opposed to the sizes, shapes and positions of scatterers available from structural methods. The ability to reconstruct material properties from a series of wave measurements is extremely valuable in a range of applications as it potentially allows diagnostic technology with superior sensitivity and selectivity. Breast cancer, for example, is stiffer and hence of higher sound velocity than the surrounding tissue, so reconstructing velocity from ultrasonic measurements could allow cancer detection. Using this concept, breast ultrasound tomography has the potential to significantly improve the cost, safety and reliability of breast cancer screening and diagnosis over mammography, the gold-standard. Key to unlocking this potential is the availability of an accurate, fast, robust and high-resolution algorithm to reconstruct wave velocity. This thesis introduces HARBUT, the Hybrid Algorithm for Robust Breast Ultrasound Tomography, a new imaging approach combining the complementary strengths of low resolution bent-ray tomography and high resolution diffraction tomography. HARBUT's theoretical foundation is explained and applied to simulated and experimental, in-vivo, breast ultrasound tomography data, confirming that it generates a step change in image quality over existing techniques, revealing lesions that would not be visible on a mammogram. This thesis also shows how, by combining data from many slices, the out-of-plane resolution can be significantly improved compared to treating each slice independently. HARBUT is applied to alternative problems including guided wave tomography, which aims to quantify the remaining wall thickness of a potentially corroded, inaccessible plate-like structure. Thickness estimates within 1mm for a 10mm nominal thickness plate were demonstrated for both simulated and experimental data. The thesis finally investigates HARBUT's performance with limited view configurations, and introduces VISCIT, the Virtual Image Space Component Iterative Technique, which accounts for the missing data, significantly improving the reconstructed image.

Hutt, T. 'Towards Next Generation Ultrasonic Imaging', Mechanical Engineering Department, Imperial College London, 2011

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Recently the use of ultrasonic arrays for imaging defects in metal components has become economically attractive in Non-Destructive Testing. Given a certain array, the image quality strongly depends on how the measurements are process into an image. The current state-of-the-art imaging algorithm in actual use is delay-and-sum beamforming, which has a resolution capability that is fundamentally limited by the physical approximation used to describe how waves interact with matter.

This thesis explores the practical use of alternative non-linear “super-resolution” imaging algorithms that use more accurate physical models, and can theoretically achieve unlimited resolution. This is made possible by utilising additional sources of information contained within the measurements, in particular the small amplitude multiply scattered signals.

The distribution of information contained in the measurements, and utilised by the imaging algorithms is studied in the context of information capacity of signals. We discover some insights into the limits of imaging which depend on the signal-to-noise ratio.

The accuracy of non-linear imaging algorithms can be strongly dependent on the accuracy of the measurements. Therefore several experiments are performed to assess their performance in practice. The experimental implementation of these methods poses a number of challenges, including removal of the incident field, and compensating for array element directivity.

Super-resolution capability is demonstrated in a highly attenuative medium for the first time. To further improve the image quality we explore the possibility of using mirror reflections. This gives an increase in the effective aperture. We perform simulated and experimental reconstructions of a complex scatterer and find that the completeness of the image is improved.

The mirror interface also allows quantitative speed-of-sound imaging of penetrable scatterers using the HARBUT algorithm. This is tested experimentally for the first time. 

Jaime Alberto Parra Raad, ‘Modelling and Design of Shear Wave EMATs’, Mechanical Engineering Department, Imperial College London, 2022

Jaime Alberto Parra Raad (pdf) 

In this thesis, two physical phenomena were investigated: the mechanical load generation and wavefield radiation of Electro-Magnetic Acoustic Transducers (EMATs). The conceptual results obtained in this work addressed two major research questions: how the transduction of EMATs and the properties of materials affect the EMAT's wavefield generation, and how orthogonal shear waves can be used to detect and characterise surface-breaking defects. The outcome of these two research questions were tested in the design of an orthogonal and co-located coils shear wave EMAT and in the design of a shear wave phased array EMAT.

EMATs, are transducers that by means of electromagnetic forces excite mechanical loads in metallic materials. EMATs are often utilised in the Non-Destructive Evaluation, NDE, of a material to detect and characterise the material anomalies. Compared to their counterpart, Piezoelectric transducers, EMATs have several properties that make them a desirable means of transduction for NDE applications. EMATs are contactless and special designs can tolerate high temperatures. Hence, EMATs do not require direct contact with the inspected material to perform the NDE inspection nor cooling system to keep them operational during a NDE procedure. Nevertheless, the use of EMATs in NDE inspections is not standard procedure in industry. This is because there are concerns over the strength and orientation of the mechanical loads that EMATs produce, and the lack of knowledge about the effects that the magnetic properties of the inspected material have on the wavefield generated with EMATs. To address these concerns, the excitation mechanisms of EMAT transduction were studied on different metallic materials. The three excitation mechanisms -the Lorentz force, the magnetisation force and the magnetostriction force- of a shear wave phased-array EMAT were estimated on paramagnetic, diamagnetic, ferromagnetic and magnetostrictive materials. The results obtained showed that by designing an EMAT to maximise the Lorentz Force mechanism, an EMAT can maximise the strength of the mechanical loads it excites and yield a consistent wavefield in the inspected material.

The interaction of orthogonal and co-located shear waves with surface-breaking defects was also studied in this thesis. The study focused on the detection and characterisation of surface-breaking defects in isotropic and anisotropic materials. To execute the mentioned analysis, the design of an EMAT that can simultaneously excite two co-located shear waves with orthogonal polarisation was proposed. Once the wave interaction with the defects was understood, a methodology that by means of the two shear waves can gauge the inspected material, detect the presence of a defect, characterise the principal shear direction of the inspected material and characterise the height of the surface-breaking defect was presented.

Jarvis, A. J. C., 'Simulation of Ultrasonic Monitoring Data To Improve Corrosion Characterisation within High Temperature Environments', Mechanical Engineering Department, Imperial College London, 2013

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Practical applications which involve analysing how waves scatter from objects with complex shapes span countless scientific and engineering disciplines. Having been the focal point of much research over the past century, many different techniques for simulating such interactions are in common use throughout literature; however there is still an opportunity to improve upon the balance between accuracy and efficiency offered by the most commonly implemented methods. A simulation based on the scalar wave distributed point source method is proposed, exhibiting a large improvement in computational efficiency when compared to the finite element method, and providing greater accuracy than the Kirchhoff approximation by including phenomena such as multiple scattering, surface self-shadowing and edge diffraction. The technique is applied to the problem of simulating how ultrasonic pulses reflect from rough surfaces; the practical application being wall thickness monitoring in high temperature and corrosive environments. Results show that the reflected pulse can take any number of forms, depending on the specific shape of the scattering surface, which can have a dramatic impact on the accuracy of the thickness measurement. Conclusions are drawn about the stability of various time of flight algorithms under conditions of increasing surface roughness. Potential thickness error metrics are also proposed with the aim of estimating measurement uncertainty based on signal shape change. The great efficiency of the simulation technique is further demonstrated by applying it to three dimensional scattering scenarios which would be impossible to carry out using any other method, leading to the proposal of a correction procedure capable of converting results gained in two dimensional geometries to more closely resemble three dimensional results based on the specific transducer and rough surface characteristics. Simulation validation is carried out by comparison to experimental results in both two dimensional and three dimensional scattering scenarios, showing agreement within the experimental error bounds of the shear horizontal ultrasonic waveguide transducers used by the wall thickness sensor. Alternative high temperature structural degradation monitoring applications are also proposed and experimentally verified using an array of waveguide transducers, providing inspection solutions for thermal fatigue crack growth and hydrogen attack.

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Jarvis, R. ‘Current Deflection NDE for Pipe Inspection and Monitoring’, Mechanical Engineering Department, Imperial College London, 2018

The detection of corrosion on insulated and/or coated pipes in the oil and gas industry remains a challenge. Routine inspection, which is commonly achieved with in-line tools known as "pigs", is not possible where there is any risk of the pig becoming stuck. There are thousands of kilometers of pipe worldwide deemed ``unpiggable'' whose safety must be ensured using Non-Destructive Evaluation (NDE) external to the pipe if potentially catastrophic failure is to be avoided.

Many NDE techniques lack sufficient sensitivity due to the coating thickness producing a high standoff distance between the pipe and the sensor and therefore require costly and time-consuming removal of the coating. A method capable of detecting and/or monitoring of defects (e.g. one-third-wall depth corrosion) while leaving the insulation/coating intact would be highly attractive. This thesis documents the development of a technique in which a low-frequency AC current is directly injected into the pipe at distant locations, and perturbations in the magnetic field caused by "current deflection" around defects are measured using solid-state magnetic sensors. Two methods of applying this novel technique were investigated. Firstly, scanning the sensors to measure perturbations in the field and screen for defects, and secondly, permanently installing sensors outside the pipe for Structural Health Monitoring (SHM).

A Finite Element (FE) model has been developed and used to investigate the practical challenges that are faced by the technique and how these may be overcome. The sensitivity of the technique for defect detection by external pipe scanning in a practical scenario has then been evaluated using a model-assisted Probability of Detection (POD) framework that combines the measurements of the signal from an undamaged pipe with synthetic damage profiles and contributions from general corrosion and sensor misalignment. The results indicate that good performance is expected for damage detection by scanning above a typical insulation thickness with just a few amps of injected current.

A similar framework has then been used to evaluate the sensitivity of the technique as an SHM solution which suggests excellent corrosion detection performance with the permanent installation of inexpensive magnetic sensors. The technique has potential advantages over competing methods in both scanning and monitoring modes and there are many opportunities for future development.

