Research on crack classification method and failure precursor index based on RA-AF value of brittle rock.

• An improved shear test is used to replace the traditional direct shear test to avoid the interference of the traditional direct shear test on AE signal acquisition. • The rock fracture process has an obvious piecewise variation characteristic and a significant increasing period. • The AE data (RA-AF values) collected in Brazil splitting test and improved shear test are coupled to determine the tensile and shear crack boundaries of rock. The mechanical and AE information of the whole process of rock damage and fracture are obtained by uniaxial compression test, Brazil splitting test and improved shear test on marble, granite, limestone and sandstone. The internal relationship between the damage and failure mechanism of brittle rock and AE signal is revealed based on these tests. Moreover, in this paper, the improved shear test is used to replace the traditional direct shear test to avoid the interference of the traditional direct shear test on AE signal acquisition. The experimental results show that the rock fracture process has an obvious piecewise variation characteristic and a significant increasing period. A variety of AE signals are screened by coupling the main frequency and its amplitude, and the values of b and CV (r) can be used as the precursor signals of rock critical failure. Based on KDE method and K-means algorithm, the AE data (RA-AF values) collected in Brazil splitting test and improved shear test are coupled to determine the tensile and shear crack boundaries of four kinds of rocks. Based on the determined boundary of AE data between tensile and shear cracks, the corresponding precursor index CV (r) of rock failure is proposed. When the value of CV (r) increases to more than 1, the rock is close to failure or failure has occurred. [ABSTRACT FROM AUTHOR]