Micropolar hypoplasticity for persistent shear band in heterogeneous granular materials

Persistent shear bands in heterogeneous materials develop progressively, rather than instantaneously. For a meaningful capture of this deformation pattern, the band must develop naturally and freely up to the peak load and well into the softening regime. We use a new micropolar hypoplastic framework to capture the development and propagation of a persistent shear band in a rectangular specimen of silica–concrete sand with spatially varying density. The constitutive framework includes a characteristic length as a regularization parameter, as well as a couple stress representing the angular momentum generated by particle spins. We use computed tomography (CT) imaging and digital image processing to quantify the spatial density variation of the sand, and a void ratio-dependent constitutive model to capture the effect of variable density. Results of the numerical simulations demonstrate the capability of the proposed micropolar hypoplastic framework to track the development and propagation of a persistent shear band in a heterogeneous sand up to the peak load and well into the softening regime.