Size effect on the mechanical properties and fracture evolution of layered sandstone: a new perspective from DIC

The inherent bedding structure of layered rocks causes their size effects to differ from those of homogeneous rocks, limiting the applicability of existing theories in engineering design. Laboratory testing, the digital image correlation (DIC) method, computed tomography (CT), and scanning electron microscopy (SEM) are used to investigate the size effect on the mechanical properties and fracture evolution of layered sandstone. The differentiation rate (DR) and differentiation acceleration (DA) are introduced to quantify the full-field strain dispersion. The relative slipping displacement (RSD) along the bedding plane is calculated to quantify the local shear deformation. The results show that a strong negative correlation exists between the specimen size and compressive strength in layered sandstone. The DIC analysis results show that, on the one hand, abrupt changes in DR and RSD occur earlier with increasing specimen size. On the other hand, in small specimens, full-field deformation and local slip exhibit a strong correlation, which significantly weakens in larger specimens. This phenomenon reveals that the asynchrony between full-field deformation and local slip in layered sandstone tends to increase with increasing size. The concept of the fracture process zone (FPZ) is introduced and verified with the help of CT and SEM techniques.