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 semi-circular 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.

为了研究花岗岩样品在高温处理和液氮（LN ₂）冷却后的断裂韧性，对包含纯I型断裂的半圆弯曲（SCB）样品进行了一系列断裂试验。在测试过程中，采用了声发射（AE）技术来监控损坏过程。实验结果表明，总体而言，经LN ₂处理的花岗岩样品的断裂韧性随样品初始温度的升高而降低，除了在25-200°C的范围内，断裂韧性随温度的升高而略有增加。LN ₂和加热后花岗岩样品的微观结构通过扫描电子显微镜（SEM）观察冷却。在200°C的LN ₂冷却的花岗岩样品中观察到孤立的，短而分散的微裂纹，这可能是由于钝化了预裂纹而导致断裂韧性提高的原因。在用LN ₂冷却后，温度较高的样品的AE计数会更早出现，这意味着随着加热温度的升高，用于引发裂缝的所需能量会降低。LN ₂后温度相对较低的样品（本研究中为200°C）冷却仍然表现出典型的脆性破坏，这可以通过载荷曲线的直线下降和弹性应变能的强烈释放来得出结论，但是，对于温度较高（目前为400°C）的样品，观察到了从脆性到延性的转变。研究），其AE计数也可以在峰后阶段进行监控。在温度较高的LN ₂冷却的样品上，会出现许多局部故障点，并伴随着大量的AE异常，这是预裂纹与热应力引起的缺陷之间联系的结果。此外，经过三点弯曲试验后，温度较低的LN ₂冷却样品的断裂路径将比温度较高的LN ₂冷却样品的断裂路径更直。