変動荷重下での遅れ破壊き裂進展開始寿命に及ぼす予荷重の影響

概要

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The effects of partial or perfect unloading on the crack growth initiation life and the threshold stress intensity factor in delayed failure under repeating load were investigated on the Precracked specimens of SNCM 439 steel quenched and tempered at 473 K. The main results obtained are as follows: (1) The crack growth initiation life and the threshold stress intensity factor in delayed failure under repeating load were markedly increased by partial or perfect unloading method, the effect of which was larger when the stress intensity factor range superposed became smaller. (2) The reason why the delayed failure strength under repeating load was increased by both prestressing methods can be explained by the decrease of effective stress intensity factor at the precrack tip, which will suppress the repeating slip and the corrosion reaction and thus prevent the invasion of hydrogen into material. There exists a relation of η=R_u^m at R_u≥0.7 between the decrease rate of effective stress intensity factor η(=K_<m・eff>/K_<ma> and the unloading stress ratio R_u(K_<ma>/K_p), where K_<ma> and K_<m・eff> are the average stress intensity factor (apparent value) and the effective one after partial unloading, respectively, K_p is the stress intensity factor at preloading, and m is a constant. (3) The change of fractographs supports the conclusion that the effective stress intensity factor at the crack tip is decreased by partial unloading.
1985-04-15