SOME COMMENTS ON PLASTICITY POSTULATES AND NONASSOCIATIVE FLOW RULES

The plasticity postulates of Drucker and Ilyushin do not in general ap ply to materials whose mechanical behavior is described by non-associa tive flow rules, although for many stress and strain cycles and under many loading conditions the corresponding work inequalities defining t he postulates are not violated. This is illustrated in this paper by c onsidering pressure-dependent dilatant materials subjected to combined shear-pressure, add triaxial compression loadings. Both, hardening an d softening ranges of material response are analyzed. It is shown that , depending on the relationship between the dilatancy factor and the i nternal friction coefficient, the net work of added stresses in some t ypical closed stress cycles can be either positive or negative, while the net work in the corresponding closed strain cycles is in each case positive. Constitutive implications and the relationship between plas ticity postulates and actual material stability are also discussed.