Experimental research on deformation and fracture characteristics of shale under cyclic loading

The previous researches have studied the deformation and fracture characteristics of shale containing natural cracks subjected to uniaxial compression. In order to further indicate how the deformation and fracture characteristics are influenced under coupling effect of confining pressure and cyclic loading, the equipment of MTS815.03 is employed to conduct loading and unloading cyclic triaxial compression tests. The experimental results show that under triaxial cyclic loading, the main failure mode of the shale samples containing natural cracks is a mixed failure, performing tensile-shear propagating mode, and having at least one tensile crack propagating the natural crack for each sample. With the gradually increasing confining pressure, the peak strength rises linearly. For the entire shale sample already subjected to 11 cycles of loading-unloading, the peak strength increases by 9.98%~25.03%; whereas for the shale sample with crack already subjected to 15 cycles, the peak strength increases by only 1.47%~6.98%. For the sample with crack under the same cycle loading, the elastic modulus is larger than the deformation modulus, the unloading elastic modulus is larger than the loading elastic modulus, and the unloading deforamtion modulus is larger than the loading deformation modulus. Simultaneously, the larger the damage-area coefficient F, the larger the difference between the unloading elastic modulus and the loading elastic modulus. With the increase of the cyclic times, the loading deformation modulus and the unloading deformation modulus firstly increase and then monotonously decrease; whereas, the elastic moduli of both loading and unloading exhibit increasing initially then decreasing gradually with wave-shape. The serious variant of elastic modulus is the direct evidence that the inner structure coordinates in local locations. The research may provide constructive references for the mechanical investigation of crack propagation to form complex crack net.