A study of crack initiation and source mechanism in the Brazilian test based on moment tensor

It has been widely accepted that the Brazilian test can be used to obtain the indirect tensile strength of geo-materials. However, the effects of the contact conditions and the heterogeneity of the rock affect the micro-failure of the disc and the accuracy and feasibility of the Brazilian tensile strength (BTS), which typically leads to deviations from the results of the direct test. This study uses active and passive ultrasonic techniques and a three-dimensional flat-joint model (FJM3D) to quantitatively investigate the effects of these factors on the location and source mechanism with a Brazilian test, both in the laboratory and with numerical tests. During the laboratory tests, eight channels were used to record full waveform data continuously, and a total of 1090 and 928 acoustic emission (AE) events were located successfully under line loads and non-line loads, respectively. Both cases showed that the crack initiation points occurred away from the centre of the disc, indicating that the true tensile strength value may be somewhat underestimated. Three classification methods based on moment tensor inversion were used to quantitatively study the AE source analysis, which revealed that tensile micro-cracks that contain some deviatoric components predominantly occurred in micro-failure events. With the use of the FJM3D, the effects of the contact conditions and the heterogeneity of the rocks were analysed in detail. Four types of numerical tests were performed with different contact conditions and degrees of heterogeneity for comparison, indicating that the heterogeneity of rocks has a remarkable effect on the distribution of horizontal stress. This microcosmic and quantitative perspective can provide new data that can be used to modify the Brazilian test to obtain more accurate tensile strength values.