Predictions of fatigue crack growth in aluminium alloy 2024–T351 using constraint factors

A substantial portion of the service life of aerospace structures is characterised by highly variable loading conditions. Prediction of the fatigue life of flawed structural components requires an adequate description of the transient behaviour of the fatigue crack growth rate following a change in loading conditions. Prediction of fatigue crack growth following the application of single tensile overloads was performed using a modified strip-yield model. A plastic constraint factor, α, was incorporated into the model. Initially it was assumed that α was constant throughout the crack growth process. Then a variable constraint factor was introduced. The modified strip-yield model was used to investigate the effects of using a constant and variable constraint factor in the prediction of simple overload fatigue crack growth tests performed on aluminium alloy 2024–T351. Results from constant amplitude fatigue tests, for different thickness, R-ratio and stress level, were used to develop an empirical constraint factor equation. This equation was subsequently used in the prediction of the overload fatigue tests. Improved simulations were obtained compared to the use of a constant constraint factor. Furthermore, additional improvements in the prediction of overload retardation effects were obtained by activating a constraint-loss regime (α=1.0 or plane stress conditions) immediately after the overload. The constraint factor remained equal to one until the crack grew out of the overload plastic zone, after which α would revert back to the value calculated from the constraint equation.