Study of the Relationship between Mode I Fracture Toughness and Rock Brittleness Indices

Mode I fracture toughness (KIC) and rock brittleness are important properties that influence many rock engineering applications. Due to the difficulties in determining KIC experimentally, previous studies have investigated the relationship between KIC and rock brittleness indices. However, only rock brittleness indices (based on strength parameters) and KIC obtained from Chevron Bend and Short Rod test methods were considered. In this paper, regression analysis was carried out to investigate the relationship between KIC and rock brittleness using literature data collected from different rock types and core KIC test methods under level I and static test conditions. Rock brittleness was assessed using ten indices based on strength and pre-peak elastic parameters. The results showed that elastic-based indices were not good predictors of KIC, while strength-based indices correlated well with KIC. A comparison with previous studies revealed that the correlations between KIC and strength-based indices were significantly sensitive to the rock type, i.e., soft or hard, and the KIC test method. However, a brittleness index, based on both strength and pre-peak elastic parameters, was found to be the best index to predict KIC because of its lower sensitivity to the test method and rock type.