Mechanical characteristics of single cracked limestone in compression-shear fracture under hydro-mechanical coupling

This paper creates single cracked specimens by the prefabricated crack in the middle of a standard cylindrical specimen of limestone. The MTS815 rock mechanics test system was used to perform conventional triaxial compression tests on single cracked limestone specimens under different confining pressures and water pressures. The theory of linear elastic fracture mechanics is used, the fracture mode of single-cracked limestone is analyzed and the rock compression-shear fracture criterion under hydro-mechanical coupling is deduced and verified experimentally. The results show that the deviatoric stress-strain curve of single-cracked limestone has a unique double stress peak behavior of high front and low back (crack inclination angle beta = 15 degrees) and low front and high back (crack inclination angle beta = 45 degrees). The crack initiation stress and peak strength of single crack limestone show a significant decreasing trend with increasing water pressure and increasing with increasing confining pressure. The highest values of crack initiation stress and peak strength for specimens with crack inclination angle beta = 75 degrees, secondary for specimens with beta = 15 degrees and smallest for specimens with beta = 45 degrees under the same confining pressure and water pressure. Based on the fracture mechanics theories, the stress intensity factor of open crack and open crack closed by force increases with the increase of water pressure. The effective stress intensity factor and mixing factor increase with the increase of the crack inclination angle. In addition, the advantage angle beta m is detected to be related to the friction coefficient of the crack surface, and the range of values of the advantage angle of the open crack subjected to pressure to closure is obtained to vary between 45 degrees and 67.5 degrees. Additionally, the advantage angle for the open crack is 45 degrees, which indicates that the specimen crack inclination angle with beta = 45 degrees is the easiest to initiate cracking, and the conclusion is in concordance with the test results.