抄録
Small crack growth characteristics under variable loading were investigated on high strength steel SNCM439 in room air and distilled water. Under repeated two-step variable loading in room air, the crack growth rates of small cracks at lower stress and higher stress levels decreased and increased respectively, as compared with those at constant stress amplitude. In distilled water, on the other hand, the decrease in crack growth rate at lower stress level was not observed below a crack length of 0.5mm, while the crack growth rate at higher stress level decreased as compared with those at constant stress amplitude, which was attributed to crack branching. In order to clarify the effect of crack length on retardation behavior, tests were conducted under Hi-Lo two-step loading at constant stress intensity factor range (ΔK=11MPa√m→7MPa√m). It was found that the retardation cycles in both environments increased markedly with increasing crack length until they reached a constant value corresponding to long crack data, but it was smaller in distilled water than in room air.
