NONLINEAR RATE-DEPENDENT DEFORMATION UNDER COMPRESSION DUE TO STATE-VARIABLE FRICTION

Observations of strain-rate effects in brittle failure under compressi on have been interpreted as suggesting that sub-critical crack growth plays an important role in the deformation process. We suggest an alte rnate interpretation in which strain-rate effects are attributed to ra te- and state-dependent friction along parent cracks. Using independen tly measured friction law parameters we demonstrate that a wing crack model incorporating state variable friction replicates the observed de pendence of the stress-strain curve on applied strain rate. Model resu lts are essentially equivalent to those of an earlier model incorporat ing sub-critical crack growth, suggesting that despite their different conceptual foundations, the validity of either model can not be deter mined on the basis of the observed strain-rate effects. However, only the rate-dependent friction model reproduces the observed unsteady gro wth of wing cracks.