New Fastener Doubles as Crack Sensor

Working with Stanfordresearchers, Alcoa is developing aircraft fasteners that also function assensors capable of detecting crack propagation in multilayer compositestructures.

The technology could reduce inspection frequencies for wingstringers by one-half. Fatigue cracks forming at fastener holes are a commonform of airframe damage.

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

In one example of the technology, a sensor film is embeddedon the shank of an aircraft fastener, such as a 1.5 inch shank fastener from AlcoaFastening Systems. An eddy current is applied to the sensor. The sensor'scircuit is established by a coating applied to the fastener and the conformablefilm.

Alcoa told Design Newsthat the specific materials' technology is proprietary.

The sensor circuit includes an active conductor to inducethe electrical response, and a passive conductor to sense the inducedelectrical response. The active and passive conductors are wound around anouter diameter of the mechanical coupler to form an alternating winding patternof active and passive conductor lines.

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

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

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

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

The new fastener is an example of a focus on aircraftassembly technology for Alcoa since it acquired fastener specialist Huck in2000. In 2002, Alcoa acquired Fairchild's fastener business and formed Alcoa Fastening Systems. Otheracquisitions followed, and Alcoa is now the world's largest producer ofaircraft fasteners. Alcoa is ramping up fastener production capability in Chinaand other rapidly developing countries.

"Our fasteners aren't the nuts and bolts you buy at Lowes orHome Depot," Christopher told analysts in New York last month. "To give you oneexample, we have developed a one-inch diameter titanium fastener used on the787 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 thedecision to be the industry leader in joining dissimilar materials," saysChristopher. "One issue that we knew would come up was lightning strike. Whenyou drill through metal, you get a nice hole. With composites, it's serrated."

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

Lightning protection of composite structure is more complexbecause of the high resistance of carbon fibers and epoxy, the multi-layerconstruction and the anisotropic nature of the structure.

Inherent conductivity of metallic fasteners coupled with thelarge number of fasteners used in planes creates a high probability oflightning damage on fasteners.

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

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

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

More than ninety percent of Alcoa's assembly systems arespecialty structural fasteners and 55 percent of them are either patented orproprietary.