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

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.