衝撃過大荷重による疲労き裂進展の遅延現象について

Abstract

As one of the on-going studies to investigate the fatigue crack growth characteristics in impact fatigue, the fatigue crack growth behavior resulting from the single application of impact over-load was investigated by using Dual-Phase steel with ultimate tensile strength of about 800MPa as an experimental material. Discussions were made on the retardation of crack growth rate, crack opening behavior after over-loading and fractographical aspect in comparison with the results obtained through the application of single static over-load with peak stress hold time of 3min.The major conclusions obtained are summarized as follows:(1) Retardation of crack growth rate after impact over-loading was not so distinct when compared with the case after static over-loading.(2) The values of crack opening ratio after impact over-loading were higher than after static over-loading.(3) Fractographical observations showed that abrasions were caused on the fractured surfaces after impact over-loading, but not so after static over-loading. The following speculation on the crack tip plastic deformed zone was needed to explain the peculiar crack growth behavior after im- pact over-loading as mentioned above. In comparison with the condition of the plastic zone constructed by static over-load, the size of the zone constructed by impact over-load is smaller, but the strength of this small plastic zone is higher, in other words, the higher density of slip bands is attained under impact over-load.