SHEAR BAND FORMATION IN GOSFORD SANDSTONE

New experimental results show unequivocally, and for the first time in a material with cohesion, that shear band formation under plane-strain conditions occurs in the hardening regime; that is, when the hardening modulus is still positive. Initial localization is inferred to be accommodated by rigid-body grain motions through the matrix, followed by only minor grain cracking occurring before peak stress. Evaluation of the hardening modulus at shear band initiation is made on the basis of derived friction and dilatancy angles for this strongly non-associated material. This modulus is greater than the critical hardening modulus derived from a Mohr-Coulomb type yield function as well as greater than that derived from a Drucker-Prager yield function. We infer that these constitutive models require modification.