An in-depth understanding of the thermomechanical properties of rocks is fundamentally important in many fields of geotechnical engineering. However, the microcracking mechanisms of thermally treated granite under mode I loading are very complex. To investigate the effect of thermal treatment on the microcracking behavior, we perform mode I three-point bending tests on a set of pre-notched semi-circular specimens. The specimens are pre-heated to different target temperatures (i.e. 50 °C, 100 °C, 150 °C, 200 °C, 400 °C, and 600 °C), which are then naturally cooled down to room temperature. Acoustic emissions are monitored during the loading tests to provide clues on the microcracking processes. The fracture process zone (FPZ) development features before the initiation of macrofracture are interpreted through analysis of the spatial-temporal evolution of AE events. Based on the AE signatures, we identify the microcracking behavior transition phenomenon as the thermal treatment temperature increases from 150 °C to 200 °C, which is related to the development of thermal microfractures. As the transition occurs, we observe (1) relatively lower load levels at the beginning of the rapid FPZ development phase; (2) longer rapid FPZ development duration; (3) larger size and maximum event density for fully-developed FPZs. Taking the microscopic observation into consideration, we propose an extended conceptual model to describe the FPZ evolution before the unstable fracture propagation in crystalline rocks. The present findings provide useful insights into the microcracking behavior in geoengineering practices where the host rocks are subjected to temperature changes.

深入了解岩石的热机械性质在岩土工程的许多领域中至关重要。但是，热处理花岗岩在模式I加载下的微裂纹机理非常复杂。为了研究热处理对微裂纹行为的影响，我们对一组预先开槽的半圆形试样进行了模式I三点弯曲测试。将样品预热至不同的目标温度（即50°C，100°C，150°C，200°C，400°C和600°C），然后自然冷却至室温。在负载测试过程中会监测声发射，以提供有关微裂纹过程的线索。通过分析AE事件的时空演化解释了大断裂开始前的断裂过程带（FPZ）发育特征。基于AE签名，我们确定了随着热处理温度从150°C升高到200°C而发生的微裂纹行为转变现象，这与热微裂纹的发展有关。随着过渡的发生，我们观察到（1）在快速FPZ开发阶段开始时相对较低的负载水平；（2）FPZ快速发展期更长；（3）较大的FPZ具有更大的规模和最大的事件密度。考虑到微观观察，我们提出了一个扩展的概念模型来描述在不稳定的裂缝在结晶岩中传播之前的FPZ演化。