Investigation of dynamic fracture characteristics of frozen red sandstone using notched semi-circular bend method

Considering that fluctuations in temperature can cause variations in both the internal structure as well as the mineral composition of rocks, their fracture characteristics must be impacted accordingly. With the exponential development of geotechnical engineering in cold regions, it is urgent to study the influence of the sub-zero temperature environment on the mechanical properties and dynamic properties of rocks. In order to investigate the influence of sub-zero temperature gradient on the dynamic fracture characteristics of rocks, red sandstone was used for the preparation of notched semi-circular bend specimens. First, a water-saturated machine and a sub-zero temperature incubator were utilized to pretreat the rock for 48 h, conducive for both satiation and freezing processes. Subsequently, the dynamic tests were carried out utilizing an improved split Hopkinson bar experimental system with a cryogenic sub-system. Concurrently, the striker velocity was modulated by setting distinctive air pressures, following which the rock was loaded at various loading rates. The test results demonstrate that the fracture toughness of the rock has an evident loading rate effect, and the fracture toughness proliferates exponentially with the increase in the loading rate. In the event that the loading rate is certain, the fracture toughness of the rock primarily increases gradually and then expeditiously over the course of advancement from room temperature to −20 ℃. Contradictorily, the rock fracture toughness diminishes abruptly with plummeting temperature. Analysis of the rock fracture process, accommodated by a high-speed camera, revealed that the fracture process of the rock at distinctive temperatures is fundamentally equivalent, and the crack propagation speed is negligibly influenced by the temperature. Furthermore, the rock fracture mode was analyzed by employing a scanning electron microscope (SEM) system. The SEM images of the rock depicted that the fracture of red sandstone at sub-zero temperature is predominantly intergranular fracture and cement tearing, accompanied by a trace of transgranular fracture. Meanwhile, the experimentation also revealed that the number of micro-cracks in the rock significantly multiplied when the temperature declined to −25 ℃, illustrating that sub-zero temperature has a deteriorating effect on the rock. Conclusively, the influence mechanism of temperature on the internal structure of the rock is discussed, and it is assumed that the change in the internal structure of the rock is the collaborative effect of thermal expansion-cold contraction and ice-water phase transition. The interpretation of this study has substantial reference significance for the further consequential analysis of frigidity on the fracture properties of the rock.