The Growth of Small Cracks in 4340 Steel

The objectives of this work were to investigate crack initiation characteristics and small-crack growth behavior for 4340 steel and to evaluate the ability of a closure-based crack-growth model to predict fatigue crack growth rates and total life for this alloy. Single-edge-notched tension specimens were used to obtain the small-crack growth rate information via an acetate replica technique. In addition to constant amplitude loading, tests were conducted using the Felix/28 variable amplitude spectrum (a shortened form of a standard loading sequence for 'fixed' or semi-rigid helicopter rotors). The small-crack growth rates were compared to those for large cracks grown under similar loading conditions. Netallurgical features associated with crack initiation are discussed. For 4340 steel under constant amplitude loading at R = 0.5 and 0 and for the Felix/28 spectrum loading, no small-crack growth was observed below the large-crack threshold. A slight small-crack effect, growth below the largecrack threshold, was observed at R = -1. For all loading conditions, growth rates measured from the small-crack specimens were slightly slower than those measured from large-crack specimens at a given stress-intensity factor range above the large-crack threshold. Fatigue life was found to depend on the size and type of initiation site, especially for the Felix/28 loading sequence. A semi-empirical crack-growth model incorporating crack-closure effects was used to predict crack growth rates and total fatigue life of notched specimens. An initial defect, typical of those identified in this steel, was assumed as the initiation site and size under both the constant amplitude and spectrum loadings. For all loading conditions, except R = -1, reasonable agreement was found between measured and predicted values for both crack growth rates and fatigue life. For R = -1 loading conditions, the model tended to predict rates which were slightly slower and lives which were slightly longer than those measured experimentally.