Multiscale Richtmyer-Meshkov Instability Experiments to Isolate the Rate Dependence of Strength at Ultrahigh Strain Rates

A series of experiments to isolate strain rate effects on strength made use of an important observation about scaling Richtmyer-Meshkov Instability (RMI) perturbations to change the strain rate. In each experiment, a copper target was shocked to 25 GPa on a gas gun. Each individual target had three regions of sine wave perturbations with the same non-dimensional sine-wave amplitude h₀k, but the dimensional scale spanned a factor of about five with wavelengths of 65, 130, and 320 mm. In such an experiment, all three regions see identical shock loading and very similar amounts of plastic deformation, but the strain rates in the instability scale with the inverse of the wavelength. The instability growth and arrest were measured with Photon Doppler Velocimetry and showed a clear trend of the maximum instability velocities increasing with increasing wavelength. The experiment was repeated three times with different values h₀k spanning a factor of three to explore different levels of instability growth, and each experiment showed similar velocity trends. The strengths estimated from the experiments increased by about 70% when the strain rate increased from about 8.7 × 10⁶/sec to 3.3 × 10⁷/sec. When plotted vs. strain rate along with strength data from the literature up to about 10⁴/sec, the “upturn” in strength magnitude and the slope of the dependence are both striking. The location of the upturn does not quite agree with extrapolations from inconsistent literature observations at lower rates.