NDE corrosion metrics for life prediction of aircraft structures

The repair and rectification of corrosion damage continues to be a major factor in the life cycle costs of aircraft. The present approach to corrosion control is based on ‘Find-and-fix’. Once an area of corrosion is found, rectification is carried out as soon as possible. This often involves the blending away of corrosion damage and the re-protection of the blended area. Where the depth of corrosion is beyond the permissible level (in the case of wing and fuselage skins this is typically 10% of the skin thickness) it is necessary to replace components. The ‘Find-and-fix’ approach can result in aircraft being taken out of service for long periods of time between regular maintenance schedules. Attempts are being made to move away from the ‘Find-and-fix’ approach and to manage the corrosion so that it can be rectified during scheduled maintenance periods. Cole, Clark and Sharp(1) put forward the concept of an ‘Identify-and-manage’ approach to corrosion on military aircraft. Similarly, Peeler and Kinzie(2) have outlined the USAF shift towards an ‘Anticipate-and-manage’ approach to airframe corrosion control. The two approaches are essentially the same and are aimed at significantly reducing aircraft non-availability and at decreasing maintenance costs. The move from a ‘Find-and-fix’ approach requires a careful assessment of the effects of the corrosion on structural integrity of the aircraft. To achieve this, reliable models are needed which can be used to predict the long-term effects of corrosion on properties such as static strength and fatigue life. Significant progress has been made towards this goal through research being conducted in the US, Canada and Australia. Models allowing the effects of pitting and exfoliation corrosion on fatigue life and the role of corrosion on the degradation of lap joints are currently under development. Eventually these will allow the impact of different levels of corrosion on structural integrity to be assessed and will permit the time at which rectification must be carried out to be determined. The ‘Identify-and-manage’ or ‘Anticipate-and-manage’ approaches will only be possible if effective non-destructive evaluation (NDE) methods are available to detect, characterise and monitor the corrosion that is present within the airframe. The aim of the present project is to assess the capabilities of a range of NDE techniques for detecting and quantifying corrosion present in aircraft structures. As part of this programme, the initial step has been to establish the corrosion metrics that need to be determined so that the ‘Identify-and-manage’ approach can be followed. The metrics will depend on the particular models that are being employed to estimate the residual life of the structure. This paper presents the results of a review of the some of the models and tools that are currently under development and the basic corrosion data or corrosion metrics that are employed. The results from an initial feasibility study of pitting corrosion and crevice corrosion coupons are also presented.