Comparative Analysis of microcracking behaviors and associated acoustic emission characteristics in sandstone subjected to compression-induced and direct tensile stresses

Rocks often undergo both direct tensile actions and indirect tensile actions induced by compression due to tectonic movements and human engineering activities, leading to the gradual accumulation of microcracks accompanied by acoustic emission (AE) events. However, to date, the differences in the microcracking behavior and associated AE of rocks under these two tensile conditions, as well as the underlying micromechanical mechanisms, have not been thoroughly understood. Here, we conducted a direct tensile test on a dog-bone-shaped sandstone sample and a Brazilian splitting test (representing compression-induced tension) on a disc-shaped sample. Corresponding nonlinear particle-based discrete element models were developed to simulate the tensile stress-displacement responses and macroscopic failure modes of the two sandstone samples. Results showed that sandstone under direct tension exhibits significantly lower strength than under compression-induced tension. In the disc-shaped model, microcracks appeared progressively at dispersed times, accompanied by high-frequency AE signals, mainly concentrated near the longitudinal symmetric axis, adjacent to the loading point. Conversely, microcracks in the dog-bone-shaped model appeared abruptly and were concentrated within a narrow range near the horizontal symmetric axis, with microcracks and energy release uniformly distributed, and fractures tending to be perpendicular to the loading direction. Compared to compression-induced tension, the dog-bone-shaped model exhibited smaller AE counts, energy release, and AE signal frequency, but a larger b-value, more than three times as high. Micromechanical analysis revealed that these differences are primarily due to significant variations in tensile/compressive contact force chains and strain energy distributions in particle assemblies under direct and compression-induced tension.