Fracture process zone in granite: a microstructural analysis

Shear fracture propagation in rock is accompanied by localized microcrackin g in a process zone surrounding the fracture tip. We investigated the crack microstructures along experimentally formed shear fractures from four gran ite samples (uniaxial compression tests). Five transects across a macroscop ic fracture were inspected optically in transmitted light. Five hundred thi rty-two photomicrographs were taken from seven study areas along each trans ect. We determined length, width, density, and orientation of open cracks a nd their assignment to intra-, transgranular, or grain-boundary cracks. Cra ck density decreases with increasing distance to the macroscopic shear frac ture and toward the fracture tip. The highest crack densities correlate wit h the maximum number of acoustic emissions. Most cracks enclose a small ang le (0-20 degrees) with the macroscopic shear fracture. Intragranular cracks are more abundant than transgranular and grain-boundary cracks. The number of transgranular cracks increases towards the macroscopic shear fracture, but the number of grain-boundary cracks decreases. The decrease in crack de nsity with increasing distance to the fault is accompanied by a change from strongly preferred crack orientation in the fault core to a random crack d istribution away from the fault. Fracture process zone widths range from 2. 1 +/-0.8 mm (Ag51r) to 5.6 +/-1.9 mm (Ag18r). The ratio of process zone wid th to fault length is approximately 0.04-0.07. This observation agrees with observations from natural fault zones. The fracture surface energy ranges from 0.2 to 1.2 J. This corresponds to < 10% of the total strain energy.