Cyclic loading effects on the strength and fatigue properties of argillaceous siltstone across various characteristic stress intervals

To examine the strength and fatigue characteristics of argillaceous siltstone under varying upper limit loads, cyclic-to-monotonic loads and fatigue failure tests were carried out with the upper limit load in different characteristic stress ranges. At upper limit loads between crack initiation stress and damage stress, the anti-deformation capacity and peak strength of argillaceous siltstone improved considerably after cyclic loading. Rock fatigue failure occurred when cyclic loads surpassed the stress causing damage, and the strain at the control point on the uniaxial compression (UC) curve served as a benchmark for identifying rock fatigue failure. The fatigue strength of the argillaceous siltstone was around 80-89% of the UC strength. The logarithm of fatigue life showed a negative linear correlation with upper limit load. In addition, deformation characteristics and modulus of elasticity also confirmed the weakening and strengthening phenomenon of argillaceous siltstone at various upper limit loads. On the basis of the macroscopic mechanical behaviour and microscopic fracture features of argillaceous quartz sandstone, the weakening and strengthening mechanism of argillaceous siltstone at a range of upper limit loads was revealed. The results indicate that increased uniformity of crack size, reduced degree of fragmentation and increased effective bearing surface contribute to rock strengthening, while crack development tends to become localized and an increase in fracture scale leads to rock weakening.