Lower Limit of Cumulative Fatigue Damage of 0.25%C Annealed Steel under Two-Step Loading.

Abstract

Rotating bending fatigue tests were performed using smooth and precracked 0.25%C annealed steel specimens. The initiation and propagation of fatigue cracks were successively observed by the replication method, and the cumulative fatigue damage under two-step loading was investigated. Precracked specimens with four different precrack lengths, 150, 300, 450 and 700μm, were examined and the S-N curves for the precracked specimens with arbitrary crack lengths were estimated. The fatigue damage defined by Miner's rule under two-step loading was estimated from the crack propagation curve, the S-N curve of the smooth specimen and the estimated S-N curve for the precracked specimen. The estimated average value of the lower limit was 0.97, but it was 0.60 when the scatter of the experimental data in each specimen was considered.