Micro-mechanism of mechanical hysteresis of crystalline rock: Insights from time-integrated X-ray computed tomography and digital volume correlation

Under repeated loading and unloading, the mechanical response of crystalline rock exhibits hysteresis. To elucidate the linkage between macroscale hysteresis, microscale deformation, and microstructural evolution in crystalline rock, we perform in-situ time-integrated X-ray Computed Tomography (X-CT) test to image the microstructural evolution along a complete loading-unloading hysteresis loop followed by reloading until failure of Blue Hone granite. The microstructural evolution is quantified by the development of geometries and orientations of segmented voids. The evolved accumulative strain fields are calculated by Digital Volume Correlation (DVC) to characterize distribution of micro-scale deformation and reveal the interaction between strain localization and microstructural evolution and build relationship across length scales. The orientation distribution of first principal strain is analysed to characterize the influence of microstructural evolution on overall deformation. The results show that the macroscale hysteresis comes from the microscale hysteresis in dilation zones, which are highly correlated with high shear strain zones. Crack-like voids (perpendicular, inclined and parallel to axial load) in dilation volumes exhibit prominent hysteresis compared to those in contraction volumes, resulting in the delayed strain releasing of dilation volumes. The evolution of inclined crack-like voids confirms that the hysteresis mainly results from the newly developed inclined crack-like voids (shear cracking). After reloading to the same stress at the onset of unload, a further evolution of damage and strain localization is observed, while the sample deforms in a higher efficiency way to accommodate applied stress. This observation is discussed with the insight of rock fatigue.