Fatigue of engineered metallic materials using small-crack theory under constant- and variable-amplitude loading

摘要

The current paper reviews application of an improved FASTRAN life-prediction code to predict fatigue behavior of notched coupons made of an aluminum alloy and a titanium alloy. For each material, fatigue-crack-growth-rate data were determined over a very wide range in rates from threshold (small-crack region) to near fracture on standard crack-growth specimens. In the low-rate regime, small-crack data were used on the aluminum alloy, whereas a compression pre-cracking constant-amplitude method was used on the titanium alloy. A characteristic microstructural flaw size was determined from fatigue-test data on notched coupons under constant-amplitude loading. These data were then used to predict fatigue behavior of similar notched coupons under various spectrum loadings. For variable-amplitude loading conditions, an improved "rainflow-on-the-fly" subroutine was used in the life-prediction code. Comparisons are made between previous life predictions and those made with the current code under variable-amplitude loading. In general, the new code produced longer fatigue lives than older versions. The new code worked well using the same initial microstructural flaw size for a given material under both constant- and variable-amplitude loading.