Experimental and numerical investigation into the methods of determination of mode I static fracture toughness of rocks

Abstract

Fracture toughness, as a measure of resistance against crack instability and propagation, is one of the most important parameters in fracture mechanics of rocks and other solid materials. Different methods have so far been proposed in order to determine mode I fracture toughness. In practice, however, considerable differences between their results have been reported. In this study, six conventional tests, including the Single Edge Round Bar Bending test, the Chevron Bend test, the Semi-Circular Bend test, the Straight Notch Disk Bend test, the Brazilian disk test, and the Flattened Brazilian Disk test have been performed on a uniform specimen of gabbro rock for determination of mode I fracture toughness. A notable maximum difference of 42% is observed between the results.The tests are categorized into three groups in terms of their similarities. Several factors which affect the prediction of fracture toughness are investigated by both the numerical and experimental evidences. Distribution of the fracture process zone around the crack tip, is numerically computed using the 3D finite element analysis and based on both the “normal tensile stress” and the “von-Mises” criteria. According to the investigated features, the Chevron Bend test is the most reliable test for determination of the fracture toughness. Furthermore, the distribution of fracture process zone around the crack tip in this test is non-uniform which leads to over estimation of fracture toughness predicted by the effective crack theory.