Effects of delamination on fatigue crack growth retardation after single tensile overloads in 8090 AlLi alloys

The transient growth rate response after a single tensile overload was examined for two different crack tip conditions, with and without delamination cracks in 8090 AlLi (T8) alloys. Single peak tensile overloads of 70% and 100% were applied under constant ΔK conditions for base line stress intensity ranges of 12 MPa√m at the stress ratio of 0.1. The overloads were performed either at 295 K or at 77 K to introduce delamination cracks ahead of the overloaded crack tip. ‘Steep’ recovery was obtained for the post-overload fatigue crack growth in the ligament with no delamination cracks. In contrast, when the fatigue crack grows in the ligament containing a large number of delamination cracks ‘gentle’ growth rate recovery was exhibited, i.e., the small growth rate region became wider. SEM observation found the existence of the single-banded dark and smooth area in parallel with the crack growth front in the delamination-less specimen; while, in the specimen including many delaminations, islands of the dark and smooth areas were dispersed on the post-overload fatigue fracture surface. The dark and smooth area is a conclusive piece of evidence for the occurrence of a considerable extent of contact and rubbing of the fracture surface. The local laminar splitting in the non-fatigue-cracked ligament changes local stress state from plane strain to near plane stress at the crack tip. The local stress state becomes non-uniform along the crack growth front. This may provide the growth of a slant crack surface to introduce a considerably large roughness-induced crack closure in a certain length of the post-overloaded crack growth. This is the reason for the prolonged retardation, i.e., characteristic ‘gentle’ recovery in the delaminated specimens.