DETERMINATION OF MARBLE FRACTURE TOUGHNESS BY USING VARIABLE-CRACK ONE-SIZE SPECIMENS

One-size flattened disc specimens with identical centre hole and different initial crack lengths are used to determine marble fracture toughness. Based on the size effect law proposed by Bazant, the dimensionless energy release rate function g(α0) is curve fitted with the data from the finite element analysis. It is concluded that the range requirement for the brittleness numbers β in the size effect law is satisfied, if the function g′(α0)/g(α0), which is related to the specimen shape, varies from 1.0 to 5.8, this condition is equivalent to that the dimensionless initial crack length α0=a0/R varies from 0.28 to 0.51. According to the analysis of experimental results, the real fracture toughness which is independent of specimen size and the length of fracture process zone cf are given. The present method of determination of rock fracture toughness avoids the difficulties in specimen preparation and testing, and there is no need to use larger specimens or two or more than two types of specimens for getting a sufficient range of brittleness numbers β.