EFFECTS OF DELAMINATION ON FATIGUE-CRACK GROWTH-RETARDATION AFTER SINGLE TENSILE OVERLOADS IN 8090-AL-LI ALLOYS

The transient growth rate response after a single tensile overload was examined for two different crack tip conditions, with and without del amination cracks in 8090 Al-Li (T8) alloys. Single peak tensile overlo ads of 70% and 100% were applied under constant Delta K conditions for base line stress intensity ranges of 12 MPa root m at the stress rati o of 0.1. The overloads were performed either at 295 K or at 77 K to i ntroduce 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 fatigu e crack grows in the ligament containing a large number of delaminatio n cracks 'gentle' growth rate recovery was exhibited, i.e., the small growth rate region became wider. SEM observation found the existence o f the single-banded dark and smooth area in parallel with the crack gr owth front in the delamination-less specimen; while, in the specimen i ncluding 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. T he local laminar splitting in the non-fatigue-cracked ligament changes local stress state from plane strain to near plane stress at the crac k tip. The local stress state becomes non-uniform along the crack grow th front. This may provide the growth of a slant crack surface to intr oduce a considerably large roughness-induced crack closure in a certai n length of the post-overloaded crack growth. This is the reason for t he prolonged retardation, i.e., characteristic 'gentle' recovery in th e delaminated specimens.