Carbon Fibre Sheet

Look under hi-tech mat. £8 - £45 per sheet. Tend to be long and thin, meant for model aircraft use.

Look under rods and tubes. £25 per sheet, 6" x 13", three thicknesses, meant for RC cars. I have in general stopped buying from fibretech as they are too expensive, but ymmv.

even more expensive, said to be top quality though

4" square 0.5 mm sheet for £1.50.

eBay

etc, google

Did you consider mixed carbon/kevlar sheet? Much less brittle than pure carbon sheet, tougher.

Thanks for all the links I'll have a browse.

Nope. Is it as rigid as pure Carbon? Don't really want flexing if I can help it.

I have shares in Du Pont, so I would of course agree with that...

When working for them, I once had an opportunity to visit the plant in Londonderry where they make the Kevlar fibre. Looked like ordinary synthetic polymer fibre, quite thin, whizzing round various bobbins at lightning speed, but they told me they had to take serious precautions to keep body parts well away - it's stronger than steel, and would take someone's arm off in a heartbeat.

David

Kevlar/carbon comes close to carbon fibre in stiffness - it's advertised as having "the stiffness of carbon, the toughness of kevlar, the strength of both".

It is not quite as stiff as carbon, perhaps 3/4 as stiff - but for most purposes carbon fibre laminate is stiffer than needed anyway.

Carbon fibre has a huge elastic modulus, but usually there is little use for it. For most applications the thickness needed to give the required ultimate tensile strength, and especially to give sufficient toughness, is far more than the thickness needed to provide the required rigidity.

Quite often, carbon fibre is actually too stiff - it doesn't flex and spread loads, permitting huge loads to build up locally which break the laminate. Pure carbon fibre laminates tend to brittleness.

I have no experience with RC cars, although I have heard that they do need lots of stiffness for good control, so I can't directly advise on suitability - maybe you could ask on a rccars group?

Having said that, kevlar/carbon does come close to carbon in terms of rigidity. It ain't gonna bend a lot. And one thing I will guarantee - after a crash a kevlar/carbon fibre sheet will be in much, much better condition than a similar weight carbon fibre only sheet would be. It's a whole lot tougher :)

It will probably cost a bit more though.

I think you probably want carbon for stiffness, but I seem to recall that Kevlar is very good in having a bit more ductility left in the fibre when the resin matrix fails, and so provides a less catastrophic failure in the event of sudden and serious damage.

I was told somewhere down the line that British mine-sweepers had Kevlar composite hulls. Kevlar fibres are plastic rather than glass or carbon fibre so they have some more give (its actually a polyamide - for the chemists). The fact that a flailing strip of it will take your arm off is no more surprising that saying the nylon cord on a strimmer will take your arm off. Arms aren't designed for that sort of abuse !

Steve

In article , Steve Richardson writes

It's a polyaramid fibre, subjected to a special heat treatment which partially pyrolyses it. It does combine good flexibility with high UTS.

This was pretty thin thread, think sewing cotton; I don't think nylon would be even in the same league. Obviously anything can damage you if it is moving fast enough, this stuff could cut you up slowly like a cheese wire (than which it is I believe stronger).

David

Nooo - we can't have that. I trained in metallurgy and can't sit by while someone tells me some plastic is stronger than steel wire - thats definitiely got me rooting in the cupboard. Where I found something that might be very useful -

"A practical guide to composites" - this was produced by Multi-Sport Composites of Bolton in 1995 (are they still there I wonder - phone number was 01204 494184), and I paid the princely sum of £20 for it. If you are into making things with composites then this is a must have book - and of course the company was then ready to sell you the pre-preg or whatever you want to make the do-dads. It covers design, lay-up, molds, ovens - you name it.

Anyway - I digress - back to my defense of steel wire against tiddly little plastic fibres. First lets sort out the mechanical properties (where a Pascal (Pa) is one Newton per square metre).

