Damage characteristics of sandstone with different crack angles subjected to true triaxial cyclic loading and unloading

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Cracks in the rock are among crucial defects affecting its deformation and strength characteristics. Particularly in underground or mining engineering, the initiation and development of cracks due to cyclic unloading are critically affected by the fracture of the rock mass. In order to scrutinize the mechanical behavior of rocks with different crack angles, a true triaxial cyclic loading and an unloading test are carried out on sandstone samples with prefabricated cracks of various angles. The cracked sandstone samples do not considerably expand under action of triaxial stresses, commonly exhibit non-ideal elastic characteristics, and mainly undergo shear failure. As the crack angle grows, the tensile properties become pronounced, and the fracture surface morphology of the cracked samples presents a huge difference. The change rules of the irreversible axial and lateral strains are apparently different, and the rate of change of the irreversible lateral strain in terms of the applied stress is more sensitive than that of the axial irreversible strain. The energy evolution law of the hysteresis loop formed in the sample cyclic load test has no evident influence on the crack angle of the sample; however, the larger the crack angle of the sample, the more energy consumption during failure. Based on the equivalent irreversible strain and the hysteretic loop energy dissipation indicators, the damage law of sandstone samples can be described. It is also revealed that the damage rule determined according to the equivalent irreversible strain index is more conducive to characterizing the initial characteristics of damaged samples. Additionally, the damage rule determined based on the hysteretic loop energy dissipation index would better describe the damage rule associated with the understudy samples. There are obvious discrepancies in the morphological characteristics of the fracture surface in the scanning electron microscope images of the typical fracture surface of the samples with various crack angles, indicating that the damage and destruction of the sample acted upon by the shear and tensile shear are wholly dissimilar.