Damage evolution of tuff under cyclic tension-compression loading based on 3D digital image correlation

We have investigated the damage evolution and crack propagation of tuff rocks under axially cyclic tension-compression (T-C) loading with different amplitudes. The field of axial strains in the rock surface was obtained by three-dimensional digital image correlation (3D-DIC) method. The apparent strains were used to analyze the damage evolution along with the nucleation and propagation of microcracks until rock failure. The experimental results demonstrated that a local damage zone (LDZ) was formed when the loading amplitude exceeded a certain value, and the rock sample was prone to tensile failure under T-C cyclic loading. Based on arrangement of the virtual extensometers on the field of axial strains, the axial stress-strain curves, maximum axial strain, secant moduli, and dissipated energy evolution were obtained as a function of cycle number. It was observed that the variations in these mechanical parameters and dissipated energy significantly increased in the T-C cycle test with the increase in the loading amplitude. Particularly, due to obvious accumulation of residual compressive strain during the first cycle, the maximum tension stress increased, secant modulus decreased, and the dissipated energy evidently increased under tension during the second cycle. Furthermore, the mechanical parameters and dissipated energy evolution as a function of cycle number inside and outside the LDZ under 80% amplitude confirmed that the deformation mainly occurred inside the LDZ and the energy mainly dissipated for the nucleation and propagation of tensile microcracks vertical to the loading direction inside the LDZ during rock failure process.