Stick-slip like behavior in shear fracture propagation including the effect of fluid flow

Shear-based fracture propagation in fluid-saturated porous materials is investigated using a displacement-pressure formulation that includes acceleration and inertial effects of the fluid. Pressure-dependent plasticity with a nonassociated flow rule is adopted to realistically represent the stresses in the porous bulk material. The domain is discretized using unequal order T-splines and cast into a finite element method using Bezier extraction. An implicit scheme is used for the temporal integration. The solid acceleration-driven fluid flow reacts to stress waves, but it results in pressure oscillations. Adding fluid acceleration terms dampens these oscillations and increases the fluid pressure near the fracture tips. By simulating a typical shear fracture case, it is shown that stick-slip like, or stepwise, fracture propagation occurs for a high permeability, also upon mesh refinement. The acceleration driven fluid flow results in a build-up of pressure near the fracture tip. Once this pressure region encompasses the fracture tip, propagation arrests until the pressure has diffused away from the crack tip, after which propagation is resumed and the build-up of pressure begins anew. This results in a stick-slip like behavior, with large arrests in the fracture propagation. Stepwise propagation related to the initial conditions has also been observed, but disappears once the fracture length exceeds the size of the region influenced by the initial conditions.