Research on size effect of fracture toughness of sandstone using the center-cracked circular disc samples

Abstract

In order to investigate the size effect and reason of rock fracture toughness, the split loading test was carried out in the GCTS rock mechanics testing machine using the center-cracked circular disc (CCCD) samples with diameters of 50 mm, 100 mm, 150 mm and 205 mm, respectively. The lengths of fracture process zone (lFPZ) were determined by the maximum normal stress criterion. The size effect of the lFPZ , fracture toughness test value (KIc), and fracture energy (Gf) were analyzed. The influence of the lFPZ on the size effect of the KIc was revealed. The crack growth paths and failure characteristics of the samples were discussed. The results show that the lFPZ and the KIc increase linearly with the sample size, and there is obvious size effect within the range of the test. The KIc and the lFPZ do not always increase linearly with the size in a larger size range, but gradually stabilize by extrapolation analysis. The lFPZ increases with the size, which means the release of stored energy increases with the size corresponding to the size effect of the Gf, and in turn leads to the size effect of the KIc. It is helpful to deeply understand the cause of the size effect reason of rock fracture toughness.