Development of Non-Destructive Inspection Techniques for Cold-Worked Repairs of Corroded Metal Holes
Abstract
Repair of corroded bolt holes on US Navy Aviation (NAVAIR) aircraft, especially those with composite plus metal structures, consumes a large amount of resources and requires specialized techniques to confirm that repairs will prevent fatigue cracking of metal stru ctures in flight. Prior work conducted under an Office of Naval Research (ONR) grant (N00014-18-1-2540) addressing this problem foun d that the use of rivetless nut plate (RNP) fasteners, with 5% expansion upon being inserted into corroded bolt holes, successfully restored fatigue life, even with some residual corrosion remaining in the bolt hole after preparation for repair. However, for this technique to successfully transition to NAVAIR use, non-destructive inspection (NDI) methods are needed to verify that the repairs do not result in other hidden features which would prevent their return to flight.To enable RNP corrosion repair technology, ident ification and validation of appropriate NDI techniques is required. To date, NAVAIR uses a combination of labor-intensive visual ins pection and eddy current NDI. The latter technique has been optimized primarily for crack detection; the only provision for corrosio n is the criteria placed on eddy current noise used to establish that a hole can be inspected. Consequently, advanced NDI methods need to be developed to characterize the desired corrosion metric pit depth. Furthermore, NAVAIR has expressed concern that eddy current inspection cannot detect the presence of cracks growing from corrosion pits. Therefore, inspection methods are required to discern and differentiate between cracks and corrosion.Moreover, monitoring of corrosion and cracks beneath the RNP will be requir ed to ascertain the continued integrity of the hole. Accordingly, methods are needed to inspect for cracks and corrosion under RPN b ushings in the hole.The contractor proposes to develop multi-frequency eddy current inspection methods to meet these challenges, wh ich can reduce or eliminate the response of corrosion-induced surface roughness while enhancing the detection of pits and cracks ema nating from this surface corrosion. Multifrequency eddy current is based on the principle that depth of penetration is frequency-dep endent. Lower frequencies produce deeper penetration and are, therefore, more sensitive to deep pits and cracks than higher frequenc y eddy current inspections. On the other hand, both high and low frequencies are sensitive to surface roughness. An optimized inspec tion setup allows surface effects to be subtracted between the two frequency responses, enabling the detection of a pit or crack und er the surface corrosion. The proposed work will also determine how multi-frequency approaches can be used to characterize and diffe rentiate between corrosion and cracks, both with and without RNP bushings. This major inspection challenge requires fundamental rese arch on Hall-effect and Giant Magneto-Resistance (GMR) sensors, which have been shown to improve this situation by measuring the res ultant magnetic field actor proposes to explore the use of Hall Effect and/or GMR sensors to solve this fundamental inspection issue. Validation will be a orroded holes will be compared to analysis predictions of life based on NDE results for crack depths. Inspection procedures will be providedto enable transition of the RNP repair technology. At the end of the proposed work, the contractor will deliver a report o n its findings and instructions on how the multi-frequency method, in combination with the new sensors, did/did not meet the goals o f the program, as well as make recommendations for future NDE method development.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 07, 2021
- Source ID
- N000142112715
Entities
People
- Wally Hoppe
Organizations
- Office of Naval Research
- United States Navy
- University of Dayton