Crack initiation and propagation from frictional fractures

An extensive experimental work has been conducted on pre-cracked rock-model materials. Prismatic gypsum specimens have been prepared with three pre-existing closed flaws. All the flaws have a constant length of 12.7 mm and are parallel to each other. Different specimens are prepared by changing flaw angle, spacing and continuity. The specimens are loaded in uniaxial compression until failure. In each test the nature of any new crack (tensile or shear), initiation stress and angle, and coalescence stress and pattern are recorded. Three different types of cracks have been observed: wing cracks, coplanar shear, and oblique shear cracks. These are the same types of cracks observed from open flaws. The most significant difference between open and closed flaws is the stress at which initiation occurs. For the same geometry initiation is systematically higher for closed flaws than for open flaws.