The influence of liquid nitrogen cooling on fracture toughness of granite rocks at elevated temperatures: An experimental study

In order to investigate the fracture toughness of granite samples after elevated temperature treatment and liquid nitrogen (LN2) cooling, a series of fracturing tests were performed on semicircular bend (SCB) specimens containing pure mode-I fracture. The acoustic emission (AE) technique was employed to monitor the damage process during the test. The experimental results indicate that, overall, the fracture toughness of LN2-treated granite samples decreases with the increase of sample?s initial temperature except at the range of 25?200 ?C where the fracture toughness shows a slight increase with the increase of temperature. The microstructures of the granite samples after heating and LN2 cooling were observed by scanning electron microscope (SEM). Isolated, short and scattered microcracks were observed within LN2-cooled granite samples with 200 ?C, which potentially accounts for the increased fracture toughness by blunting the pre-crack. The AE counts occur earlier for samples with higher temperature after cooling by LN2, which means the required energy for fracture initiation decreases with the increase of heating temperature. Samples with a relatively low temperature (200 ?C in the present study) after LN2 cooling still exhibit typical brittle failure, which can be concluded by the straight drop down of loading curve and intensive release of elastic strain energy, however, a transition from brittle to ductile behavior was observed for samples with higher temperature (400 ?C in the present study) whose AE counts can be also monitored in the post-peak stage. Many local failure points accompanied with unique large AE counts occur on the LN2-cooled samples with higher temperature, which is the result of the connection between the pre-crack and the flaws induced by thermal stress. Besides, the fracture paths of LN2-cooled samples with lower temperature after three-point bending test will be straighter compared with those samples with higher temperature.