Abstract: To determine the deformation and fracture properties of brittle shale containing natural cracks sampled from Pengshui region of Chongqing City, uniaxial tests under uniaxial cyclic loading have been conducted using the rock mechanics test system RMT–150C. The experimental results show that: (1) Under the combined effect of cyclic load and cracks, the existence of cracks can weaken the local properties, accelerate the cracks propagation and promote the failure of the shale, resulting in the reductions in yield stress, fracture stress and peak strength; namely, the peak strength reduces by 13.7%―58.3%. (2) The axial strain shows a closed “sharp-leaf” shape hysteresis loop with a “sparse-compact-sparse” arrangement, whereas the transverse strain displays a 8-shaped hysteresis loop upward opening with a “compact-sparse” arrangement. The curve of transverse strain versus cyclic number can be divided into four phases: initial deformation phase, constant speed deformation phase with a low speed rate, constant speed deformation phase with a high speed rate, and accelerating deformation phase. The onset of transverse strain within the initial phase can be referred to as the sign of the completion of the pre-existing cracks propagating and coalescing into the new-forming cracks, and then transferring to next phase; a subsequent mutation occurrence can be considered as the omen of whole failure. (3) The main fracture morphologies of shale containing pre-existing cracks are can be categorized into two distinct types: tensile-shear cut-through mode and tensile cut-through mode. Both modes contains at least one tensile crack which cut through the pre-existing crack. (4) During the elastic phase, there is a linear relationship between the effective modulus and the damage area coefficient F, and the larger the damage area coefficient , the lower the effective modulus. (5) Under the cyclic condition of low stress level, irreversible deformation develops slowly, and the curves of axial strain versus times always stay in the initial deformation phase, while the specimens remain stable; under the cyclic condition of high-stress level, after the specimens have experienced the three phases, failures can be observed in the specimens. Under the cyclic condition of a stress level close to the theoretical peak strength, deformation curves directly reaches the acceleration phase and several cycles of loading can result in fracture of the specimens

Key words: shale, damage area parameter, cyclic loading, natural crack, deformation characteristics, fracture characteristics