Heating-path dependency of fracture behavior of sandstone subjected to thermal treatment

Abstract

The thermal effect has a negative impact on the strength and fracture properties of rocks, and much related research has been carried out around heat treatment methods such as temperature levels, heating rates and cyclic thermal shocks. In fact, the temperature history to which rocks are subjected is often complex, and the effects of this accumulation of thermal damage on the rock cannot be ignored. In this work, we have carried out three-point bending tests on sandstone subjected to thermal treatments under conventional and complex heating paths based on digital image correlation (DIC) method. The results show that the P-wave velocity, peak load and fracture toughness all tend to decrease with increasing temperature under conventional heating paths, and that 600 °C is the sandstone threshold temperature for strong and weak transformation. Under complex heating paths, a constant temperature of 200 °C for two hours and further heating to the target temperature does not weaken the mechanical properties of the sandstone, whereas a constant temperature of 400 °C for two hours and further heating results in a significant reduction. The variation trend of crack opening displacement (COD) with axial load shows that the cracks transition from slow opening in the early loading stage to rapid propagation near the peak load and develop into decelerated extension in the later stage. Finally, the crack propagation process was refined by determining the jump points of the maximum principal strain at different positions on the crack propagation path. At room temperature, the sandstone cracks at approximately 0.8Pm and propagates at a higher rate, exhibiting clear brittle characteristics. Both the crack initiation load and propagation rate show a significant decrease with increasing temperature after 200 °C, with ductile characteristics gradually becoming apparent.