Understanding the evolution mechanism and classification criteria of tensile -shear cracks in rock failure process from acoustic emission (AE) characteristics

• Five failure stages of rock were divided by accumulative AE hits line method, and crack closure stress ( σ cc ), crack initiation stress ( σ ci ) and crack damage stress ( σ cd ) were determined. • A criterion for crack classification is proposed based on AE spectral parameters (Ad, FW), and a straight line dividing tensile and shear cracks was obtained in the density map of RA-AF.. • Based on the evolution characteristics of crack scale and type, the failure modes of different rocks were determined and verified by the failure images. This study aimed to investigate the crack evolution and failure mechanism of rocks by conducting uniaxial compression tests (UCT) and Brazilian indirect tension tests (BITT) using four categories of rock specimens. Real-time acoustic emission (AE) signals were detected and recorded during rock failure processes by an AE monitoring system. The study focused on the accumulative AE hits curves, stress–strain curves, and other AE energy characteristics to divide the failure process into five stages, and determine the stress thresholds. Furthermore, the transition trend of rock crack types was analyzed based on RA-AF data. A novel classification criterion of crack types based on clustering and statistical theory was proposed, and the crack boundaries of the four rocks were determined. Additionally, the tension to shear ratios of the rocks in descending order were fine-grained granite, granite, yellow sandstone, and white sandstone. Finally, the crack development trend and failure characteristics of the rock were analyzed using the crack scale and crack dividing line. The study found that the failure mode of the rock could be predicted accurately by AE parameter analysis at Unstable Crack Growth (Stage-IV).