Rock Crack Initiation Triggered by Energy Digestion

Abstract

The stress intensity factors with the kinds of critical models are usually obtained from the strength test on laboratory specimens to characterize the rock strength and fracture-ability. However, crack initiation phenomenology for rock material has not been comprehensively understood. In this study, energy theory—an essential character of material property changes—is drawn to evaluate rock crack initiation and study the correlation between rock strength and energy digestion. Uniaxial compression test was conducted on different hardness coefficients rock samples under quasi-static loading conditions and the results of energy evolution, non-linear cumulative digestion, and stored ultimate energy were obtained and compared with a series of results for sandstone. A novel algorithm for the evaluation of rock crack initiation has been proposed and the concept of energy digestion index (EDI) using the ratio of ultimate energy over accumulated energy has been developed to characterize the rock strength. The result shows that crack initiation is related to the maximum growth rate of energy cumulation. Simultaneously, the increasing rate of variable elasticity modulus of rock material gets the peak. The defined EDI has a negative correlation with the rock material's strength and mechanical properties. The relationships between final strength and EDI at crack initiation were also discussed and an assessment of the remaining strength life is introduced using the predicted value by fitting the EDI, leading to a new understanding and exploration for in-situ monitoring of rock damage and failure prediction.