過大過小応力負荷条件下の微小き裂の進展 過大過小応力の振幅と繰返し頻度の影響

Abstract

Rotating bending fatigue tests were conducted on S55C steel under two-step spectrum loading, where nH cycles of the overstress σH were followed by nL cycles of the understress σL in subsequent blocks.Crack initiation and growth lives in spectrum loading tests were much shorter than the estimated values based on the linear cumulative damage rule when the damage given by the understress was neglected. The stress amplitude at 60% of the endurance limit of the steel was judged effective to make both of crack initiation and growth lives shorter when the number of cycles of the understress repeated in a block was large enough. A larger value of nL gave shorter crack initiation and growth lives until nL reached about 5×105, and beyond this a larger value of nL did not give shorter lives, when nH was fixed. A smaller value of nH gave shorter crack initiation and growth lives. The crack growth amount at the understress was large, because the number of cycles at the understress was larger compared to that at the overstress, though the crack growth rate at the former was low from about one fifth to one tenth of the growth rate at the latter.