"Both of these effects increase the risk of the fasteners sparking during a lightning strike, which could be disastrous if it happened near the wings that hold the fuel tanks," said Raskind. "Various strategies have been developed to avoid these issues, including tight-fitting sleeves to deal with the machine-induced micro texture, and dielectric patches to prevent arcing from the fasteners during a lightning strike."
The issues related to lightning strike protection in composite structures have been pretty well handled, said Hempstead. Fasteners for components made of carbon composites must be bonded to a conductive material that can dissipate the charge. A lot of work is going into how to do this for permanent fasteners in more cost-efficient and weight-efficient ways.
Alcoa Fastening Systems has done extensive research to understand how a joint behaves differently with composites versus metallics, said Gurrola. The load distribution on a fastener in a composite versus a metallic joint is very different. The company has designed its newer fasteners to accommodate this difference, and to be optimized for the different behavior of the joint. In addition, since composites aren't set up well to harmlessly dissipate energy from lightning strikes over the surface of the aircraft, fasteners are being designed to do that job.
For example, Alcoa's Flite-Tite pin fastening system provides electrical contact with the structure so electricity can flow better from the composite. Another is the Eddie-Bolt, which installs in a more compatible way because it doesn't have prevailing torque. "Prevailing torque is a characteristic of most threaded fasteners that makes the whole thing want to turn as you torque the nut," said Gurrola. The FC43 is a high-strength structural panel fastener that's compatible with composite or metallic structures. For joining aluminum with composite, the Extended Performance Lockbolt fastening system has a titanium collar.
Alcoa is working on new fastener materials, such as higher-strength titanium alloys. The typical titanium alloy in aerospace fasteners is Ti6Al4V, with an ultimate shear strength of 95ksi. "For most applications, that strength works just fine," said Gurrola. "But as you get into larger diameter fasteners with some areas that need higher strength, we've been looking at 108ksi or higher." Alcoa is also investigating lower-cost alternatives to titanium, and higher-strength aluminum-lithium alloys. Those alloys have two benefits over conventional materials. They can directly replace some standard aluminum alloy fasteners and increase strength, or because of their higher strength, they can replace some steel alloy fasteners.
The shift toward increased use of composite materials means fewer fasteners will be needed per airplane, although inflationary pressures will be greater for aerospace fasteners than for fasteners used in other applications, said Raskind. "The relatively small number of suppliers making fasteners for use with composite materials, the more heterogeneous nature of these products, and the lack of effective substitutes means that price competition for these fasteners will be less intense."
Also because composites require fewer fasteners, the shift toward increased composite use by airplane manufacturers means that fastener suppliers will sell fewer units. The Freedonia Group predicts that lower sales volume may also reduce the economic viability of large R&D projects for these products.
The lightning strike issue isn't about frequency so much as it is about catastrophic results. If you've only got a (for example) 1% chance of something happening, but that something has catastrophic results--people dying, lawsuits--then that's something you've got to protect against, or at least not encourage, in your materials and assembly process selection.
I never thought that lightening strikes on aircraft was so common. I read that it happens 2 times per year on average, per airplane. I have seen electrical discharge responsible for fastener loosening and in some cases, ejecting.
There is a downside to composite pieces, price. Bolting parts together will always be around. I designed a mechanical system that ended up having over 60 bolts.. it was cheaper than with none, that was for sure.
Glad you liked the article. The whole issue of the grounding of composites used in aircraft has been widely misunderstood, so I thought it was a good idea to include some clear discussion on that issue. Could you clarify your question about comparisons between fasteners for composites and fasteners for metal? What sort of comparisons do you have in mind?
Excellent post Ann. I know the longevity of any fastener is dependent upon the application and use. Relative to composite fasteners, do we know how they "stack up" relative to metal fasteners? I have seen no data that tries to correlate life cycles of either type. Great point also about the grounding of composites. I know this must be a huge issue but not talked about too much in the literature.
That's true of course. The question is, given an increase in composite use, whether fasteners will be used in high enough quantities in repair to make up for the lower overall quantities in manufacturing.
UK-based Plastic Logic and French company ISORG have created what the pair tout as a first in flexible printed electronics: a large area, conformable, organic image sensor printed on plastic.
New versions of BASF's Ecovio line are both compostable and designed for either injection molding or thermoforming. These combinations are becoming more common for the single-use bioplastics used in food service and food packaging applications, but are still not widely available.
The 100-percent solar-powered Solar Impulse plane flies on a piloted, cross-country flight this summer over the US as a prelude to the longer, round-the-world flight by its successor aircraft planned for 2015.
GE Aviation expects to chop off about 25 percent of the total 3D printing time of metallic production components for its LEAP Turbofan engine, using in-process inspection. That's pretty amazing, considering how slow additive manufacturing (AM) build times usually are.
A $1,500, hand-operated, bench-model, plastic injection machine crowdsource-funded via Kickstarter can be used to mold small, quality, plastic parts inexpensively, on demand.
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