Slip propagation along frictional discontinuities

Slip initiation and propagation along non-homogeneous frictional surfaces are investigated by loading specimens of gypsum in biaxial compression. The specimens used in the tests are composed of two individual blocks with perfectly mated contact surfaces. The contact surfaces have on their upper half a frictional strength smaller than on their lower half. This creates a “weak” surface on the upper half and a “strong” surface on the lower half. Four test series are performed using specimens with different surface characteristics along the contact surface. The experiments are conducted by applying first a normal stress across the frictional surface and then increasing the shear stress until final debonding and slip along the strong surface occur. The magnitude of the normal stress used in the experiments ranges from 0.7 to 15 MPa (about 50% of the unconfined compression strength of the material). Slip starts on the weak area and, as the shear stress is increased, propagates towards the strong area. Full slip along the weak area introduces a sharp transition between the area of the discontinuity that has slipped and the area that has not. This transition creates a large concentration of stresses which can be treated within the framework of fracture mechanics as a mode II frictional crack. With further loading, rupture occurs through the strong area as an unstable process that coincides with failure. The results show that the critical energy release rate GIIC is a good indicator of the rupture. However, GIIC is not a material property. It depends on the normal stress applied, on the frictional characteristics of the interface, and on the critical slip required for the transition from peak to residual. A slip initiation model is proposed based on experimental observations and on fracture mechanics theory and is incorporated into a finite element method code. Predictions with the model compare very well with experiments.