The GAO report questioning the safety of repair and maintenance procedures for Boeing 787 Dreamliner composites sparked a lot of discussion on Design News. That report was the result of a painstakingly detailed review of certification processes followed by the Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA), a search of the technical literature, and interviews with repair and maintenance experts, FAA and EASA officials, and Boeing representatives.
The 50-page report contains a wealth of information about the history of composite usage in aerospace, the differences in nature between composites and the default material, steel, used for structural airframing, and the differences between their respective test procedures.
The first production Boeing 787 Dreamliner sits parked in Mukilteo, Wash., prior to its delivery to Japan's All Nippon Airlines in September 2011.
For instance, the report states (on page 4): "The material properties of composites make them more resistant to fatigue and corrosion than metal, which leads to lower maintenance costs." At least two different types of carbon fiber composites are used in the 787, carbon laminate and carbon sandwich. The laminate layers multiple sheets of carbon fibers on top of each other, while the sandwich layers these sheets on top of a honeycomb structure.
Much of what's known about these materials comes from research by the federal government. The National Aeronautics and Space Administration (NASA) has done fundamental and applied research on composites since the 1970s and 1980s. NASA's studies explored the basic properties of advanced composites and the effects of in-flight service and environmental exposure of components. More recent NASA research has studied the aging and durability of advanced structural materials, including composites, used in aircraft.
Thanks, Alex, that's a good question. For carbon fiber-based composites, the figure I saw quoted most often was that virgin fibers cost about 10 times as much as their recycled equivalents. Since virgin fibers cost more than $10,000 per ton, recycled fibers would represent quite a cost savings.
My very first job out of college was in the structural test lab of a military aircraft company. I got to perform fatigue tests on graphite/ epoxy composite hat / skin sections, and residual strength tests on battlefield damage and subsequent repair.
I know all the other aircraft companies were doing the same type of tests at the time. Is all of this data unusable? I'm puzzled why there seems to be a gap in the data as described in these articles.
Composites have been used in race cars and racing sailboats for a long time. Sailboats in many cases have been exstensivly insrumented (particularly during an Americas Cup cycle) to understand fatique cycles and degredation. There are many carbon fiber based racing sailboats now approacing twenty years old. These have been subject to high slamming loads, structural stresses and extensive UV/atmospheric exposure. It would seem that some of the research would certainly cross over into the aviation field. I also seem to remeber when working on the F14 in the early 1970s that there were a number of Boron composit pieces.
Of course previous research is still applicable - but the world of civilian aircraft is very different from the world of military aircraft in terms of duty cycle, inspection frequency, and design lifetime. (The same goes for racing vehicles as compared to street vehicles).
You can do all the research and testing you want, but the only way you really know whether your design works is by putting it out in the field for a long period of time. This is basically what the GAO report is pointing out.
Thanks Dave, that's a good summary of the problem. It's only field testing that will show up certain types of problems. A lot of research on very similar materials for military planes exists, but the key word is similar; they're not identical.
Composits do have a bit of history, just not in aircraft, it seems.
I know that for many years I have read about methods for monitoring the health of structural composits that have used a variety of fibers embedded into the composite parts during fabrication. All of the articles claimed that this method was wonderful and quite an improvement upon anything else that had ever been done. So the question becomes one about the actual utilization of these methods of monitoring composite health. Are the methods as good as originally claimed? Do these methods work at all? Are these methods just expensive enough that they are not utilized at all? Does anybody else recall reading all of those wonderful articles? Would anybody have any actual experience with this monitoring approach?
It would be good to hear some comments from those closer to the product than I am.
The were first used in military aircraft, though they've been used in commercial aircraft for nearly as long. The difference in the Dreamliner and other recent planes, such as the Airbus 350, is the sheer amount of composites in the plane, and the amounts in primary structures, ie, wings and fuselage.
As to methods for monitoring their health, I'd like to know more about them, too, since that seems to be a key point in the GAO report. William, can you tell us more about what those methods are, or point us towards some of those articles?
None of the articles are recent, but they were in mainstream publications like Design News and Machine design. Some of them may have appeared in NASA Tech Briefs. The common thread of most articles is that composit parts would include a fiber or a conductor in their construction that would be affected by any strain or delamination in the composite component, and that this effect could be measured by some external system. Glass fibers seem to have had the more recent spotlight as potential stars in the monitoring business, at least that is my recollection. None of the reports went into really extensive details, so I am not able to help much there. My opinion is that it would hbe a huge challenge to create a composite part and be able to preserve a glass fiber for monitoring purposes. But possibly it could be done.
So now you know as much as I can recall about this means of monitoring the health of composite materials.
I'm sure Design News has covered structural health monitoring of composites in the past, but I couldn't find anything in the archive. However, here's a good article from Composites World which describes some of the many approaches which have been tried.
The fact that there are so many (radically different) approaches to structural health monitoring of composites suggests to me that none of the approaches is perfect; after all, if any one of them were particularly good, we wouldn't be hearing about the others. But maybe this field is new enough that these things haven't been sorted out yet.
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