Are Large Carbon Fibre structures Airworthy ?

[Double Zero]

Forgive me for asking a question which one would have thought answered long ago, but a recent posting on ' Rumours & News' basically critisizing the metal BAe 146 after a lightining strike, + personal experience, still make me wonder about this.

I was whisked at relatively high speed to an R.A.F. base where the then latest fighters, first with large parts of the structure inc. whole wing ( I believe at that time the largest carbon fibre structure in the world ) had just suffered the first lightning strikes.

The aircraft ( 2 were involved simultaneously I seem to remember ) were grounded, with slightly puzzled & worried looking technicians going over the wings - where the strike tracking was clearly visible - with ultra-sound.

This ties in with another trial I photographed, where ejected bullet link cases were hitting the aft fuselage ( alloy ) and horizontal stabilisers, ( carbon fibre ).

The problem is that even after a mild impact, carbon fibre acts rather like plywood, delaminating on the inside with no external sign of trouble.

This fact caused a lot of worry with the fighter, but being a military aircraft I presume such risks were acceptable - though personally I reckon a real warplane needs to be able to take a few dings without causing panic.

Since there is also a theory around that Carbon structures actually attract lightning, and / or are difficult to bond or protect from that and life's little minor impacts, is this material REALLY suitable for airliners, or are people being seduced by the ' light weight & strength' figures on the drawing board rather than real life ?

A material considered for advanced fighters just before carbon fibre was de rigeur is lithium alloy - this sounds a lot more sensible to work with 'in the field' to me, including for airliners, even if it's not quite as light as C.F, & it doesn't require huge autoclaves & coddling !

I would add that I am not in the aviation industry now, and am most certainly NOT talking Boeing V. Airbus or that nonsense.

I have sailed high performance sailing boats with carbon fibre rudders & hulls though, where I would guestimate the loadings are not that disimilar re. water at relatively very low speeds - even if they feel high - and air at high speeds - the results were not encouraging there either, and if I were skipper of an expensive racing boat, rather hoping to make the finish - especially on long distance races, I would very firmly decline a carbon rudder.

Any opinions welcome...( especially any agreeing with me, naturally )

DZ

[Re-entry]

Well this guy agrees with you anyway.

Boeing/aerospace | Fired engineer calls 787's plastic fuselage unsafe | Seattle Times Newspaper (http://seattletimes.nwsource.com/html/boeingaerospace/2003889663_boeing180.html)

[SNS3Guppy]

Carbon fiber structures can be made lighter than aluminum and stronger than steel. Scaled Composites still has a panel, not even carbon fiber but e-glass, which is over 30+ years old, I believe. When last I was there, it was mounted outside in the sun, as it has been for decades, with visitors invited to jump up and down on the panel, to show it's still strong.

The way carbon fiber structures suffer damage and transmit loads is different than other materials, as is the repair and inspection process. Every bit as much as wood is different than metal. I learned to fly in wood and fabric aircraft. I've flown very efficient composite aircraft, and have been a mechanic and inspector, as well as pilot for all my adult life and my professional career. I'll also admit that glass structures aren't my forte, which is the case for many technicians (and pilots). Let's face it; most of us are comfortable with metal because it's what we've used for so long.

Glass offers the ability to conform to very complex shapes without a great deal of additional supporting structure, but at the same time it also offers the ability to include reinforcement with substantial weight savings when done properly. It also offers some significant advantages when it comes to repair.

A large composite structure isn't simply a lay-up, like on a surf board. It often involves windings and other construction techniques which make an increadibly strong part. What isn't entirely understood today is the fatigue life limits for such structures, though more is learned daily. A particular concern is long term pressure vessel cycles; the regular pressurization of the structure over an extended period of time. In theory fiberglass and carbon structures and other epoxy-matrix materials have a much more extensive fatigue life, but there's a lot of variability in this, from design to material to application to environment...etc. Carbon structures require different inspection techniques, newer ones of which are still being developed.

