SEM in situ investigation on thermal cracking behaviour of Pingdingshan sandstone at elevated temperatures

P>Although many studies on thermally induced cracks in rocks have been carried out by indirect investigations of samples at atmospheric conditions in macro/microscale, scanning electron microscope (SEM) in situ investigations of thermal cracking are rare, especially at the elevated temperature conditions in vacuum chamber of a SEM. In this paper, SEM in situ investigations of the thermal cracking behaviour of Pingdingshan sandstone from 25 to 300 degrees C indicate that there are two thermal cracking threshold temperatures for the sandstone because of the effect of elevated temperature, namely, grain boundary cracking behaviour when the temperature is lower than 125 degrees C and intragranular cracking behaviour when the temperature is from 150 to 300 degrees C. The effects of mineral thermal mismatch capability on deformations of sandstone microstructure cannot be ignored. Three thermal cracking models have been observed, namely, grain boundary, intragranular and mix thermal cracking. Experimental results have also indicated that the threshold temperature of most of grain boundary thermal cracks is lower than that of intragranular and mix thermal cracks of grain boundary and intragranular. The relevant fractal models of thermal cracking have been used to quantitatively explain these phenomena successfully. The statistics analysis indicates that the amount of thermal cracking increase as the temperature increasing. However, the density of thermal cracks of in situ results is approximately equal with the existing results after cooling in literatures. Most of thermal cracks occurred at about 90 degrees tilted to the direction of pre-load. In addition, the effect of pre-load is significant when the temperature is lower than 250 degrees C, and the effect gradually reduce when the temperature is higher than 250 degrees C. The thermal cracking behaviour is affected not only by thermal expansion mismatch and thermal expansion anisotropy of mineral, but also by the shape of mineral grains and by the direction of pre-load.