Failure and Mechanical Behaviors of Sandstone Containing a Pre-existing Flaw Under Compressive–shear Loads: Insight from a Digital Image Correlation (DIC) Analysis

Flaws play a critical role in mastering the mechanical responses and failure behaviors of rock masses in practical engineering. In this study, a series of compressive–shear tests were performed on sandstone samples containing a single flaw to systemically investigate the effect of flaw geometrical parameters and die angle on the cracking process using digital image correlation (DIC) analysis. The results indicate that the peak force of specimens is significantly affected by the geometrical parameters of the pre-existing flaw and the die angle. The peak compressive–shear force decreases with increasing flaw length, width, inclination angle and die angle except for the flawed specimen with the die angle of 15°. Aided by DIC analysis, it is found that the initiation of the anti-wing cracks and secondary cracks lags behind the wing cracks. The crack initiation and propagation of wing crack are dominated by the tensile strain, while the crack initiation mechanism of the anti-wing crack and secondary crack is mixed tensile–shear dominated. A novel crack dominant parameter (CDP) is proposed to quantitatively analyze the effect of the maximum principal strain and shear strain at the aforementioned key points. The crack behaviors are dominated by tensile stress, mixed tensile–shear stress and shear stress when CDP > 2, 0 < CDP < 2 and CDP < 0, respectively. With increasing flaw length, width and die angle, it became easier for the flawed specimen to coalesce along the shearing plane and the corresponding final failure mode became concise with a lower degree of fragmentation. The findings of this paper would facilitate a better understanding of the failure behavior of rock masses containing flaws under compressive–shear loads.