Fracture analysis of Beishan granite after high-temperature treatment using digital image correlation

•Thermal effect on fracture behaviors of granite is investigated using DIC technique.•High-temperature treatment significantly affects the FPZ characteristics.•Thermal effect on fracture toughness measurement is analyzed.•The thermal-induced damage mechanism is revealed by microscopic observation. A systematical knowledge of the fracture characteristics and thermal damage mechanism of granite at elevated temperature is an important issue in the high-level nuclear waste disposal and geothermal energy exploration field. To investigate the influence of high temperature on the fracture behaviors and characteristics of Beishan granite, three-point bending tests were conducted on specimens subjected to different high temperatures, i.e. 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 800 °C, and specimens at room temperature (25 °C) were also tested for comparison. An innovative technique called digital image correlation (DIC) was used in this study to trace the fracture process, and important fracture parameters, such as the fracture process zone (FPZ) size and critical opening displacement, were measured by full-field displacement and strain measurement. The experimental result illustrates that thermal treatment has a significant influence on the fracture behavior of the Beishan granite. Comparisons reveal that the enhanced ductile feature, a linear decline in the peak load and a rapid increase in critical crack mouth opening displacement is observed with the increase of temperature from 200 °C to 600 °C. Moreover, the increasing temperatures also reduce the applied load for fracture initiation. The FPZ length and critical opening displacement are significantly affected by the thermal treatment, while the FPZ width almost does not change with the increasing temperatures. The fracture toughness for Beishan granite shows a decreasing trend with the increase of temperature from 25 °C to 300 °C, while a valid fracture toughness value cannot be obtained for granite specimens subjected to temperature over 300 °C due to the existing of a large FPZ above the notch tip. Finally, the thermal-induced damage mechanism is also revealed by optical microscopic observation, and the thermal-induced microcracks are considered as the essential cause for the change of the fracture behavior of Beishan granite.