Anisotropic fracture behavior and corresponding fracture process zone of laminated shale through three-point bending tests

Understanding the anisotropic fracture behavior and the characteristics of the fracture process zone (FPZ) under size effects in laminated rocks, as well as its role in rock fracturing, is crucial for various engineering applications. In this study, three-point bending tests were conducted on shale specimens with varying bedding angles and sizes. The anisotropic characteristics and size effects of fracture parameters were revealed. A comparative analysis was performed on the evolutions of FPZs computed using size effect theory, digital image correlation (DIC), and linear elastic fracture mechanics. The results divulged that: (i) With increasing bedding angles, there is a noticeable decrease in apparent fracture toughness (KICA), apparent fracture energy (GICA), and nominal strength (sNu). When the bedding angle of shale is less than 45 degrees, the crack propagation and fracture parameters are mainly influenced by the matrix. Contrary, shale with bedding angles greater than 60 degrees, the crack propagation and fracture parameters are mainly controlled by the bedding. When the bedding angle is between 45 degrees and 60 degrees, the fracture propagation evolves from permeating the matrix to extending along the bedding; (ii) The fracture parameters exhibit significant size dependent behavior, as KICA and GICA rise with increasing specimen size, but sNu falls with increasing specimen sizes. The fracture parameters align with the theoretical predictions of Bazant size effect law; and (iii) The lengths of DIC-based FPZ, effective FPZ, and inelastic zone follow Wshape variations with bedding angle. The dimensionless sizes of FPZ and inelastic zone decrease with specimen size, indicating a size effect. Furthermore, there is a negative relation between KICA and the dimensionless size of the FPZ, while sNu is positively correlated to the dimensionless size of the FPZ. This highlights the essential role of the FPZ in the size effect of rock fracture. The bedding angle exerts an influence on the FPZ, subsequently affecting the anisotropic fracture and size-dependent behavior of shale. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/