Application of small-crack theory to aircraft materials

Over the past two decades, studies on the growth of small cracks have led to the observation that Fatigue life of many engineering materials is primarily "crack growth" from microstructural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using "small-crack theory" under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (Delta K-eff) under constant-amplitude loading. Modifications to the Delta K-eff-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using the crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens. Results from the tests acid analyses agreed well.