Experimental investigation of anisotropic fatigue characteristics of shale under uniaxial cyclic loading

The influence of combining the inherent anisotropy and fatigue loading on shale's mechanical behaviors is rarely investigated. To study the anisotropic fatigue behaviors of shale, uniaxial fatigue tests under different cyclic stress limits and loading frequencies are conducted using shale specimens with 7 different bedding orientations. The anisotropic characteristics of the fatigue failure mechanism, stress-strain behavior, cycle numbers, irreversible strain and energy evolution of each hysteresis loop are analyzed. The combined effects of bedding and fatigue loading can cause crack branching and coalescence, which generates a crack network. Maximum and minimum fatigue cycle numbers are obtained when the bedding orientations are 90 degrees and 75 degrees, respectively; correspondingly, the maximum and minimum energies of each hysteresis loop are obtained. The irreversible strain and energy of each hysteresis loop decrease, become approximately constant and then increase before failure when the bedding orientations are 0 degrees, 15 degrees, 30 degrees, 45 degrees and 90 degrees; this rate is approximately constant for the bedding dip angles of 60 degrees and 75 degrees. Increasing either the loading frequency or cyclic stress limit increases the energy and irreversible strain of each hysteresis loop as well as decreases the cycle number. Fatigue damage indicators defined by the secant modulus, irreversible strain, dissipated energy and accumulated hysteresis loop area cannot reflect the effects of shale's inherent anisotropy. Multiscalar or tensorial damage factors, which can describe the anisotropic fatigue damage features of shale, need be further studied.