Quantitative characterization of the fracture behavior of sandstone with inclusions: Experimental and numerical investigation

To investigate the effects of inclusions on the mechanical properties of rock with holes, sandstone specimens with square openings were prepared, considering the absence and presence of inclusions as two cases. The mechanical mechanism of crack evolution during the loading of the specimens and the effect of inclusions were quantitatively studied using the digital image correlation (DIC) technique and the rock failure process analysis (RFPA). The presence of inclusions reduced the stress of the sandstone specimen with an opening and hence increased its peak strength and elastic modulus. Based on the relative movement trend of the measuring points on both sides of new cracks, the crack types were divided into direct tensile cracks, relative tensile cracks, and shear cracks, as also confirmed by the damaged element distribution obtained by the RFPA2D. The strain values of the typical monitoring points along the potential crack propagation paths were extracted, and it was found that tensile cracks were initiated at the upper and lower boundaries of the opening and in the middle of the inclusions, then propagated vertically along the axial loading direction. Comparing the strain measurements of the two types of specimens showed that inclusions reduced the horizontal strain on tensile cracks and inhibited the initiation and development of cracks. The differentiation rate of the strain field was proposed to characterize the evolution of the degree of the overall differentiation of the strain field. This indicator was found to have a substantial spike at the time of crack initiation, thereby enabling the quantitative identification of the crack initiation stress. Compared with the specimens without inclusions, the specimens with inclusions had a higher crack initiation stress, which corresponded to a smaller differentiation rate of the strain field.