Unusual Failures Associated with Cracks That Nucleated from Corrosion Damage in Fatigue Tests on AA 7085-T7452 Specimens.

The paper is the first to highlight what appears to be a unique and unusual failure mechanism that is associated with cracks that nucleated from corrosion pits in aluminium alloy (AA) 7085-T7452 specimens that were tested under a variable amplitude load spectrum. In this study, cracks initially nucleated at corrosion pits and first grew as would be expected, namely at ninety degrees to the surface and perpendicular to the applied load. However, after reaching a depth of approximately 2 mm, these various Mode I cracks transformed into what can be best described as interlayer cracks with their surfaces at an angle of approximately ninety degrees to the initial fatigue crack surface. Analysis of the failures revealed that the maximum value of the stress intensity factor at which this phenomenon occurred, which we have defined as KIL, was substantially less than the fracture toughness for this material. As such, failure was not due to classical Mode I failure, but rather due to K exceeding what we will term KIL. Despite the unusual failures, it was found that, up to the point where this phenomenon occurred, the crack growth versus cycles histories could be reasonably accurately predicted using the small crack growth equation developed by the authors in a prior study on AA 7085-T7452 specimens with a fastener hole.