londonsean69
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| posted on 20/4/09 at 10:34 AM |
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Carbon vs Carbon Kevlar
OK, can't seem to find much on this, so if anyone has any linky links they would be most appreciated, otherwise, let the thought process
begin.
I have 'acquired' a vacuum pump with all the associated materials (peel ply, breather fabric etc.), this is for vacuum bagging of Carbon
Fibre to remove excess resin and get better consolidation of the laminate layers.
I am going to have a bash at making a few choice bits, but am undecided on what material I need.
Carbon, or Carbon/Kevlar??
In what situation would you go for the mixture? I seem to remember a lot of motorbike parts were CF/K, rather than CF - things like mudguards and
huggers.
Would I be right in thinking that the bit of kevlar gives it a higher impact resistance, to things like stones being flung up?
Any help greatly appreciated
Sean
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tegwin
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| posted on 20/4/09 at 10:39 AM |
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Its worth noting that carbon by its nature is very brittle.... If you just have a carbon layup and allow it to flex it WILL crack...
If you add some kevlar into the mix it gives it would allow it to flex before it cracks...
That and the fact that kevlar is a lot cheaper than CF.... so use the kevlar to get the strength, and CF to get the looks!
------------------------------------------------------------------------------------------------------------------------
Would the last person who leaves the country please switch off the lights and close the door!
www.verticalhorizonsmedia.tv
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Mr Whippy
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| posted on 20/4/09 at 10:42 AM |
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mind that kevlar won't bond to itself and so you'll need to have something between each layer of it. Kevlar has low abrasion resistance,
carbon slighty better but the best is glassfiber. All depends on what resin your using really, some epoxys set harder than others
if it was for a wing I'd do carbon outside to look good, kevlar in the middle and GRP on the side that takes the stones
[Edited on 20/4/09 by Mr Whippy]
Fame is when your old car is plastered all over the internet
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brianthemagical
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| posted on 20/4/09 at 10:47 AM |
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Is that the vacume kit off ebay per chance?
Kevlar is tougher, or less brittle, so will take higher impacts.
Kevlar has a higher tensile strength, but lower tensile modulous, so for min weight, carbon is better, min volume kevalr, but only in tensile
strength.
Stiffness wise, carbon is better than kevlar, so if you're making single layer components, carbon is better, but there's no point going to
the effort of not using twin layer with a core. You could then go for inside carbon for stiffness, out for kevlar. Depending on which way the material
will bend.
As for linkys, there is very little i could find, and i've looked alot. The best i could find as a starting point is compotition car composites,
but it's not too good. Other than that try and find some SAE papers.
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smart51
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| posted on 20/4/09 at 11:17 AM |
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Kevlar is cheaper so people like to dilute their laminates but keeping carbon on the top for the look.
Kevlar is more stretchy and not as brittle to will withstand impacts better. The kevlar takes the impact load so the carbon layers don't crack
either.
Kevlar doesn't cut nearly as well as carbon so leaves tatty frayed edges. Carbon on the outer layers is nice and neat.
Kevlar is a nasty yellow colour
There are plenty of benefits from a mixed laminate.
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Mr Whippy
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| posted on 20/4/09 at 11:32 AM |
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yeah if you look on the web for premade carbon panels, virtually all are a mixture of different fabrics, most common is a glass fiber core with carbon
outer
Fame is when your old car is plastered all over the internet
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twybrow
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| posted on 20/4/09 at 11:39 AM |
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Quite simply, if you don't understamd enough about the differences, its probably not a material you should be using. Kevlar is a PITA to work
with and not worth the hastle IMHO. If you have exposed fibres, it suck up water and will delaminate pretty quickly - this is a big problem with using
Kevlar (aramid).
I would use a layer of glass (preferably S-Glass) on both surfaces, which will aid with impact and flexure. As you have a vac pump, look at doing
infusion rather than just vac bag - well worth the extra effort!
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londonsean69
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| posted on 20/4/09 at 11:57 AM |
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For starters, yes, it was the pump off of Fleabay - I thought why not give it a go.
These answers have confirmed pretty much what I thought.
I think I'm going to have to get hold of some and just have a practice (it's going to be messy)
Cheers
Sean
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brianthemagical
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| posted on 20/4/09 at 12:08 PM |
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If you're new to it, probs best trying some GFRP first, rather than wasting good carbon.
Unless you know the specific strengths of the materials you're using, and how to design well the components you're making, then you
won't be able to get the lightest possible cmponent from the CFRP, in which case, it's pointless making a complete CFRP component, may
aswell save some pennies and mix it with other laminates. Also, uless you plan on hitting some low flying bricks or the like, it's doubt full
you'll crack CFRP. If you're just after the bare carbon look, then, as others have said, carbon on the surface, GFRP underneath. As to
AFRP as the inner surface of arches, how many GFRP archs get damaged, and will the repair costs outweigh the initial Aramid costs?
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londonsean69
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| posted on 20/4/09 at 03:14 PM |
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True enough, I could well imagine carbon needs a substantial whack to get it to break (obviously if it is not stupidly thin).
What I actually meant in my initial post though, was the use of carbon/kevlar hybrid fabric, where the 2 are interwoven.
It would seem to be the best (or maybe worst) of both worlds.
Sean
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britishtrident
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| posted on 20/4/09 at 03:51 PM |
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Carbon fibres are ultra britlle should only be used it properly engineered structures where the streeses have been properly analysed with a
composites FEM program---- Carbon really isn't suited to use on its own without Kevlar and/or glass fibres in the mix.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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britishtrident
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| posted on 20/4/09 at 04:00 PM |
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quote:
Originally posted by londonsean69
True enough, I could well imagine carbon needs a substantial whack to get it to break (obviously if it is not stupidly thin).
Quite the reverse --- carbon fibres are very fragile only strong on pure tension.
With a CF aircraft wing even dropping a spanner or screwdriver on it can cause unseen damage which seriously affect the strength.
Most things we think of as CF are acutally mainly Kevlar & glas and some CF.
The toughness of F1 monocoques in impact situations is down to the use of kevlar.
A good example of a structure that is mainly CF are F1 front suspension wishbone and pull rods ---- which are very strong indeed in their designed
loading mode but notoriously fragile if they take loading in the wrong direction.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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britishtrident
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| posted on 20/4/09 at 04:01 PM |
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quote:
Originally posted by Mr Whippy
mind that kevlar won't bond to itself and so you'll need to have something between each layer of it. Kevlar has low abrasion resistance,
carbon slighty better but the best is glassfiber. All depends on what resin your using really, some epoxys set harder than others
if it was for a wing I'd do carbon outside to look good, kevlar in the middle and GRP on the side that takes the stones
[Edited on 20/4/09 by Mr Whippy]
5 star answer 100% on the nail
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Benonymous
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| posted on 23/4/09 at 06:11 AM |
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Another aspect of composites is that both the laminating resin and the reinforcements can be subject to UV damage from sunlight. Maybe not such a big
problem in Blighty but there is a Civic near where I live that has got a sooper dooper carbon bonnet on it. It looked good to begin with but four
years on and it's looking really tatty. The resin will yellow and Kevlar in particular will actually degrade structurally as the UV breaks down
the fibres at a molecular level. The outcome of this damage would be tricky to determine at a structural level. If you wanted to test a part
you'd have to stress it which may cause further damage. You'll see all CF aircraft and aircraft parts painted (usually white to minimise
heating from sunlight) not put on display like "OOOH look at my carbon wings!!" Just another factor to consider with composites....
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