Role of weak flaws in nucleation of shear zones: an experimental and theoretical study

This paper investigates the role of inherent weak flaws in the formation of plastic zones in deforming solids to understand the development of geological shear zones. Physical experiments were carried out on polymethylmethaacrylate (PMMA) models containing single and multiple circular cylindrical flaws under compression, maintaining plane strain condition. Models with single flaws show development of shear zones against a flaw in the form of conjugate sets with an average dihedral angle of 84° and oriented at an angle of 42° to the bulk compression direction. The shear zones are generally tapered, with increasing width away from the flaw. In models with multiple flaws, shear zones nucleated against individual flaws, which propagated and coalesced with one another, forming through-going, band-like shear zones with inclination varying from 35° to 53° with the bulk compression direction. With an increase in flaw concentration, the through-going shear zones defined persistent conjugate sets. We applied the plane theory of elasticity for numerical simulations of ductile shear zones under the influence of a single circular weak flaw. The pattern of shear zones yielded from numerical runs grossly matches with those observed in physical model experiments. Theoretical analysis demonstrates that the presence of a flaw promotes nucleation of shear zones at a bulk stress below the yield strength of matrix. This critical stress is a non-linear function of the flaw–matrix competence contrast, and decreases asymptotically with increasing competence contrast.