hb88
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Composite Crashworthiness

Sat Apr 01, 2006 12:39 am

I was reading the other day that a metal aircraft will generally provide superior crashworthiness over other types of construction. As I understand it, for a metal airframe impact energy is absorbed by progressively collapsing the metal structure, as opposed to splintering or shattering upon impact as exhibited by composite structures.

Does the 787 embody some new type of composites design philosophy to avoid this issue? Or has composite construction advanced so that this is not a problem?

I understand that there has been some work done on aircraft underfloor structures for impact absorption, but as far as I'm aware the 787 underfloor structure is fairly standard.

Any comments or input?
 
pilotdude09
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RE: Composite Crashworthiness

Sat Apr 01, 2006 1:25 am

Quoting Hb88 (Thread starter):
I'm aware the 787 underfloor structure is fairly standard.

Remember its not just the 787 that is made of composite its the A380, A350 and 747-800
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khobar
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RE: Composite Crashworthiness

Sat Apr 01, 2006 1:28 am

Quoting Hb88 (Thread starter):
was reading the other day that a metal aircraft will generally provide superior crashworthiness over other types of construction. As I understand it, for a metal airframe impact energy is absorbed by progressively collapsing the metal structure, as opposed to splintering or shattering upon impact as exhibited by composite structures.

An interesting question.

One assumes you are not talking about crashes that require DNA tests on the resulting goo.

1) Mythbusters did some interesting testing on the crash crouch - some said the brace position was designed to provide for a quick kill rather than a long drawn-out injury that would be more expensive for the insurance company. What was interesting about the testing is how fragile the economy seats actually are. They said it was possible that this was part of the design to absorb impact forces, but the collapsing seats resulted in broken femurs in the test subjects thus most likely negating any benefit they provided in absorbing the impact force. Still, it demonstrates that not just the fuselage is in play for energy absorption.

2) http://www.mscsoftware.com/support/library/conf/auc99/p00399.pdf
 
roseflyer
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RE: Composite Crashworthiness

Sat Apr 01, 2006 1:38 am

Quoting Hb88 (Thread starter):
Does the 787 embody some new type of composites design philosophy to avoid this issue? Or has composite construction advanced so that this is not a problem?

It is a really complicated science, but I will try my best to give a bit of an explanation.

First off all most metals are somewhat ductile meaning that you can bend them without them breaking. Stronger metals are less ductile and are more likely to just fail and fracture (in sheer, torsion, tension, etc). Metals usually will deform under high forces. This deformation absorbs energy and thus makes them better for protecting occupants in a crash. This is why your car has crush zones in the engine and trunk where the car is designed to collapse.

Composites on the other hand aren't ductile. They are just as strong, but they will fracture rather than bend. The 787 is using composites that can be forced in many directions due to the laminar design that is done in multiple directions. This helps them be strong in normal use in the ways that they should be strong through normal use. However the fact that a single layer of composite is only strong in one direction means that if you smash the side of the plane with a baseball bat, it is more likely to fail by facturing rather than receiving a dent.

As far as surviving a crash. The composites are just as strong and should be equally safe. They will deform in a different way, but the structure should break just as high strength aluminum would. It isn't necessarily less safe, but it would be more costly to repair.
If you have never designed an airplane part before, let the real designers do the work!
 
hb88
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RE: Composite Crashworthiness

Sat Apr 01, 2006 1:50 am

Quoting Pilotdude09 (Reply 1):
Remember its not just the 787 that is made of composite its the A380, A350 and 747-800

Yes, but it differs in that the bulk of the airframe is composite whereas the the aircraft you note aren't completely composite. I think the 380 fuselage is still mainly metal and this is the component which would be the relevant one in an impact.

Quoting RoseFlyer (Reply 3):
The composites are just as strong and should be equally safe. They will deform in a different way, but the structure should break just as high strength aluminum would.

This partly answers my question. Is it something inherent in the specific composite structure/fabrication that makes the composite design deform like an isotropic metal structure and hence provide resilience?
 
Markhkg
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RE: Composite Crashworthiness

Sat Apr 01, 2006 2:12 am

Quoting Khobar (Reply 2):
They said it was possible that this was part of the design to absorb impact forces, but the collapsing seats resulted in broken femurs in the test subjects thus most likely negating any benefit they provided in absorbing the impact force.

I thought one of the interesting things with the mythbusters demo was that the show inadvertantly showed a significant problem with seats during crashes: seat attachments. In the mythbusters tests, you can see the seat attachments to the floor fail. This was probably due to the fact they weren't attaching them properly to their wood base, but it is interesting to note that in survivable accident, the failure of the seat attachments can cause seats to "pile up" during the impact-- these incidents have occured in real life, thereby impeding emergency egress.

Also, it's interesting to note that the "proper" brace position doesn't have universal concensus throughout the world. Each country says theirs is the best.  Wink
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A319XFW
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RE: Composite Crashworthiness

Sat Apr 01, 2006 2:18 am

Quoting RoseFlyer (Reply 3):
The 787 is using composites that can be forced in many directions due to the laminar design that is done in multiple directions. This helps them be strong in normal use in the ways that they should be strong through normal use.

