On the measurement of shear-strength in quasi-isentropic loading

It has been a subject of discussion that the rate of loading of a material determines the strength it appears to exhibit. This observation has implications for understanding and modelling the deformation mechanisms involved in high-strain rate and shock-loading. Discussion has focussed upon the response of various metals. The loading has ranged from prompt shock to pillow loads (graded density impactors) which give uniformly rising pulses over varying times. One variant upon such loading is to allow the stress to rise in steps to a final Hugoniot level which averages to a lower rate. Clearly, while the applied strain-rate during the rise in the steps is still of the same order as that in shocks, the loading is more quasi-isentropic in nature. As a means of observing the strength of a material, the lateral stress in the sample may be monitored along with the longitudinal stress therefore allowing a direct measurement (in uniaxial strain) of the difference between these two values. Potential mechanisms explaining the experimental results and comments on the interpretation of the experimental diagnostics and design is given.