Variability on nucleation and growth of short fatigue cracks due to material anisotropy in aluminum 2024-T351

Early fatigue life behavior is important for the prediction of residual useful life of aerospace structures via computational modeling. In particular, the influence of rolling-induced anisotropy on fatigue properties has not been studied extensively, but it is likely to be an important effect. Therefore, fatigue behavior of a 2024-T351 plate was studied using notched uniaxial samples with load axes along either longitudinal or long transverse directions. Interrupted fatigue testing at stresses close to yielding was performed to nucleate and propagate short cracks, and local nucleation sites were located and characterized using optical and electron microscopy techniques. Results show that crack nucleation occurred due to fractured particles for longitudinal samples, while either debonded or fractured particles led to nucleation for transverse samples. Crack nucleation from debonded particles reduced life till matrix fracture because sharper flaws were generated when compared to those from fractured particles. Longitudinal samples experienced multisite crack initiation because of preferential fracture of inclusions for that loading direction. Conversely, long transverse samples showed reduced particle fracture, which eliminated multisite cracking. Crystallography of individual grains also played a role related to bulk plasticity in the grains, as estimated by their individual Taylor factors, and to environmental effects.