Significance of AE crack monitoring in fracture toughness evaluation and non-linear rock fracture mechanics

This paper describes results of experimental research in which the fracture toughness of a granite is determined by means of the J-integral approach and an acoustic-emission (AE) technique. Compact tension specimens and three-point bend specimens of various sizes were tested, and the effects of specimen size and loading condition on the fracture toughness were examined. Concurrently with the fracture toughness tests. AE monitoring was conducted, and the relation between the AE characteristics and fracture behavior was examined. Energy measurements of AE signals and frequency spectral analyses were made to characterize the AE signals. The AE measurements were used to determine a fracture toughness evaluation point and to obtain the critical J-integral value, denoted by J_iAE. It is demonstrated that the J_iAE value is reasinably constant for a wide range of specimen sizes and specimen types. Based on the result, the fracture toughness of the granite K_Ic was then computed from the J_iAE value using the conversion equation under the assumption of linear elastic response. A comparison of linear elastic fracture toughness values with K_Ic showed that the J-integral approach combined with the AE technique can be applied to determine a valid fracture toughness of the granite. Finally, a reason for the specimen size dependency observed in the linear elastic toughness value is discussed on the basis of the experimental results.