Investigation of meso-failure behavior of rock under thermal-mechanical coupled effects based on high temperature SEM

It is extremely important to study and understand the deformation behavior and strength characteristics of rocks under thermal-mechanical (TM) coupling effects. Failure behavior and strength characteristics of Pingdingshan sandstone were investigated at room temperatures up to 300°C in an internally heated apparatus and tensile load through meso-scale laboratory experiments in this work. 33 experiments have successfully been conducted for Pingdingshan sandstone. Experimental results indicated that the tensile strength increased slowly with temperatures from 25°C to 100°C, and then sharply jumped from 100°C to 150°C, and finally decreased slightly with temperatures from 150°C to 300°C. And about 150°C is the threshold temperature of strength and thermal cracking. At low temperatures (25°C–150°C), sandstone strength is determined by relatively weak clay cement. However, at higher temperatures (150°C–300°C), because of the strength enhancement of clay cement, sandstone strength is controlled by both mineral particles and clay cement. The effects of cement clay, micro-cracks closing, and thermal cracking were the possible reasons for our detailed analysis. In addition, the typical fracture position maps and nominal stress-strain curves indicated that the temperature had strong effects on the failure mechanism of sandstone. The fractograph implied that the dominant fracture mechanism tended to transform from brittle at low temperatures to ductile at high temperatures.