Experimental study of energy dissipation characteristics and crack coalescence modes of cracked sandstone under different cyclic loading paths

This study performed uniaxial compression tests and cyclic gradient loading tests with both an increasing and a constant lower stress limit on cracked sandstone. Test results showed how the different cyclic loading modes changed deformation modulus, energy dissipation, and crack coalescence modes in the cracked sandstone samples. For the rock deformation analysis, a deformation modulus was used to explain why a sample with a smaller crack inclination angle underwent less deformation when subjected to the same cyclic loading path. The tests showed that, compared with uniaxial compression, the two kinds of cyclic loading increased the specimen's deformation modulus. Under cyclic gradient loading with an increasing lower stress limit, the dissipated energy densities of the samples show a "step-like" increased, and the overall growth trend is roughly logarithmically. However, for the other cyclic loading path with a constant lower stress limit, the samples' dissipated energy densities increased exponentially as the number of cycles increased. In addition, a block energy dissipation density rate K was defined, and the deformation of the samples before fracture instability could be divided into stages defined by K. In terms of crack coalescence modes, six crack types were defined. The comparison and analysis of the test results according to these defined crack types showed that, compared with uniaxial compression, cyclic loading increased the number and types of cracks that developed in the rock samples and more surface spalling occurred.