True Triaxial Yielding and Hardening of Rock

Abstract

Analysis of test results for intact and granulated marbles (cemented and noncemented granular materials) adds to the indications that strain softening does not express a real material behavior; it highlights similarities in stressstrain relations and confirms equality of a stress and a plastic strain invariant. The physical meaning of the parameters of a yield equation, which express energy dissipating in frictional and nonfrictional processes and define the deviation from the normality condition, is shown. A hardening rule that takes into account friction and dilatancy is formulated from yield relations and from an experimentally proved consistency condition. Variation of constitutive parameters and evolution of various yield states is considered together with application of the hardening rule. Variation of the normality condition deviation angle along yield states obtained under low and very high confinement is constitutively modeled. Meridian and deviatoric sections of yield surfaces, obtained by low and high pressure experiments and substantiated by results compiled from the literature, are considered. A polyaxial yield equation and hardening rule are proposed and experimentally verified.