Experimental Study of Cracking Characteristics of Kowloon Granite Based on Three Mode I Fracture Toughness Methods

Mode I fracture toughness is an important material parameter of rock. To date, a number of laboratory methods have been developed to measure the mode I fracture toughness. Although the fracture toughness values measured by different methods have been compared in many previous studies, the effect of the specimen configuration and pre-existing notch shape on the fracture toughness value and the associated cracking characteristics have not been comprehensively studied. In the present study, the failure modes and mode I fracture toughness of Kowloon granite, a major type of granite in Hong Kong, are studied experimentally based on the chevron bend (CB), semi-circular bend (SCB) and cracked chevron notched semi-circular bend (CCNSCB) methods. The fracture toughness measured by the SCB test is much lower (56.5% lower) than that measured with the CCNSCB test. The measured CB fracture toughness is between the SCB and CCNSCB results. The failure modes of specimens containing the chevron notch (CB and CCNSCB specimens) are found to differ from those containing the straight-through notch (SCB specimens), especially with respect to the post-failure behavior. The cracking mechanism of the SCB and CCNSCB is studied macroscopically and microscopically. The results show that the straight-through notch and chevron notch, which are the most commonly used notch shapes adopted in various mode I fracture toughness tests, have significant effects on the cracking behavior and crack characteristics. Further investigation on the thin sections prepared from the specimens after the mode I fracture toughness tests provides us with new insights into the microscopic characteristics of tensile cracks in granite under mode I loading condition.