Estimating and Interpreting an "Average'' Strength from Richtmyer-Meshkov Instability Experiments.

Richtmyer-Meshkov Instabilities (RMI) have recently been used to estimate metal strength at strain rates of about 10\textasciicircum 7/s. RMI experiments involve shocking a metal interface with sinusoidal perturbations that invert, grow, and possibly arrest subsequent to shock in a way very sensitive to deviatoric strength. All published RMI-based strength estimates report a strength averaged over the duration of the experiment interrogated by the diagnostic. Since strain, strain rate, temperature and pressure all affect strength and all vary in the experiment, what does ``average'' strength mean, and is it useful for calibrating a high-rate constitutive model? In this study, we use a series of numerical simulations to establish the regimes and extent of those variables to which the instability is most sensitive. We assess assigning the strength estimate to a point in (strain, strain rate, temperature, pressure) space and using that in to aid in calibration of a strength model. We then apply the findings to experimental RMI data on copper at five different perturbation sizes so we can attempt to fit a PTW constitutive model and reproduce the data. Finally we compare the estimate of average strength to other estimates of high-rate strength in copper.