Influence of thermal treatment on dynamic mode Ⅱ fracture properties of rocks using the short core in compression (SCC) method

Abstract

The effect of thermal-induced damage on the dynamic mode II fracture properties is crucial for evaluating the long-term safety and stability of underground rock-related infrastructures. Thus, it is significant to quantify systematically and accurately the dynamic mode II fracture parameters of rocks under elevated temperatures. Recently, the short core in compression (SCC) has been successfully adopted to measure the dynamic mode II fracture toughness KIIC of rocks due to its easy preparation and simply assembly. However, the dynamic mode II fracture parameters of rocks under various high temperatures have not been systematically determined. In this study, the dynamic SCC specimen was adopted to investigate the influence of thermal treatment on the dynamic mode II fracture parameters of a homogeneous biotite gabbro (BG). The SCC specimens were thermally treated at 25 °C, 100 °C, 300 °C, 500 °C and 700 °C. The P wave velocity and CT value were measured to characterize the thermal-induced damage within the BG specimens. The XRD analysis was employed to examine chemical changes in BG treated at elevated temperature. A split Hopkinson pressure bar (SHPB) apparatus was used to conduct the dynamic SCC tests and the momentum-trap system was utilized to ensure that the dynamic energy absorbed by the SCC specimen can be accurately determined. The SCC specimen was sheared along the upper and lower notch-tips, and the fracture surface of the SCC sample is relatively smooth. The failure pattern is valid mode II fracture. For five different temperatures, the dynamic KIIC of BG increases linearly with the loading rate. At the same loading rate, the dynamic KIIC of BG specimen treated at the temperatures lower than 300 °C is much higher than that without thermal treatment. With the increase of the heat-treatment temperature, the dynamic KIIC of BG decreases gradually under the same loading rate. The combination of the thermal expansion, the phase transformation and the oxidation in BG could generate the increase of the number of microcracks and the growth of microcracks, further reducing the dynamic KIIC of BG at high treatment temperatures. Furthermore, at the same temperature, the dynamic fracture energy of BG demonstrates the rate dependence. Another important observation is that, at the same loading rate, the dynamic fracture energy of BG increases when the temperature increases from 25 °C to 100 °C, whereas the dynamic fracture energy of BG decreases with the increase of temperature when the temperature is higher than 100 °C.