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14 November 1996 Detection of cracks under cladding using magneto-optic imaging and rotating in-plane magnetization
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Conventional nondestructive inspection (NDI) of steel components, using magnetic particle, flux-leakage or near surface ultrasonic methods, is rendered difficult by the presence of protective coatings such as paint or stainless- steel cladding. Thick-section nuclear reactor pressure vessels (RPVs), having as-welded cladding up to 0.25 inches thick are a case in point. Here, magnetic particle techniques do not work and ultrasonic techniques are difficult to apply because of cladding roughens and variable elastic properties in the cladding and the weld interface. An NDI technique that is essentially unaffected by standard thicknesses of protective coatings would be a major advance. Magneto-optic imaging is one such technique. While conventional magneto-optic/eddy current imagers (MOIs) are a proven technology in the NDI of nonferromagnetic conductors, they do not possess a self-contained method for magnetizing steel. The purpose of this work was to develop methods for producing rotating, in-plane magnetization and then combine this technology with magneto-optic imaging to produce a self-contained instrument capable of real-time imaging of cracks in steel through protective coatings. We successfully demonstrated rotating in-plane magnetization using special 'quadrature' magnetic-yokes designed to accommodate both flat and cylindrical steel surfaces. The yokes were attached one at a time, to an MOI of reduced size, and the combined system was placed on a sample of the appropriate curvature containing cracks. A two-channel power amplifier was used to drive the yoke coils in quadrature. The resulting crack images were found to be much less sensitive to liftoff than conventional magnetic particle or flux-leakage NDI. In particular, cracks in steel were successfully imaged through 0.125 inches of stainless-steel cladding, making it likely that images of cracks in steel under even thicker cladding should be possible after further development of the technology. Unlike conventional magnetic particle or flux- leakage NDI, where crack orientation is important, rotating in-plane magnetization renders crack orientation irrelevant. Finally, because of these successes, the new NDI technology for steel should find many important applications.
© (1996) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Gerald L. Fitzpatrick, Richard L. Skaugset, David K. Thome, and William C.L. Shih "Detection of cracks under cladding using magneto-optic imaging and rotating in-plane magnetization", Proc. SPIE 2947, Nondestructive Evaluation of Utilities and Pipelines, (14 November 1996);

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