Application of Local Fracture Criterion Approach for Dynamic Fracture Toughness

Abstract

Influence of dynamic loading on the fracture toughness of steels has been investigated analytically and experimentally. Stress singularity at a crack tip and applicability of fracture mechanics parameter have been discussed with dynamic FEM analysis. An augmentation of the stress field due to the increase of loading rate could be successfully normalized with the yield strength referred to the rate-temperature parameter; R at a particular location near the crack tip. The Weibull stress obtained by the stress integration around the crack tip for the applied stress intensity factor K could be also normalized with the reference yield strength independent on the loading rate. These results imply that the dynamic fracture toughness can be described with R parameter at a specified location near the crack tip.The transition behavior of fracture toughness Kc (Jc) under static, quasi-static and dynamic loadings were investigated for four kinds of steels with different strengths. Lower strength steel showed more significant embrittlement due to the increase of the loading rate, as is previously reported. Predictions of the fracture toughness under quasi-static and dynamic loading was examined with the Local Fracture Criterion Approach. As a measure of loading rate, R parameter evaluated from elastic strain distribution was taken for simplification. The experimental fracture toughnesses Kc (Jc) smaller than 100MPam1/2 were well described with the simplified prediction. The predicted values for the larger fracture toughness, however, were too conservative. This may be due to the constraint loss around the crack tip and/or the heat effect by plastic work and/or the dynamic effect on the cleavage fracture strength.