Temperature dependences and rate effects on Mode II fracture toughness determined by punch-through shear technique for granite

Abstract

Most studies of thermal effects on rock fracture have focused on Mode I, while the temperature-dependence and loading rate effect on Mode II fracture are rarely involved. We present the static and dynamic punch-through shear (PTS) tests, implemented on the INSTRON 1342 and the modified split Hopkinson pressure bar (SHPB) system respectively, as the approaches for determining the Mode II fracture toughness (KIIC) of PTS specimens at a series of thermal treatments and loading rates. As any macroscopic characteristic of rock is related to its microscopic or mesoscopic structures and properties, the SEM technique is thus applied to observe the mesoscopic morphology of heated and tested specimens. The fracture mode in the static and dynamic PTS tests is verified to be the dominant Mode II by fracture surface SEM observations. The results show that static KIIC (KIICS) increases with the thermal treatment temperature while static Mode I fracture toughness (KIC) is almost unchanged within 200 °C, and they all decrease substantially from 200 °C to 600 °C. Moreover, the dynamic KIIC (KIICD) at each thermal treatment temperature appears obvious loading rate effect. The strengthening effect of high loading rate on KIICD is almost unchanged below 400 °C, while the thermal effect on KIICD/KIICS becomes apparent when the temperature reaches 600 °C. The SEM observation displays that almost no cracks initiating within 200 °C, and the growth of obvious intergranular and transgranular cracks commence at about 400 °C and 600 °C, respectively. Besides, reticular cracks start to generate at 400 °C. As such, the mesoscopic characteristics at different thermal treatment temperatures are in accord with the variations of physical and mechanical parameters.