Possibility of shear type fracture in viscoplastic material under confining pressures

Transition from opening-mode wing crack growth to shear type fracture has been investigated by conducting triaxial compression tests under confining pressures, using epoxy resin cylindrical specimens. The epoxy resin used exhibited viscoplastic deformation characteristics under confining pressures, which is expected to simulate the nonlinear deformation observed commonly for rocks at great depths and to provide useful insight in understanding of the fracture transition in rocks. Under the range of confining pressures, 10 MPa-30 MPa, an array of opening-mode wing cracks were initiated from a preexisting inclined penny-shaped crack, as the axial stress was increased. Only the extension of a wing crack was observed to take place under no confining pressure condition. It was shown that the growth of the wing cracks were suppressed when higher confining pressures were applied and shear type fracture consisting of several wing cracks was induced as the deformation progressed. The experimental observation suggests the possibility of occurrence of shear type fracture in such a homogeneous material under higher confining pressures, and the mechanical interaction between cracks plays a crucial role in the generation of shear type fracture rather than the inhomogeneity of rocks.