Fatigue Resistance of 787-composites

[CONSO]

Just an interesting short video of how many cycles completed on 787 simulating a fight- takeoff - landing, etc

Testing the 787, looking for cracks (http://www.king5.com/story/tech/science/aerospace/2015/12/01/testing-the-787-looking-for-cracks/76639566/)

[Whiskey Zulu]

How do they incorporate the additional stresses caused by excessive external temperature ranges experienced in-flight and on the ground?

[Volume]

We do have composite aircraft around for half a century now, not a single accident so far could be attributed to fatigue.
Nevertheless we have seen cracks developing and growing when the fibre orientation has been poorly selected. Those result from unintended stresses, because otherwise there would be fibres in that direction. The funny thing with composites is, sometimes you need to make it softer, not stronger to prevent cracking. With metal you can not do that, hence many designers are not used to this.
Simulating real in-flight environment in the laboratory is a real challange, but so far no relevant influence could have been identified. We are however still quite conservative with these materials (this is why there is no revolutionary weight saving), it will be interesting to see how far we can push the boundaries without getting hurt.

[arearadar]

So we become the guinea pigs ?

[Tourist]

All new technologies require guinea-pigs.

That is just a fact of life.

You can try things out in the lab, but eventually you need to try it with real people.

Seat belts had Guinea-pigs. Airbags. ABS. Parachutes. ILS. TCAS.

Man up.

[lasernigel]

We do have composite aircraft around for half a century now, not a single accident so far could be attributed to fatigue.

But isn't that just wings etc. Isn't the 787 the first commercial jet with a composite fuselage? Not forgetting the smoking batteries!

[A and C]

The fuselage is just a pressure vessel and wound fiber pressure vessels have been fitted to airliners since the late seventies, composite fuselage technology has been around for even longer. The 787 is simply a combination of a number of well understood technologies.

All this composite technology is only new to the more inward looking people in what is a very conservative industry.

[Volume]

But isn't that just wingsActually the wing should see more load cycles than the pressure hull, which basically sees one per flight cycle.
The fuselage is generally more critical on metallic designs because of the potentially instable crack growth (aka the balloon pop). We find more fatigue cracks in other areas of the aircraft, the big difference is that we have some time to find them, as they grow slowly and controlled.

One detail I sometimes think about, is the fact that in metal aircraft design standard fasteners are never the fatigue critical components. Cracks start in the structural parts, typically at fastener holes. If the structural parts become fatigue resistant, are we going to experience fatigue to bolts and rivets some day?

[jmmoric]

Composite fuselage? How would it react to explosive decompression? And would it be stronger against explosive devices than an all metal one?

It won't exactely crack like metal would, and could be more durable in those situations, but do we have an idea?

[CONSO]

Composite fuselage? How would it react to explosive decompression? And would it be stronger against explosive devices than an all metal one?Puuuhleeese ! drop the term explosive re decompression and replace with ' rapid" Even in metal airframes, the whole section does NOT ' explode " :mad:

Check with mythbusters as to what happens when an reasl explosive is used to blow out a significant portion of skin.

AS to the never ending bit re composite ' tanks' barrels, tubes, fuselages, et ad infinitum ' exploding or being somehow ' magical " and or "new"

In the 1960's ( and possibly earlier) composite rocket motor cases were used on solid propellant missiles such as Minuteman. Such rocket motor cases are subject to higher pressure, higher temperature, and flight stresses well above aircraft parameters.

Yet still they myth of "fiberglass" car bodies ( corvette ?) bodies exploding on impact still persist. No doubt due to the condition of stretched and broken fibers of the broken parts. Knowledge and materials have improved somewhat over the last 50 years :eek:

[MrSnuggles]

In theory a composite fuselage could be able to stretch enough to withstand the tearing apart that is seen with metals.

In theory that is. I don't know if we ever will see such a thing in flight.

Reason for this is that if you place the fibers in the correct angle to the air flow, the fibres can stretch pretty far without breaking. The limiting factors would be the properties of the resin and the true length of the fibre. This is related to what Volume said about making the composite softer to get more strength.

However, since decompression can happen anywhere there is no catch-all possibility regarding the fiber position relative to the air flow and thus it is very unlikely that we will ever see a decompression resistant fuselage. I'm guessing it would be possible for those boxes transporting cargo in some distant future though. Afaik the boxes from the Lockerbie testing were metal and thus very heavy, that's why no airline wanted them. Composite cargo boxes would be much lighter.

[Lonewolf_50]

We are however still quite conservative with these materials (this is why there is no revolutionary weight saving), it will be interesting to see how far we can push the boundaries without getting hurt.
Just so long as I'm not on the flight in the plane where they pushed it just a hair too far, OK? :E
The funny thing with composites is, sometimes you need to make it softer, not stronger to prevent cracking. Materials science for the win. :ok: One is reminded of about 50 years ago in the auto industry how new glass manufacturing processes arrived at windshields were less likely to shatter: you could say the glass was less stiff in the newer windshields.

[Duralumin]

The highest profile material failure in recent times was cracking of the new 7449 aluminium alloy in A380 wing ribs, luckily this was not catastrophic. It is quite likely that this alloy (which is from the late 90s) is less well studied than the composite in the 777 wing.

I am curious as to why there should be any suspicion about the use of composites. How many metallic crash helmets are there? How much safer are modern formula 1 cars then their metallic forebears? How often do you hear of composite hulled boats breaking up?

