A STUDY OF THE FRACTURE PROCESS ZONE IN ROCK (CRACK PROPAGATION, FRACTURE TOUGHNESS, ACOUSTIC EMISSIONS, ULTRASOUND)

A study of the fracture process zone was conducted to gain an understanding of the physics of crack propagation in rock. The influence of microstructure was ascertained by examining granite of small and large grain size. A conceptual model of the fracture process was developed, and the effect of the inelastic region on the measurement of fraction toughness was described qualitatively with the proposed model.;The grain size seems to govern the extent of the inelastic region. For Charcoal granite, which has an average grain size of 1 mm, a process zone was observed to form a single multiconnected region within the macrocrack. A damage zone occurred in front of an effective crack tip. But as average grain size increases to 10 mm as for Rockville granite, crack growth becomes even more of a linking process, where pre-existing microcracks form a large damage zone.;The proposed model considers an effective crack to be composed of a traction free length and a ligament process zone, with a damage zone existing at the tip. The effective crack defines the length necessary to achieve the equivalent fracture resistance when using linear fracture mechanics. Thereby, the energy consumed in the process zone is included. Conventional methods, together with the effective crack length concept, were employed to calculate the apparent fracture toughness of both granites. The effects of the inelastic zone on toughness testing were explained by the proposed model.;Nondestructive testing techniques of ultrasonic probing and acoustic emissions were utilized to measure the inelastic region. Charcoal and Rockville granite were tested in the double cantilever beam and double-edge-notched geometries. The major acoustic activity was located along previously fractured surfaces; this energy was released from unbroken ligaments within the crack plane. Attenuation of surface waves delineated both ends of the inelastic region. The beginning of the process zone was interpreted by a threefold increase in transmission of ultrasound from an open crack to a partially closed crack; the end of the damage zone was found by comparing attenuation before and after fracture.