Ultrasonic Monitoring of the Fracture Process Zone in Fracture Toughness Tests of Granite

A microcracking fracture process zone is known to accompany tensile macrocrack propagation in rocks. The fracture process zone with nonnegligible length is suggested to be largely responsible for the dependence of fracture mechanics data on the specimen size and geometry. In this paper, an ultrasonic timing method is employed to detect the extent of the fracture process zone (FPZ) during fracture toughness tests of granite, where the travel time of longitudinal waves propagating through fractures is monitored using 500-kHz transducers with commercially available equipment. It is demonstrated that the travel time technique can locate the tip of the FPZ continuously during fracture experiments. The growth behavior of the FPZ observed by the ultrasonic method is shown to agree with the previously published results of numerical analysis that is based on the tension- softening model. Finally, a method is developed for determining the critical J-integral value, J_c on the basis of the length of the FPZ, and the measured J_c values are compared with those obtained by Li’s proposed multiple-specimen technique. It is shown that the present method permits a valid fracture- toughness value to be evaluated by use of a single fracture specimen.