Corroded Bolt Hole Repair Guidelines
Abstract
This project addresses the number one maintenance issue for the Navy’s F-18 aircraft, corroded fastener holes in the airframe caused by galvanic corrosion arising from carbon skins installed on aluminum substructure. This problem is so severe that adding massive amounts of manpower seems to have had little effect in restoring readiness, and now the Navy is preparing to scrap 130 aircraft because they are too expensive to maintain. Part of the problem is inherent in the carbon-over-aluminum design, so scrapping these aircraft will not help the remainder of the similarly constructed F-18A-D’s. The problem could be worse for Super Hornets and F-35’s, which have three times as much carbon-over-aluminum as the legacy Hornets. Clearly a better long term, widely applicable solution is needed to help keep squadrons and flight decks full with fighters. Since the structural designs are fixed the solution has to lie in the repair method. The current method requires custom assessment, analysis, and repair of each corroded area. A simple, quick, common, reliable, and low-cost repair solution is needed to restore the life of corroded holes to the as-new condition, and is the objective of the proposed program. The effort being proposed herein will employ a newly developed thin-wall, rivetless nut plate retainer which is cold worked in place. The bushing portion of this product has a wall thickness of 0.010” and therefore will require only minimal clean up / reaming of the hole for its installation. The advantage of this approach is that most of the material will be left to carry load as originally intended. Thus the edge distance is reduced very little compared to current repair methods and components, which require substantial hole oversizing. Any remaining corrosion pits, while possibly damaging to fatigue life, are much less harmful than the fastener hole itself, and their potential deleterious effects will be overcome by the imposed compressive residual stresses. The proposed program will first characterize corrosion observed in the field, and develop laboratory methods to reproduce this corrosion. We will conduct fatigue testing of 7050-T74 aluminum samples (common for F-18 airframes) with and without laboratory-induced corrosion, and with and without nut plates installed, providing sufficient data to demonstrate the life impact of the nut plate repair compared with pristine (new) and corroded holes. The fatigue spectrum will be representative of wing bending loads. Finite element analysis will be conducted to simulate the nut plate cold expansion installation process, helping understand the evolution of the residual stress field and effects of key installation parameters, such as mandrel force. X-ray diffraction measurements of the surface residual stresses will benchmark the models. Finally, the test data also will demonstrate the reliability and repeatability of the proposed simplified installation procedure, helping address a quality control issue associated with current hole cold working techniques, which are highly dependent on maintainer skill and care. The outcome will be a demonstrated, effective, low-cost bolt hole corrosion repair method. In addition, we will create a straightforward hole repair manual that will guide depot engineers in repair method selection (new nut plate versus conventional oversizing) for the observed corrosion conditions. The resulting fatigue life from this corroded hole repair method is expected to be greater than the life of the hole when the aircraft was new. It will eliminate the need, in the vast majority of the cases, for any custom engineering analysis or bushings. The installation method will be simple for maintainers and highly repeatable. Thus time in the depot will contract, speeding the return of aircraft to the fleet. Finally, the method will be widely applicable to other Navy air vehicles, providing an efficient means to address any future bolt hole corrosion that might develop.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 26, 2018
- Source ID
- N000141812540
Entities
People
- Dennis Buchanan
Organizations
- Office of Naval Research
- United States Navy
- University of Dayton