Experimental study of pure shear fracture in rock-type materials

Based on the criterion of local symmetry, under pure shear a fracture in rock-type materials will gradually grow to the direction where the local mode II loading is smaller. Eventually a pure mode I fracture occurs. However, once the specimen size is small and the condition changes from small scale yielding to large scale yielding, it is a challenge to predict the fracture direction by quantitative evaluation of the local shear influence. In this study, to predict the fracture direction, three types of specimens were tested to create the loading of pure shear: the angled edge crack semi-circular specimens under three-point bending (SCB), the centrally cracked Brazilian disk (CCBD) specimens, and double edge notched compression (DENC) specimens. The tested materials are the marble and graphite. Experimental results are analyzed to obtain information of the local shear influence by a parameter eta, which is defined on non-symmetric distribution of opening displacements along the fracture process zone (FPZ). The minimum value of eta is set to 1, which suggests a pure mode I fracture. When eta is larger than 1, a larger eta corresponds to a larger shear influence. After the first fracture kink, three different potential fracture directions are identified in this study: (i) it can change to where the local shear influence is smaller, as observed in the SCB specimens, because parameter eta decreases from a finite value (larger than 1) to 1; (ii) it can maintain the current fracture direction, as observed in the CCBD specimens, because parameter eta maintains a finite value within a range 1.5-2.5; (iii) it can turn to where the local shear influence is larger, as observed in the DENC specimens, because parameter eta increases from a finite value (larger than 1) to the infinity. This study clearly demonstrates the complexity of fracture process and the importance of the local information to understand fracture mechanism under large scale yielding.