Finite‐Discrete Element Methods‐Based Analysis of Layer Thickness Effects on Mechanical Behaviour and Fracture Mechanisms in Anisotropic Shale

ABSTRACT Layer thickness in shale arises from sedimentary variability and governs the density of weak bedding planes that act as primary pathways for fracture development. Although many studies address bedding orientation, the influence of layer thickness on mechanical behaviour and fracture mechanisms of shale remains under‐examined. This paper employs a finite‐discrete element method (FDEM) anisotropic shale model to systematically investigate how layer thickness influences shale's anisotropic mechanical and fracturing behaviours from a micro‐crack viewpoint. The results show that the sensitivity of layer thickness shows great anisotropy. The mechanical behaviours, fracture mechanism and brittleness of shale inclined at 45° and 90° are significantly affected by layer thickness, while shale with 0° inclination remains unaffected. The crack initiation stress ( CI ) threshold is independent of both layer thickness and bedding inclination, offering a robust input for future anisotropic constitutive models of layered rocks. By contrast, the crack damage stress ( CD ) threshold of shale at 45° and 90° rises with increasing thickness, indicating an earlier onset of unstable fracture growth under higher bedding densities. Furthermore, a novel brittleness index, BI _C , has been proposed. It relates brittleness to the CI / CD ratio. Validation using both laboratory and numerical simulation data shows that BI _C reliably evaluates shale brittleness. As BI _C is derived from uniaxial compressive strength tests alone, it is more practical than conventional indices for future assessments of rock brittleness. In addition, compared with conventional brittleness indices, BI _C captures the effects of shale anisotropy more effectively, showing larger and clearer differences.