Eddy Current Inspection Of Threaded Fasteners

Utilities often use liquid penetrant or magnetic particle methods for surface examination of threaded fasteners. These methods require extensive surface preparation to achieve a reliable and accurate exam. Anti-seize lubricant is often applied to fastener threads to improve plant operation. High temperature behavior of the anti-seize lubricant leaves a residue which complicates surface preparation requiring grit blasting and/or wire brushes to obtain adequate examination surfaces.

In this project, an eddy current technique requiring minimal surface preparation was developed to inspect thread roots for surface cracks. Eddy current inspection offers many advantages over traditional surface methods. Minimal surface preparation reduces the overall inspection time thereby reducing inspection cost. Fastener inspection is often performed in radiological controlled areas; therefore, reduced inspection time is also a health benefit. Eddy current methods do not require developers or liquid suspended ferromagnetic particles, thus consumable material use is reduced. For fasteners in radiological controlled areas, reduction in consumable material use reduces the generation of mixed hazardous waste.

Several materials issues were investigated to aid in the development and verification of an eddy current technique. Initially, the thermal behavior and electromagnetic properties of the anti-seize lubricant were investigated to determine the feasibility of an eddy current technique. In addition, electromagnetic properties of common fastener materials such as 4340 steel were also investigated. A system to detect and size circumferential cracks in thread roots was developed and demonstrated on fasteners with service induced flaws. Signal analysis methods were also developed for determining crack length and depth. Capable of detecting and sizing circumferential cracks in thread roots, this eddy current technique provides an alternative to traditional surface methods for examination of fastener thread roots.

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