Experimental analysis of crack propagation in granite

An experimental approach is used to demonstrate the concept of an effective crack length with double-edge-notched specimens of Charcoal and Rockville granite. Crack propagation in rock is characterized by microcracking around the crack tip and interlocking along a portion of the crack; this region is called the fracture process zone, and together with the fraction free length defines the effective crack length. Twenty closed-loop, strain-controlled fracture tests were conducted on Charcoal and Rockville granite. Crack growthwas monitored with a travelling behaviour of both granites, it is suggested that the process zone is larger for the larger grain-sized rock (Rockville). Conventional linear elastic fracture mechanics techniques are used to calculate the apparent fracture toughness of Charcoal granite at various crack lengths. In addition, a J-integral expression is derived for the double-edge-notched geometry in terms of the area under the load-displacement record. The shortcomings of both analyses are discussed. An attempt is made to explain the inadequacies by including the process zone in the calculations of the fracture toughness.