Fracture behavior and damage evolution model of sandstone containing a single pre-existing flaw under discontinuous fatigue loading

The process of disaster incubating in deep rock mass engineering is mainly related to the fracture behavior and damage mechanism of the rock mass. This paper proposes a new damage model based on Lemaitre's equivalent strain hypothesis with improved Harris distribution and laboratory discontinuous fatigue loading test (DFLT). Firstly, discontinuous fatigue loading tests were carried out on the jointed sandstone specimens and acoustic emission (AE) was used as a research tool. Based on the b-value and AF-RA theory, the failure mode of specimens was further explored in terms of the crack evolution process and crack classification. Meanwhile, digital image correlation (DIC) techniques were used to analyze the evolution of the deformation field on the surface of the specimen. Subsequently, the fracture morphological characteristics of the rock masses were analyzed from a microscopic perspective by scanning electron microscopy (SEM) techniques. The investigations found that in a short time, the low-level constant load will lead to the interlocking of particles inside the rock mass, reduced incremental coupled damage, and enhanced plasticity “with” reducing the coupled damage increment and enhancing the plasticity, thereby inhibiting the damage growth rate. In addition, as the stress level increases, the friction between the grains intensifies, the cuttings and roughness of the fracture surface increase, the fatigue softening effect is enhanced, and the specimen gradually transforms from brittle failure to ductile failure. At the same time, the proportion of stress-induced tensile cracks and shear cracks gradually increased, resulting in the specimen finally presenting tensile-shear composite failure.