Fracture Mechanics Study on Process Zone in Brittle-Microcracking Materials

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The objective of this research is to develop a fracture mechanics model for characterizing the process zone growth in brittle-microcracking materials. A microcracking fracture process zone is known to accompany tensile macrocrack propagation in quasi-brittle materials such as rock, concrete and ceramics. The fracture process zone is suggested to be largely responsible for the dependence of fracture mechanics data on the specimen size and geometry.In this study, we examine the fracture process of granites. An ultrasonic timing method was employed to detect the extent of the process zone during fracture toughness tests of granites. The travel time of longitudinal waves propagating through fractures was monitored using 500-kHz transducers with a commercially available equipment. It was demonstrated that the travel time technique can locate the tip of the process zone continuously during fracture experiments. The ultrasonic wave measurements revealed that the process zone length is not a m … More aterial property but dependent on specimen size, indicating the importance of characterizing the process zone growth for the fracture toughness evaluation. The tension-softening model is chosen to describe the development of the process zone in the granites. The J-based technique, which has recently been proposed by Li et al. for concrete, was used to determine the tension-softening curve. A boundary element analysis was carried out to simulate the process zone growth in the fracture toughness specimens based on the tension-softening relation. The growth behavior of the process zone observed by the ultrasonic method was shown to agree with the results of numerical analysis. This supports the use of the tension-softening model to characterize the process zone growth in the granites. A method was developed for determining the critical J-integral value, Jc on the basis of the process zone length, and the measured Jc values were compared with those obtained by Li's proposed multiple-specimen technique. It was shown that the present method permits a valid fracture-toughness value to be evaluated by use of a single fracture specimen. Less