Synopsis: Doubly Shocked
A shock wave is a pressure jump that propagates at supersonic speed. In solids, an impact or other sort of shock can generate two waves: an “elastic” compression wave followed by a separate, slower “plastic” wave that irreversibly deforms the material. At high impact intensity, the elastic wave was thought to disappear, resulting in a single plastic wave. But in a paper in Physical Review Letters, Vasily Zhakhovsky of the University of South Florida, Tampa, and his colleagues now report simulations of high-pressure shock waves, in which a thin layer of elastic compression “paves the way” ahead of the advancing plastic wave front.
Prior models assumed that such a “two-zone” state should not exist, since any elastic zone would presumably be overridden by plastic deformations at the leading edge of a strong shock wave. However, previous simulations were not able to follow the wave propagation in detail over long periods of time. The reported new molecular dynamics technique avoids this problem by simulating the shock wave inside a moving window. Material “flows” in from one edge of the window and out the other, as you would see if you filmed a wave while surfing on top of it. For a certain range of pressures, the authors identified an elastic compression zone that preceded the plastic deformation zone by a constant average distance that approaches a micron. The authors claim this could explain results of recent experiments studying shock waves in aluminum. – Michael Schirber