The effect of asymmetric damage on dynamic shear rupture propagation II: With mismatch in bulk elasticity

We investigate asymmetric rupture propagation on an interface that combines a bulk elastic mismatch with a contrast in off-fault damage. Mode II ruptures propagating on the interface between thermally shocked (damaged) Homalite and polycarbonate plates were studied using high-speed photographs of the photoelastic fringes. The anelastic asymmetry introduced by damage is defined by ‘T’ and ‘C’ directions depending on whether the tensile or compressive lobe of the rupture tip stress concentration lies on the damaged side of the fault. The elastic asymmetry is commonly defined by ‘+’ and ‘−’ directions where ‘+’ is the direction of displacement of the more compliant material. Since damaged Homalite is stiffer than polycarbonate, the propagation directions in our experiments were ‘T+’ and ‘C−’. Theoretical and numerical studies predict that a shear rupture on an elastic bimaterial interfaces propagates in the ‘+’ direction at the generalized Rayleigh wave speed or in some numerical cases at the P-wave speed of the stiffer material, P_(fast). We present the first experimental evidence for propagation at P_(fast) in the ‘+’ direction for the bimaterial system undamaged Homalite in contact with polycarbonate. In the ‘−’ direction, both theory and experiments find ruptures in elastic bimaterials propagate either at sub-shear speed or at the P-wave speed of the softer material, P_(slow), depending on the loading conditions. We observe that the off-fault damage effect dominates the elastic bimaterial effect in dynamic rupture propagation. In the ‘C−’ direction the rupture propagates at sub-shear to supershear speeds, as in undamaged bimaterial systems, reaching a maximum speed of P_(slow). In the ‘T+’ direction however the rupture propagates at sub-shear speeds or comes to a complete stop due to increased damaged activation (slip and opening along micro-cracks) which results in a reduction in stored elastic potential energy and energy dissipation. Biegel et al. (2010-this issue) found similar results for propagation on the interface between Homalite and damaged Homalite where rupture speeds were slowed or even stopped in the ‘T−’ direction but were almost unaffected in the ‘C+’ direction