So far as lightening protection, certainly the structure may be bonded for conductivity, generally by installing diversion strips or a conductive matrix around or through the part, much like the diverter strips on a composite radome. A matrix part is by it's very nature one big resistor, which doesn't transmit electricity well. This offers both advantages and disadvantages. I've experienced lightening strikes on several occasions in airplanes which resulted in everything from minor to substantial damage; often the most significant damage has occured not in the composite structures, but in the metal...with holes burned through control surfaces, engine damage, even burns melting holes in the fuselage. Most often little more than a burn at an entry point, and one at an exit point...but clearly no structure is immune.

Is it "airworthy?" Airworthy is a two-prong description of part quality. On the one hand it means meets approved data...it is what it was engineered to be, and is maintained in that condition. The other is more esoteric, but more critical, yet harder to define...is it safe? This is perhaps the crux of your question...are carbon structures safe? Yes. Can they fail? Yes. Do they have limitations? Yes. Will we see more and more use of these materials in future construction? Most certainly.

Even with advanced carbon fiber materials, remember that the strength of the part isn't the carbon fiber...but the epoxy in which it's embedded. The combination of the different materials when placed together equals the strength. Take away one, you have either a flexible, flimsy (hard to cut) fabric or stringy set of threads, or a gooey mess than hardens into a chunk of quasi-plastic with no inherent strength of it's own. It's the specific application of the two, both in their combination and in their combination in the properly designed part, which determine what's safe and what isn't.

[Double Zero]

Thanks, Re-Entry & SNS3Guppy,

I'm used to fibreglass ( which I think may be getting confused with here ) & the various lay-up techniques, epoxies & load-spreading mats & rovings, but what really concerns me about carbon fibre is, it can LOOK fine, but be shattered / delaminated inside, and is basically a brittle material.

In a crude way I would from observation describe carbon fibre as a 'dry' material - after all it's baked -, with glass reinforced plastics ( fibreglass ) being 'wet' and more forgiving ( and heavy ) , though there are obviously similarities ...

Re. the chap complaining about 'crash-worthiness', we all know it's a moot point whether most, say, airliner crashes are survivable; I don't fancy being in amongst mangled alloy, but I can't see carbon fibre offering any energy absorbtion at all, just shattering.

DZ

[SNS3Guppy]

Carbon structures, fiberglass structures, and other composite matrix structures aren't the only materials that can look fine on the surface yet have trouble out of sight. The most basic building material, the one we've been using in airplanes long before metal and glass, is wood...and that can appear perfecton the surface but be completely rotted beneath. Further, it doesn't lend to ultrasonic detection or other forms of nondestructive testing that we take for granted on most structures today.

The traditional method for testing glass structures, believe it or not, is the coin-tap test. Tapping with the edge of a coin, one listens for a change in the sound. It doesn't sound very high tech, but it's also very effective.

Glass structures don't lend well to many types of NDT testing, either. However, this doesn't mean that they can't be tested, or maintained.

Heat and vacum bagging and pressure moulding are various methods of establishing a cured part in a particular form or shape, but in the end the result is the same; a compromise between the advantages and weakneses of the matrix and the cloth or thread fiber balanced agaisnt stiffness, flexibility, fatigue resistance, weight, and overalll strength when considering the complexity of the part.

One of my flight jobs involves wearing a helmet. I had mine custom constructed by a well known helmet manufacturer using kevlar, instead of their usual glass layup. I elected to go with the added expense for increased strength and puncture protection...and a couple of years ago I got to put it to the test. All I'll say about that was that it was worth the investment.

Where a glass mat surfboard might blow apart under enough stress, the same isn't true of many fiberglass structures or carbon fiber (kevlar, spectra, etc) structures. In fact, often they remain surprisingly intact. You might be surprised at the resistance to failure of a composite part vs. a typical metal semi-monocoque structure that we see in an aluminum airplane.

You may be surprised too, at the amount of glass and composite structures flying around today...which do very well indeed.