Jones, R. 'Use Of Microwaves For The Detection Of Corrosion Under Insulation', Mechanical Engineering Department, Imperial College London, 2012

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Corrosion Under Insulation (CUI) is a widespread problem throughout the oil and gas industry, and is a major cause of pipeline failure. CUI occurs on pipelines fitted with thermal insulation; the insulation itself is protected from the environment by a layer of metallic cladding and sealed to prevent water ingress. This cladding can deteriorate from age or become damaged, allowing the ingress of water into the insulation, which allows corrosion of the external pipe surface to initiate. This corrosion can proceed at an accelerated rate due to the elevated process temperature of the pipe, compromising the integrity of the pipeline. The detection of this type of corrosion is an ongoing problem for the oil and gas industry, as the insulation system conceals the condition of the pipe. Therefore, there is a requirement for a long-range, screening inspection technique which is sensitive to the first ingress of water into the insulation, in order to provide an early warning of areas of a pipeline at risk from CUI.

This thesis describes the development of a new inspection technique which employs guided microwaves as the interrogating signal. Such guided microwaves provide a means of screening the length of a pipeline for wet insulation, by using the structure of a clad and insulated pipeline as a coaxial waveguide to support the propagation of electromagnetic waves. Areas of wet insulation will create impedance discontinuities in the waveguide, causing reflections of the incident microwave signal, allowing the water patches to be detected and located. The performance of such a guided wave inspection system is intrinsically linked to the signal-to-coherent-noise ratio (SCNR) that can be achieved. Therefore, the value of the SCNR that the technique is capable of achieving is of central importance to this thesis. The excitation system is optimised to maximise the SCNR, whilst the effect of typical pipeline features such as bends, pipe supports and the various types of insulation which can be used, are studied to quantify the effect on the SCNR.

A wide variety of methods are employed throughout the development of the guided microwave technique described in this thesis. Theoretical methods are employed in the initial stages to enable the development of a model to describe electromagnetic wave propagation in the large coaxial waveguides formed by pipelines. Numerical simulation techniques are employed when there are too many parameters to study for experimentation to be a viable option, and to study complex problems for which no analytical solution exists. Experiments are conducted in the laboratory using a model setup which employs metallic ducting to represent an insulated pipeline. These experiments are performed to demonstrate the practical feasibility of the technique, and to study pipeline features in a controlled environment. Finally, experiments are performed in the field on a section of real industrial pipeline, in order to validate the accuracy of the model experimental setup in representing conditions which exist on real pipelines.

The main findings of the thesis are that it is possible to excite a guided microwave signal in a large coaxial waveguide with a high SCNR. Experiments revealed that the technique is highly sensitive to the presence of water in the waveguide. Measurements of the effect of different types of insulation demonstrated that rockwool causes a very low attenuation of the microwave signal, while polyurethane foam insulation has a slightly higher attenuation coefficient. An investigation into the effect of bends determined that, whilst significant mode conversion occurs at a bend, the transmission coefficient of the TEM mode is high for typical bend angles and bend radii in small diameter pipes. The behaviour of the signal at a typical pipe support was also examined; the reflection from the support was minimal, whilst the transmission beyond the support remained relatively high. Whilst there is still further work to be done before this technique can be applied in the field, the major aspects of practical implementation that could affect the technique have been investigated here, and the results consistently indicate the feasibility of the technique for long-range screening of insulated pipelines for water.

Juluri, N 'Inspection Of Complex Structures Using Guided Waves', Mechanical Engineering Department, Imperial College London, 2008

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Experimental observation has shown that a compression wave (similar to the Lamb wave S0) travelling along a weld between two plates is strongly guided by the weld and so does not decay as quickly as it would in a plain plate. This phenomenon is attractive for Non Destructive Evaluation (NDE) of welds because it may offer the potential to inspect long lengths of welds using a wave which travels along the weld and is guided by it. In order to understand the phenomenon, studies have been carried out on a variety of structures. Finite element, semi analytical finite element simulations and experimental measurements of waves propagating along these structures have revealed the physics behind the phenomenon.

Studies have been conducted on structures where a medium, in which the wave is slower, is embedded in a medium, in which the wave is faster, and from these studies it is understood that a trapped mode is generated in a medium when it is embedded in a faster medium. It is also understood that this trapped mode decays less than the S0 mode in a plain plate because of its one dimensional propagation, and can potentially be used to inspect long lengths of slower medium from a single location. Numerical and experimental studies proved that the trapped mode exists in the weld and in the region near the weld and therefore it is also possible to inspect defects in the region near the weld or heat affected zones using the trapped mode.

The trapped mode generated in the slower medium decays as it leaks bulk longitudinal and shear waves into the surrounding faster medium. The attenuation of the trapped mode in the slower medium increases as the impedance difference between the slower medium and the surrounding faster medium decreases and it shows zero attenuation at frequencies where the trapped mode is slower than both bulk longitudinal and shear waves.

This thesis discusses the nature of the trapping effect, illustrates the effects, and proposes its potential for practical NDE of welds and other geometric features where a slower medium is embedded in a faster medium.

Khalili, P. 'Alternative techniques for detection of inaccessible pipe corrosion', Mechanical Enigeering Department, Imperial College London, 2018

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Testing for corrosion in the petrochemical industry has always been a significant challenge which takes up a large portion of the operating expenditure. Whereas major advancements have been made for the detection of general corrosion, inspection at inaccessible locations, such as at pipe supports, remains a demanding prospect; this signifies the need for an alternative technique, capable of dealing with various surface conditions encountered when testing at such locations including weld patches, T-joints, surface roughness and coatings. Long range guided waves are commonly used to detect relatively severe defects in plain sections of pipe but are less suited to inspection at supports because the support itself gives significant reflection. The reflection coefficient at the support reduces with frequency so it would be beneficial to test at higher frequencies, which can also improve the sensitivity of the test to smaller, pitting-type defects.

Following the attractive properties of the Higher Order Mode Cluster (HOMC) proposed by Balasubramaniam et al. (IIT Madras), this research starts by investigating the nature of the mode cluster and shows that the features of this method are essentially those of the A1 mode in the high frequency-thickness regime. The study then goes on to investigate the possibility of exciting a single mode Lamb wave with low dispersion at a frequency-thickness of around 20 MHz-mm. Excitation of the A1 mode was considered because of its relationship with HOMC and due to its non-dispersive nature and low surface motion at such frequencythickness products; this makes it attractive for inspection at supports since it will be unaffected by the support itself and also by surface roughness and attenuative coatings.

The thesis then explores the relative ability of different transducer types for single mode excitation in the medium and high frequency-thickness regimes; here the practical feasibility of exciting the A1 mode at around 20 MHz-mm, in spite of its low surface motion, is investigated. Next, a systematic performance analysis of the A1 mode compared to the existing inspection techniques is carried out and, finally the sensitivity of this technique to realistic 3-D pitting-type holes is established. The thesis shows that the A1 mode is an attractive tool for the detection of localized, sharp, severe defects that will be missed by standard, lower frequency guided wave testing.

Michael Leung,  ‘Probabilistic approach to structural integrity assessment of fatigue damage using permanently installed monitoring systems’, Mechanical Engineering Department, Imperial College London, 2020

Michael Leung

This thesis aims to develop strategies for integrating frequent onload data obtained from permanently installed monitoring systems with probabilistic structural integrity methods in order to produce real-time, uncertainty-quantified diagnostics and prognostics for fatigue damage. The proposed strategy is broadly divided into two phases: defect detection, and defect growth monitoring. For the defect detection phase, a framework for evaluating the detection capabilities of PIMS is first proposed. This is essential to qualifying PIMS for industrial applications, and forms the basis of quantifying its value for structural integrity assessment. The framework is then utilised to address the well-recognised compromise between area coverage and sensitivity of PIMS. By combining information on the spatial sensitivity of PIMS and the spatial uncertainty of defect location, the detection capabilities of specific combinations of monitoring systems and components can be quantitatively compared. A novel approach to incorporate measurements from PIMS into structural integrity assessment is subsequently proposed. The ability of PIMS to recursively eliminate the possibility of there being substantial damage in the monitored component is demonstrated, which proves to be an effective way of maintaining confidence in its structural integrity. This framework will therefore help promote the adoption of PIMS for damage detection in suitable engineering applications. For the defect growth monitoring phase, the ability of PIMS to produce accurate rate measurements is exploited to perform remnant life predictions using the Failure Forecast Method (FFM). A statistical analysis comparing the conventional inspection-based approach to the FFM approach is performed, demonstrating the ability of the FFM approach to estimate more accurately the remnant life of the monitored component. A novel method for using the FFM under non-constant amplitude loading conditions is also developed and validated. This enables the use of the FFM in more complex loading conditions, thereby advancing its potential uses in real-life applications.

Long, R. ‘Improvement of ultrasonic apparatus for the routine inspection of concrete’, Mechanical Engineering Department, Imperial College London, 2000

Long, R (pdf)

The most common application of ultrasonic testing in civil engineering is to determine the velocity of sound in concrete, which is related to concrete quality. This thesis addresses some of the limitations of current commercial apparatus used for determining ultrasonic pulse velocity in concrete. The intention is to recommend improvements to enhance the reliability of measurements and make application more convenient.

The velocity of sound in concrete measured by commercial apparatus is known to vary with the path length being tested. Attenuation of sound in concrete, commercial transducer characteristics, and determination of signal transit times have been investigated. From this study, a function has been derived to correct measurement errors.

Commercial equipment is calibrated by coupling the transducers to a reference bar and setting the apparatus display to a time value stamped on the bar. To validate the time value, an experimental and finite element study have been carried out on wave propagation in a finite length of bar. To aid interpretation of data, signal-processing techniques have been investigated that are suitable for the evaluation of wave velocities in dispersive systems. Results suggest that the time value corresponds to a relatively low energy component propagating at the longitudinal bulk wave velocity. Reliable calibration can be achieved when the apparatus recognises the component, which is dependent on the acoustic coupling made by the transducers to the reference bar.
Currently, viscous couplant must be applied between the transducer face and the concrete surface under test to facilitate signal transmission. Consistent coupling is difficult to achieve and couplant application and removal proves time consuming and inconvenient. Alternative coupling has been investigated, one technique that looks promising is rubber coupling. Contact models have been derived to predict the deformations of rubber coupled devices when loaded onto rough surfaces and thereby predict signal transmission. Experiments and predictions suggest that dry rubber coupling of transducers using a hand held device might not be feasible. However, more convenient coupling has been achieved when wetting a prototype rubber coupled membrane device with very little water.

