Fracture Toughness and its Relation to Initial Tangent Modulus of Granitic Rock

This paper describes the relationship between fracture toughness and initial tangent modulus, i. e., Young’s elastic modulus in tension, of granitic rock. ASTM standard specimens for notched three-point-bend and compact tension tests, which contained various levels of water or were subjected to thermal cycle, were used to correlate fracture toughness to Young’s modulus. Furthermore, fracture toughness tests were carried out at high temperature and in pressurized water environment to evaluate the fracture and deformation behaviors under a simulated condition of geothermal reservoir. Based on the linear elastic fracture mechanics, elastic modulus was precisely measured for granitic rock samples by means of a compliance calibration technique prior to fracture toughness tests. Fracture toughness was determined by means of AE technique from an abrupt increasing point of AE activity. The results obtained in this study indicate that a significant increase in fracture toughness (K_iAE) appears with increasing elastic modulus (E) and fracture toughness has a simple linear relationship with elastic modulus. Theoretical consideration based on the atomic scale fracture model by Gilman can give an adequate explanation to this relationship between K_iAEand E. It is suggested that the effect of water content on the elastic modulus and fracture toughness can be interpreted as the result of internal friction on the micro-crack plane. An optical microscopic study led to a conclusion that the decrease in elastic modulus and fracture toughness with increasing temperatue is due to the increase in thermally-induced micro-crack in the rock sample. The result obtained at high temperature/pressurized water environment may be attributed to highly activated chemical reaction at the crack tip.