Abstract:

In order to investigate the energy evolution and failure characteristics of single fissure carbonaceous shale under drying-wetting cycles, intact and fissure carbonaceous shale samples with fissure angles of 30°, 45°and 60° were fabricated respectively. MTS815 rock mechanical test system was used to conduct triaxial compression tests under different drying-wetting cycles. The influence of drying-wetting cycles on the strength, failure mode and energy evolution of single fissure carbonaceous shale were studied. The results show that the elastic energy and dissipated energy at crack initiation stress, damage stress and peak stress present exponential relationships with drying-wetting cycles. The elastic energy and dissipated energy at crack initiation stress and dissipated energy at damage stress are less sensitive to drying-wetting cycle, while the sensitivities of elastic energy at damage stress, and elastic energy and dissipated energy at peak stress are relatively high. The failure mode of carbonaceous shale is dominated by drying-wetting cycle and fissure angle, in which the drying-wetting cycle is the main controlling factor, and the fissure angle is the secondary controlling factor. It is found that tensile shear failure occurs in dry rock sample with fissure angle of 30°, while the dry rock sample with fissure angle of 45°and 60° are subjected to shear failure. With the increase of the number of drying-wetting cycles, the macroscopic length of the main crack increases, the density of secondary cracks increases, and the failure mode transforms to shear-tension composite failure. With the increase of the number of drying-wetting cycles, the energy storage level at crack initiation stress K_ci and the energy storage level at damage stress K_cd increase gradually. The higher the energy storage level at crack initiation and damage stress is, the more likely the crack initiation and rock damage occur. K_cd can be used as an warning indicator of rock failure. A larger K_cd indicates that the rock is more vulnerable to failure.

Key words: triaxial compression, single fissure carbonaceous shale, drying-wetting cycle, energy evolution, failure characteristics