The fracture process zone in granite: evidence and effect

Evidence of the fracture process zone and its effect on fracture toughness were examined for Charcoal and Rockville granite, average grain sizes of 1 and 10mm. Fracture tests were conducted on wedge-loaded, double cantilever beam specimens, 500mm long × 200 mm wide × 40 mm thick. The seismic techniques of ultrasonic probing and acoustic emission were utilized to measure the inelastic region. The beginning of the process zone was interpreted by a three-fold increase in transmission of ultrasound from an open crack to a partially closed crack; the end of the damage zone was found by comparing surface wave amplitudes before and after fracture. Following these procedures, the inelastic region in Charcoal granite was about 40 mm; whereas in Rockville granite, a region over 90 mm long was estimated. The major acoustic activity was located within the macrocrack, being released from unbroken or interlocked crystals. The experimental evidence indicated an effective crack to be composed of a traction free length and a ligament process zone, which was observed to form a single, multiconnected region within the macrocrack. The effective crack was used for an R-curve calculation, so that the energy consumed in the process zone was included in an approximate manner by elongating the crack length. A model of crack propagation in rock, consistent with measurements of the inelastic region, describes the effect of a process zone on fracture toughness testing.