M-SCOT

Aging aircrafts are experiencing high level of maintenance costs to reduce the risk due to structural effect of existing corrosion damage for which the propagation rate cannot be predicted with a sufficient level of reliability. Indeed, in service, aircraft can suffer from cumulative corrosion and fatigue damage. Therefore, the evolution of corrosion in aluminium alloys, in particular the propagation of intergranular corrosion, remains not only a problem of fundamental interest but is also a key point in aircraft risk and reliability analysis as corrosion damage and mainly intergranular corrosion has a detrimental effect on integrity of aircraft structures by promoting fatigue crack initiation. The concept of M-SCOT is driven by the important need in developing strategies for validating engineering scale tests that would allow the detrimental effect of an intergranular corrosion defect to be evaluated and the remaining lifetime of the corroded structural parts to be quantified. In M-SCOT, it is proposed to revisit laboratory scale tests completed by micro-environmental scale simulations. To illustrate how the propagation rate of intergranular corrosion defects and therefore the validation of tests (at various scales) is strongly dependent of the role of the microstructure, M-SCOT project will be focused on the typical case of the propagation of the intergranular corrosion of AA2024 as it is largely described from the phenomenological point of view in literature and represents a significant cause of structural damage in ageing civilian and military aircrafts. The final scientific objective is to reduce the lack of predictive data for intergranular corrosion which is a bottleneck in implementation of operational management of existing defects in structures. The scientific and technical work plan has been defined to cover all the relevant scales for this type of corrosion to develop an engineering testing methodology based on a mechanistic (laboratory scale tests) and kinetic understanding of intergranular corrosion propagation rate (modelling at the microstructural scale). The key deliverables will be to predict quantitative estimations of the intergranular corrosion rate propagation on the basis of coherent results of tests (tests on specially designed specimens or using simulation based testing), conducted at microstructural scale in simulated environments, at laboratory scale in controlled environments and finally at engineering scale in standardized atmospheric environments (natural marine, salt spray, immersion-emersion exposures). It is planned to demonstrate that the prediction of the intergranular corrosion rate results from the benchmarking of the average and maximum penetration depths obtained from testing on metallic specimen compared to the instantaneous propagation rate defined from modelling. Beyond the immediate project, the final technical deliverable will be structured as an application guide which will helps engineers to improve and rationalize their knowledge on corrosion testing, to propose durable and damage tolerant designs. The engineering models developed will predict the performance of structural parts of aircrafts during service life. This demand is and will be more and more important as metallic materials (aluminium alloys) are and will be combined with composite materials by hybrid joining generating specific features of corrosion damages.