FATIGUE-LIFE PREDICTION METHODOLOGY USING A CRACK-CLOSURE MODEL

This paper reviews the capabilities of a plasticity-induced crack-clos ure model and life-prediction code, FASTRAN, to predict fatigue lives of metallic materials using small-crack theory. Crack-tip constraint f actors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of t he effective-stress-intensity factor range (Delta K-eff) under constan t-amplitude loading. Some modifications to the Delta K-eff-rate relati ons were needed in the near-threshold regime to fit small-crack growth rate behavior and endurance limits. The model was then used to calcul ate small- and large-crack growth rates, and to predict total fatigue lives, for notched specimens made of several aluminum alloys and a tit anium alloy under constant-amplitude and spectrum loading. Fatigue liv es were calculated using the crack-growth relations and microstructura l features like those that initiated cracks for the aluminum alloys. A n equivalent-initial-flaw-size concept was used to bound the fatigue l ives for the titanium alloy. Results from the tests and analyses agree d well.