Transition from a crack-type to a supershear-type to a spall-type mode of separation for tensile loading of an elastic solid with a weak interface

Dynamic mode I crack growth in a sheet with an edge pre-crack subject to remote impact tensile loading is investigated experimentally and computationally. Separation is constrained to occur along a weak interface directly ahead of the pre-crack tip. The experiments are carried out on a PDMS sheet composed of two sheets glued together to make the weak surface in front of the pre-crack. The thickness and composition of the glue are varied to provide different cohesive properties. In the calculations, the sheet material is represented by an isotropic hyperelastic constitutive relation and the weak interface is represented by a zero thickness cohesive surface with the cohesive traction related to the displacement jump across the interface. The calculations are in qualitative agreement with the experiments for the propagation speed, the shape of the opening along the interface and general features of the deformation distribution in the material. Both the experiments and the calculations indicate that a characteristic length scale, associated with the cohesive response of the interface plays a key role in affecting the propagation speed and the mode of separation. When the cohesive length scale is sufficiently small, propagation is crack-like and the propagation speed does not exceed the Rayleigh wave speed. An increased value of the cohesive length scale leads to a propagation speed that exceeds the shear wave speed. Transition to a spall-like separation mode occurs when the opening traction on the remaining ligament reaches the cohesive strength of the interface. A cohesive interface with a larger value of the work of separation can have a faster separation speed than one having the same cohesive strength but a smaller value of the work of separation. For calculations with loading imposed on the faces of the pre-crack, so that propagation occurs into unstressed material, the propagation speed does not exceed the Rayleigh wave speed even for a very weak interface.