Fracture mechanical properties of shale and macro-meso-micro multi-scale fracture surface characteristics

Abstract

The presence of bedding planes imparts pronounced anisotropy to the mechanical behavior of shale, fundamentally influencing its response to external stress. This anisotropic behavior is critical in determining the fracturing characteristics and overall mechanical performance of shale in engineering applications, particularly in resource extraction and stability evaluations. In this study, fracture tests were conducted on shale specimens with varying bedding angles (0°, 30°, 60°, and 90°) using the notched semi-circular bend (NSCB) method. The influence of the bedding angle on fracture toughness and failure pattern was systematically investigated. Additionally, multi-scale fracture surface morphology characteristics were analyzed through 3D optical scanning, ultra-depth field microscopy, and scanning electron microscope (SEM), enabling a comprehensive evaluation of the structural effects of bedding angles. The results indicate that fracture toughness decreases with increasing bedding angle, crack propagation becomes more stable, and the dispersion of fracture toughness diminishes. The failure pattern observed can be categorized as follows: tensile failure across the bedding plane (0°), shear failure along the bedding plane with mixed failure across the bedding plane (30°), shear failure along the bedding plane or tensile failure across the bedding plane (60°), and tensile failure along the bedding plane with mixed failure across the bedding plane (90°). These distinct failure patterns underscore the critical influence of bedding angle on fracture mechanisms. Moreover, the multi-scale failure characteristics exhibit significant correlation and consistency. The fractal dimension and joint roughness coefficient (JRC) initially increase and decrease with increasing bedding angle. Based on parameters such as asperity height, slope angle, and aspect direction, quantitative morphology characterization confirms that 30° specimens exhibit the highest surface complexity. A strong correlation is observed between the fractal dimension and the standard deviation of morphology descriptors, indicating robust geometric consistency across scales. These findings provide compelling evidence for the intrinsic link between macroscopic mechanical response and microscopic fracture surface morphology, offering critical insights into the multi-scale evolution of shale failure mechanisms and furnishing a theoretical foundation for designing and optimizing fracturing strategies in anisotropic shale formations.