# Determination of Fracture Mechanism and Mode II Fracture Toughness of Red Sandstone Subjected to Compressive-Shear Loading

## TL;DR

This study improves the measurement of mode II fracture toughness in red sandstone by considering crack initiation angles and proposing a modified method.

## Contribution

A modified Mohr–Coulomb criterion incorporating fracture angle and Mode I stress intensity factor is proposed for accurate mode II fracture toughness determination.

## Key findings

- Red sandstone exhibits distinct fracture mechanisms under varying loading angles, including tensile spalling, shear fractures, and tensile fractures.
- Traditional mode II fracture toughness methods are inaccurate due to crack initiation angle deviation from the initial crack plane.
- A refined framework is established for mode II fracture toughness determination under compression–shear conditions.

## Abstract

Mode II fracture toughness is an important material parameter of rocks, but accurate measurement of this parameter is still a challenge in rock fracture mechanics. This study aims to modify the mode II fracture toughness of red sandstone measured through shear box testing by emphasizing the critical role of crack initiation angle. Experimental tests combining fracture trajectory scanning and digital image correlation reveal distinct fracture mechanisms of red sandstone under varying loading angles: tensile spalling dominates low angles, and shear fractures emerge at medium angles, while tensile fracture initiates from the rock bridge center at high angles. Although shear fracture initiates from the notch tip, its initiation angle deviates from the initial crack plane, invalidating traditional mode II fracture toughness determination methods. A modified Mohr–Coulomb criterion incorporating fracture angle and Mode I stress intensity factor is proposed to correct the significant errors of traditional methods, and this study establishes a refined framework for mode II fracture toughness determination under compression–shear conditions.