Multiple-scale crack propagation characteristics and failure precursor identification of freeze–thawed sandstone during loading

To study the effect of freeze–thaw temperature change rate on the crack propagation characteristics and failure precursor in freeze–thawed sandstone, uniaxial compression tests were simultaneous monitoring for acoustic emission (AE) and microseismic (MS) signals. The results demonstrate that increased temperature change rates resulted in accelerated crack propagation, earlier rock failure, and lower brittleness; the failure mode changes from tensile to shear-tensile mixing; the proportion of tensile cracks decreases from 90.9% to 70.8%; and shear cracks increase from 9.1% to 29.2%. A comparative analysis of AE and MS provided insights into the evolution of crack propagation at multiple scales, enabling the classification of crack types and their relationship with propagation scale. Based on the precursor characteristic of the original waveform and time-domain curve in AE and MS signals, the advantageous areas of early-warming indicators for rock failure were identified. Compared with traditional indicators, the precursory indicators calculated using MS b-values and AE energy rates can obtain a larger early warning window, with maximum windows of 20.52%–29.29% and 7.09%–13.73% in high initial damage rocks and low initial damage rocks, respectively.