Predicting fatigue crack initiation due to the presence of intergranular corrosion in extruded 7075-T651 aluminium alloy

摘要

The management of various forms of corrosion that are present in both civilian and military aircraft is an increasing burden as they age. One particularly insidious form of corrosion is laminar Intergranular Corrosion (IGC) due to the small surface corrosion present, even though the long, sharp fissures can grow parallel to the surface up to and over 5 mm in highly extruded materials. This thesis investigates the fatigue effects of this peculiar form of IGC, particularly focussing on fatigue crack initiation. Its aim is to determine the mechanism behind the early fatigue failure due to IGC and to develop a model that can predict the knock-down factor for the number of cycles to a 1 mm fatigue crack, which is termed “crack initiation”. This thesis uses the example of IGC on the AP-3C Orion maritime surveillance aircraft, which is currently an issue for fleet operators world-wide as the unknown fatigue effects results in any IGC found being completely removed, leading to significant delays during maintenance. A secondary aim of this thesis is to use these results to develop a tool that can be used by fleet operators to assist in lifeing the AP-3C Orion in the presence of IGC. A series of constant-amplitude fatigue tests on specimens with a high stress concentration (of 3.0 for a plate-with-hole) showed that IGC reduced the number of cycles to crack initiation and that this reduction was proportional to the depth at which the fatigue crack initiated. Fractography showed two types of initiating features on the corroded specimens. Fatigue cracks initiated at either a corrosion pit at the bore of the hole, or at corroded inclusions situated along the path of the IGC fissure. In contrast, fatigue cracks in the un-corroded specimens initiated at the hole corners A Monte Carlo model, combined with finite element analysis, was used to simulate the fatigue specimens. The Monte-Carlo model created representative IGC paths within simulated fatigue specimens. It then added simulated pits and corroded inclusions at locations along the surfaces of the predicted IGC paths. As a final step it created script files that were submitted to ABAQUS to complete the analysis. This Finite Element analysis showed that IGC only slightly changed the stress concentration at the specimen’s hole. In contrast, pits and corroded inclusions significantly increased the stress concentration. This change explains the two different fatigue initiation locations observed in the fatigue tests, which were based on a competition between the stress concentraitons of these two mechanisms. The results of the FE analysis and the fatigue tests were combined to produce a model to predict the knock-down factor of the number of cycles to a 1 mm fatigue crack of a population of likely pit sizes and corroded inclusion locations. This model can then be used by fleet operators world-wide to assist in lifeing the AP-3C Orion to attempt to reduce the maintenance load currently experienced.