A Case Study of Corrosion Fatigue in Aluminium Casing Bolt Holes

Abstract

Multiple cracks were observed around the steel outlet guide vane bolt holes of an aluminium (A2618) casing during overhaul. Contact witness marks of the bolted steel vanes and corrosion scale/fibrils were observed around the bolt holes. Fractographic examination of the four longest cracks revealed multiple arrest bands suggestive of a fatigue mode of propagation. All cracks examined had initiated from multiple origins located on the inner surface of the rear fan case, and not from the bore of the bolt hole, even if the crack had breached the bolt hole. SEM examination of the fractures revealed copious amounts of oxidation and/or contamination at the crack origins, with some isolated corrosion pitting. The cleaner parts of the fracture exhibited a feathery morphology, with irregularly spaced micro banding suggestive of a high cycle fatigue (HCF) mode of propagation. Metallographic examination through the crack origin revealed corrosion pits propagating inwards from the fracture surface, but no pitting in the adjacent break-open surface. A small amount of chlorine was found within the pits by EDX. The evidence suggested that the cracks had initiated due to corrosion in the vicinity of the vane contact witness marks and then propagated by HCF.