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kbuf737
Topic Author
Posts: 765
Joined: Thu Dec 04, 2003 2:22 pm

Carbon/Composite Fatigue?

Sun Jun 06, 2004 4:47 pm

Just as we have seen metal fatigue happen in some planes, is there a possibility that we will see carbon or composite fatigue, or are these very well engineered and highly tested products?
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dl757md
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Joined: Mon May 24, 2004 9:32 am

RE: Carbon/Composite Fatigue?

Sun Jun 06, 2004 6:03 pm

I think we've already seen it in AA587, the starship, and the B-2. All of these have had composite fatigue issues. The industry has been developing NDT inspection methods of finding defects for years and is also developing standards for the inspection frequency of composites.
757 Most beautiful airliner in the sky!
 
Airtractor
Posts: 26
Joined: Sun Apr 25, 2004 9:36 am

RE: Carbon/Composite Fatigue?

Mon Jun 07, 2004 9:27 pm

As with standard metal airframes, composite airframes do fatigue aswell, just in different ways. A normal metal wing for instance can corrode and weaken with years of service due to water contamination, flight stresses, dissimilar metals used in construction ect.
A carbon composite airframe is no different. With years of service and flight stresses they tend to, in certain areas develop very small cracks in the epoxy resins and coatings used in their construction. Although these cracks usually do not compromise strength, the do however allow water to be "wicked" into the structure thus causing the weakness.
Unlike cracks in metal components, these cracks cannot be found with conventional inspection techniques and great care must be used in looking for them. A common method of detecting a fatigued composite component is by the way of the "tap hammer". The AME will go over the wing skin for instance with a small light weight hammer "tapping" lightly on the surface. Any unseen damage will give off a different tap from undamaged areas.

Hope this helps.

Kev.
In memory of Agnes & Aaron, you're always with me when I fly.
 
QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

RE: Carbon/Composite Fatigue?

Fri Jun 11, 2004 8:42 am

Sorry to bump this back up, but I just read an article in Flight International on composites and their characteristics in crashes. Apparently there are concerns that in a crash composites 'smash' instead of bend or tear like metal, and that tiny airborne particles are released which damage all sorts of things in your body - mainly, of course, your lungs. This would obviously be quite dangerous for any surviving passengers and rescue crew. The article said that large-scale tests haven't really been done, and that both Airbus and Boeing claim that that particle problem is non-existant. However, Airservices Australia claims it certainly is a problem, and as a result they're working out new emergency procedures for the imminent arrivals of both the A380 and 7E7 (both obviously contain quite a bit of composites).

Can anyone shed more light on this?

Cheers,
QantasA332

(The article also mentioned that, in terms of fatigue, composites are quite good; relaminating is one of the only main things that needs to be done now and then.)
 
msllsmith
Posts: 387
Joined: Fri Feb 20, 2004 3:25 am

RE: Carbon/Composite Fatigue?

Sat Jun 12, 2004 1:48 pm

Airtractor ,

Jeez....let's hope these fellows have good hearing!.....I mean finding studs is one thing.......

I'ld like to read more about this.
There's nothing more beautiful than flying into the dawn.
 
liamksa
Posts: 301
Joined: Mon Oct 29, 2001 1:13 pm

RE: Carbon/Composite Fatigue?

Sat Jun 12, 2004 2:11 pm

An Aussie company has developed a concept called comparitive vacuum monitoring (CVM) which is aimed at catching fatigue cracking. Airbus and Boeing are both interested in its potential for application to composite materials.

Here's an article for those interested:
http://www.theaustralian.news.com.au/common/story_page/0,5744,9804820%255E23349,00.html
 
airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

RE: Carbon/Composite Fatigue?

Sat Jun 12, 2004 10:53 pm

A normal metal wing for instance can corrode and weaken with years of service due to water contamination, flight stresses, dissimilar metals used in construction ect.


I believe fatigue is a different issue than corrosion/dissimilar metals/water contamination. With respect to design, fatigue analysis doesn't typically deal with deterioration due to these factors.

With respect to composites, its extremely difficult to detect imperfections caused by fatigue. Delamination due to fatigue that are not indicated at the edge of the material are almost impossible to detect using commonly available methods.

Also, composite structures give little warning before failure. There is no "plastic" state as exibited with metal structures that have been fatigued. If you overstress metal, it may stick around long enough to get the airplane down, but composites tend to fail explosively. They don't fail "a little bit" like metal can...  Smile

Some work is being done in ultrasound technology, but I imagine it will be quite some time before you see this commonly used in the field.

Critical composite panels are currently manufactured with quite conservative safety factors until the maintenance industry catches up with the technology.

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