In addition to repairing composites in aircraft structures, figuring out whether they have been damaged in the first place was a top priority in a GAO report that critiqued the growing use of composites.
We have reported before on this site on self-monitoring adhesives that could predict aircraft structural failures. Now GKN Aerospace, one of Boeing's composite aerostructure suppliers, is researching a class of coatings that may make fractures and other damage visible under certain nonvisible wavelengths.
GKN Aerospace has partnered with a UK university in an 18-month collaborative research venture that's in its first quarter, Max Brown, head of engineering at GKN Aerospace's Filton engineering center, told us.
Inspection of composites used in aircraft such as the A350 XWB, shown here in Getafe, Spain, during
initial horizontal tailplane assembly, may be assisted by research into coatings
that will make damage visible at certain wavelengths.
(Source: Airbus)
"Our collaborative coating research is aimed at supplying inspection techniques for the future maintenance and repairability of composites," said Brown. "Today, it may not be immediately obvious to an inspector of composite material as to what level of damage has been experienced, if any. What we envisage is a more intelligent inspection capability. For example, when inspectors are looking at a wing structure, they could look in the right wavelength range, and any areas that have damage above a certain preset threshold will clearly display that damage."
By the end of the program, GKN expects the coating to reach TRL (technology readiness level) 2 or 3. That would indicate that the research team has proven the basic chemistry and would then be able to move to detailed conversations within the engineering community about development for specific applications. "In an additional 18 to 24 months or so, we would probably be approaching TRL 5 or 6." When a technology reaches TRL 6, it's ready for a demonstration -- in this case, a flight trial.
GKN's goal is to design specific microspheres held within a coating. On impact, if damage has occurred, the microsphere capsule wall fractures, and the chemical reactions produce a signature within the coating that is visible at specific wavelengths.
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?
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 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.
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....
TJ, that's funny, using whiteout to detect cracks and delams. I bet it worked great. But I doubt if that would work on CFR composites or even glass-reinforced composites. Damage on these, especially CFR, is invisible to the naked eye and techniques for detecting it different from those used for detecting same in traditional materials. You are right, I carefully did not reveal the wavelength since I honored the company's request in order to get this much published.
You say damage-detecting coatings have been around for awhile, but not using non-visible wavelengths. Do you mean that damage-detecting coatings *for these composites* have been around for awhile? Please inform us if you know!
Thanks for the info, Ann. So that means that the ability to utilize this detection technique will be proprietary, but I guess it also indicates that the state of the technology is at the point where other composite makers should be able to do this too, at least eventually. (That's unless there's only a very narrow class of coatings which are amenable to the detection process, and they're patented or trade secret.) Anyway, I guess the upshot is that this is not going to be anywhere near as industry-widee as I assume. At the same time, it opens up the idea that, with technology advancing, maybe the FAA can move towards some specificity in its composites directives.
Alex, thanks for thinking industry-wide again. I agree, the technology is certainly in the early stages and it makes me wonder how many other coatings manufacturers or composite airstructure makers are conducting similar research under the radar, possibly even in partnership with each other. It might make more sense from an industry standpoint to develop and commercialize something that can be applied by all airstructure manufacturers and regulated by the FAA. But that also assumes that it can be applied in an aftermarket scenario and still work properly. I get the impression that GKN's coating needs to be "baked" in, either literally or figuratively, in order to do its job. But that could also be because they are not a coatings manufacturer.
Chuck, someone knows a lot about the subject, and I wish I did. I've already spent quite a lot of time surfing and snooping around on the Web, but it's quite difficult to find out anything aside from what's in that GAO report, and Boeing is less than forthcoming. I assume this is for security and/or market competition reasons. I'm checking the MRO schools' websites for course descriptions, e.g., but not much luck so far. The thing to remember, in general, is that repair techniques have existed as long as composites in aircraft have existed, but for some time it was all military. Then they entered the commercial aircraft sector, but not, I repeat not, in primary structures. Their use in primary structures has changed everything.
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.
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.
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