Thermally-induced microstructural alteration drives auxetic transition in crystalline rock: Effects on microcracking and acoustic emission

Crystalline rocks exhibit a negative Poisson's ratio (NPR) after thermal damage, transitioning from a positive Poisson's ratio (PPR), though the micromechanisms driving this auxetic behavior and its effect on microcracking and acoustic emission (AE) remain unclear. To address this problem, unconfined compression tests on original and thermally damaged Suizhou granite were conducted and associated grain-based models (GBMs) with compression-hardening were established using the discrete element method. Results showed that compressive stress-strain curves of original and thermally damaged samples were dominated by PPR-related lateral extension and NPR-related contraction, respectively. GBMs accurately simulated the auxetic transition observed in the experiments for both sample types. Calibrated parameter analysis reveals that greater degradation of the grain-scale compressive versus shear modulus during thermal damage reduces their ratio below 1.0, causing the auxetic transition. Microstructural heterogeneity led to the formation of compressive and tensile stress concentration zones in the synthetic samples under compression. In the synthetic original sample, the PPR effect caused synchronous increases in localized compressive and tensile stresses with increasing compressive strain, promoting the formation and evolution of tensile microcracks. Conversely, in the synthetic thermally damaged sample, the NPR effect resulted in an increase in compressive stress while suppressing tensile stress as compressive strain increased, facilitating the formation and evolution of shear microcracks. Compared to the synthetic original sample, the synthetic thermally damaged sample showed later initiation of microcracks and AE events, shorter durations of activity, fewer events, and lower average and cumulative energy release. Additionally, it displayed a higher b -value.