Fatigue-Life Prediction Methodology Using a Crack-Closure Model

This paper reviews the capabilities of a plasticity-induced crack-closure model and life-prediction code, FASTRAN, to predict fatigue lives of metallic materials using small-crack theory. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (ΔKeff) under constant-amplitude loading. Some modifications to the ΔKeff-rate relations were needed in the near-threshold regime to fit small-crack growth rate behavior and endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched specimens made of several aluminum alloys and a titanium alloy under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys. An equivalent-initial-flaw-size concept was used to bound the fatigue lives for the titanium alloy. Results from the tests and analyses agreed well.