In previous methods, a symmetric set of differently oriented layers were required. "For example, the top four layers were oriented in one direction, and the bottom four in another," said Kelkar. "So you needed eight layers of composites to avoid bending and warping. Now you don't need as many layers or even an even number of layers, and the composite is much flatter."
Also, in previous composite structures, resin collected in large pockets. Resin is weak, and fibers are strong, so resin pockets must be minimized to produce a stronger material. "With this structure and process, we can minimize the resin pocket areas, so they are almost five times less than in previous composites," he said. "This has made the new composite's strength uniform."
Bowers said the resin pockets are reduced by using a nonwoven material created with a small, fine carbon thread. "All these carbon fibers are laid down flat-oriented in one direction and stitched together. Then another layer of fibers is laid down on top of the first at a slightly different angle and stitched together and then infused with resin. This process lets the composite be flatter than can be achieved by weaving."
The proper angles for layup are critical, and so are the number of plies, he said.
Advaero's HVARTM process reduces the viscosity of the resin infused into the carbon fiber material, so it doesn't move the threads, according to Bowers. "Instead, it flows evenly into the carbon fibers, so the space between fibers is kept to a minimum, allowing everything to lie flat."
Kerkal said conventional composites that are subjected to enough loading fail where the large resin pockets are located. Once failure starts, the composite breaks into two pieces. "In the new fabric, the resin pockets are very small and distributed all over the composite, and it's at those many points that failure starts." Instead of breaking into two pieces when it fails, the new composite breaks into hundreds of small pieces -- somewhat like safety glass.
This nonwoven architecture seems like it would have some real potential. Would it address some of the issues you've been writing about in terms of the challenges and concerns around addressing composite failures when aircraft are in the field? Also, I'm wondering about what's involved in creating and supporting the manufacturing process for a material like this. It seems like an entirely new approach and I'm wondering what kind of hurdle that could be for companies already invested in tools and processes specifically designed to support the use of composite materials in their products.
Ann, This kind of development just underscores the fundamental need and desire for lighter but very strong materials. Research and new product development in this area is vital enabling technology moving ahead on so many fronts when it comes to more efficient designs. Thanks.
Thanks, Beth, great questions. The main issues of concern about CFR composites have been detecting failure. Whether this material addresses that problem remains to be seen. One could hypothesize that because its failure modes take longer to reach breaking point and are more spread out throughout the fabric because of its different structure, that those failures might be more visible, using the safety glass analogy. Regarding process, fewer layers to achieve the same strength imply it will be faster, as well as saving a great deal more energy.
I'll email my contacts at Advaero to see if they can address those questions.
Again another bad PR piece. Nothing new here I can see as I've been doing this way for 35 yrs!! It's basic common sense among those who do it.
While these examples are mostly true, no one does them because they are not cost effective or just plain dumb!!
Anyone who uses woven CF just doesn't know enough to use it. CF is great only in tension, compression where it is very good. But only if it stays perfectally straight. If bent as a weave it becomes very expensive springs!! And just not much tension or compression strength in springs of CF. So why do it?
Anytime you see woven CF cloth the only reason it is there is CF hype as once woven, has less strength than FG done right at 10% of the cost of CF.
As for the resins we have many that do the same thing as the example. Selective examples like used are just a form of disinformation.
Great article, Ann. Are they able to quantify the tensile capacity (in psi or ksi) of this material versus that of conventional woven carbon fiber fabrics? Or is it too early to give a solid number? Also, does the higher tensile capacity place it in a different set of applications? A threefold increase is an amazing jump.
When I first read Jerry's post, I thought he was being a bit of a Luddite. But after re-reading the article, I think he may have some valid points. The key thing is that the test was a tensile-pull-to-failure test. That is not at all indicitave of how CF structures are stressed. If the fiber orientation is almost parallel to the pull test axis, of course it will yield better results compared to a woven or cross hatch layup. I would like to see a variety of comparative test-to-failure scenarios (axial twist, bending, compression, etc.) between this layup method and traditional woven layups. I'm not saying this design doesn't potentially have merit, but the test shown is not conclusive evidence of this technique's validity in real world utilization.
This sounds like Unidirectional Carbon Fiber that has been available for decades. Is the stitching part supposed to be the "Special" feature that we're supposed to take note of? Sorry, but I don't see anything special in this article (for those that don't know Unidirectional is much stronger in a pull test than woven, because all the fibers are oriented in the direction of load).
I heard back from Greg Bowers at Advaero. Here is his response:
While I am not familiar with the referenced specific failures of composites in the field, most non-impact failures are a result of delamination of the fabric layers. There is a developing scanning technology that will allow detection of defect or delamination failures in-situ. These delaminations are generally initiated in the resin rich areas of composites. Since resin is not as strong as the carbon fiber the cracks will start there and propogate until eventual failure. Advaero's HVARTM infusion process coupled with the small diameter carbon fiber of Chomarat's NCF fabric significantly reduce the space between fibers and therefore reduce the resin rich areas. Typical woven carbon fiber fabric is crimped in the weaving process thereby increasing pockets of space in the fabric and in the laminate interface. More space....more resin rich pockets. Repair of damage to composites in the field can be a challenge, especially larger repairs. Advaero is developing a new nanofiber technology that we feel will provide a stronger repaired area yet it will be field capable.
Regarding tooling required for new composites: There will be new or
modified tooling required to make the NCF fabric but not radically
different from today's processes. Advaero's HVARTM process is a
modification of well known resin infusion processes being used today and requires only small modifications to existing tooling. In some cases, HVARTM has the potential to replace the very expensive and energy consuming autoclave processes. While this may result in some new tooling the cost is easily offset by the reduction in manufacturing costs.
Chuck, that kind of detail is in the journal article (subscription required). I agree, it's quite a hefty increase.
Here's what Greg Bowers said about that:
As a follow up to some of your bloggers there were additional material tests conducted on the NCF material. Those results were published in the JEC Asia October 2011 magazine [article]. A blogger mentioned the application of fiberglass but fiberglass' strength to weight is inferior to carbon and therefore you see most aircraft using carbon fiber instead of fiberglass.
Ann, Gregg's response hits at the real potential benefit of this methodology, namely decreasing the resin to fiber ratio and getting the fibers in as close of proximity as possible. The smaller fiber size is the crutial factor I believe. An example is wire rope design that gets stronger when the individual wire sizes are reduces and multiplied but the overall diameter stays constant. I reiterate my earlier post that the test cited does not accurately represent the gains possible in actual use. I imagine if one took virgin carbon fibers without any resin matrix and subjected them to the same test the results might be very similar.
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