Crack initiation, propagation and coalescence from frictional flaws in uniaxial compression

An extensive experimental program has been conducted on pre-cracked specimens of a rock-model material to investigate crack propagation and coalescence from frictional discontinuities. Prismatic gypsum specimens have been prepared with three pre-existing closed cracks (flaws). The flaws all have a constant length of 12.7 mm and are parallel to each other. Different geometries are obtained by changing the angle of the flaws with respect to the direction of loading, the spacing, and the continuity of the flaws. In the experiments, three different types of cracks have been observed: wing cracks, coplanar shear, and oblique shear cracks. These are the same types of cracks observed with open flaws. Crack initiation occurs simultaneously at all the tips of the flaws for wing and shear cracks. Mean crack initiation stress is higher for secondary cracks than for wing cracks. The differences however decrease as the flaws are oriented at smaller angles with the direction of loading. The types of coalescence (i.e. the type of cracks and crack pattern that link two flaws) from closed flaws are similar to those from open flaws. However, the type of coalescence observed in a specimen with open flaws is not necessarily produced when using the same geometry but with closed flaws. The most important conclusion reached in this research is that the fracturing processes in open and closed flaws are similar. Friction along the flaws increases the initiation and coalescence stress and favors linkage through shear cracks.