Experimental study on triaxial unloading mechanical properties and acoustic emission response of shale with different water contents

Water is one of the most significant factors influencing the mechanical properties of rocks. Triaxial unloading acoustic emission (AE) tests were performed to study the unloading mechanical characteristics and damage evolution mechanism of shale with various water contents. The damage evolution characteristics of shale under triaxial unloading conditions were analyzed using AE time series parameters. The results show that when the water saturation coefficient rises, the number of AE rings and the energy value of shale decrease. Based on the results of the AE test, the ratios of the crack initiation stress and damage stress to the peak stress are 0.4 ~ 0.6 and 0.9, respectively. The closure stress, crack initiation stress, damage stress, and other characteristic stresses determined by the stress axial strain method and the AE method all decrease with the increase of the water saturation coefficient, demonstrating that water has a significant impact on the mechanical characteristics of the shale during the unloading damage and damage process. Based on the test results, a statistical damage constitutive model is established, and it uses the AE cumulative ringing count to represent the damage variable. The calculated results of the model are in good agreement with the experimental data, indicating that the established model can more accurately depict the evolution process of shale unloading damage and failure under different water contents. The study's findings can be used as a reference for determining the extent of rock damage caused by water-rock interaction.