Steel piano wire - Strength 2.8 GPa, modulus 210 GPa Du Pont PRD49 (Kevlar polaramid) - Strength 2.7 Gpa, modulus 130 Gpa High strength carbon fibre - Strength 3.5 GPa, modulus 230 GPa High modulus carbon fibre - Strength 2.5 GPa, modulus 350 GPa Glass fibres (E-Glass) - Strength 2.5 GPa, modulus 72 GPa Glass fibres (R-glass) - Strength 3.1 GPa, modulus 86 Gpa

So its not worth changing your piano to plastic strings !

Of the fibre types above, Kevlar is the only one that has a negative coefficient of thermal expansion (i.e. it shrinks on heating), the resin matrix in which it is bonded tends to expand. So with varying temperature the Kevlar composite may not be so dimensionally stable as the fibres fight the resin matrix.

If you take into account the density, then you can work out the strength per unit weight - and Kevlar comes out top in this, and carbon fibres wins in stiffness per unit weight.

All of this combines to make Kevlar composites favoured where you don't want a sudden load to cause a catastrophic failure, it will take more deformation and fail more progressively than carbon fibres. Good for canoes and mine-sweepers. If you want dimensional stability and stiffness then carbon fibre is for you (and you can mix the two if you want to get the best of both worlds). If you want cheap - you can still get glass fibre (about a tenth of the price of Kevlar or carbon fibre).

Maybe someone can let me know if Multi-Sport still exist.

Steve

They seem to be still there. See

Gents

I have just noticed that the book "A practical guide to composites" is available on eBay for a Buy-It-Now price of £20 (same price as I paid 11 years ago).

Steve

In article , Steve Richardson writes

Thanks Steve. I guess I must have been thinking of tensile strength per unit weight...

BTW, would piano wire be stronger and stiffer than mild steel wire?

David

Hi Steve, it is also available for £19.95 at

Oh yes - very much stronger, but similar stiffness. You see your mild steel wire will tend to stretch elastically to start with, then beyond the yield stress it begins plastic deformation (that is permanent deformation). It probably has an almost identical stiffness up to the yield point (the yield point might be a fifth of that of piano wire). Beyond the yield point it is taking a permanent set. Piano wire probably goes almost right up to the failure stress without any plastic deformation.

The actual yield stress for the mild steel wire will depend how much cold work has been put into it. The more it is rolled or drawn through a die at cold temperatures the higher the yield point will be. The elastic modulus stays the same, it just means you can bend it further and still have it spring back - so its not quite as simple as just knowing the amount of carbon.

Steve

In article , Steve Richardson writes

Interesting; thanks Steve

David

David,

Just to follow up on the previous off-thread digression, I came across some numbers for the effect of cold work on mild steel and thought they might be interesting. As you probably know, you can't change mild steel properties much by heat treatment, but you can get a range of properties if you combine cold work (like drawing a wire) with heat treatment.

Although this dates from 1945 its still valid.

Annealed Mild Steel (0.01%C) Yield stress 137 MPa (thats when it goes from elastic to plastic deformation) Breaking Stress 290 MPa (nominal stress, i.e. not allowing for necking of the test specimen) Elongation at failure 26%

The same steel cold drawn (84% reduction) - so lots of cold work - same thing would apply if cold rolled. Yield stress 614 MPa Breaking Stress 614 MPa Elongation at failure 0%

In this condition heat treatment will relax it back. So anneal it at 420C and you get : Yield stress 541 MPa Breaking Stress 552 MPa Elongation at failure 8%

So you can get a range of properties from mild steel, but its still fairly pathetic compared to piano wire. I don't know how much cold work is present in bright mild steel bar but a commercial steel supplier will quote the properties, and thats what counts. It benefits the strength considerably as you can see, and it does lock in compressive stresses in the surface which are beneficial from a fatigue perspective, but which can lead to it distorting if you machine one side only.

Steve

In article , Steve Richardson writes

Steve,

Thanks very much for taking the trouble to set this out, I found it very interesting. Apologies to others for hi-jacking the thread!

David