Acoustic emission and micro-rupture characteristics of rocks under Brazilian splitting load

Considering the polycrystalline and anisotropic features of rock, its mechanical failure actually involves the generation, propagation, and penetration of internal micro-cracks until an ultimate macro-fracture is achieved. The nucleation and propagation of cracks emits energy outward as elastic waves referred to as acoustic emission (AE). The close relationship between AE signals and the rock fracture mechanism has been demonstrated. Many instability and failure processes in underground engineering are induced by the effects of tensile stress on tunnels and chambers or local damage to the rock structure. Several compression experiments show that the main fracture mode of rock is tensile failure. Thus, investigations of rock AE characteristics under tensile failure and the effects of the rock fabric on crack propagation patterns are of great significance. This study assesses the signal characteristics AE and its relationship with the micro-rupture mechanisms in granite and marble under tensile stress. Herein, an MTS-322 rock mechanical test system was employed to carry out Brazilian split tests, and a scanning electron microscope was employed to carry out micro-morphological analysis of rupture surfaces. According to the trends of RA and AF, the distribution of crack modes-tensile and shear or mixed patterns in both rock types and its fracture strength depend on the rock fabric. By contrast, the evolution process of crack propagation appears to depend on the softening process. Although the rock fracture signals are mainly in the range of 400−499 kHz and 100−199 kHz, the variation trend of peak frequency shows significant differences at different failure stages. At the microtopographic level, granite mainly shows three micro-morphologies, including laminated, stepwise, and smooth planar patterns. Marble is mostly smooth polyhedrals. The signals at 400−499 kHz may be inferred to be mainly generated by fractures in the k-feldspar and marble minerals, while those at 100−199 kHz are mainly produced by discontinuous separation among quartz mineral particles and slipping among mineral particles in the compaction stage.