Quantitative characterization of cracking and debonding behaviors of sandstone containing an inverted U-shaped inclusion.

To better investigate the reinforcement effect of inclusions on inverted U-shaped openings, sandstone specimens with an U-shaped opening were fabracation, considering two cases of filled and unfilled opening. Uniaxial compressive tests combined with digital image correlation (DIC) monitoring were performed on these specimens. Three quantitative methods were introduced to decipher DIC measurements. The inclusion has a significant effect on enhancing the strength and elastic modulus of the specimens but cannot change the ultimate failure mode. The strain distribution around the opening was analyzed by extracting the strain data. The presence of inclusions changes the strain distribution and reduces the concentration level, thereby causing a change in the crack initiation position and a reduction in the crack initiation stress. The change rate of variance (CROV) was proposed to describe the strain field evolution quantitatively, which is found to exhibit abnormal features in response to cracking events. The alteration in the cracking process caused by the inclusion changes this response feature, and the first cusp in CROV associated with crack initiation is defined as the precursor point. A novel approach was used to reveal the debonding mechanism between rock and inclusions. The relative opening zones are concentrated around the sidewalls, and the relative squeezing zones are located at the opening top and bottom. The crack initiates from the relative opening zone and, in turn, changes the initiation area from the relative opening state into the relative squeezing state.