Grayscale evolution characterization of the strain field and precursor identification in sandstone containing multiple flaws after freeze–thaw treatment

To investigate the mechanical properties and fracture mechanism of rocks containing multiple flaws after freeze–thaw (F-T) treatment, sandstone specimens with multiple flaws were prepared and subjected to F-T treatment and uniaxial compression tests. The global strain field on the specimen surface during the loading process was calculated using the digital image correlation (DIC) method, and four grayscale parameters were introduced to quantitatively characterize the evolution of the strain field. Then, the differentiation rates (DRs) of these parameters were defined, and the correlation between these parameters and the rock fracture state was analyzed. The F-T cycling deteriorated the mechanical properties and increased the strain concentration of the specimens. The calculation results of the horizontal strains at the measurement points along the crack propagation path showed that the crack initiation of the specimens after F-T treatment occurred earlier, and their brittle failure characteristics were less pronounced. The specimen fracture process was successfully described by combining DIC and statistical grayscale parameters. The evolution pattern of each grayscale parameter of the specimens was generally similar before and after F-T treatment in that it exhibited disorderly fluctuation first, then steady development, and finally accelerated change. The grayscale parameters changed abruptly at the initiation of wing cracks and antiwing cracks, resulting in sharp points in the DR vs. strain curves. These points can be defined as precursor points P₁ and P₂ for identifying the deformation and fracture state of the rock. The F-T cycling led to the delayed occurrence of P₁ and the advanced occurrence of P₂.