"Depending on the chemistry of the capsule wall, we intend to grade the wall structure so it fractures at different energy levels corresponding to different amounts of impact," Brown said. "Some coatings could be created for applications with damage that occurs at low energy levels of impact, and others could be developed for damage that occurs at higher energy levels of impact. Alternately, we could develop a single coating with different signatures that looks different depending on different energy levels of impact. For now, we are looking to develop one coating that displays one energy level of impact. Once you've proved the technology works for one energy level, there's no reason you couldn't dial up a different wall structure for a different energy level."
Both internal and external coating applications could be developed with this technology. For example, composites inside the aircraft are exposed to a less severe environment than those on the leading edge of a wing, so the types of energy events in these environments may be very different. "As we approach the end of the first 18 months, we may therefore have certain research strands that aim toward external coatings and others that aim toward internal coatings."
The research targets glass-fiber and carbon-fiber composites. Since GKN provides aerostructures, the coatings would be provided as added functionality to its current and future product portfolio.
Very cool piece of development and one that would certainly benefit broader use of composites. The idea that a coating could deliver intelligent inspection capabilities is in some ways out there, but then again, in keeping with steady pace of technological advances. In many ways, the development strategy makes perfect sense. Do you have a sense of how difficult or how unique it is to develop a single coating with different signatures that can appear different depending on different energy levels of impact?
Thanks for the article Ann! It is great to see this technology being commercialized and incorporated into engineering materials. I was involved in the development of diagnostic coatings which, when excited and viewed under specific wavelengths, provided surface information of temperature, pressure, strain, and cracks. The coatings were applied to the surface of the completed unit for testing. I'm delighted to learn of continued development of both surface and internal coatings during component manufacturing.
One of our biggest surprises came from using composite repair material when applied to traditional metals (aircraft aluminium). Our coatings were used to inspect the performance of a "composite Band-Aid" that could be used to field dress a fatigue crack until the panel could be replaced. The difficulty was that the mechanical performance of the composite material was so superior to the original alloy that the repair site was often a greater point of additional fatigue cracking in the original metal because of the sharp differences between the materials. I imagine things will be better and far superior when all of the components are made out of advanced composites in the first place.
Coatings such as Stresscoat have been used in experimental stress analysis for decades. It seems like a no-brainer to use something like this for structural health monitoring. Of course, it's easy to say that something is a no-brainer after someone else has already come up with it. I'm just surprised that nobody came up with something like this sooner.
Beth, I understand that designing these different coatings is equally simple, whether one coating detects one energy level or multiple energy levels. Creating the actual coating may be a different story, but that wasn't entirely clear. In any case, GKN said it plans to sell the coatings as an integral part of its composite aircraft structural components, not as a separate product line.
Thanks, William and Dave, for sharing your experience in this area. It surprises me that using composites as a repair material for aluminum didn't strike anyone as not a good idea, since their properties are so different. To my limited knowledge so far, repair materials for composites are supposed to pretty closely match the material they are replacing.
The whole subject of using coatings to monitor structural health does seem obvious, doesn't it? I notice that Stresscoat does not appear to address composites. The big deal about them is the fact that damage can be invisible, hence the attempts to make it visible under other wavelengths. And yes, you would think that research such as GKN's would have already occurred, and perhaps it has. Theirs was not easy to find, so it's possible there's other such research going on quietly.
Dave, You just articulated the point that I was trying to make so much better than I did. It does seem like a no-brainer, especially if the technology has been around for a while. I'm wondering what hurdles there were preventing this from being put to use in any significant form prior to now. Or maybe it's that there wasn't a formal market for something like this given that composite materials are just now becoming so dominant in aerospace design.
I don't think the technology has been around for awhile, at least not for composites. The idea may have been. But there's a big difference between realizing one can use coatings to assist in detecting damage--the no-brainer aspect--on one hand, and on the other figuring out exactly what coatings, how they should work, how to apply them without causing other problems, etc. Since GKN is a supplier of composite airstructures and since their scientist describes redesigning a coating at the microsphere level, I would suspect that what's taken some time is the process of figuring out details of how to make and implement such a coating. Even at this point before the 18 months + another 18-24 months before commercialization, they gave a quite coherent description of the basic idea. Yet it will likely take 3+ years before a flight test is likely. So the R&D involved is not trivial.
This is indeed a significant development because it holds the promise that there will be a cost-effective, easily implementable, repeatable, industry wide technique for inspect composites. This is going to be critical important not only to prevent in-flight failures, but also to gather life (MTBF) data on how different composite structures actually perform on commercial aircraft, particularly on primary structures like wings. (A primary structure in aerospace terms refers to a part where, if it fails, the plane will no longer be flyable. So for example you can survive a rip in the fuselage, but not the loss of a wing.)
Alex brings up two important points. First, since this proposed coating or class of coatings will be available only as an inherent part of a composite airframe structure sold by one company, it won't be available for other composite airframe structures sold by other manufacturers. I've already heard of one other project targeting a similar purpose but using an entirely different chemical and behavioral model. That means competition among different types that work in different ways. So actually there may not be much in the way of industry-wide techniques.
Second, it does provide a great opportunity to gather MBTF data. Even if it's coming from airstructures using entirely different coating types, the data should be comparable about how composites break.
I've seen simple things like White-Out used during materials testing to detect cracks and delaminations.
The damage-detecting coatings themselves I believe have been around for a while. The trick to which this article alludes is the non-visible wavelengths that would be used. THAT is a good idea. Damage-detecting coatings that the flying public can see are not confidence building.
The article carefully did not state which spectrums would be used, whether infrared or ultraviolet. I might consider watching the wing with my IR scanner in the future....
Oak Ridge National Laboratory has developed a surface preparation method to improve joining carbon composites with aluminum, with potentially far-reaching ramifications for high-volume industrial applications.
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