University of South Florida
USF physicists discover super-elastic shock wave[09.28.2011]
TAMPA, Fla. -- A new and better way to observe how high speed, powerful shock waves move through solids -- and how the solids consequently respond -- has been developed by University of South Florida physicists.
The traditional view is that when subjected to shock waves, solids at relatively low pressures exhibit an elastic state in which the crystalline lattice eventually returns to normal but, upon exceeding a threshold pressure, enters an irreversible plastic state. With greater time and length scale resolution for observation offered by a new atomic-scale computer simulation method called “moving window” molecular dynamics, researchers have been able to observe that solids under great compression by high intensity shock waves also exhibit a “super-elastic state” which, according to conventional view, should not exist.
Such phenomenon is difficult to observe using standard molecular dynamics, or even in experiment.
This research was carried out by physicists at the USF and their colleagues at the Landau Institute of Theoretical Physics in Chernogolovka, Russia and at the Naval Research Laboratory (NRL) in Washington, D.C. Their study was funded by the National Science Foundation within the Materials World Network program and the Office of Naval Research, both directly and through NRL. The simulations were performed using NSF funded Teragrid supercomputer resources and the results are published in the current issue of Physical Review Letters and spotlighted at the American Physical Society Physics website.
“Shock waves propagate through solids at supersonic speeds,” said first author Vasily Zhakhovsky, a research associate professor in the USF Department of Physics. “If shock waves are weak, they produce an elastic state. However, if they are powerful enough they can produce an irreversible plastic state in solids. By examining high intensity shock waves using the “moving window” molecular dynamics simulation technique, we were able to discover a ‘super-elastic state’ in these waves. Contrary to the traditional view, we have found that such a shock wave has a two-zone structure: a leading elastic zone followed by a plastic zone, both moving at the same speed. The elastic front is over-driven by the plastic front, but it is not overrun.”
According to Ivan Oleynik, professor of Physics at USF, who directed the research, “because the new method simulates the shock wave propagation in the reference frame moving together with the wave, it decouples time and length scales, thus allowing simulations for an indefinite time. The extra observation time gave us an opportunity to study with greater resolution the complicated structure of materials response affected by the shock wave, which includes complex dynamics of dislocation creation and multiplication.”
According to Oleynik, the time scale of traditional, piston-driven simulation methods was inadequate.
“That method could not expose the super-elastic state behavior in solids under extreme compression,” he said. “To see this effect, you need long time and large length scale resolution. The new simulation method we developed has greater resolution, giving us the opportunity to observe a complete structure of the shock wave, including the leading super-elastic zone which, due to its metastability, eventually decays into the plastic state.”
As demonstrated by the image, two-zone super-elastic-plastic wave possesses rich internal structure: the synchronization of super-elastic and plastic zones occurs via ultra-short elastic pulses emitted by the plastic shock front. Both the elastic and plastic fronts move with the same speed, with a fixed net thickness that can extend to microns.
“The metastable super-elastic state within two-zone elastic-plastic shock wave structure is a general phenomenon, which was observed by us by sending high-intensity shock waves through various materials including aluminum, nickel, gold and diamond,” Oleynik said.
To provide experimental proof of the existence of the super-elastic-plastic two-zone structure, the time delay between arrivals of the elastic and plastic fronts at the back surface of sample solid will have to be measured with several picoseconds (one-trillionth of a second) resolution. The theoretical team is currently working with experimental collaborators at Lawrence Livermore National Laboratory and Russian Academy of Sciences to observe such super-elastic state within the two-zone shock wave using femtosecond (one quadrillionth of a second) lasers.
“Our work is an important step toward a better understanding of the fundamental mechanisms of shock-induced plasticity, thus providing a new insight into materials behavior at extreme conditions,” Oleynik said.
The authors suggest that the two-zone, elastic-plastic single wave phenomenon should be considered in any future study of shock-induced elastic-plastic transitions. Their discovery could aid in developing atomic-scale mechanisms of initiation and propagation of condensed-phase detonation. Such fundamental understanding is urgently sought to devise safe but powerful explosives.
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