Abstract: In order to investigate the mechanical properties and failure mechanisms of the rock containing a single hole and double cracks, we used the 3D sand printing technique to prepare the rock-like specimens. Digital image correlation (DIC) method was employed to non-contactly monitor the deformation field of the specimens during the compression process. By calculating the convariance matrix of the horizontal, vertical and shear strains, we introduced the effective variance of the strain field to quantify and identify the failure behaviors of the specimens. The test results indicate that the mechanical properties of the standard 3D sand printed specimens are similar to those of natural sandstones, and the test data show a low dispersion. Therefore, they can be classified as a rock-like material. Due to the inclusion of the cracks, the mechanical properties of the specimens are degraded. The compressive strength and elastic modulus of the specimens each of which contains a single hole and double cracks are reduced by 8.04%-38.91% and 14.44%-27.78%, respectively, compared with those of the specimens each of which merely contains a single hole. Based on the DIC results, three basic types of cracks are identified successfully, i.e. tensile crack (Mode I), shear crack (Mode II) and mixed tensile-shear crack (Mode I-II). All the specimens each of which contains a single hole and double cracks show mixed tensile–shear failure (Mode I-II). The coalescence patterns between the hole and the cracks are influenced by their horizontal distance, and can be classified into tensile coalescence, rotation coalescence and mixed tensile-shear coalescence. The dispersion of the strain field can be quantified by the effective variance of strain field comprehensively. The effective variance of the strain field is close to zero at the initial crack closure stage and the elastic deformation stage. The cracks propagate in different manners after initiation. Based on the effective variance of the strain field, a quantitative method to identify the type of crack was proposed. The cracks can be identified as tensile crack, mixed tensile-shear crack, and shear crack, respectively, when the growth rates of the effective variance fall into the ranges of 0.72×10-2-1.89×10-2, 2.34×10-2-3.59×10-2, and 9.63×10-2-32.40×10-2, respectively.

Key words: rock mechanics, hole, crack, digital image correlation (DIC) method, effective variance, crack identification