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 charac