2001 Volume 50 Issue 3 Pages 255-262
Fatigue crack growth behavior of aluminum alloy under three-step varying load conditions was investigated. The load patterns used in this study consist of high low two-step load (KH/KL) and additionally inserted middle level (KM). The crack growth behavior was observed fractographically, and the results were correlated with two typical phenomena observed under two-step loads; one is the retardation phenomenon due to load change, and another is the steady crack growth behavior where linear cumulative law of crack growth rate based on ΔKeff can be applied. It was revealed that the crack growth law derived under two-step load could be applied to the estimation of crack growth rate at KM. But, the crack growth rate at KL depends on the K-values at KH and KM, although the trend of the crack growth behavior at KL under three-step load is qualitatively similar to the result observed at KL under two-step load. Accordingly the analytical crack growth law derived in authors previous papers under two-step load conditions was extended to make it applicable to the results obtained in the present paper. The estimated results based on the modified method well agree with the experimental results.
