Microstructural compression-hardening and tension-softening effects on compression-induced tensile microcracking and acoustic emission in bimodular rock materials: A distinct element study

Rock materials typically experience microstructural compression-hardening and tension-softening (CHTS) deformation, thereby exhibiting tension-compression elastic asymmetry and inherent bimodular mechanical behavior. However, how this CHTS regulates stress conversion, microcrack evolution, and acoustic emission (AE) within rock materials under compression-induced tensile loading remains unclear. Here, using the distinct element method, we develop an advanced grain-based model (GBM) incorporating CHTS (CHTS-GBM), and benchmark it against the conventional linear elastic (LE) GBM (LE-GBM) with equal tensile and compressive moduli. We perform simulations of uniaxial compression, direct tension, and Brazilian splitting on marble samples. CHTS-GBM accurately reproduces the nonlinear responses under all three loading paths, whereas LE-GBM deviates considerably due to the absence of asymmetric moduli and nonlinearity. Analyses of stress distribution, force chains, microcracking, and AE reveal the controlling role of asymmetric and nonlinear contact behavior on progressive tensile microcracking and AE characteristics. Notably, relative to the LE-GBM, CHTS-GBM exhibits low-energy microcracking, a higher b -value, gradual energy release, and progressive damage evolution. In contrast, LE-GBM, devoid of nonlinear constraints, promotes the formation of more tensile contact forces, more high-energy through-going microcracks, and more abrupt failure. Moreover, CHTS-GBM can be degenerated to LE-GBM, demonstrating its theoretical compatibility. Overall, this study reveals the inherent physic-mechanical law governing the unity and opposition of tensile and compressive behaviors of bimodular rocks. It elucidates how grain-scale contact nonlinearity and tension-compression elastic asymmetry dominates microcracking processes and spatiotemporal evolution of AE and further clarifies the intrinsic CHTS mechanisms. These results underpin refined numerical simulation of compression-induced tensile instability in rocks.