Oh and why isn't this thread in Tech Log

[tdracer]

Composites do not 'fatigue' in the same way that metals do. Metals such as steel have a nearly infinite fatigue life, so long as you stay away from the plastic deformation area - aluminum is somewhat unique it that it does eventually fatigue even if you are operating well away from the plastic deformation area. Corrosion is a different matter - many fatigue failures in metal structures are really due to corrosion. In contrast, composites are pretty immune to corrosion, what eventually happens is the binding resin start to break down (which can sometimes be more of an 'age' issue than 'stress' issue - the resin gets old and brittle)
The one big area of concern with composite structure is that it basically does not plastically deform. Over the years, several aircraft have been able to land safely after exceeding structural limits because the metal structure bent but didn't break. Composites will simply break when the structural limit is exceeded, so composite structure need to be designed with that in mind. This was demonstrated rather dramatically when Formula 1 first started using composite constructions - in a big wreck the chassis would pretty much shatter. Now they design for that and the result is the safest race cars ever built.

[Wageslave]

Composite fuselage? How would it react to explosive decompression? And would it be stronger against explosive devices than an all metal one?

Reading between the lines I'd surmise that someone doesn't understand the meaning of "explosive decompression" if they combine it in the same sentence with discussion of explosive devices. There is, of course, no connection between the two at all.

After all, how does any fuselage react to explosive decompression? It just relaxes and goes, "Aah!" doesn't it? Is that dramatic in some way?

A bit more engineering/science please, a bit less inaccurate media induced technical (in)expertise?

[rottenray]

How do they incorporate the additional stresses caused by excessive external temperature ranges experienced in-flight and on the ground?

The same way rhey do with AL - they don't. Many more cycles instead.


So we become the guinea pigs?

First, cavia porcellus is rarely used in research. Next, where have you been hiding? The paying customer is in the research loop of virtually every vended product - food, drug, appliance.

[blind pew]

I had the pleasure of driving the Lak test pilot in South Africa a decade ago.
He was partially crippled after a high speed, low level composite wing failure (flutter) which was due to fatigue.
He stated that they used Russian composites at the time but had based their structural calculations on the West's research figures.
The glider had passed all of the flight testing but the structure had failed iirc after two years of heavy use.

[nonsense]

... Metals such as steel have a nearly infinite fatigue life, so long as you stay away from the plastic deformation area - aluminum is somewhat unique it that it does eventually fatigue even if you are operating well away from the plastic deformation area.

Steel (and many other materials) may fail by low cycle fatigue (https://en.wikipedia.org/wiki/Fatigue_%28material%29#Low-cycle_fatigue), where repeated plastic deformation such as bending a wire coathanger rapidly causes failure.

But if you think simply avoiding plastic deformation will protect your steel structure from fatigue failure, then high cycle fatigue (https://en.wikipedia.org/wiki/Fatigue_%28material%29#High-cycle_fatigue) will come as a nasty surprise (http://www.n56ml.com/corvair/flexplate/problem.html).

Aluminium is not at all unique; it is steel which is unusual in exhibiting a fatigue limit (https://en.wikipedia.org/wiki/Fatigue_limit), a stress level about half that required for plastic deformation, below which it does not accumulate fatigue damage.

Typically if a steel structure will withstand over about 10 million cycles, it is not loaded past its fatigue limit and should last indefinitely:

https://upload.wikimedia.org/wikipedia/commons/thumb/3/30/S-N_curves.PNG/350px-S-N_curves.PNG

When you consider that an engine running at, say, 1700 RPM, will accumulate about 100,000 cycles per hour, 10,000,000 cycles in 100 hours (roughly 2000 miles for a car in city traffic), you begin to appreciate why we don't make crankshafts in aluminium.

[Tourist]

The one big area of concern with composite structure is that it basically does not plastically deform. Over the years, several aircraft have been able to land safely after exceeding structural limits because the metal structure bent but didn't break.

However, the metal in the Merlin helicopter frame shatters like glass it turns out.

The one that beat itself to death at Culdrose from the hover is well worth a look at. It's failure mode looks closer to glass than anything else...

[EEngr]

Seat belts had Guinea-pigs. Airbags. ABS. Parachutes. ILS. TCAS.

Man up. Bring it on. I'm ready.

https://steynian.files.wordpress.com/2013/06/guinea.png

[Mach Stall]

I have to take issue with several posts here regarding metals. Steel is hardly some outlier metal or alloy in terms of having a fatigue endurance limit (basically the stress at which theoretically infinite cycles will not result in rupture).

In fact, many aluminum alloys at various tempers, titanium alloys at various agings, beryllium and other alloys all have specified fatigue endurance limits.

Here is a simple representative ranking of the ratio of fatigue strength to density of a sample of engineering metals/alloys:

titanium 10-2-3: 833,000 inches
beryllium hot-pressed: 597,000 inches
Ferrium M54 steel: 528,000 inches
aluminum 2024, T-4: 200,000 inches

You can see why titanium alloy 10-2-3 is a preferred alloy for aircraft landing gear.

Carbon fiber reinforced polymer (CFRP) is an extremely nice material, but it has been hyped to the point that many people are surprised to learn that it (only!) saves on the order of 10%-35% weight in most aircraft component applications vs aluminum alloys. It does have downsides versus metals, though, including being less rugged in service wrt external damage and wear.