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

抄録

Rotating bending fatigue tests were conducted on S55C steel under two-step spectrum loading, where n_H cycles of the overstress σ_H were followed by n_L cycles of the understress σ_L 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 n_L gave shorter crack initiation and growth lives until n_L reached about 5×10⁵, and beyond this a larger value of n_L did not give shorter lives, when n_H was fixed. A smaller value of n_H 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.