- 2018 Dynamic crack growth is analyzed numerically for a planar bimaterial block with an initial edge crack subject to mode II impact loading along the side of the block. The crack is constrained to grow along a weak interface directly ahead of the initial crack tip. A cohesive constitutive relation, that relates tractions and displacement jumps, is specified across the weak interface. The normal traction along the interface is taken to be a linear function of the normal displacement across the interface while the shear traction is taken to be a nonlinear function with the shear traction approaching zero for large displacement jumps, thus allowing mode II crack propagation. The material on the impact side of the interface is taken to be a linear elastic (actually hypoelastic) solid while the material on the other side of the interface is taken to be an isotropically hardening elastic-viscoplastic solid. Both plastically incompressible and plastically compressible solids are considered. Results are presented for the effects of plastic deformation on the evolution of near crack tip fields, on the crack speed history, and on the tractions that develop at the interface. The effects of various parameter variations are also explored. It is found that plasticity can result in tensile normal tractions developing on the interface in the crack tip vicinity and that the transition to an intersonic crack speed via a daughter crack mechanism can take place in the presence of small amounts of plastic straining.