Determination method of fatigue strength and precursors of fatigue failure of porous weakly cemented siltstone

In order to study the fatigue behavior of porous weakly cemented siltstone, a quantitative calculation method of characteristic stress was established based on uniaxial step loading and unloading tests of siltstone. Then uniaxial cyclic loading and unloading tests of constant amplitude were carried out to study the volumetric strain, bulk compliance, and acoustic emission evolution of siltstone where the upper limit stress was in different stress ranges. The new method separates volumetric strain into the linear elastic strain of solid-phase dominated by rock skeleton, the non-linear elastic strain of gas-phase dominated by void, and the irreversible plastic strain. The peak points of gas-phase and plastic volumetric strain are taken as crack closure stress and crack initiation stress, besides the points of gas-phase volumetric strain and total volumetric strain changes from positive to negative are taken as crack damage stress and absolute dilatation stress, solving the problem that the calculation of characteristic stress is affected by subjective factors. The crack damage stress and absolute dilatancy stress determined by the new method can be regarded as the threshold stress of low-cycle and high-cycle fatigue failure. When the upper limit stress is lower than the crack damage stress, the continuous friction and slippage between the skeleton particles and the microstructure rearrangement under cyclic loading and unloading result in the specimen being gradually compressed. As the times of cyclic loading-unloading increase, the residual volumetric strain, bulk compliance, and cumulative acoustic emission count tend to be stable. When the upper limit stress is higher than the crack damage stress, due to the generation of a large number of cracks, the residual volumetric strain and bulk compliance continue to decrease while the cumulative acoustic emission count continues to increase, resulting in a fatigue failure of the specimen. The change of bulk compliance from positive to negative can be regarded as the early precursor of fatigue failure. The number of acoustic emission events N_AE and k are approximately in a logarithmic-linear relationship. On this basis, it is proposed slope b_k to describe the fracture pattern. The sudden increase of b_k can be regarded as the early precursor and critical precursor of fatigue failure.