A lot has been said about the fatigue and structural benefits afforded by the FTI ForceTec rivetless nut plates in structural assemblies, however a benefit that the product brings that is often overlooked, is its ability to minimize the susceptibility to corrosion and to enhance the load-carrying ability and fatigue life of the product when operating in a corrosive environment. 

Traditional riveted and adhesively bonded nut plates have been shown in service to be very susceptible to galvanic, inter-granular and fretting corrosion.  As with any aerospace assembly, good surface protection is essential to seal against corrosive elements.  Conventional nut plates are riveted to the surface through satellite rivet holes adjacent to the main fastener hole.  Such a configuration leads to a high stress concentration and can result in fatigue crack initiation.  Additionally the holes penetrate the corrosion protection scheme and leave the alloy end grain susceptible to inter-granular and exfoliation corrosion.  Adhesively bonded nut plates similarly leave the end grain of the fastener hole exposed to the same conditions, as well as the surface it is being bonded to has to be abraded to ensure a clean metal surface bond.  The ForceTec rivetless system on the other hand eliminates the rivet holes and the high interference fit if the expanded nut retainer effectively seals the end grain of the fastener hole and eliminates this primary source of end grain corrosion.  ForceTec can also be conveniently installed with wet primer in the fastener hole.

A number of corrosion environment tests have been conducted to evaluate the relative corrosion resistance of ForceTec made from different materials and with different corrosion resistant coatings on the retainer. In an FTI 1000-hour salt fog test conducted to ASTM B-117 standard by an independent lab, stainless steel retainers which were passivated or coated with IVD (Ionized Vapor Deposition) aluminum were compared to riveted and bonded nut plates. These were installed into a 2024 aluminum alloy plate representing typical commercial aircraft cargo bay sidewalls. The plate was primed with BMS-10-11V, type 1, Class A, Grade E primer and coated with Dydrol for corrosion protection. A series of different nut plates including NAS 1068 riveted nut plates, an adhesively bonded nut plate and the different finish ForceTec nut plates, were installed in the test plate to current published specifications. All holes were treated with alodine after drilling and prior to installing the nut plates.

Image 1: Test specimen in corrosion chamber ready for 1000 hour salt fog test

The panel was subjected to the salt fog test and checked at periodic intervals: 96, 200, 500 and 1000 hours.  At the same time two separate panels with the cadmium plating and Alumi-Plated ForceTec nut plates were also evaluated.  These panels are shown in Image 1. At the conclusion of the test, the panels were washed and evaluated for corrosion.

The test verified what commercial aircraft operators had observed. All the riveted nut plates had severe corrosion both on the front and backside surfaces as well as through

the bores of both the rivet and fastener holes (see Image 2). The bonded nut plates had surface corrosion around the heads and backside of the retainers; however one of the nut plates dis-bonded and washed off revealing extensive corrosion under the base of the nut retainer. Some of the bare passivated ForceTec retainers had some surface blistering around the head of the retainer but none on the backside. The IV-coated retainers showed no direct surface corrosion on either the front or backside of the retainer except for an area adjacent to the head of one retainer that appeared to have been contaminated by corrosive element runoff from an adjacent riveted nut plate.

Image 2: Severe corrosion around the riveted nut plates

Image 3: ForceTec IVD retainers after 1000-hour salt fog test

In the separate plates containing the cadmium and Alumi-Plated ForceTec retainers there was no evidence of corrosion at the conclusion of the 1000-hour test (as seen in Image 3).

In an independent corrosion evaluation conducted in Australia, anodized aluminum ForceTec retainers were compared to the same riveted and bonded nut plates. In this case corrosion was artificially induced using a solution called EXCO to accelerate corrosion in the bores of the different nut plate configurations. The objective of this test was to gain a better understanding of the resistance to corrosion of the three different nut plates and to conduct low load transfer fatigue tests of 7075-T651 aluminum alloy specimens in the presence of the induced corrosion. Testing was conducted at three different stress levels representative of aircraft service environments.

