Because of the Roadrunner supercomputer’s unique capability, scientists are for the first time attempting to create atomic-scale models that describe how voids are created in materials, mostly metals, how they grow, and merge; how the materials may swell or shrink under stress; and how once broken bonds might reattach, and they’re doing it at size and time scales that approach those of actual experiments, so that the models can be validated experimentally.
Using the reliable SPaSM (Scalable Parallel Short-range Molecular dynamics) code, adapted to run on Roadrunner, Tim Germann of DOE's Los Alamos National Laboratory is studying the physics of how materials break up, called “spall,” and how pieces fly off, called “ejecta,” from thin sheets of copper as shock waves force the material break apart.
“Our multibillion-atom molecular dynamics code is providing unprecedented insight into the nature of the critical event controlling the strength of materials, a fundamental long-standing problem in materials science,” said Germann.
Some phenomena that can lead to “spall failure” as the material breaks apart, take place at precisely the time and length scales which were inaccessible to both simulation and experiment, and so have typically been described by “trial and error” models that could never be directly verified.
Steady advances in both experimental and simulation techniques — and supercomputer performance, culminating with Roadrunner — have closed this gap and are now enabling both simulations and experiments to probe shock deformation at between 1 and 10 microns, and at nanosecond time scales. Spall failure and the ejection of material from shocked metal surfaces are problems that have attracted increased attention both experimentally and theoretically at Los Alamos. Models are required that can predict both when a material will fail, and the amount of mass ejected from a shocked interface with a given surface finish and strength.