The effects of thermal treatments on the fatigue crack growth of Beishan granite: an in situ observation study

In some temperature-related rock engineering projects, rocks are subjected to cyclic loading. A deep understanding of the influences of temperatures on rock fatigue mechanisms is of great engineering significance. In this research, in situ observations of the fatigue crack behaviors of Beishan granite after 50–600 °C thermal treatments were conducted using a scanning electron microscope equipped with a loading system. The continuous cracking processes under cyclic loading were recorded. It was found that the thermal treatments resulted in thermal cracking, as well as weakening of the mineral grain boundary strength, which accounted for different crack behaviors under cyclic loading. The observations revealed that as the temperature elevated, the cracking patterns had gradually changed from the forward propagation of new cracks to the continuous opening of the pre-existing cracks. In the low temperature cases (50–150 °C), it was found that the fatigue cracks were initiated at the notch roots, then propagated along the direction which was approximately perpendicular to the maximum tensile stress, which was less affected by the temperature conditions. In regard to the intermediate temperature cases (200–300 °C), it was observed that the fatigue cracks propagated in the manner of crack bridging between the pre-existing cracks and the intergranular cracks which had formed by mineral grain decohesion. In high-temperature cases (400–600 °C), the pre-existing cracks were observed to gradually open and few newly formed cracks were found. In addition, in the 50–250 °C cases, the crack growth rates were in good agreement with the variant Hartman–Schijve equation. In this study, the observed dependence of the parameters in the variant H-S equation on the micro-structure characteristics was discussed. The decreasing threshold of ΔK for the crack initiation, along with the increasing sensibility of the crack growth rate to ΔK, was attributed to the changes in the micro-structure which could be quantitatively characterized by the linear crack densities.