Investigation of three different cooling treatments on dynamic mechanical properties and fragmentation characteristics of granite subjected to thermal cycling

Underground thermal engineering such as geothermal exploitation usually involves the response of impact loads due to the drilling operation and blasting load. This study investigated the dynamic mechanical properties and fragmentation characteristics of granite sub-jected to thermal cycling under natural cooling, water cooling, and liquid nitrogen cooling. Firstly, split Hopkinson pressure bar tests were performed on granite under three different cooling methods to study the dynamic mechanical parameters with thermal cycles. Sub-sequently, the damage factors were calculated to evaluate the damage on granite and the relationship between damage and stress. Finally, the granite fragments after impact were sieved, and the fractal dimension and average fragment size were introduced to analyze the frag-mentation characteristics. The results demonstrate that the degradation of dynamic mechanical properties mainly occurs in the first four thermal cycles; with the increase of damage factor, the stress decreases linearly under the three cooling methods, and the damage caused by liquid nitrogen cooling to granite is the most significant, followed by water cooling. Fragments of granite change from axial splitting failure to a more complex composite failure mode. In addition, with the decrease of the average fragment size, the fractal dimension of the granite gradually increases, resulting in the higher fragmentation degree and the better uniformity of fragmentation.