Due to rock mass being commonly subjected to compressive or shear loading, the mode II fracture toughness is an important material parameter for rocks. Fracturing in rocks is governed by the behavior of a nonlinear region surrounding the crack tip called the fracture process zone (FPZ). However, the characteristics of mode II fracture are still determined based on the linear elastic fracture mechanics (LEFM), which assumes that a pure mode II loading results in a pure mode II fracture. In this study, the FPZ development in Barre granite specimens under mode II loading was investigated using the short beam compression (SBC) test. Additionally, the influence of lateral confinement on various characteristics of mode II fracture was studied. The experimental setup included the simultaneous monitoring of surface deformation using the two-dimensional digital image correlation technique (2D-DIC) to identify fracture mode and characterize the FPZ evolution in Barre granite specimens. The 2D-DIC analysis showed a dominant mixed-mode I/II fracture in the ligament between two notches, irrespective of confinement level on the SBC specimens. The influence of confinement on the SBC specimens was assessed by analyzing the evolution of crack displacement and changes in value of mode II fracture toughness. Larger levels of damage in confined specimens were observed prior to the failure than the unconfined specimens, indicating an increase in the fracture resistance and therefore mode II fracture toughness with the confining stress.
The fracturing in laboratory-scale rock specimens is often characterized by the deformation of the inelastic region surrounding the crack tips, also known as the fracture process zone (FPZ) (Backers et al., 2005; Ghamgosar and Erarslan, 2016). While the influence of the FPZ on mode I fracture in rocks has been extensively investigated, there are limited studies on FPZ development in rocks under pure mode II loading (Ji et al., 2016; Lin et al., 2020; Garg et al., 2021; Li et al., 2021).