Materials Design Graduate Research Prize

Beñat Gurrutxaga Lerma wins the Materials Design Graduate Research Prize 2012

The Materials Design Graduate Research Prize for 2012 has been awarded to Beñat Gurrutxaga Lerma for his work on dislocation mediated plasticity at very high strain rates that is breaking new ground in dislocation dynamics. His project is supervised by Dr Daniele Dini and Dr Daniel Balint from the Department of Mechanical Engineering, Dr Daniel Eakins from the Institute of Shock Physics and Prof Adrian Sutton from the Department of Physics. However his supervisors all admit that they have had to work hard to keep up with him!

Beñat exposed a spectacular failure of conventional, quasistatic, dislocation dynamics approaches to model plasticity induced by a shock front as it propagates through an elastic continuum at the speed of sound. In such approaches no account is taken of the finite time required for the stress created by a dislocation nucleated behind a shock front to be transmitted to other parts of the specimen. As a result dislocation sources are nucleated ahead of the shock front by fields generated instantaneously by dislocations behind the shock front. It is plainly wrong that dislocations are generated ahead of a shock front because no elastic signal can travel faster than the speed of sound. This fundamental failure of all quasistatic approaches to dislocation dynamics rules them out as acceptable methods to simulate shock physics experiments.

Building on the pioneering work of Xanthippi Markenscoff, Beñat has set about the development of a new dislocation dynamics, which solves the time-dependent equations of elasticity. He has derived the time-dependent elastic fields for the creation and annihilation of straight edge dislocations. Together with Markenscoff’s time-dependent solutions for the elastic fields of edge dislocations moving at non-uniform velocities these are the fundamental building bricks of a new 2D elastodynamic approach to dislocation dynamics. This has been pains-taking work, which is now being written up in a long paper for the Proceedings of the Royal Society.

Commenting on the most challenging aspect of his research, Beñat says, “Elastodynamics have been around for 150 years, and I barely knew the basics when I began, so there was quite a lot to catch up with. Besides, the field equations I use offer a closed form solution expressed in the form of the time derivatives of elliptic integrals of the first, second and third kind that are full of singularities. Solving them analytically was out of question, and the numerics have involved quite a lot of tweaking.”

He is now writing the first code to simulate real dislocation dynamics. He has already shown that with his code he does not see the spurious creation of dislocations ahead of a shock front. He has also developed an ingenious strategy for parallelizing his code in the time domain rather than the space domain. These achievements in just the first nine months of Beñat’s PhD are truly outstanding. Beñat himself cites the most satisfying aspect of his work as “the fact that we are pretty much in the dark at the moment: all we know is that we have set foot on shore, and that the land exists. So exciting!”

Looking to the future, Beñat has no doubt that there is much work to be done: “There are many challenges ahead. On one hand, right now we are still not able to account for relaxation processes arising from lattice compressions; anisotropy is still work in progress. On the other hand, dislocations are set to move non-uniformly at unusually large speeds; there is no experimental data or simulations describing how this motion really occurs, but there is a lot of work suggesting effects often neglected at low strain rates (such as electronic effects) might be relevant.”