Influence of weak inclusions on the cracking behavior of a jointed rock mass containing an opening: insights from DIC-based approaches

To gain deep insights into the reinforcement effect of weak inclusions on jointed rock masses containing an opening, natural sandstone specimens containing multiple non-persistent joints and an opening were prepared, and two different cases, unfilled and filled openings, were considered. Digital image correlation (DIC) was applied to characterize the deformation fields of specimens during loading. Two measures extended from the DIC, the displacement vector and multivariate measure of strain dispersion, were proposed to identify and quantitatively analyze the cracking mechanism. Weak inclusions were found to improve the mechanical properties. The displacement vector fields around the newly formed cracks were calculated, and then, four types of cracks were recognized: direct tensile cracks, relative tensile cracks, shear cracks, and mixed cracks. Based on the fracture nature of cracks, nine types of crack coalescence among joints and six categories between openings and joints were summarized. The influence of weak inclusions on the cracking behavior of jointed sandstones was further clarified. Many more cracks were detected in the specimens containing a filled opening, which was validated by the fractal dimensions of the ultimate fracture surfaces. The differentiation rate of the effective variance (DREV) was defined to describe the dispersion of full-field strain data. The DREV was extremely close to 0 at the initial compacting and elastic deformation stage and then exhibited anomalous responses to cracking events. The first peak in the DREV–strain curve represents crack initiation, which can be regarded as a precursor.