Theoretical and Experimental Study on Dynamic Mode II Fracture Toughness Measurement of Rock Using a Shear‐Box Method

Drill‐blasting method is the main construction technology for long tunnels, and the impact loads would promote natural cracks embedded in the rock mass initiation and propagation. Dynamic fracture toughness is a key mechanical parameter for characterizing rock failure under high loading rates. Available literature on dynamic fracture toughness measurement for rock has predominantly focused on Mode I fracture, whereas experimental determination of dynamic Mode II fracture toughness commonly relies on tests with confining pressure, resulting in increased experimental complexity. In this study, a shear‐box (SB) method is first proposed to determine the dynamic Mode II fracture toughness of brittle rock, and the fracture type (FT) criterion is employed to verify the occurrence of true Mode II fracture. Both experimental results and theoretical analysis demonstrated that Mode II fracture occurs in the SB method, and the dynamic Mode II fracture toughness of red sandstone exhibits a progressive increase as the loading rate rises.