Characteristics of fracture changes and fatigue failure signals for siltstone under cyclic loading

Changes in the mesoscopic fracture characteristics and critical signs of fatigue failure of siltstone due to macroscopic deformation under different limit loads during cyclic loading have been investigated. The fatigue strength of the siltstone is approximately 80%–85 % of the uniaxial compressive strength. The fatigue strength can be identified as the point of changes in the deformation and AE parameters of the siltstone. When the upper limit load is less than the fatigue strength, deformation is predominantly axial compression with minor damages to the siltstone. The residual strain, AE count, AE energy and AE peak frequency undergo two-stages of change. When the upper limit load is higher than the fatigue strength, the specimen undergoes fatigue failure. The upper limit load follows a logarithmic relationship with the fatigue life. The rock deformation is governed by lateral expansion, and a network of cracks on the damaged specimen formed are predominantly penetrating "X-shaped" shear cracks and supplemented by secondary shear cracks. The residual strain, AE count and AE energy show a three-stage process. The peak frequency tends towards the dominant frequency band (low- and mid-frequencies), and the peak frequency of the dominant frequency band increases and then decreases. The strain at the control point of the uniaxial compression curve can be used as a reference value for predicting the fatigue failure of the siltstone. The continuous decrease of AE b-value and the sudden increase of AE RA/AF can be regarded as critical signs of fatigue failure of the siltstone. In addition, when siltstone changes from the damaged fracture phase to the fatigue failure phase, there will be a noticeable critical slowing down phenomenon. Therefore, the sudden increase in the variance and autocorrelation coefficient of AE characteristic parameters (AE count, AE energy and RA/AF) can be considered as a precursor of rock fatigue failure.