FAA-NASA Symposium on the Continued Airworthiness of Aircraft Structures

This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using “small-crack theory” for various materials and loading conditions. Crack tip constraint factors, to account for three-dimensional stateof-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (∆Keff) under constant-amplitude loading. Some modifications to the ∆Keff -rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate smalland large-crack growth rates and to predict total fatigue lives for notched and unnotched specimens made of two aluminum alloys, a titanium alloy, and a steel 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 and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well.