Lowe, M. J. S. ‘Plate waves for NDT of diffusion bonded titanium’, Mechanical Engineering Department, Imperial College London, 1992

Lowe, M. J. S (pdf)

Diffusion bonding, the joining of two surfaces by the diffusion of material across the interface, has the attractions of very high strength and minmal distortion of the components. Recent developments of the diffusion bonding process in the aircraft industry have further exploited the process by the diffusion bonding and superplastic forming of sheets of titanium to create cellular structural components. Along with these developments has been the necessary research into inspection methods for quality control during production. An important inspection problem is the detection of a brittle layer of a phase of the titanium alloy which can occur at the bondline if air is present during bonding.

This thesis presents an evaluation of the potential of using ultrasonic plate waves for the detection of the presence of such a layer. The principle is that differences in the acoustic properties of the layer with respect to the adherends will affect the modal properties of wave propagation along the joint. Thus the presence of the layer could be detected by the measurement of a selected propagating mode.

A theoretical model is developed for the prediction of the modal properties of wave propagation along a layered plate. The model is applicable to plate systems of any numbers of layers of isotropic viscoelastic materials and can describe either free wave propagation or leaky wave propagation, when the plate is assumed to be immersed in a fluid or solid. The model predicts the velocities, frequencies and attenuations of the propagating modes as well as the distributions of displacements and stresses.

The acoustic properties of the brittle phase are measured and the model is used to predict the plate wave properties in good and defective joints. Two approaches are considered, one involving Lamb waves which occupy the full thickness of the joint and the other involving interface waves which travel along the brittle layer. The optimum modes and conditions for testing are identified and their sensitivities are compared with conventional normal incidence testing. It is found that both approaches show some sensitivity in principle to the presence of the layer but it is concluded that in practice it is not likely that either will offer advantages over normal incidence testing.

Ma, J. 'On-line Measurements Of Contents Inside Pipes Using Guided Ultrasonic Waves', Mechanical Engineering Department, Imperial College London, 2007

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There have been extensive demands from industries to determine information about the contents inside pipelines and it would be a great benefit if on-line measurements could be made. Guided ultrasonic wave measurements can potentially fulfil such a purpose since they are non-intrusive and can be carried out from outside of the pipe wall. This thesis investigates the principles and develops new guided wave techniques for two specific applications.

The first application relates to the fluid characterisation inside pipes. A new guided wave technique is developed to measure the acoustic properties (bulk sound velocity and shear viscosity) of fluids inside pipes. It is based on the measurements of the velocity dispersion and attenuation of guided longitudinal modes in the pipe. It allows the fluid properties to be characterised without taking samples out of the pipe and can be employed both when the pipe is completely filled or when the filling is local. In the latter case, the technique is exploited as a pipe 'dipstick' sensor dipped into the fluid to be measured. The dipstick sensor has the advantages that the velocity measurement requires a single pulse echo measurement without the need for knowing the depth of immersion of the pipe into the fluid.

The second application is for sludge and blockages detection in long-range pipelines. Existing techniques have the limitations that the sludge position needs to be known a priori and the area to be inspected needs to be accessible. Two guided wave techniques have been developed which allow the sludge or blockages to be detected remotely without the need to access the specific location where the pipe is blocked, nor to open the pipe. The first technique measures the reflection of guided waves by sludge or blockages which can be used to accurately locate the blocked region; the second technique detects sludge by revealing the changes to the transmitted guided waves propagating in the blocked region or after it. The two techniques complement each other and their combination leads to a reliable sludge or blockage detection. Various types of realistic sludge or blockages have been considered in the study and the practical capabilities of the two techniques have been demonstrated.

Marty, P. ‘Modelling of ultrasonic guided wave field generated by piezoelectric transducers’, Mechanical Engineering Department, Imperial College London, 2002

Marty, P (pdf)

The thesis investigates some aspects of the fundamental science necessary for the development of piezoelectric sensors for use in integral structural inspection systems based on ultrasonic Lamb waves. It is particularly concerned with the analysis of the electromechanical interaction, the process of generation of Lamb wave modes and the design of permanently attached transducers such as PVDF-based interdigital transducers (IDT) or ceramic-based piezoelectric strips.

Interdigital transducers developed for use in smart structures are now at the stage where practical applications on plate and pipe structures are being considered. For such a transducer to be used, it is necessary to understand exactly the electromechanical interaction and the internal scattering phenomena governing their performance. An analytical investigation into the interactions that occur between mechanical fields and electric quantities is presented. This model is developed for a simple transducer design, a single-strip transducer under plane strain conditions. A computer model for predicting the acoustic field generated by a given voltage applied to the transducer and vice-versa is presented. This model is developed on the basis of normal mode theory and perturbation methods, providing flexibility and physical insight. Intermediate calculations as well as final results are validated using the finite element model developed in parallel with this work. Since the analytical model is based on assumptions mainly related to the perturbation methods, these are discussed and limits of the model as well as its eventual extensions are drawn.
The thesis is also concerned with a numerical analysis based on the finite element method. A finite element formulation that includes the piezoelectric or electroelastic effect alongside the dynamic matrix equation of electroelasticity and its reduction to the well-known equation of structural dynamics, based on a strong analogy between electric and elastic variables, is presented. It is shown how these equations were incorporated in an already existing finite element code. In parallel with validation, results are produced to identify several important features that are not taken into account in the analytical model. Results are presented for IDTs and checked against experimental data when measuring displacement field amplitudes using a laser probe.

Milne, K. ‘Studies into the vibro-enhancement of penetrant inspection and the ultrasonic inspection of diffusion bonds , Mechanical Engineering Department, Imperial College London, 2010

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The four years of my Engineering Doctorate were divided between two projects: vibro-enhanced fluorescent penetrant inspection and ultrasonic non-destructive evaluation of titanium diffusion bonds. The project was originally entitled ‘Vibro-Enhanced Non-destructive Evaluation’. Prompted by work previously carried out at Imperial College and Bath University into two

vibration-based techniques - vibro-acoustic modulation and thermosonics - the aim was to look at vibration as a means of enhancing non-destructive evaluation techniques that are conventionally used to detect fatigue cracks in aeroengine components. The premise was that the reliability of conventional methods was limited by the width of the fatigue crack and that vibration could be used to actively open the crack, thereby improving reliability. A literature review of conventional non-destructive evaluation techniques indicated that the reliability of both fluorescent penetrant inspection and ultrasonics was related to crack width, although the relationship between crack width and reliability is different for the two techniques (see Part I,
Section 5.1. of this thesis).

The project plan was to concentrate upon the effect of crack width on the reliability of fluorescent penetrant inspection and the potential of vibration as a means of actively enhancing the penetrant inspection for the first two years. The relationship between crack width and ultrasonic response and the potential of vibro-enhancement would be pursued in the final two years, once a suitable application had been identified and the requisite equipment had been purchased. 

Vibro-acoustic modulation is an example of a nonlinear ultrasonic inspection. During the literature review into nonlinear ultrasonic inspection for the detection of fatigue cracks, I was also exposed to literature on the nonlinear ultrasonic inspection of bonded interfaces, such as diffusion bonds, inertia welds and adhesive joints. As the work on fluorescent penetrant inspection was coming to an end, the Rolls-Royce plc Technology Acquisition team for Non-destructive Evaluation identified a pressing need to develop an improved ultrasonic inspection for titanium to titanium diffusion bonds. Nonlinear ultrasonic inspections of bonded interfaces have been reported by several authors in the academic literature. However, it was clear that a nonlinear solution would require considerable resource and a working solution could not realistically be achieved in the timescales requested. Also, several options for improving the conventional ultrasonic inspection through signal processing methods were identified. The research that had been carried out on the ultrasonic inspection of diffusion bonds between pieces of highly textured forged Ti-6Al-4V was limited and therefore there were areas where the basic understanding of the problem could be improved. This project had clearly defined requirements, a strong industrial pull and a dedicated funding stream. For these reasons, it was decided to focus on this project during the final two years of my Engineering Doctorate programme.

Huang-M-2021-PhD-Thesis‌

Huang Ming 'Elastic wave propagation in polycrystals', Mechanical Engineering Department, Imperial College London, 2020

Scattering occurs as elastic waves propagate through a random polycrystalline medium, exhibiting scattering-induced attenuation and velocity dispersion. These behaviours carry bulk information about microstructure and can therefore be used for microstructure characterisation. The purpose of this thesis is to gain knowledge about the behaviour of elastic waves in polycrystals in order to facilitate the characterisation of microstructure. This thesis contributes mainly in six aspects.

First, a theoretical second-order approximation (SOA) model is developed to calculate the scattering-induced attenuation and velocity dispersion of plane elastic waves in random polycrystals. This model provides solutions of second-order accuracy in material inhomogeneity that are valid across all scattering regimes and partially account for multiple scattering. It applies to statistically equiaxed and elongated grains of arbitrary crystal symmetries, with decoupled geometric and elastic statistics represented respectively by the two-point correlation (TPC) function and the elastic covariance. A simple Born approximation, with a reduced accuracy considering only single scattering, is formulated based on the SOA model, and analytical asymptotes are derived for the low-frequency Rayleigh and high-frequency stochastic regimes.

Second, a three-dimensional (3D) finite element (FE) method is advanced to solve the wave propagation problem in the time domain. This method uses grain-scale spatial representation, in significant sample volumes of large numbers of grains, to describe polycrystalline materials. It captures the exact interactions of waves with grains without low-order scattering approximations. The numerical errors and statistical uncertainties of the FE method are minimized to deliver very accurate calculations of attenuation and phase velocity. The TPC function of the FE model is accurately determined and incorporated into the SOA model to enable a direct comparison of both models.