So far as what happens in a crash, well...far too many variables are involved. We can design a structure to withstand known or expected inflight loads, based on load paths and values. We can't begin to predict what may happen to a structure during a crash because we have absolutely no idea what may be done to the structure during the crash, or where the loads may be imposed. A structure may be extremely strong in tension, but not compression...and a compression load, or bending load, or shear load, may cause it to fail very quickly.

We recently saw the destruction of a B747 in a crash; the flight deck remained intact where the remainder of the structure was predictably destroyed. I'd have bet such a thing would never have happened; I was as surprised as anyone at the fact that not only did the flight deck remain intact, but the eight souls on board survived. That was a metal airplane, of course. What would have happened had it been carbon fiber? We can't possibly say, but I would suspect that the integrity of the structure would certainly fare as well, if not better.

There are so many directions the discussion could be taken ranging from flame propogation to the ability or inability of rescuers to cut through the structure during an extrication. All valid issues, but the question you posed was one of airworthiness. Certainly if the structure is designed properly and aerodynamically, composite structures have been key parts of airplanes for many decades now, and will continue far into the future.

Carbon fiber is just one more construction material; something to be learned, but not feared. It's used because it's advantages outeigh it's disadvantages. Metal structures fatigue, and have their own disadvantages, but also have advantages. Same for glass and carbon. Just another material, with some fantastic possibilities.

[Double Zero]

SNS3Guppy,

I am well aware of how many 'composite' aircraft, or parts thereof, are whirling around us as I type.

With all respect, I can't help but get the impression you're thinking more along the lines of modern fibreglass techniques rather than actual carbon fibre.

Re. your mention about traditional wood 'looking perfect but destroyed internally', that's exactly my concern about carbon fibre - and for example after a slight impact with a handling platform, otherwise inconsequential birdstrike etc, how many airlines will turn out a team with ultrasound ?

In that respect especially, military aircraft will probably be better maintained.

You're quite right about the 'coin test' on GRP ; I tap with my knuckles on my boat to locate bulkheads for trestle points ( the stickers wash away ) !

[SNS3Guppy]

I'm thinking of both, because carbon materials and glass are interchangable in construction and use in many cases. You seem to be under the impression that carbon fiber must be baked; this is not the case. Often it's used as a prepreg or lay up material in the same way that E or S glass or mat is used. It may be used in a number of different matrix compounds to make a finished part. Wound structures and heated, autoclaved, baked, and pressure formed parts are only some of the ways in which it may be utilized.

Testing processes that work on fiberglass don't necessarily work on wood, and that includes the tap test.

Delamination issues are particular to lay-ups, which are glass structures, or layups involving graphite and carbon fiber composites such as kevlar, etc. Sometimes the fix is as simple as injecting a filler such as certain types of epoxy, whereas other times it may involve cutting and grinding, and replacing the section with a patch, plug, core, or layup (or series thereof), or simply patching the outer or inner radius where the failure has occured.

You probably don't need to worry much about spent casings striking the wings of an airline aircraft.

Wood differs somewhat in that techniques useable for detecting problems in glass, layups, or other parts and structures don't necessarily reveal the problem in the wood. In fact, cutting a cross section may be the only way to see it. This isn't the case with glass necessarily, and I use the reference "glass" to be inclusive of all synthetic layups and parts/structures, not just glass fibers. That would include carbon fiber. I used the wood as an example of other materials that have long traditionally been used in aircraft which share similiar problems, but in the case of wood, an even greater drawback or limitation.

Wood, incidentally, also offers an advantage in that it has a nearly limitless fatigue life; it bends and bends, without fatiguing...one of it's better properties that made it suited for a long time to use in propellers, among other things. Glass enjoys similiar properties, with greater stiffness and strength.

[FlightTester]

Re. the chap complaining about 'crash-worthiness', we all know it's a moot point whether most, say, airliner crashes are survivable; I don't fancy being in amongst mangled alloy, but I can't see carbon fibre offering any energy absorbtion at all, just shattering.

