Technical report | Equivalent Crack Size Modelling of Corrosion Pitting in an AA7050-T7451 Aluminium Alloy and its Implications for Aircraft Structural Integrity
Ageing military aircraft fleets are becoming the norm as fleet managers try to extend operational life without compromising safety. This has led to substantial world-wide research into ageing aircraft and the implications of corrosion and multi-site damage on aircraft residual strength and fatigue life. This report details part of DSTO's research program into the effect of pitting corrosion on aircraft structural integrity. The report focuses on the F/A-18 structural aluminium alloy AA7050-T7451 and its susceptibility to developing large pits. The report emphasises that with the present design philosophies of Safe-Life and Damage Tolerance, the major corrosion problem areas on aircraft will be secondary structure or non-fracture critical structure. The report also shows the applicability of the Equivalent Crack Size approach to assessing corrosion. This approach currently appears to be the best approach to assessing pitting corrosion and its effect on aircraft structural integrity.
The high cost of aircraft maintenance, which is focused on the repair of corrosion damage, could be substantially reduced if we understood and could predict the effect of corrosion on fatigue and fracture and could therefore avoid unwarranted maintenance actions. This has the potential to greatly reduce the cost of corrosion management in the Royal Australian Air Force (RAAF) fleet while simultaneously increasing aircraft availability.
Improvements in materials technology have reduced many of the corrosion problems of stress corrosion cracking and exfoliation. However, the demand for thicker sections of high strength aluminium structure has increased the relative impact of pitting corrosion. The research discussed in this report is part of a larger Defence Science and Technology Organisation (DSTO) research program looking at all RAAF aircraft and the susceptibility of their fracture critical components to pitting corrosion. These include 7050-T7451 for the F/A-18 and 7010-T7651 for the BAE SYSTEMS Hawk Mark 127. Within the overall Equivalent Initial Flaw Size/Equivalent Crack Size (EIFS/ECS) approach, each material and aircraft has a unique set of problems.
This report examines the research conducted on 7050-T7451 and how corrosion pitting could influence the fatigue life of components in RAAF aircraft manufactured from this alloy. The report shows that corrosion pitting causes not only a reduction in time to failure at a certain stress but also up to a 50% reduction in the fatigue threshold. The report also shows that at the high stresses seen by many of these fracture critical components, pitting corrosion is no worse than the ion vapour deposition (IVD) treatment used in production. It appears that the major area for concern with regard to pitting corrosion is secondary structure. Pitting corrosion can effectively reduce the life of these types of components to below the conservative Safe-Life of the component.