Then, the SOA and FE models are used to study the propagation of plane longitudinal waves in polycrystals with statistically equiaxed grains and greatly differing inhomogeneities. Attenuation exhibits fourth- and second-power dependences on frequency in the Rayleigh and stochastic regimes, while phase velocity is nondispersive in both regimes. Attenuation and phase velocity also show proportionalities to material inhomogeneity, and in the Rayleigh regime, the difference between the SOA and FE models is quadratically related to inhomogeneity for both attenuation and velocity.

The fourth contribution relates to using the SOA and FE models to study plane longitudinal wave propagation in polycrystals with statistically elongated grains. The models are found to agree very well with each other for the studied polycrystals over a wide frequency range. In the Rayleigh regime, attenuation and phase velocity exhibit dependencies on the fourth- and zeroth-power of frequency, show respective proportionalities to the effective
volume of the grains and the mean grain radius in the direction of propagation, and both manifest a proportionality to the mode-converted elastic scattering factor. In the stochastic regime, attenuation and phase velocity show dependencies on the secondhand zeroth-power of frequency, demonstrate positive and negative proportionalities to
the mean grain radius in the direction of propagation, and both are proportional to the same-mode elastic scattering factor.

Subsequently, a practical problem is addressed to represent the actual TPC statistics of polycrystals by a single exponential. A variety of potential parameters are identified for the single exponential and their goodness is evaluated by using the SOA and FE models. It is found that the effective grain radius is an optimal choice for the single exponential to represent the microstructure of a range of polycrystals with greatly differing grain uniformities and to achieve a fairly accurate calculation of attenuation and velocity.

The last contribution concerns the theoretical discovery of three modes for elastic waves in polycrystals. For either longitudinal or transverse propagating waves, three solutions are found in the far-field approximation and the SOA model, indicating that three modes may co-exist. A further study by the spectral function approach reveals that the two non-dominant modes mostly have negligible energy in comparison to the dominant mode.

Morbidini, M. ‘A Comparison of the Vibro-Modulation and Thermosonic NDT Techniques', Mechanical Engineering Department, Imperial College London, 2007

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Thermosonics and vibro-acoustic modulation (VAM) are two recent nondestructive testing (NDT) techniques which show potential to improve the detection of interfacetype defects (fatigue cracks in metals and delaminations in composites) compared to traditional NDT methods. These two novel "active" techniques have in common the excitation of structural vibrations of the test-sample to generate significant displacement of the defect interfaces. In thermosonics this is achieved by high power ultrasound applied to the specimens by high power sonotrodes (20 or 40 kHz). In VAM the defect is vibrated by the excitation (by impact or by an electro-magnetic shaker) of the lowest frequency modes of the structure. The diagnosis of damage is done by using different "pick-up" methods. Thermosonics exploits the local generation of heat, due to friction and/or stress concentration, which can be imaged by modern infrared (IR) thermal imaging cameras. VAM uses the modulation of an additional interrogating ultrasonic field, which is the result of a nonlinearity introduced by the defect in the previously linear and undamaged structure.

In this Thesis, an extensive experimental study of both NDT techniques is presented. The general aim of the study has been to assess and then compare the sensitivity of both methods as a function of defect size as well as their rapidity and reliability of application. Laboratory specimens with fatigue cracks of variable size were prepared and quantitative measurements were taken using both methods. VAM tests were carried out at different amplitudes of the low frequency modulating wave and for a wide range of ultrasonic frequencies (70 - 230 kHz) to find those at which the sensitivity to damage was maximum. A damage index is proposed to evaluate the crack severity. Thermosonic experiments were carried out varying the amplitude of the input ultrasound to study the correlation between vibration level and temperature rise. This was achieved through the characterization of the extra damping introduced in the specimens by the crack. Based on this information, an algorithm is proposed for the prediction of the thermosonic signal from vibration records.

The results of the experiments show that VAM has a relatively limited sensitivity to small cracks, because it is affected by coherent noise generated at the supports of the specimen. On the other hand, thermosonics can potentially detect smaller cracks if a sufficient vibration amplitude is excited in the specimens. Hence, the reliability in detecting small cracks using thermosonics was studied further and led to the definition of practical calibration and testing procedures that allow us to detect reliably any cracks. 

Neau, G. 'Lamb waves in anisotropic viscoelastic plates. Study of the wave fronts and attenuation', Mechanical Engineering Department, L'Universite de Bordeaux, 2003

Neau, G (pdf)

The properties of the lamb waves propagating in viscoelastic anisotropic media are studied. The dependencies of the phase velocity, the attenuation, the energy velocity, and the beam deviation on the frequency and on the phase front direction are described. The energy considerations taken into account enable a more precise study of the energy propagation of the Lamb modes. Thus, the dispersion curves are not plotted any longer for a given phase front direction but for a chosen observation direction. The attentuation of the guided waves along the ray direction is also detailed. An experimental illustration of the described properties (energy and phase velocities, skewing angle, attenuation) is carried out on undirectional carbon-epoxy and glass-epoxy plates.

Pavlakovic, B. ‘Leaky guided ultrasonic waves in NDT’, Mechanical Engineering Department, Imperial College London, 1998

Pavlakovic, B (pdf)

This thesis concentrates on the development of a general purpose model of ultrasonic wave propagation in leaky cylindrical structures and the integration of this model with finite element modelling so that effective ultrasonic non-destructive testing (NDT) techniques can be developed. The analytical model that has been developed provides information on the guided modes that can exist in a wide range of infinitely-long, multi-layered, isotropic and transversely isotropic, Cartesian and cylindrical systems. The discussions in this thesis concentrate on the complicated case of cylindrical layers whose energy leaks into surrounding semi-infinite spaces. Using techniques developed in this thesis, the analytical wave modelling results are integrated with time domain finite element modelling to extract additional information about the behaviour of the guided ultrasonic waves, such as how they will interact with defects. This information allows non-destructive testing strategies to be developed for many challenging applications. One such application that motivated much of the work on this wave propagation model is the inspection of post-tensioned bridges. The recent unexplained collapse of this type of bridge in Wales emphasised that there are currently no inspection techniques that are reliable, practical, and inexpensive enough to routinely evaluate the integrity of this type of bridge. The model that is developed in this work has been applied to this inspection problem to evaluate the possibility of propagating guided waves down embedded steel tendons to evaluate the condition of post-tensioned bridges by looking for reflections from fractures or loss of section due to corrosion. Several modes that would allow reasonable amounts of the tendons to be tested from their ends have been identified and confirmed experimentally on realistic test specimens. These modes can provide valuable information on the integrity of the anchorages, a sensitive region that cannot currently be inspected.

Pettit, J. R. 'Modelling the Ultrasonic Response From Rough Defects Using Efficient Finite Element Modelling Techniques', Mechanical Engineering Department, Imperial College London, 2015

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The work of this Engineering Doctorate addresses the research and development of efficient Finite Element (FE) modelling techniques for calculating the ultrasonic response from rough defects for Non-Destructive Evaluation (NDE) applications specific to the nuclear power generation industry. The project has been carried out in collaboration with Imperial College London and Rolls-Royce allowing for the transfer of novel academic research into an applied industrial context.

Within the UK nuclear power generation industry, one of the fundamental principles of regulation and operation is a robust safety culture where the highest levels of quality assurance are applied to safety critical components. This principle places a requirement on NDE to deploy reliable and accurate inspections to ensure the structural integrity of the plant and its components.

To achieve this goal, modelling techniques can be used to aid in the design and justification of ultrasonic NDE inspections. For smooth, relatively large defects, analytical methods can provide an accurate scattering solution; however, for more realistic rough defects, the limitations of these methods are only applicable for specialised cases of roughness.

Defects which possess rough surfaces greatly affect ultrasonic wave scattering behaviour. Ultrasonic NDE inspections of safety-critical components rely upon this response for detecting and sizing flaws.

Reliable characterisation is crucial, so it is essential to find an accurate means to predict any reductions in signal amplitude. An extension of Kirchhoff theory has formed the basis for many practical applications; however, it is widely recognised that these predictions are pessimistic owing to analytical approximations. As a result, NDE inspections can be overly sensitive, meaning that small and insignificant indications are incorrectly classed as being potentially hazardous defects. This increases the likelihood of making false-calls and incurring unnecessary expenditure to the programme.

A numerical full field modelling approach does not fall victim to such limitations, and therefore, FE modelling techniques have been developed to deliver a non-conservative methodology for the prediction of expected back-scattering from rough defects. This has been achieved in two parts: improved performance of absorbing boundary methods for use with commercial FE codes, and application of domain linking algorithms to NDE inspection problems. This thesis presents the development of these methods and their application to industrial NDE inspections. Ultimately, the findings of this work will aid in establishing more reliable, less conservative, reporting thresholds for the inspection of power plant components, reducing false call rates and therefore any unnecessary expenditure.

Phillips, R. A. ‘Efficient numerical modelling of the ultrasonic scattering from complex surface-breaking defects’, Mechanical Engineering Department, Imperial College London, 2019

Phillips, R.A. (pdf)

‌Ultrasonic testing is routinely used in the nuclear power generation industry to assess the structural integrity of plant components. The regulatory nature of the industry means that ultrasonic inspection procedures require substantiation through experimental trials and semi-analytical simulation, which can be costly and time-consuming. Furthermore, due to the complex defect and component geometries that are commonly encountered in the industry, the generation of evidence that accurately represents real inspections can be challenging. Numerical modelling techniques offer an effective alternative for substantiating inspections as they can accurately simulate the ultrasonic scattering from complex geometries, yet such techniques lead to increased computational cost. Hybrid techniques, which combine both semi-analytical and numerical methods, offer an approach to rapidly simulate an entire ultrasonic inspection whilst maintaining the ability to simulate the scattering from complex defects. However, there is a lack of experimental validation for extant hybrid techniques, particularly for three-dimensional simulations, and they have not been widely applied to the simulation of surface-breaking defect inspections, which are commonly encountered in both manufacturing and in-service inspections.

