A Modified Fracture Criterion for Brittle Rocks Under Mixed‐Mode I/III Loading

ABSTRACT In geotechnical and geo‐energy engineering, rock masses are commonly subjected to complex stress conditions involving mixed‐mode I/III loading. Available criteria are primarily based on tensile stress or strain, and can only be used to predict Mode I fracture; the criteria used to predict Mode III fracture under mixed‐mode I/III loads generally neglect the influence of T‐stress. This study proposes a modified mixed‐mode I/III fracture criterion for brittle rocks to predict both fracture mechanism and crack initiation angle. Experiments were conducted on edge‐notched disk bending (ENDB) and double‐edge notched disk compression (DNDC) red sandstone specimens under different loading conditions to validate the proposed criterion. Results show that the modified mixed‐mode I/III fracture criterion can accurately predict both Mode I and Mode III fracture. The fracture mechanism is of Mode I under pure shear, tensile‐tearing and pure tearing loads, whereas the DNDC specimen exhibits Mode III fracture under compression‐tearing loading.