Investigation on the characteristics of fracture process zone under cyclic loading: Insights from macro-mesoscopic analysis

Cyclic disturbance is common in engineering which accounts for the main reason for fractures and plays an important role in the fracture process zone (FPZ), leading to unclear characteristics at the macro-mesoscopic level. It is thus essential to focus on the macro-mesoscopic analysis of fracture characteristics in FPZ subjected to cyclic loading. In this paper, a series of tests are conducted on sandstone to quantify the macro-mesoscopic features of FPZ. The experimental results indicate that the cyclic loading has an important effect on fracture toughness, the correction of nonlinear fracture toughness based on FPZ is reduced at different amplitude levels. Detailed scanning electron microscope (SEM) observation revealed the scenario of the damage zone indicating the connection between macroscopic fracture properties and mesoscopic mechanism. Additionally, it is found that more intergranular cracks were produced on the cycling failed surface, which is opposite to static rupture. A cohesive zone model (CZM) was introduced to characterize the constitutive relationship of tensile fracture, combined with digital image correlation (DIC) method, the evolution law of FPZ can be divided into three stages. Moreover, 3D laser scanning technology was applied to calculate and analyze the parameters of failed surfaces, namely, fractal dimension D, asperity height, and slope angle which were positively correlated with surface roughness. Furthermore, the joint roughness coefficient (JRC) increases almost linearly with decreasing amplitudes.