This thesis has extended the functionality of a three dimensional hybrid technique to simulate the inspection of surface-breaking defects. Validation evidence demonstrates that the hybrid model accurately predicts the ultrasonic scattering from a well-characterised reflector. An arbitrary transduction system has been incorporated to the model, enabling the model to be more broadly applied to the simulation of ultrasonic inspections of plant components. Furthermore, validation on a real defect provides a high level of confidence that the model can be used to assess and quantify the performance of inspections where qualification becomes challenging. This work has made a clear step forward in the use of simulation for inspection qualification and further work is required to fully optimise the simulation methodology.

Pialucha, T. P. ‘The reflection coefficient from interface layers in NDT of adhesive joints’, Mechanical Engineering Department, Imperial College London, 1992

Pialucha, T. P (pdf)

The structural integrity of adhesive joints is known to be dependent on the properties of the adhesive (cohesive properties) and the properties of the adherend/adhesive interface (adhesive properties). Despite a substantial research effort worldwide there is no currently available nondestructive technique to test for interfacial defects in adhesive joints. However, ultrasonic methods have been identified as the most promising techniques for these purposes. It is therefore desirable to assess their suitability.

This thesis presents an evaluation of the ultrasonic reflection coefficient method and, in particular, the oblique incidence method, for the nondestructive characterisation of adherend/adhesive interfaces in bonded joints. The technique uses two ultrasonic transducers inclined at an angle, operating in a pitch-catch mode, with respect to the tested joint.

A theoretical model is developed which is capable of accurate predictions of reflection and transmission coefficients from isotropic multilayered, viscoelastic plates, excited at normal and oblique incidences by ultrasonic transducers of finite sizes. Experiments are performed on simple model systems for the theory validation. The measured reflection coefficient amplitudes are found to be within 5% of the predicted values.

Theoretical and experimental work is carried out to find the optimal arrangement of the probes, frequency range and type of reflection in order to achieve maximum sensitivity to changes in the adherend/adhesive interfaces. It is found that the oblique incidence techniques can offer a substantial increase in sensitivity to interfacial properties over the current standard inspection techniques, but the results obtained indicate that the improvement is unlikely to be sufficient for the technique to be used as a new reliable nondestructive procedure

Ptaszek, G. S. 'Investigation and Development of Transient Thermography For Detection of Disbonds in Thermal Barrier Coating Systems', Mechanical Engineering Department, Imperial College London, 2012

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This thesis has explored the use of transient thermography for the detection of disbonds of minimum diameter 2mm located in a thermal barrier coating (TBC) system whose surface may be unpainted. The technique, the type/size of the defect and also the condition of the TBC system for the inspection has been specified by Alstom Power Switzerland, the sponsor of the EngD project.

As for other Non-Destructive Testing (NDT) techniques, reference test specimens are required for calibration, but unfortunately, real disbonds are very difficult to use because it is difficult to control their size, and larger ones tend to spall. Flat bottomed holes are commonly used, but these over-estimate the thermal contrast obtained for a defect of a given diameter. The thesis quantifies the differences in thermal response using finite element analysis validated by experiments, and proposes a form of artificial disbond that gives a better representation of the thermal responses seen with real defects. Real disbonds tend to have a non-uniform gap between the disbonded surfaces across the defect, and the effect of this on the thermal response is evaluated using finite element simulations. It is shown that the effect can be compensated for by adjusting the diameter of the calibration defect compared to the real defect.

Surfaces of inspected specimens are usually covered by a black, energy absorbing paint before the transient thermography test is carried out. Unfortunately, this practice is not acceptable to some turbine blade manufacturers (including the project sponsor) since thermal barrier coatings are porous so the paint is difficult to remove. Unpainted TBC surfaces have very low emissivity, and after period of service their colour changes unevenly and with which also absorptivity and emissivity changes. The low emissivity gives low signal levels and also problems with reflections of the incident heat pulse, while the variation in emissivity over the surface gives strong variation in the contrast obtained even in the absence of defects. The thesis has investigated the effects of uneven discolouration of the surface and of Infra Red (IR) translucency on the thermal responses observed by using mid and long wavelength IR cameras. It has been shown that unpainted blades can be tested satisfactorily by using a more powerful flash heating system assembled with an IR glass filter and a long wavelength IR camera. The problem of uneven surface emissivity can be overcome by applying of the 2nd time derivative processing of the log-log surface cooling curves.

Rajagopal, P. 'Pseudospectral Collocation Method for Viscoelastic Guided Wave Problems in Generally Anisotropic Media', Mechanical Engineering Department, Imperial College London, 2016

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In Non-Destructive Evaluation (NDE) applications guided waves are attractive to perform rapid inspections of long lengths and large areas. However, they are complicated, therefore it is important to have as much information and understanding about their physical properties as possible in order to design the most efficient and robust inspection process as well as to draw the correct conclusions from the measurement results. The main piece of information to gain insight into the guided wave’s properties is dispersion curves which, for isotropic structures such as plates and cylinders, have been available for many years. There are many robust algorithms which are currently used to compute them: finite element simulations, partial wave based root finding routines (PWRF) and semi-analytical finite element simulations (SAFE). These methodologies have been generalized and also used to study and compute dispersion curves of more complicated anisotropic materials though the range of tractable cases was limited.

Although robust, all these approaches present several challenges, mostly computational, such as missing modes (PWRF), the so called ”large-fd” problem (PWRF), artificially increased stiffness (FE, SAFE) or improvement of dispersion curve tracing routines (FE, PWRF, SAFE). In addition, when studying complicated anisotropic materials with a low degree of symmetry or unusual axes configurations where propagation does not take place along any of the principal axes, PWRF routines are frequently unreliable and one must resort to specific SAFE simulations which also present their own challenges and, depending on the SAFE scheme used, can yield spurious modes which need to be carefully filtered. Recently, Pseudospectral Methods (Galerkin and Collocation schemes), were introduced in the field of elastic guided waves, providing a powerful, yet strikingly and conceptually simple alternative to the above algorithms by successfully finding the dispersion curves in isotropic structures and in some simple anisotropic problems.

However, a systematic and general approach for accurately and robustly computing dispersion curves of guided waves in anisotropic media, up to the most general case of triclinic symmetry, has not yet been developed. The goal of the work presented in this thesis is to develop such a tool by means of the Pseudospectral Collocation Method (SCM) and to take advantage of its particular features to make it as robust as possible.

Firstly, a PSCM scheme is developed for computing dispersion curves of guided waves in anisotropic elastic media by finding all the frequencies for a given value of the real wavenumber. The results are validated with the existing literature as well as with the results provided by the software DISPERSE developed in the NDT group at Imperial College London. Many of the most remarkable features of the PSCM (spectral accuracy, speed, and its failure to miss modes for instance) are already observed in this simple, yet important, class of problems in elastic media.

Secondly, guided waves in viscoelastic anisotropic media are studied. In this case, modes present attenuation due to material damping which is reflected in the wavenumber being complex. In order to handle complex wavenumbers the PSCM schemes developed for elastic materials are appropriately extended by means of the Companion Matrix Method. It will be seen that, apart from lowly attenuated propagating modes, all the other highly attenuated modes are found, yielding the full three-dimensional spectrum of the problem under consideration. Moreover, when the PSCM schemes for viscoelastic media are used to compute the dispersion curves of guided waves in an elastic medium, all the remaining, imaginary as well as complex, roots of the elastic problem which were not computed by the simpler PSCM elastic schemes are found, providing the full three-dimensional picture of the dispersion curves.

These PSCM schemes, as any other of the aforementioned approaches, only find pairs (ω, k). If dispersion curves are to be plotted, those pairs must be linked correctly in order to plot the desired dispersion curves, which is non-trivial when crossings amongst modes occur. Motivated by this, an investigation of the parity and coupling properties of guided wave solutions is carried out in detail for all crystal classes. This investigation provides a robust alternative to conventional tracing routines and avoids the problem of mode crossings by exploiting the parity and coupling properties of the solutions.

Finally, the most complicated problems involving embedded structures are investigated by including a Perfectly Matched Layer (PML) in the previously developed PSCM schemes for viscoelastic media. The dispersion curves for leaky and trapped modes in an isotropic elastic plate and in a similar cylinder immersed in an infinite ideal fluid are found, showing very good agreement with the results given by PWRF routines in a large range of frequencies. Last, but not least, an illustration of a two-dimensional PSCM scheme is presented to study a vibrating membrane. The results are compared with the available analytical solution showing again excellent agreement.

Rajagopal, P. 'Towards Higher Resolution Guided Wave Inspection: Scattering Studies', Mechanical Engineering Department, Imperial College London, 2007

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This thesis presents work contributing to the development of ultrasonic guided wave NDE inspection systems with improved resolution. Guided waves today are well established in the rapid inspection of large structures. The approach taken so far has been to develop screening tools to maximize coverage; the methods yield precise information about the exact location of defects but only an approximate estimate of the severity of defects. However there are many applications where the areas of concern are not accessible, and reaching them for a secondary accurate inspection may not be possible or involve considerable cost. Therefore there is much interest in improving the resolution of guided wave NDE towards direct defect sizing. Two possible approaches are being considered to achieve this, using either multiple modes at high frequency-thickness or single mode array imaging at low frequencies. The work reported here concerns the understanding of the interaction of guided waves with defects so that an appropriate approach can be selected and implemented. A review of the basics of elastic wave scattering from defects is first presented in order to introduce methods used and effects encountered later in the work.

