Magneto-Optic/Eddy Current Imaging of Subsurface Corrosion and Fatigue Cracks in Aging Aircraft

Abstract

The rate of production of new aircraft, as well as the availability of funds to purchase them, are limited. As a consequence of these two factors, only a small percentage of the many thousands of aging aircraft in existence today (both civilian and military), will be replaced by new aircraft in the foreseeable future. Because replacement is not an option in many cases, the only practical way to maintain operations is to extend the useful service lifetimes of these aircraft by means of proper maintenance, repairs and upgrades [1]. An important component of proper maintenance is periodic inspection for subsurface corrosion and fatigue cracking in airframes.Nondestructive inspection (NDI) methods used for this purpose [2] include, among others, visual examination, eddy current inspection, x-ray and ultrasonic inspection, and more recently, neutron radiography. Although each of these inspection methods can successfully detect subsurface corrosion and cracks, they all have limitations.Conventional visual examinations can detect large surface-breaking cracks or extensive corrosion-induced "bulging" or "pillowing" of visible surfaces. However, this method generally cannot detect small surface-breaking cracks or tiny surface displacements associated with subsurface cracks or areas of corrosion where no pillowing occurs. Furthermore, even in areas of significant pillowing, other reliable easy-to-use NDI methods must still be used to verify the existence of corrosion.Conventional eddy current and ultrasonic methods are essentially point-contact inspection methods, which significantly limits inspection speed and makes these methods very tedious. Through-transmission x-ray and neutron radiography inspections have limited applicability because the source and receiver of radiation must be on opposite sides of the inspection surface. A notable exception to this constraint is the successful, but relatively expensive, x-ray backscatter technique.

Non-destructive techniques for corrosion detection: A review

“…The response of the Faraday magneto-optic sensor to weak magnetic fields generated by EC to a defect in the inspected component is used to create an image. 105,106 This technique is fast and reliable with relative insensitivity to the lift-off. Data interpretation is relatively simple since this is an imaging technique with a low signal-to-noise ratio.…”

Nondestructive evaluation of a crack on austenitic stainless steel using a sheet type induced current and a Hall sensor array

“…Conversely, the real time inspection of a crack in a large area (e.g., a circle area with a diameter of 75 mm) at a high spatial resolution is an advantage of MOI (magneto-optical/eddy current imaging) [3,4], [5]. MOI was developed for use in inspecting fatigue cracks and corrosion in aluminum alloys of aged aircraft.…”

Motion-based filtering of magneto-optic imagers

“…MOI inspection is up to 10 times faster than conventional eddy current methods because of its large inspection area and the ease of inspection and interpretation [2,3]. However, MOI images are severely corrupted by a characteristic serpentine pattern associated with domain walls in the sensor.…”