Staged Moduli: A Quantitative Method to Analyze the Complete Compressive Stress-Strain Response for Thermally Damaged Rock

The ultrasonic method and destructive test were combined to examine sandstone specimens taken from underground construction field in the Mount Taibai of Qin-ling Mountains, middle part of China. Staged moduli of the four stages during the uniaxial compression of sandstone after temperature varying from 25 to 1,000 degrees C were defined, through which the complete stress-strain curves of sandstone were studied quantitatively. Thermal damage of sandstone after different high temperatures was analyzed based on the thermal damage factor (TDF) defined by the modulus of compact stage. The temperature-sensitivity coefficient (TSC) was proposed to describe the sensitivity of TDF to temperature as temperature level varied. Research suggests that the compression process of thermally damaged sandstone is of prominent staged characteristic. The strain of compact stage increases significantly in a near-linear style as temperature rises up. For temperature above 400 degrees C, the ratio of compaction strain to peak strain increases to more than 50 percent. Changing rules of the four-staged moduli with temperature differs widely, among which the modulus of compact stage has a strong relativity with longitudinal wave velocity. The TDF defined by wave velocity loses sight of the change in density and Poisson's ratio, avoiding the defect of which, the defining method based on modulus of compact stage is of greater veracity. Within the range of 25-200 degrees C, the TSC is largest and the thermal damage of sandstone is more sensitive to temperature. The results of this article have some guiding significance to rock engineering in high-temperature environment.