A simple implementation of the high-frequency multimodal approach, in which the input consists of a single fundamental mode while the multiple-mode scattered signal permits separation into component modes, is then considered. Finite element simulations and theoretical analysis are used to study the interaction of the fundamental antisymmetric Lamb wave mode A0 and the fundamental torsional pipe mode T(0,1) with long but part-thickness planar cracks, in this context. The results show that the reflection due to both modes is more sensitive to shallow cracks than at lower frequencies. The reflected A0 and A1 modes in plates and T(0,1) mode in pipes emerge as the ‘best modes' for discrimination between shallow and deep cracks since their amplitudes have a uniform relation with the crack depth. Also, knowledge of effects such as regions of little or no mode conversion and the extent to which the reflections of the different modes differ, emerge as powerful ways of obtaining useful additional information about defect dimensions.

In view of promising trends from parallel work at the Imperial College NDT Group using low-frequency array imaging methods, the rest of the thesis focuses on the interaction of cylindrical crested low-frequency SH0 waves with finite cracks in thin plates. Finite element simulations are used to obtained trends which are subject to experimental confirmation and analysis. Since guided SH waves in thin plates correspond to torsional modes in pipes, the results obtained help clarify the physics of scattering so that imaging methods may be better formulated and developed. The simpler case of through-thickness cracks is first taken up and the influence of the crack length, monitoring position and the angle of incidence on specular reflection as well as diffraction are studied. The insights obtained are then used to understand the scattering from the more general part-thickness crack case.

The through-thickness crack studies show that low-frequency scattering of the SH0 mode is strongly affected by diffraction phenomena, leading to focusing of energy by the crack in the backscattered direction. The diffracted field itself consists of components arising from primary diffraction from the crack tips (or edges) and multiple reverberations of Rayleigh-like waves traveling along the crack length. The amplitude of the primary diffraction can be estimated from known solutions to canonical bulk SV wave diffraction problems. The angular behaviour of the reflection is highly directional, with strongest fields in the specular direction, while the specular reflection itself is strongest when the central ray of the incident beam bisects the crack face at 900. The trend of the scattering as observed from part-thickness crack results is identical to that from through-thickness cracks of the same length; the actual values differ only by a frequency dependent scale factor, provided the cracks are small compared to the radius of the incident wavefront. Thus the understanding obtained for scattering from through-thickness cracks may well be sufficient to deal with the part-thickness case also.

From the guided wave imaging perspective, these results help obtain the far-field values for a given operating frequency-thickness and crack length. The directionality of the reflected field informs the possibilities for imaging, but imposes a limitation on the extent to which the resolution of inspection can be improved by low-frequency methods.

Ribichini, R. 'Modelling Of Electromagnetic Acoustic Transducers', Mechanical Engineering Department, Imperial College London, 2011

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At present, the dominant technology for transducers in the field of Ultrasonic Non- Destructive Testing is piezoelectric. However, some industrially important applications, like the inspection of components operating at high temperature or while in motion, are difficult tasks for standard piezoelectric probes since mechanical contact is required. In these cases, contactless NDT techniques can be an attractive alternative. Among the available options, Electromagnetic Acoustic Transducers (EMATs) can generate and detect ultrasonic waves without the need for a physical contact between the probe and the test object, as their operation relies on electromagnetic, rather than mechanical coupling. Since EMATs do not require any coupling liquid, the experimental procedures for inspection set-up are simplified and a source of uncertainty is eliminated, yielding highly reproducible tests that make EMATs suitable to be used as calibration probes for other ultrasonic tests. A further advantage of EMATs is the possibility of exciting several wave-modes by appropriate design of the transducer. Unfortunately, EMATs are also characterized by a relatively low signal-to-noise ratio and by a complex operation relying on different transduction mechanisms that make their performance dependent on the material properties of the testpiece.

The present work aims to develop a numerical model including the main transduction mechanisms, the Lorentz force and magnetostriction, that can be employed as a prediction tool to improve the understanding of EMAT operation. Following an overview on the historical development of EMATs and their models, the theory describing EMAT operation is presented. The governing equations are implemented into a commercial Finite Element package. The multi physics model includes the simulation of the static and dynamic magnetic fields coupled to the elastic field through custom constitutive equations to include magnetostriction effects. The model is used to quantitatively predict the performance of a magnetostrictive EMAT configuration for guided waves without employing arbitrary parameters. The results are compared to experimental data providing a validation of the model and insight on the transduction process. The validated model, together with experimental tests, is exploited to investigate the performance of different EMAT designs for Shear Horizontal waves in plates. The sensitivities of each configuration are compared and the effect of key design parameters is analyzed. Finally, the model is used in the evaluation of the performance of bulk wave EMATs on a wide range of steel grades. Experimental data interpreted via numerical simulations are employed to investigate the relative weight of the transduction mechanisms, with implications on the applicability of EMATs on the range of steels usually encountered in inspections.

Russell, J Thesis (pdf)

Russell, J. ‘The development and implementation of advanced ultrasonic phased array technology’, Mechanical Engineering Department, Imperial College London, 2010

Ultrasonic inspection is the primary technique for the detection of planar flaws within the nuclear industry. Current inspections are typically limited to the application of rigid wedge, single element transducers to components with regular surface geometry. This thesis addresses some of the limitations of this approach and develops new methods and techniques that are suitable for application in an industrial environment.

Inspection modelling is a cost effective technique for inspection design and qualification; it can be used to aid understanding of the inspection process, and provides valuable insight into inspection data interpretation. In this work semianalytical and numerical modelling tools have been used to accurately and efficiently simulate the ultrasonic inspection of large complex components.

Modelling tools have also been applied to aid in the design of a membrane coupled conformable phased array device. The membrane coupled array is a low-cost, robust device that uses a conventional phased array coupled to the outer surface of the component under test via a water path encapsulated by low-loss membrane. Early design of this device was performed by Long and Cawley at Imperial College, London. The work reviewed in this thesis discussed the further development of this membrane coupled device and its application for the inspection of a specific target application component.

The target application is a safety critical, thick-walled stainless steel section of pipework that contains an austenitic weld. The current inspection of this component is carried out by first removing the component weld cap and then mechanically raster scanning a large number of rigid wedge single element transducers. Weld cap removal is an expensive time consuming task that can lead to excessive wall thinning and the exposure of small surface breaking defects. The motivation for this work is to provide complete inspection of the weld and heat affected zone with the weld cap in place. It is also designed to improve inspection speed and accuracy, and to reduce the amount of user intervention required to complete the inspection in a hostile environment.

Inspection results from the 2nd and 3rd generation membrane coupled device on multiple test-pieces representative of the target application are presented. These results demonstrate that the membrane coupled device can be used to detect defects in locations that cannot be inspected using conventional techniques. The amount of scanning required can be significantly reduced, simplifying the inspection and helping to improve inspection speed by over 60% in comparison to the single element approach.

Sebastian Eckel, ‘Automating defect visibility assessment in industrial radiography and realistic film noise modelling based on experimental investigations’, Mechanical Engineering Department, Imperial College London, 2020

Sebastian Eckel

Radiography is one of the main methods within the industrial field of Non-Destructive Evaluation (NDE). It is a projection imaging technique relating the subsurface information of the structure under investigation to a greyscale valued image. In order to ensure operational reliability, inspection procedures need to be qualified before the actual evaluation takes place in practice. Alongside experiments, this qualification process often involves computational tools to model various possible test setups incorporating artificial defects, producing a simulated image. Then the simulated image is assessed, which is traditionally performed subjectively by an operator deciding on the visibility of an artificial defect. If the detectability of that defect is increased, then the underlying parameters of the considered test procedure are preferred. Two crucial aspects of the stated qualification process are addressed within this thesis. Firstly, the presented work advances the simulated qualification approach to a more holistic one by not only modelling the test setup and simulating the resulting image but also by assessing the simulated image automatically. A Model Observer approach is chosen for automating the interpretation of the simulated images. This approach is based on the so-called Channelized Hotelling Observer (CHO), which was originally established in the medical field but is now transferred to NDE. Validation of the new assessment approach is done by comparing experimental psychophysical data of human image assessment to the performance of the CHO in assessing simulated radiographic images. The new method outperforms other state-of-the-art visibility models currently used in industry. Secondly, the correct simulation of the artificial radiograph is crucial for an accurate computational qualification of the test procedures. An important part of the radiograph is the underlying noise, which has a great influence on the image quality. Industrial radiography still mainly relies on films as detectors, since these are reliable and well established over decades. These films show a typical film noise granularity, which usually degrades the defect visibility. Therefore, it is crucial to incorporate realistic film noise in the simulation by a sophisticated film noise model. Currently, the industry considers only white Gaussian noise, which is unrealistic. A new noise model based on experimental investigations of real noise on film samples is presented. It is shown that very realistic artificial noise can be generated by extracting and utilising the spectral characteristics of the real noise. The method generates new and unique noise samples by randomising the phase spectrum but keeping the original spatial frequency spectrum. Furthermore, the method allows the generation of noise with local, space depending spatial frequency spectra while ensuring pixel correlation throughout the image. Various validation studies show that the noise generated by this new method is visually and statistically indistinguishable from the original noise. Additionally, a new optical setup for digitising radiographic films on the microscale is presented. This setup is based on a digital camera and image processing procedure enabling the extraction of the film noise spectra that can then be used to generate film noise realistically. Homogenously exposed and developed films are investigated by the new setup in order to generate a database of spectral properties necessary for the generation of accurate film noise. Furthermore, the new setup can efficiently classify the quality of film systems without relying on time-consuming microdensitometer measurements, which is an important secondary benefit of this new setup.

Seher, M. ' From EMAT to Image: Practical Guided Wave Tomography', Mechanical Engineering Department, Imperial College London, 2015

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The detection and characterisation of corrosion type defects on pipelines is a major challenge for the petrochemical industry, especially in regions with poor accessibility. Guided wave tomography is one feasible approach to inspect areas with restricted access by transmitting guided waves through the area and then processing the measured wave field into a thickness map of the pipeline wall, without having to take measurements at all points on the surface. The key objective of this research project is to develop, implement and test a prototype guided wave tomography system based on the A0 Lamb mode.

