Fracture process zone in granite

In uniaxial compression tests performed on Aue granite cores (diameter 50 m m, length 100 mm), a steel loading plate was used to induce the formation o f a discrete shear fracture. A zone of distributed microcracks surrounds th e tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samp les. Propagation velocity of the process zone is varied by using the rate o f acoustic emissions to control the applied axial force. The resulting velo cities range from 2 mm/s in displacement-controlled tests to 2 mu m/s in te sts controlled by acoustic emission rate. Wave velocities and amplitudes ar e monitored during fault formation. P waves transmitted through the approac hing process zone show a drop in amplitude of 26 dB, and ultrasonic velocit ies are reduced by 10%. The width of the process zone is similar to 9 times the grain diameter inferred from acoustic data but is only 2 times the gra in size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acou stic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic e vents show similar orientation of nodal planes on both sides of the shear f racture. The ratio of the process zone width to the fault length in Aue gra nite ranges from 0.01 to 0.1 inferred from crack data and acoustic emission s, respectively. The fracture surface energy is estimated from microstructu re analysis to be similar to 2 J. A lower bound estimate for the energy dis sipated by acoustic events is 0.1 J.