Impact of small-crack effects on designlife calculations

An assessment has been made of the potential reduction in calculated fatigue-crack-growth lives when the small-crack effect is considered. The assessment was based on small-crack and large-crack grwoth-rate data of 2024-T3 aluminum alloy forR=−1 andR=0 constant-amplitude loading and for a fighter-wing spectrum loading (FALSTAFF). The potential impact of the small-crack effect was assessed by comparing lives computed using only large-crack data to those computed using a combined small- and, large-crack data based. Based on life calculations, the small-crack effect would have: (1) no impact on life analyses that assume an initial crack depth large than about 0.3 mm (such as in current airframe damage-tolerance analyses); (2) only a small impact on life analyses that assume an inital depth of 0.1 mm (such as im some current durability analyses) if the large-crack thresholds are ignored; but (3) a large impact on life analyses that assume an initial depth of about 0.01 mm (such as in a total-life analysis).