Study on mechanical behavior of sandstone micro-fractureevolution under uniaxial compression test

In order to analyze the temporal and spatial mechanical behavior of micro-crack at each stage of rock deformation and explore the evolutionary relationship between micro-fracture development and macro-destruction, the sandstone samples were subjected to uniaxial compression using high-speed camera and 2 sets of PCI-II acoustic emission monitoring systems. Combining the response relationship between acoustic emission information and rock micro-fracture, the characteristics of acoustic emission evolution at each stage of rock deformation and the temporal and spatial evolution characteristics of microfracture strength and fracture scale were compared and analyzed. Moment tensor inversion was used to obtain the source of rock micro-fracture, type and direction of movement, analyze the crack development trend. The precursor information of rock failure and instability was obtained, and the mechanical behavior in the process of micro-crack initiation and evolution into macroscopic failure was revealed. The results show that the evolution characteristics of AE parameters have a good correspondence with each deformation stage of the rock, and there are three main frequency bands in the acoustic emission radio frequency domain, namely the low frequency band (90～130 kHz), the middle and high frequency band (250～280 kHz), and high frequency band (280～330 kHz). There is a positive correlation between the acoustic emission waveform duration and the scale of the micro-cracks. Before reaching the yield strength, most of the durations are mostly below 500 μs. After the yield strength, the micro-fracture strength and scale become more abundant. There is an obvious mid-frequency band (130～250 kHz) in the domain, a large number of activities with acoustic emission waveforms with a duration of more than 1000 μs appear, and the generation and evolution of cracks are transformed from a disordered state to an orderly development state, and the direction of micro-fracture movement gradually deviates towards the axial stress direction, the source of shear rupture begins to dominate the damage before the main macro-crack appear. Three characteristic phenomena of active mid-band acoustic emission, concentrated occurrence of high-intensity large-scale micro-fractures, and orderly development of micro-fractures are used as precursor information for evaluating rock failure and instability.