NONDESTRUCTIVE EVALUATION FOR CORROSION IN AGING AIRCRAFT .1. INTRODUCTION, ULTRASONIC AND EDDY-CURRENT METHODS

In both military and civil aircraft operations, where man?, aircraft are suffering the effects of corrosion, there is an increasing need to detect and characterise corrosion at an early stage. It is unlikely that one all-encompassing large-area high-resolution corrosion inspection method for ageing aircraft will ever be attainable. However, a common strategy utilising several different NDE techniques, may already be possible, covering many aircraft types. In principle, a large-area detection technique could be employed to highlight suspect areas and then the appropriate characterisation method used to assess the area and severity of corrosion. This paper describes the results of an intercomparison of currently available and future non-destructive evaluation (NDE) techniques for corrosion in ageing aircraft. Ultrasonic techniques can detect disbonds, provide quantitative information about first-layer thickness (to a resolution of 30 microns) and about the roughness of surfaces, but ultrasound is limited to the first one or two layers in a structure. Eddy current methods cart detect plate separation due to the presence of corrosion products, and material thinning in the first or deeper layers. X-radiographic images of less than 3% corrosion are possible in metal thicknesses of 4 mm or less. Thicker areas could require the use of neutron radiography if the area is free of sealant and variable paint thicknesses. Further developments of both ultrasonic and eddy current methods should increase the speed and ease of inspection, whilst improving the ability to quantify the depth and extent of corrosion.