Fatigue Failure Characteristics of Sandstone Under Different Confining Pressures

Rock fatigue behaviour, including the fatigue threshold stress (FTS), post-peak instability and strength weakening/hardening during cyclic loading, is of paramount significance in terms of safety and stability assessment of underground openings. In this study, the evolution of the foregoing parameters for Gosford sandstone subjected to systematic cyclic loading, in the pre-peak and the post-peak regimes at different stress levels and under seven confinement levels ( σ 3 / UCS avg ) was evaluated comprehensively. The results showed that the FTS of rocks decreases exponentially from 97 to 80%, when σ 3 / UCS avg increases from 10 to 100%. The brittleness of rocks under monotonic and cyclic loading conditions increases with an increase in σ 3 / UCS avg when σ 3 / UCS avg ranging between 10 and 65% (known as the transition point). For higher confinements, however, the brittleness of rock transits from self-sustaining behaviour into ductile behaviour. The evolution of fatigue damage parameters for hardening tests showed that no critical damage happens within the specimens during cyclic loading; rather, they experience more compaction. This is while for weakening cyclic loading tests, continuous damage along with stiffness degradation was dominant. Furthermore, the variation of axial strain at failure point ( ε af ) shows that for lower confinement levels, the applied stress level does not affect the pre-peak irreversible deformation; its effect, however, becomes significant when confining pressure is high. For the specimens that did not fail in cycles, cyclic loading resulted in peak strength weakening or hardening depending on the applied stress level. The weakening effect was observed in higher confining pressures, which was mainly due to a higher amount of irreversible deformation accumulation in rocks in the pre-peak cyclic loading. An empirical model was proposed using classification and regression tree (CART) algorithm to estimate the peak strength variation of Gosford sandstone based on σ 3 / UCS avg and the applied stress level. Highlights The effect of confining pressure and systematic cyclic loading history on the failure behaviour of rocks was investigated comprehensively. The lateral strain-controlled loading method and a modified triaxial testing procedure were used to measure rocks' failure behaviour. The evolution of fatigue threshold stress, rock brittleness, and peak strength with confining pressure and applied stress level was assessed. An emp