Corrosion conditioning of the coupons showed the susceptibility of the riveted nut plates to exposed end grain corrosion in both the rivet and fastener holes. While the bonded nut plates showed less surface corrosion, the unprotected bore of the fastener holes was the primary source of corrosion damage and also the point of rapid crack growth from the corrosion pits in the fatigue test (see Image 4). The bonded nut plates had only marginally better fatigue life than the riveted style.

Image 4: Riveted nut plate showing corrosion in bore of fastener hole

The ForceTec nut plate had no corrosion in the bore of the hole or the surface (see Image 5).  Part of this could be attributed to the use of the anodized aluminum retainer and the absence of any galvanic corrosion.  The ForceTec fatigue specimens showed almost an order of magnitude better life than either of the other two nut plates, as shown in the graph below (Ssee Image 6). 

Image 5: ForceTec showing no corrosion in bore of hole

Although the failure of the specimens, in fatigue in most cases, was attributed to crack initiation from the corrosion pitting, an interesting observation from the fatigue test was the effect of fretting on crack initiation in the specimens. Fretting was evident under the nut retainer heads as well as at the faying surface of the load transfer joint.

In summary, ForceTec, with the appropriate coating against galvanic corrosion, has a definite advantage over riveted and bonded-on nut plates in corrosive environments and especially in fatigue-loaded joints.

Image 6: Even after corrosion testing, ForceTec out-performed
other nut plates for fatigue life improvement

The expanded barrel of the retainer provides protection of the exposed end grain in the bore of the hole and seals it from intrusion of the corrosive elements. This can afford considerable structural benefit in both initial manufacture and assembly and in structural rework. ForceTec has proven a convenient way to repair and replace existing riveted nut plates that may be damaged by corrosion by removing the nut plate and rivets then cold expanding the satellite rivet holes and plugging them with a solid rivet. A ForceTec nut plate can then be installed into the fastener hole. A number of aircraft rework programs already employ this repair technique, such as the F-16 FIghter Doors and current rework of the P-3 Orion fleet.

Contact us for more information about the ForceTec system for design or rework applications.

 

The E-6 is a derivative of the Boeing 707 and is flown by the United States Navy Strategic Communications Wing, whose primary mission is strategic airborne communications in support of the President, Secretary of Defense, and the Chairman, the Joint Chiefs of Staff and United States Strategic Command.

After many years of service, the Navy is conducting a Service Life Extension Program (SLEP) on the E-6 airframe, specifically, the tail, lower surfaces of the wing, and selected areas of the upper wing surfaces. A total of 16 aircraft will undergo the SLEP.

Technician using an FTI puller unit to cold expand holes on the
lower wing of the E-6 at Tinker AFB

The 566th Aircraft Maintenance Squadron at Tinker Air Force Base, Oklahoma, was chosen as the site to conduct the work.  Mechanics will remove and replace approximately 14,000 structural fasteners on the aircraft.

All the designated fasteners are methodically removed; the fastener holes inspected for evidence of damage, corrosion, or fatigue cracking; and subsequently repaired and prepped for cold expansion.  The holes are then split sleeve cold expanded to induce beneficial residual compressive stresses that will shield the holes from future cyclic fatigue loading to extend the aircraft's life, expected to the year 2040.  After cold expansion, the holes are reamed to the oversize fastener size and then fitted with the new oversize fastener. Interference fit is created in most cases.

Replacement of the horizontal stabilizer tail attachment bushings with FTI high interference fit ForceMate bushings provided several "challenges" as these are fracture critical attaching lugs.  FTI engineering conducted Finite Element Analysis of the bushed installations to optimize the benefits of ForceMate and ensure that the fatigue design requirements were met.

US Navy E-6

FTI provided considerable on-site and engineering and precision tooling support.  Aircraft #1 and #2 have been completed and the third aircraft is in work.  Dedicated on-site engineering support was provided to the mechanics during the critical installation. Mr. Dean Madden from our Dallas office along with FTI engineering staff members spent many hours ensuring the Navy and Air Force supervisors and Air Force mechanics had the support they needed as they reviewed work plans to streamline the total work package.