Surely composites are great in tension but not compression?
 
norcal
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RE: Composite Crashworthiness

Sat Apr 01, 2006 2:18 am

The fact of the matter is that if you have a major airplane crash you probably won't walk away from it.

The real question is: Why don't they make airplanes out of the same stuff they make the nearly indestructible "black boxes" out of?  Wink
 
hb88
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RE: Composite Crashworthiness

Sat Apr 01, 2006 2:21 am

Quoting NorCal (Reply 7):
The fact of the matter is that if you have a major airplane crash you probably won't walk away from it.

For a major crash, yes. But 'minor' crashes are common and survivability of a such a crash depends on the degree of protection that the airframe gives the passengers.
 
LMP737
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RE: Composite Crashworthiness

Sat Apr 01, 2006 2:21 am

Some years ago there was a Beech Starship that went off the end of the runway. From what I have heard it held together rather well.
Never take financial advice from co-workers.
 
2H4
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RE: Composite Crashworthiness

Sat Apr 01, 2006 2:31 am




Quoting RoseFlyer (Reply 3):
Composites on the other hand aren't ductile. They are just as strong, but they will fracture rather than bend.

While this may be true with certain composites, it is wrong to apply uniform characteristics to all composites.

Again and again, people make broad generalizations about composites. "Composites are brittle", or "composites shatter when impacted", or "composites are very difficult to repair". It goes on and on.

At the end of the day, any generalization about composites is meaningless until properly qualified. Saying "Hexcel M21 prepreg matrix structures exhibit brittle impact characteristics when compared to a comparable aluminum monocoque structure" is one thing. Saying "composites are brittle" is quite another.

Claiming composites are brittle or prone to fracture is no different than saying paints are green. In certain cases, sure.....but it all depends on what's inside and how it's made.





Quoting A319XFW (Reply 6):
Surely composites are great in tension but not compression?

Again, it depends. Sheets of most carbon fiber prepreg sheets exhibit excellent resistance to tension and poor resistance to compression. That said, carbon fiber manufacturers are now weaving strands of metal into sheets of carbon fiber to increase the material's resistance to compression.




2H4


Intentionally Left Blank
 
roseflyer
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RE: Composite Crashworthiness

Sat Apr 01, 2006 5:09 am

I will preface this by saying that I am talking about Glass reinforced or Carbon fiber reinforced composites. Composties by definition are just two materials put together that maintain their unique properties. You can wrap an aluminum tube in plastic and call that a composite, but that isn't what I am talking about here.

Quoting A319XFW (Reply 6):
Surely composites are great in tension but not compression?

Yes that is absolutely true. Composites are only strong in the direction of the fibers when in tension. Composites are only as strong as the matrix structure in compression. Fortunately for airplanes, there is very little compression that large sections like the skin will endure. It is hard to generalize, but many materials that you would find in an airplane are better in tension than compression.

What makes the carbon fiber strong is that there is a matrix structure of layers upon layers of carbon fiber in all different directions. This means that you have a strong structure in multiple directions meaning that you can get strength in two dimensions if you have a curved shape.

Quoting Hb88 (Reply 4):
Is it something inherent in the specific composite structure/fabrication that makes the composite design deform like an isotropic metal structure and hence provide resilience?

Modulus of Resilience isn't really something that you use to measure strength of a composite. It is more so a measure of strength that is used in composites since modulus of elasticity is pretty meaningless.

Composites using fibers reinforcement do not perform like isotropically. The A380 is using GLARE which is a composite made up of aluminum. Aluminum is the matrix which is the minimum strength of the composite and will maintain it's properties. GLARE just uses glass fibers to strengthen the aluminum so that it is stronger in certain directions, but it still will maintain the qualities of aluminum.

Quoting 2H4 (Reply 10):
Quoting RoseFlyer (Reply 3):
Composites on the other hand aren't ductile. They are just as strong, but they will fracture rather than bend.

While this may be true with certain composites, it is wrong to apply uniform characteristics to all composites.

You are right. I am wrong. Carbon fiber composites aren't very ductile. I should have said that I was referring to the 787. Some composites are going to be just as ductile as metals like GLARE. You can probably get composites as ductile as polymers, but I haven't heard of any used in the real world.
If you have never designed an airplane part before, let the real designers do the work!
 
KELPkid
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RE: Composite Crashworthiness

Sat Apr 01, 2006 5:31 am

Anyone hear of the Cirrus SR-20 and SR-22, or the Lancair Columbia? These are certified composite aircraft. The interesting thing is that they are new enough that they had to be certified under FAR 23. FAR 23 does contain several crashworthiness requirements that weren't present in CAR 3 (which most other GA aircraft were certified under).

I don't know what the exact requrements of FAR 23 are, but one of them is that all seats must be able to withstand a 26 G Impact loading without coming loose from the airframe. A NASA study in the mid-1970's on crashworthiness in GA aircraft showed that some fatalities in GA accidents were due to the seats coming loose (of course, not all  Wink ). I know the sidestick layout in the SR-20/22 and Lancair also improves occupant safety over a yoke, amongst other benefits (I've heard some have been killed when the firewall was pushed in and they were impaled on the pole in the yoke, and that that's why the 172P and later 172 models have a pad in the center of the yoke).
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