CRACK-CRACK AND CRACK-PORE INTERACTIONS IN STRESSED GRANITE

Scanning electron microscope observations of stress-induced crack growth, causing crack-crack and crack-pore interactions within individual grains of Barre granite, show that photoelastic models of cracks are essentially correct and applicable to rock. The stress placed externally on a sample of rock is not the same stress, in magnitude or direction, experienced by cracks within individual grains. Observations of crack-crack and crack-pore interactions can be used to infer the local and regional stress directions, with some ambiguity. Two basic types of microcrack linking, termed en echelon and en passant are delineated. The former is often a result of relatively large shear stress concentrations between the crack tips in the array. The latter is a combined result of the interaction of the stress fields around two approaching crack tips and the regionally applied stress. Except for some en echelon linking, almost all crack growth within grains is an extensional phenomenon. © 1979.