Evaluation of equivalent crack propagation length and fracture energy of two commonly used rock fracture toughness test configurations based on Bažant’s size effect law

Abstract

Purpose of this research is to apply the Bažant’s size effect law to investigate the effects of material properties, specimen size, normalized crack length and loading span on the equivalent crack propagation (ECP) length and fracture energy test results of cracked straight through Brazilian disc (CSTBD) specimens and notch semi-circular bend (NSCB) specimens, and to clarify the relationship between ECP length and fracture process zone (FPZ) length. The results indicate that the ECP length is positively correlated with the specimen radius, cf (the ECP length of the infinite-size specimen) and the loading span, and the influence of the normalized crack length on the ECP length has the dependence of specimen configuration. Both the apparent fracture energy and the equivalent fracture energy have size effect, and the fracture energy tested by the specimen with smaller cf are more reliable. Based on the analysis of the influence of normalized crack length and loading span on fracture energy test results, CSTBD specimen with normalized crack length of 0.4 is recommended for rock fracture toughness test. In addition, the FPZ length determined by digital image correlation technique is more accurate than that determined by stress intensity factor method and finite element model method, and the ratio of ECP length to FPZ length decreases with the increase of specimen size.