For the development of a guided wave tomography system a low-frequency, omnidirectional A0 Lamb wave Electromagnetic Acoustic Transducer (EMAT) is developed, and operates at 0.50MHzmm on a steel plate. For that, a parametric Finite Element (FE) model is implemented in a commercially available FE software and a numerical optimization process employing a genetic algorithm is set up to optimised the EMAT design for an improved A0 mode selectivity. The FE model is validated against measurements on an aluminium plate and on a steel plate. A two-step model-based design approach is proposed whereby only the Lorentz force is used in the first step for the optimisation and then in a second step, a realistic estimate of the mode selectivity can be obtained by additionally considering the magnetisation force. The optimised design fulfils the S0 suppression design requirement and is integrated into the guided wave tomography system consisting of two ring arrays.

The developed guided wave tomography system is tested on two steel pipes with smooth and well defined defect. The repeatability of measurements is assessed and the robustness of the guided wave tomography measurements to sensor position errors is investigated. It is demonstrated that there is a small influence on the thickness reconstruction for fairly large systematic and unsystematic position errors. Similar results are obtained for single sensor failures or gaps in the arrays and an increase in sensor spacing is found to increase reconstruction artefacts. With Golay complementary sequences, a signal processing technique is presented that allows for a significant increase in the data capture speed with the same performance as time averaging.

Three areas with restricted access, support locations, pipe clamps and STOPAQ(R) coatings, are considered and their influence on the thickness reconstruction is investigated relative to a reference configuration and only a small influence is found in the experiments.

Seppings, R. ‘Investigation of Ice Removal From Cooled Metal Surfaces’, Mechanical Engineering Department, Imperial College London, 2006

Seppings, R (pdf)

This thesis describes the problem of material build-up on metallic surfaces, with particular reference to ice and frozen sucrose build-up on small diameter pipes. The thesis aims to determine the mechanisms behind ice removal when transverse vibrations act upon the system over a range of frequencies. Additionally, factors such as the noise output from the system and fatigue stresses acting on the system should be minimised. The thesis will outline a system where a practical compromise between ice removal, fatigue life and noise reduction might be achieved, based upon understanding of the failure criteria.

To understand the mechanisms of failure the dynamics of the system with and without the presence of ice have been characterised over a range of frequencies. Ice
comprises a complex crystalline structure and a brief outline of its mechanical properties is described within the thesis. Special attention is paid to the relationship
between ice and frozen sucrose solution. A simple experimental system has been developed as a generic example application and results from this rig have been investigated. The same system has been modelled using a finite element program. Data derived from the finite element simulation and experimental results have been compared with data for both cohesive and adhesive
failure criteria.

Data on strain rates in excess of those previously reported is presented, showing an apparent decrease in the yield/fracture strength of frozen sucrose after the critical strain rate. Good agreement has been achieved between the derived results and results from the literature, over a range of frequencies. Hence, an understanding of the mechanisms behind ice failure has been built up. To be fully removed from the pipe the ice must fail in at least three planes: axially, circumferentially and also at the interface. Results derived from experimentation and modelling indicate the order of failure and the magnitude of the last or critical failure stress.

Simonetti, F. 'Sound propagation in lossless waveguides coated with attenuative materials', Mechanical Engineering Department, Imperial College London, 2003

Simonetti, F (pdf)

Research into ultrasonic guided wave non-destructive testing for the long range inspection of large metallic structures is now well advanced. The main advantage of this method is that a large area can be covered from a given transducer position, so avoiding expensive scanning of all the structure surface. However, in the presence of attenuative coatings the test range can be dramatically reduced. As a consequence, it is of great practical interest to characterise those modes and frequencies which minimise the guided wave attenuation.

This thesis investigates the nature of shear horizontal (SH) and Lamb waves propagating in elastic plates coated with viscoelastic layers, this geometry being also representative of coated pipelines with large diameter to wall thickness ratio. For both SH and Lamb waves the mode which exhibits the highest potential for long range inspection purposes is identified and analysed. It is demonstrated that Lamb modes provide longer propagation distance than SH waves. Moreover, it is shown that the acoustic properties of the coating play a major role in the attenuation of the guided waves. In order to measure these properties for a broad variety of viscoelastic materials, two novel techniques are developed.

The bulk velocities and attenuation of the coating may be obtained by measuring the phase velocity and attenuation of guided waves propagating in a hollow waveguide filled with the viscoelastic material. This method is feasible when the material flows sufficiently easily for the cylinder to be filled. An alternative, when the material does not flow easily, is to clamp a sample of the coating between two rod waveguides and to measure the reflection and transmission of guided waves across the sample. This has enabled the acoustic properties of the bitumen used to provide corrosion protection on pipes in the chemical industry to be measured both when it is applied in its viscous liquid state and when it has been in place for many years and become solid.

Sposito, G. ' Advances in potential drop techniques for Non-Destructive Testing' , Mechanical Engineering Department, Imperial College London, 2009

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In the field of Non-Destructive Testing, Potential Drop (PD) techniques have been used for decades, especially in the petrochemical and power generation industries, for monitoring crack growth and wall thickness variations due to corrosion and/or erosion in pipes, pressure vessels and other structures. Inspection is carried out by injecting currents in the specimen to be tested and
measuring the arising electrical potential di erence between two or more electrodes placed on its surface. The presence of a defect generally increases the resistance and hence the measured voltage drop; inversion of these data can give information on the size and shape of the defect.

However, while the principle underlying these techniques is relatively simple, some difficulties have been encountered in their practical applications. Many commercial systems based on PD methods, for instance, require the injection of very large currents in order to obtain sufficiently large signals; doubts have been raised on the stability of these methods to variations in the contact resistance between the electrodes and the inspected material. The present work aims to show that some of these problems can be easily overcome, and to evaluate the capabilities of PD
techniques for crack sizing and corrosion mapping.

After a brief review of the advantages, disadvantages and applications of the main electromagnetic methods for Non-Destructive Testing, an experimental setup for Potential Drop measurements which was developed for this work and which uses small alternating currents (AC) is described. The setup is benchmarked against existing PD systems and then used to validate a model that allows AC PD simulations to be run with a commercial Finite Element code. The results of both numerical simulations and experimental measurements are used to investigate the possibility of sizing defects of complex geometry by repeating the analysis at several different frequencies over a broad range, and of reconstructing the depth pro le of surfacebreaking defects without the need for assumptions on their shape. Subsequently, the accuracy to which it is possible to obtain maps of corrosion/erosion on the far surface of an inspected structure is discussed, and results obtained with an array probe that employs a novel arrangement of electrodes are presented. Finally, conclusions are drawn and suggestions for further research are made. 

Tippetts, T. 'Improved Reliability of Automated Non-Destructive Evaluation', Mechanical Engineering Department, Imperial College London, 2014

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In recent years, Non-Destructive Evaluation (NDE) has trended toward increased automation in data acquisition. Automated scanning has the potential to greatly increase reliability of the NDE results, but it also tends to increase the volume of data that must be inspected manually by a skilled technician. This is a time-consuming task made tedious by the fact that most of the data contains no indication of a defect. There is a great need for software that can partially automate the data analysis by prioritizing regions of interest to the inspector. This thesis describes an approach to that end, laying out a framework that is general enough to fit a wide array of NDE applications. It also describes practical considerations for the specific application of ultrasound inspection of titanium turbine discs.

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Van Pamel, A., 'Ultrasonic inspection of highly scattering materials', Department of Mechanical Engineering, Imperial College London

Ultrasonic Non-Destructive Evaluation (NDE) relies on the scattering of waves from discontinuities, such as fractures or voids, to probe media otherwise invisible to the naked eye. Whilst this has been industrially exploited for several decades within acoustically transparent materials, many materials maintain a microstructure that causes scattering of the propagating waves. This undermines the aforementioned premise as it becomes exceedingly difficult to discern the features of interest from the scattering inherent to microstructural features, thereby limiting the range of materials which can be reliably inspected, non-destructively. Experimental investigations confirm the challenges and significant shortcomings for the inspection of future industrial components where such microstructures are desirable for their mechanical properties. It is demonstrated that the rapid increases in scattering with the insonifying frequency severely limit the achievable sensitivity of conventional ultrasound techniques. A review of the latest advances in ultrasound technology, including signal processing and imaging algorithms, explore the opportunities to exceed current limitations and advance the capability of ultrasonic NDE. Establishing these advances, and those of future approaches, requires a rigorous definition of performance. In contrast to commonly adopted strategies, a novel strategy which considers the probabilities of detection and false alarms is proposed as a valuable benchmark that can be used to make objective comparisons in terms of performance between competing algorithms. Future progress will also rely on a better scientific understanding of scattering, which can be provided by powerful modelling tools. Here, Finite Element modelling is established to be very useful; it captures the complex scattering physics and allows an investigative flexibility which can provide extremely useful insights. Whereas previous studies have often been restricted to weak scattering assumptions, the present FE modelling capability now enables the study of more complex, highly scattering environments. This is demonstrated by investigating ultrasonic arrays, where through optimising their engineering, especially in terms of their configuration, significant performance enhancements are shown to be possible. These important scientific tools have enabled the assessment of the latest imaging algorithms, the optimisation of inspection configurations, and increased our understanding of scattering phenomena. Their use in the future enables wide possibilities towards further pursuing the ultrasonic inspection of highly scattering materials.

Vine, K. ‘The non-destructive testing of adhesive joints for environmental degradation’, Mechanical Engineering Department, Imperial College London, 1999

Vine, K (pdf)

This thesis presents the results of an investigation aimed at producing a non-destructive test for determining environmental degradation of adhesive joints. One of the biggest factors preventing more widespread use of adhesive joints is the adverse effect that wet environments have on their performance coupled with the lack of a reliable non-destructive technique for assessing the extent of this reduction in performance. This thesis has attempted to determine the suitability of ultrasonic techniques for detecting environmental degradation, specifically of aluminium epoxy joints typical of those used in the aerospace industry. Degradation of this type of joint is one of the factors that may determine the serviceability of many ageing aircraft. It was also hoped that a greater understanding of the mechanisms of degradation would be attained.

