Service Load and Fatigue Strength : Report 11, Effects of Gradually Increasing or Decreasing Load on the Initiation and Propagation of Fatigue Crack

Abstract

Using notched specimens of mild and medium carbon steels, the effects of a gradually increasing or decreasing load and sinusoidally varying load on the crack initiation and propagation are examined and the following results are obtained. i) The linear cumulative damage law is applicable to the crack initiation of low carbon steel. ii) The superposition law is applicable to the crack propagation under a gradually decreasing load, but is not applicable to that under gradually increasing condition. iii) The superposition law is also applicable to the crack propagation under constant load test after imposing a gradually varying load, but is not applicable to the crack propagation under constant load test after imposing a sinusoidally varying load. Moreover, the experiments about the fatigue crack propagation under varying load which were performed by our research group are arranged and the following matter is confirmed. Namely, the approaches from the structural and mechanical point of view are necessary to clarify the fatigue behaviours under varying load conditions.