Differential evolution of the anisotropic fracture characteristics of laminated rock under monotonic and fatigue loading

Abstract

To study the difference anisotropic fracture evolution of laminated rock between monotonic loading and fatigue loading, this study conducts three-point bending and real-time digital image correlation (DIC) monitoring tests on shale with 7 different bedding orientations under these two different loading paths. DIC is used to analyze the anisotropic horizontal and shear strain fields, crack dominant parameters (CDP), crack tip opening displacement (CTOP), and fracture process zone (FPZ) characteristics. The results indicate that the peak load is generally lower under fatigue loading than under monotonic loading, and the difference becomes more pronounced as the bedding angle increases. Under fatigue loading, the load–displacement curve has four stages: the elastic, stable microcrack evolution, unstable microcrack evolution, and failure stages. Three stages are observed under monotonic loading without obvious unstable microcrack evolution stages. The CDP values are smaller under fatigue loading than under monotonic loading, indicating that shear stress has a more pronounced effect on crack evolution under fatigue loading, and the shale samples are more prone to tensile-shear failure. The CTOD shows anisotropic characteristics influenced by the bedding angle. The variance of CTOD values measured for seven bedding angle specimens under monotonic load is 2.68, while it is 2.94 under fatigue load. The average length and width of the FPZ under fatigue loading are 1.2 to 1.3 times greater than those under monotonic loading, resulting in a larger damage zone. Under fatigue loading, the bedding structure accelerates the formation of microcracks and the anisotropy of shale mechanical behavior is more pronounced. This is because damage accumulates gradually with increasing load during monotonic loading, while repeated loading and unloading in fatigue loading results in significantly more cracks and crack expansion propagation.