Here's a video of some carbon fiber crashing - result a sprained ankle!
Yes it shatters, however as in any crash what matters is energy management and the important point to note about carbon fiber is that with it being incredibly strong per unit weight, it also has the capability to absorb energy and dissipate it - by shattering. It's the amount of energy that it takes to get the material to shatter in the first place that is the fundamental issue here.

Robert Kubica Crash @ Canada (http://video.google.com/videoplay?docid=-3119039867907859307)

[spannersatcx]

The bits shattering are just aerodynamic panels, the main monocoque or chasis has remained intact, it does have crumple zones to absord some of the energy but is designed to stay in one piece to protect the driver.

[FlightTester]

The bits shattering are just aerodynamic panels, the main monocoque or chasis has remained intact, it does have crumple zones to absord some of the energy but is designed to stay in one piece to protect the driver.


It's not just the aero attachments shattering - it's the front rear and side crumple zones. Yes, the monocoque is designed to stay in one piece, however part of the design of making it stay in one piece is to sacrifice the rest of the car as an energy absorbing device. So.... you'll notice that the engine, fuel tank, wheels and all the other heavy bits on an F1 car (or IRL cart) depart the fix pretty rapidly in the event of a major incident, that reduces the cars mass and therefore it's inertia. After that the crumple zones offer progressive deformation to absorb the remainder of the energy until they shatter. At which point the car has hopefully bled enough energy for the monocoque to remain in one piece.

Which goes back to my original posting regarding energy management and that CF gives a huge amount of energy absorbtion compared with a similar mass of aluminium or steel.

[Double Zero]

Thanks guys,

you have just about convinced me re.crash resistance, but what about the ' sharp impact with no external signs ' scenario I was on about in the first place ?

DZ

[FlightTester]

You've got me on that one, but.... we're probably at the same point with CF now that we were with aluminium pressurised fuselages 60 years ago with the Comet - we didn't know a lot about fatigue cracking back then and it took a couple of nasty accidents to open our eyes. Now we do routine NDT/NDI etc. to check for fatigue cracking and propogation - composite structure adheres to the same rules.

Inadvertant damage from birdstrikes etc - more difficult to quantify but I would speculate that a bird that was big enough to do visible damage to composite would also be big enough to do visible damage to a metal skinned aircraft. Likewise, if it didn't cause visible damage then the damage would probably be limited enough so that structural integrity wasn't compromised and the damage would be picked up at the next NDI.

All this is of huge interest to me as my next big project is an all composite biz jet!

[Double Zero]

Flightester,

Good luck with your project, if you feel any need for an instrumentation photographer ( cine' pods/ onboard + handheld, A-A etc ) let me know !

From the top of my head I can't think of a truly effective system to counter the 'sharp impact' problem - apart from Ultrasound testing which will probably be constant for initial flight trials, but it's the 'routine' flying which seems a possible achilles heel - I very much hope NOTHING like that of the Comet !

A few things spring to my late night even more feeble mind; maybe a paint finish which will readily show witness marks of any impact ( not just birds, but errant vehicles on the ground etc )- but I am aware of what a problem paint erosion is when using soft RAM paint, could the main structure be instrumented with stress / shock monitoring strips - solar power with battery back-up would seem a good bet for 24 hour cover - and video cameras covering the underside & upper surface & engines of an aircraft ( with a recorder of course ) have already shown their use in real time to aircrew, let alone technician playbacks during turnrounds / overnights ???

How about a shock sensor recorder, coupled with as much overall camera coverage as possible, to enable playback if a shock has been sensed ?

May well just end up like a car alarm with frequent spurious warnings, particularly unless the aircraft is hangared - but I'm sure there's a way, as I say it's late here and I'm probably not firing on all cylinders.

The camera views meanwhile could be made available to passengers too, who are generally interested in what's going on, and not as thick as some cabin crew would wish ! Certainly of use in early flight testing of course.

Just a few ramblings, but I'm glad we seem to agree in the end - DZ