Wide-range stress intensity factors for the ISRM suggested method using CCNBD specimens for rock fracture toughness tests

Abstract

Ever since the 1960s, many methods have been developed for determining the opening mode (Mode-I) fracture toughness, an intrinsic material property related to resistance to crack initiation and propagation. Unfortunately, most of these methods are not suitable for rock tests. In 1988, the International Society for Rock Mechanics (ISRM) suggested two types of specimens for rock fracture toughness tests: they were the chevron notched short rod and the chevron notched three-point bend round bar [1]. Furthermore, in 1995, the ISRM suggested a new type of specimen: the cracked chevron notch Brazilian disc (CCNBD) [2] (Fig. 1). The CCNBD can also be used to test Mode-II or mixed mode fracture toughness. This feature is similar to the cracked straight through Brazilian disc (CSTBD) (Fig. 2); however, the CCNBD is still superior to the CSTBD in waiving prefabricating a sharp crack in the specimen, since the crack can be produced automatically in the initial stage of the loading process. The formula for calculating KIC using the CCNBD specimen is [2]