Abstract:

Using self-designed tension shear auxiliary device, we carried out a tension shear test and a compression shear test under  different normal stresses between -0.28 and 3.0 MPa. We applied the acoustic emission to compare and analyze the mechanical properties and the damage failure mechanisms of the specimens for both tests. The main results are listed as below. The tension shear auxiliary device can help carry out the tension-shear test well. The peak stress varies non-linearly with the normal stress and is more sensitive to the normal tensile stress. The Hoek-Brown strength criterion can generally characterize the strength of the full stress area. The post-peak stress of the specimen in tension shear test drops severely and separates into two parts rapidly, showing the failure characteristics with more brittleness than that of the compression shear test. The morphological characteristics of the fracture plane are closely related to the direction and magnitude of normal stress. For compression shear test, the damage degree represented by the acoustic emission parameters is greater, as well as the failure degree of the fracture plane, compared with those observed in tension shear test. The frictional area and the local spalling of the fracture plane are also more obvious for the specimens in compression shear test, compared with those in tension shear test. As the normal stress increases, the duration of the acoustic emission quiet period gets longer, and the start time of the unstable crack propagation gets later. Compared with the compression one, the tension shear test shows a shorter duration time for each stage of the failure process as well as a higher failure rate. The critical points σ_cc、σ_ci and σ_cd of the shear-stress plot and the acoustic emission parameters can represent, respectively, the macroscopic and microscopic failure process of the specimen. The critical point σ_cd and the acoustic emission b value can be used as omens of rock failure.

Key words: tension shear test, strength criterion, brittle failure, acoustic emission, damage failure mechanism, omen