Dynamic micro-macro fatigue fracture under cyclic direct tensile impacts in brittle rocks

The fatigue fracture under cyclic dynamic direct tensions of brittle rock is an important mechanical characteristic index for the evaluation of geological disasters and underground engineering safety. However, most studies focus on macroscopic fracture mechanical properties, and the mechanism linking the macroscopic fracture with the microcrack growth during the cyclic dynamic direct tensile loading of brittle rocks is rarely studied. In this paper, a micro-macro fracture model explaining the stress-strain constitutive relationship is established at the last impact failure after being subjected to multiple cyclic direct tensile impacts of brittle rocks. This model is based on the wing crack extension model under direct tensile loading, the quasi-static and dynamic fracture toughness relationship, the suggested crack rate and strain rate relationship, the relationship of damage and dynamic tensile fatigue life N, the relationship of dynamic fracture toughness and dynamic tensile fatigue life N. The variations of dynamic mechanical properties of rocks with dynamic tensile fatigue life for different initial crack sizes and angles within the rocks are further discussed. The compressive strength, elastic modulus, crack initiation stress, limit crack extension length and crack extension rate descend and the failure strain ascends with an increment of dynamic tensile fatigue life in rocks. This study's results provide help for the safety and stability of the underground surrounding rocks under blasting working or seismic disasters.