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