Principles of localized strain rate, strength field and transition strain rate in cracking processes under dynamic loadings

Abstract

The cracking mode 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. In the present paper, the interaction between the rate-dependent properties and the geometry effect of pre-existing flaws is investigated and successfully explained these questions. The classical single-flaw model providing good stress concentration at possible fracture initiation and material homogeneity is used to analyze the stress, strain, strain rate, and strength fields experimentally and mathematically. The strength field in the dynamic regime is first proposed and seen as the key to cracking mode change. Based on the dynamic tests on intact specimens, the tensile strength is more sensitive to strain rate than the compressive strength. The strain rate is proved uneven in the specimen, naming the “localized strain rate effect” induced by the stress concentration around the flaw. In the analytical study, the “transition strain rate” is given as a watershed for the different fracturing behaviours. The theoretical study shows that the dynamic mechanical properties and the geometry induced stress/strain rate distribution non-uniformity should be coupled together to analyze the failure process of rocks.