Measurement and description of tensile fracture in granite

A study of the process zone in Charcoal and Rockville granites, average grain sizes of 1 and 10 mm, is conducted using the double-cantilever-beam and double-edge-notched geometries. The fracture process for mode I (tensile) opening is inferred from ultrasonic probing, where the extent of the inelastic zone is interpreted by the energy loss in surface wave transmission, and from acoustic emission, where the locations of microseismic events outline the region of interest. For Charcoal granite, a zone of major acoustic activity is observed to form a single, multiconnected region within the fracture. Crack growth in Rockville granite is more discontinuous. In general, the tensile fracture process seems to be a gradual separation of surfaces along a preferential path set up by microcracking. This development of crack propagation accounts for the dissipation of acoustic energy from unbroken and interlocked crystals within a localized process zone, the length of which may be a substantial portion of the effective crack.