New Fastener Doubles as Crack Sensor

Working with Stanford researchers, Alcoa is developing aircraft fasteners that also function as sensors capable of detecting crack propagation in multilayer composite structures.

The technology could reduce inspection frequencies for wing stringers by one-half. Fatigue cracks forming at fastener holes are a common form of airframe damage.

In the invention, a fastener couples layers of a multi-layer structure together via an opening that traverses the structure. A sensor circuit is inserted into the opening with the fastener, inducing an electrical response in a portion of the multi-layer structure adjacent to the opening. If the structure surrounding the fastener hole is damaged, the electrical response is slowed, indicating a failure.

In one example of the technology, a sensor film is embedded on the shank of an aircraft fastener, such as a 1.5 inch shank fastener from Alcoa Fastening Systems. An eddy current is applied to the sensor. The sensor's circuit is established by a coating applied to the fastener and the conformable film.  

Alcoa told Design News that the specific materials' technology is proprietary.

The sensor circuit includes an active conductor to induce the electrical response, and a passive conductor to sense the induced electrical response. The active and passive conductors are wound around an outer diameter of the mechanical coupler to form an alternating winding pattern of active and passive conductor lines.

"When you plug this in, you can see if there is a crack and if it has propagated," says Bill Christopher, executive vice president of Alcoa.

Stanford University developed the structural health monitoring (SHM) technology under a research grant sponsored by Alcoa.

Alcoa's SHM system can be used for aluminum aircraft structures as well as hybrid structures that combine carbon fiber-reinforced composite and aluminum. For example, the SHM system can be applied to the joint between aluminum ribs and carbon fiber reinforced wing skins.

Pre-production prototypes of Alcoa's SHM system are currently being tested with select customers for commercial applications.  Alcoa plans to complete comprehensive testing with select customers before SHM reaches full production.

The new fastener is an example of a focus on aircraft assembly technology for Alcoa since it acquired fastener specialist Huck in 2000. In 2002, Alcoa acquired Fairchild's fastener business and formed Alcoa Fastening Systems. Other acquisitions followed, and Alcoa is now the world's largest producer of aircraft fasteners. Alcoa is ramping up fastener production capability in China and other rapidly developing countries.

"Our fasteners aren't the nuts and bolts you buy at Lowes or Home Depot," Christopher told analysts in New York last month. "To give you one example, we have developed a one-inch diameter titanium fastener used on the 787 and A350 that can support the weight of 50 Toyota Camrys."

Alcoa's competitive strategy focuses on design engineering.

"When composites were starting to emerge, we made the decision to be the industry leader in joining dissimilar materials," says Christopher. "One issue that we knew would come up was lightning strike. When you drill through metal, you get a nice hole. With composites, it's serrated."

Voids created by uncut fibers or resin are referred to as machining-induced micro texture. They can trap excess sealant, inhibiting close electrical contact between the fastener and the composite structure. Machining-induced micro texture is associated with arcing between the fastener and the composite structure during lightning strike tests.

Lightning protection of composite structure is more complex because of the high resistance of carbon fibers and epoxy, the multi-layer construction and the anisotropic nature of the structure.

Inherent conductivity of metallic fasteners coupled with the large number of fasteners used in planes creates a high probability of lightning damage on fasteners.

"So we had to develop a sleeved fastener that allows you to have a perfectly close hole," says Christopher.

Conforming fasteners decrease the voltage drop across the interface and reduce the dielectric effect caused by the sealant, minimizing the possibility of arcing between the sleeve and the composite panel. 

Alcoa also developed the Ergo-Tech next-generation fastening system that can be installed by a single person or robotic system instead of two people. The key feature is advanced low-torque installation tooling that reduces strain on installers, making it more compatible with robotic systems, and reducing installation time and cost.

More than ninety percent of Alcoa's assembly systems are specialty structural fasteners and 55 percent of them are either patented or proprietary.