Crack Interaction with a Frictional Interface in a Rock-Model Material: An Experimental Investigation

A series of experiments on homogeneous gypsum specimens, used as a rock-model material, containing two pre-existing open flaws and a frictional interface has been performed under uniaxial compression. The specimens are 203.2 mm high, 101.6 mm wide, and 25.4 mm thick. The two flaws, with 0.1 mm aperture and 12.7 mm length, are created through the thickness of the specimen. The spacing (S) between flaws, continuity (C), and inclination angle measured from the horizontal (β) define the geometry of the flaws. The specimens discussed in the paper have three different flaw geometries: S = 0, C = − 2a = − 12.7 mm, β = 30° (a left-stepping geometry); S = 2a = 12.7 mm, C = a = 6.35 mm, β = 30° (an overlapping geometry) and S = 3a = 19.05 mm, C = 0, β = 30° (a right-stepping geometry). Unbonded interfaces with two different roughness are created by casting the specimens in two parts. The first half of the specimen is cast against a PVC block with an inclined face (i.e. 90°, 80° or 70°) with respect to the vertical axis of the specimen. The second half is then cast against the first one. Digital image correlation is used to monitor crack propagation on the specimen surface by measuring displacements during loading. The experiments indicate that the interface itself is an important contributor to new cracks and its presence in the specimens reduces crack initiation stress. Furthermore, the change in roughness and inclination of the interface affect coalescence patterns and incident crack angles.