SIZE DOES MATTER: SHOCK WAVE SIMULATIONS SHOW HOW ATOMS SLIP
LOS ALAMOS, N.M., June 26, 1998 - A computer model is giving researchers a new way to explore and understand material properties as it reveals what happens, atom by atom, when a shock wave moves through a metal.
In a paper published today in the journal Science, researchers at the U.S. Department of Energy's Los Alamos National Laboratory describe a 10 million-atom, three-dimensional simulation that shows how a shock wave moving through an idealized crystal leaves behind a mosaic of intersecting faults. The simulation provides valuable insights for materials design and engineering codes.
"We are at the stage now that our molecular dynamics simulations are large enough that we can provide serious guidance to experimentalists," said Los Alamos physicist Brad Holian. "We are on the threshold of bringing experiment and engineering together to provide insight into the fundamental physics of materials."
Holian and his colleague Peter Lomdahl slam an idealized cubic structure against what they call a "momentum mirror" that sends a shock wave along the internal lattice points of the crystal. Atoms respond according to Newton's equations of motion. The model shows how the atoms slip at an angle to the face of the crystal along preferred directions. As the stress is relieved, the slight slippage leaves behind planes that appear to be one notch out of arrangement. Slight fluctuations lead the shock along four planes, leaving cubic fault lines.
Molecular dynamics studies, especially the effects of shock waves, are fundamental to improving the physics and engineering codes necessary for the nation's nuclear weapons stockpile stewardship program.
While previous calculations modeled one tiny corner of a cube and hinted at a single slip fault, the new model represents a large enough cross-sectional area to see definitively the mosaic pattern of slippage.
Advances in computing power and the molecular dynamics code developed at Los Alamos recently have made large, 3-D simulations possible. Even with available massively parallel computers, the current model represents a wave passing through a cube only a few millionths of a meter across in less than a nanosecond. Nevertheless, the calculations bring physicists one step closer to marrying computation and engineering.
"We've looked at perfect crystals and shown they will yield in this interesting manner when the shock strength is above a threshold value," Holian said. "We've shown preexisting defects can trigger and effect the slippage. Now we need to run models of samples made with grain boundaries and different orientations that will respond at various speeds, directions and plasticity changes." The molecular dynamics calculations were all performed on a SUN Microsystems 12 node Ultra Enterprise 4000 system with 3 gigabytes of memory.
Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.
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Note to reporters: a color image of the shock wave simulation is available in JPEG format.
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