Ultrasonic techniques were identified as being the most promising for assessing the degradation of adhesive joints, with normal and oblique incidence techniques being used. Mechanical tests and surface analysis techniques were also used to quantify and explain the degree of degradation that specimens had undergone in a hot wet environment.

Several possible mechanisms of water ingress were identified. The most readily identified of these was through edge disbands, which was easily detected using ultrasonic methods. Diffusion of water through the epoxy layer was also seen to occur and could be predicted, but could not be related to a loss of toughness. With the epoxy used for this work water diffusion through the epoxy layer could not be detected non-destructively. Evidence was also seen for the penetration of water through flaws in the epoxy layer and this was thought to be the mechanism responsible for a loss in toughness across large areas of some of the specimens. Small spot disbands were seen to form around flaws in the epoxy layer. Detection of these small spots and the flaws in the epoxy layer was possible non-destructively, but only given sufficient spatial resolution. There was also some evidence to suggest that there was water ingress along the interface in some specimens. It was concluded that the most suitable technique for inspecting adhesive joints for environmental degradation is high frequency, highly focused normal incidence ultrasound.

Vogt, T. K., 'Determination of material properties using guided waves', Mechanical Engineering, Imperial College London, 2002

Vogt, T. K (pdf)

The thesis examines the possibility of using an ultrasonic guided wave system for the determination of material properties. The system considered consists of a cylindrical waveguide which is partly embedded in another material whose acoustic properties are to be determined. Two main ideas are investigated.

The first idea is based on the fact that, when a waveguide is embedded in another material, a guided wave in the waveguide will be attenuated due to the leakage of bulk waves into the surrounding material. The rate of leakage depends on both the material properties of the waveguide and the embedding material. The propagation characteristics such as the phase velocity and the attenuation can be predicted using software that has been developed in the laboratory as part of previous investigations. With these predictions it is possible to relate a measured attenuation to material properties such as the viscosity of a liquid. Experimental results show the validity of these predictions.

The second idea uses the fact that, when the waveguide enters the material under investigation, the guided wave will be scattered at the entry point due to the change in surface impedance. Since the magnitude of the reflected guided wave depends on the properties of the embedding material, it can in principle be used for materials characterisation. Finite Element (FE) modelling and a theoretical scattering analysis have been carried out in order to calculate reflection coefficients for different material properties. Both these methods agree very well with each other and with experimental results.

As one of the possible applications, the cure monitoring of epoxy resins has been investigated in more detail. Both methods have been successfully applied to the monitoring of bulk samples, yielding accurate quantitative results of epoxy shear velocity. For the monitoring of adhesive cure in automotive joints, the reflection coefficient method seems most suitable. However, it was found that the geometry of the joints influences the reflection of guided waves. This effect has been investigated using FE modelling. In this case, due to the uncertainty of the geometry in the industrial environment, the reflection coefficient can be determined only qualitatively.

Wilcox, P. ‘Lamb wave inspection of large structures using permanently attached transducers’, Mechanical Engineering Department, Imperial College London, 1998

Wilcox, P (pdf)

This thesis investigates some aspects of the fundamental science necessary for the development of an integral structural inspection system based on the use of ultrasonic Lamb waves. It is particularly concerned with the long range propagation of Lamb wave modes, the selection of suitable modes and the design of permanently attached transducers.

An analytical investigation into the effect of plate curvature on Lamb wave propagation is presented, since this is highly relevant to the inspection of structures such as pipes and pressure vessels. It is shown that when the radius of plate curvature to plate thickness ratio is greater than approximately 10:1, the effect of curvature on the propagation of lower order Lamb wave modes is negligible. Quantitative studies into the effects of dispersion on the long range propagation of Lamb waves are presented. It is demonstrated that at any point on any dispersion curve, there is an optimum number of cycles required in an input signal to maximise the spatial resolution obtainable over a particular propagation distance.

The design of inter-digital transducers (IDTs) made from the piezoelectric polymer PVDF for the transmission and reception of Lamb waves is investigated. A one-dimensional transducer model is used to investigate the frequency response of PVDF bulk wave transducers. Results from this model are used to develop various types of PVDF IDTs which work over a frequency range from 65 kHz to 2.5MHz. These transducers are shown to be able to propagate Lamb waves over several metres in structures between 1 and 13mm thick.

A model based on Huygen’s principle of superposition is developed for predicting the acoustic field from an IDT. This model has been shown to be of equal accuracy to an existing finite element model and several orders of magnitude faster. The model has also been successfully validated against experimental data and used to elucidate guidelines for the design of two common configurations of IDT.

Xi, X. 'Controlled Translation And Oscillation Of Micro-Bubbles Near A Surface In An Acoustic Standing Wave Field', Mechanical Engineering Department, Imperial College London, 2012

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The removal of contamination particles from silicon wafers is critical in the semiconductor industry. Traditional cleaning techniques encounter difficulties in cleaning micro and nanometer-sized particles. A promising method that uses acoustically driven micro-bubbles to clean contaminated surfaces has been reported. However, little is understood about the microscopic interaction between the micro-bubble and particle. This thesis explores the mechanism underlying the ultrasonic cleaning using micro-bubbles at the micrometer scale. The investigation was carried out from the perspective of bubble dynamics near a surface and bubble-particle interaction. Prior to contributing to the particle removal, micro-bubbles normally need to be transported to a target surface. The motion of a bubble was analyzed based on a force balance model for single and multi-bubble translations respectively. A good agreement is found between the observed bubble movement trajectories and the theoretical predictions. After arriving on a surface, a micro-bubble starts to disturb the flow field near the boundary through its oscillation. The characteristics of the flow field are closely related to the bubble oscillation modes. The influence of a wall on the change of bubble oscillation mode during its translation toward the boundary was studied. The relationship between bubble oscillation modes and the corresponding microstreaming around the bubble was established. The experimental results of bubble oscillation modes and the flow motion are quantitatively in good agreement with the simulation results. From a mechanic point of view, a possible ultrasonic cleaning mechanism is explained by exploring the relationship between different torques that are exerted on micro and sub-micrometer-sized particles. This estimation provides a qualitative insight into the ultrasonic cleaning process at a moderate pressure amplitude. The experimental investigation of the complicated particle detachment process requires improved test equipment to be developed in the future.

Xiaotang Gu. ‘Estimating Uncertainties in Clamp-on Ultrasonic Flow Meter Measurements', Mechanical Engineering Department, Imperial College London, 2019

Gu-X-2019-PhD-Thesis

This thesis has two objectives. The first is to investigate the relative importance of the installation related setup parameter errors on uncertainties of a fully defined reference configuration of conventional clamp-on ultrasonic flow meters (UFMs). The second objective is to improve the overall accuracy of the UFMs by using a different ultrasonic transmission method to address the major contributors found in the first objective. The first objective relates to carrying out uncertainty analysis on conventional UFMs. There is very little work published reporting on the relative importance between these installation parameters and this is addressed in this thesis. First, a reference configuration of a conventional direct-path clamp-on UFM is presented. The upstream and downstream signals of this reference UFM were simulated using 2D finite element analysis including some simplifying assumptions to simulate the effect of flow. The uncertainties were quantified by comparing simulations that contain deliberately induced parameter errors in relation to the reference configuration. The uncertainty analysis revealed that the pipe roughness was the biggest contributing factor, approximately 2% for a moderately corroded pipe (RMS 0.2mm, correlation length 5mm). This is followed by the effect of wedge angle (>1%), pipe thickness (1%), wedge properties(1%) and pipe properties (<1%). To verify the simulation results, experiments were carried out by studying the effect of horizontal separation distance between transducers and internal pipe wall roughness on flow measurement error. The experimental result for separation distance matches well with the simulated result. The experimentally measured roughness related uncertainties also have the same magnitude levels as the simulation. The second objective is to address the important contributors found in the uncertainty analysis, internal pipe wall roughness. To reduce the roughness related uncertainties, leaky Lamb wave that uses longer wavelength and lower frequency (200kHz, 5 times lower than conventional UFMs) than the conventional UFMs was chosen. The ultrasonic signals of this leaky Lamb wave UFM were simulated using 2-D finite element (FE) analysis by unwrapping the pipe and only considering the cross section as a plate. The simulation results show that the uncertainties related to pipe wall roughness of leaky Lamb wave UFMs is approximately half of that of conventional UFMs for corroded pipe walls with RMS values larger than 0.1 mm (0.2, 0.35 and 0.5 mm). This method also improves the physical limit of the velocity sensitivity (60% improvement) and interrogates the average velocity of the whole volume of the fluid that passes the inspection area. Initial experiments were carried out. The experiments verified that a pure A0 mode can be transmitted using an EMAT transducer. The experiments also successfully detected the leaky A0 mode and prove the validity of using leaky Lamb wave in flow metering.

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Yuan Liu, 'Ultrasonic grain noise and defect monitoring in large grained materials', Mechanical Engineering Department, Imperial College London, 2018

In the next generation of power stations, high temperatures will be used to improve efficiency. As creep deformation is of great concern at such high temperatures, materials with large grains are desirable due to their high creep strength. However, this brings a challenge in ultrasonic monitoring of the plants, because when the grain size is
comparable with the wavelength, scattering caused by the acoustic impedance contrast on the grain boundaries can strongly attenuate the signals from defects and significantly increase the noise, which often results in low signal to noise ratios (SNRs) and makes the defect detection difficult.
In this thesis, the potential of two techniques to overcome the low SNR problem, baseline subtraction with repeat scanning and optimising the transducer and placement for monitoring with permanently installed sensors, is investigated. In order to find the optimal permanently installed monitoring system, it is desirable to understand the noise behaviour in highly scattering regimes, which cannot be fully described with existing theoretical models. Therefore, 3D FE modelling, which can achieve it, is proposed as an alternative approach for noise prediction.