Dynamic cracking process of rock interpreted by localized strain-rate, rate-dependent strength field and transition strain-rate

The cracking mode of rock under dynamic loadings is significantly different from that under quasi-static loadings. One of the underlying mechanisms is the variation of the rate-dependent mechanical properties under different loading rates. However, the rate-dependent mechanical properties cannot explain the transition of the failure mode and the suppression of the tensile cracks under dynamic loading. In this paper, the interaction between the rate-dependent properties and the geometrical effect of pre-existing flaws is investigated and successfully explained these questions. The classical single-flaw model providing a good stress concentration at possible crack initiation positions and material homogeneity is used to analyse the stress, strain, strain rate, and rate-dependent strength fields experimentally and mathematically. The rate-dependent strength field in the dynamic regime is proposed and seen as the key to the cracking mode change. Based on the dynamic tests on intact specimens, the tensile strength is generally found more sensitive to strain rate than the compressive strength. Due to the uneven strain induced by stress concentration around the flaw, the strain rate is also uneven and proportional to the stress intensity naming the "localized strain rate effect". In the analytical study, the equations of the "transition strain rate" as a watershed for the different fracturing behaviours are given. The theoretical study shows that the dynamic mechanical properties and the geometry-induced stress/strain rate distribution non-uniformity should be coupled together to analyse the failure process of rocks.