How strong is carbon fibre?

dazzawazza
dazzawazza Posts: 462
edited November 2008 in Workshop
Last night I had just completed installation of the fork, stem and bars on by new Planet X SL Carbon. The frame was standing on the kitchen table, but when I walked away it lost balance and fell off the table. During the fall the bars swung around and hit the frame with the full weight of the bike in one spot on the top tube.
There doesn't appear to be any damage to the painted frame, but it still worries me. Could there be hidden damage caused by such a fall (cracks etc), or is the carbon fibre strong enough not to worry?
Also, when I was cutting the steerer on the fork a thin piece of carbon fibre stripped off in the centre of the tube. Should I be worried?
Thanks,

Numpty :shock:

Comments

  • softlad
    softlad Posts: 3,513
    the frame will be fine - any impact damage to any of the layers would have been very obvious through the top layer or the paint finish. I wouldn't worry about the steerer either.. ;)
  • John.T
    John.T Posts: 3,698
    My Trek has been kicked about for over 8 years now and still seems OK. I had it over 40mph today and was not worried. Carbon is a lot stronger than many people think. It is just that if you do hit it hard enough then that is that, you don't limp home on it like you could on a bent steel or allu one.
  • pliptrot
    pliptrot Posts: 582
    I had a pair of carbon bar ends which had a stripped thread in the (alu.) fastener. So they were binned. On the way there I experimented with hammers and vices to see how tough the carbon fibre was. Now, I imagine that it wasn't high quality carbon (although I don't know how such things are quantified) and so I'd bet whatever is used for frames is superior.

    I was astonished how tough the material was. I wouldn't worry about the fall - after all the only weight involved was that of the frame, stem and bars, which is very little.
  • fluff.
    fluff. Posts: 771
    http://bustedcarbon.blogspot.com/

    When carbon breaks, you'll know about it fairly quickly ;)
  • I make the stuff for a living so I can assure you how strong it is :)

    Even the cheap stuff (3k. 6k fibre) is strong, i.e 3k has 3000 filaments per fibre "bootlace" so If the carbon weave is made up of hundreds of bootlaces then you can imagine how many individual filaments that is.

    It's the epoxy resin that gives CF it's strength, on their own they don't have much in the way of strength properties but put them together in the correct quantities and it's phenominal stuff.

    I work with 12k and 24k composites for the aerospace industry, now this material is incredible (read expensive)

    You will find that when it fails it fails proper, it doesnt tend to go bendy, it breaks and shatters.
  • aracer
    aracer Posts: 1,649
    It's the epoxy resin that gives CF it's strength.
    Strange way to put it for somebody working in the industry. I'd describe it more as the CF giving the epoxy its strength, since the carbon does all the work, the epoxy just keeps it in place (and will crack before you break any fibres).
  • I always thought that they try to minimise the amount of resin in CF components for the above reasons...

    processes like pre-preg fibres and OLCV and the scott processes that are designed to remove as much resin as possible...
  • jojo90
    jojo90 Posts: 178
    CF frames are a joy to ride tho :)
  • CF on its own has very little strength, i'm talking about the raw fibres, I work with unidirectional prepreg as well as woven cloth, the UD has no lateral strength at all due to the fibres all laying in the same direction(it just pulls apart), the woven has it's fibres laying at 90° to each other so that gives it strength in both directions.

    A good reason to cut down on resin content is weight, the more resin you impregnate the heavier the component is - personally I get given a spec and I have to produce CF to a certain weight, in aerospace it is absolutely key - for one you have the safety aspect of something which could be too light (in resin) and that would affect its strength and on the other you have the weight of the component, the heavier the plane the more fuel it uses and that makes it more expensive to fly - make it too heavy and you might as well go back to using aluminium, it's a bit cheaper :roll:

    I don't proclaim to be an expert - i'm sure you'll shoot me down for the above but I work in the prepreg industry, I dont lay it up and work with autoclaving so I cannot comment too much on the finished product.
  • aracer wrote:
    since the carbon does all the work, the epoxy just keeps it in place (and will crack before you break any fibres).

    I'd say that sounds about right although they have to work hand in hand, the CF absorbs the impact so yes thats doing most of the work but without the epoxy it has absolutely no rigidity.
  • Anonymous
    Anonymous Posts: 79,667
    I thought carbon forks where solid? Are they like pipes??
  • redddraggon
    redddraggon Posts: 10,862
    willhub wrote:
    I thought carbon forks where solid? Are they like pipes??

    they are Hollow
    I like bikes...

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  • aracer
    aracer Posts: 1,649
    CF on its own has very little strength
    Yes it does - just in only one direction. The thing is the resin doesn't actually add any significant strength at all it just keeps the fibres in the right place - for a lot of construction (eg bike frames) you do want most of your strength in one direction, hence extensive use of unidirectional - the woven pattern typical of CF is usually just a cosmetic top layer. The other reason of course for using unidirectional is that you can pack the fibres in better for higher fibre to resin ratio.

    Admittedly most of my construction experience is with using woven in applications where isotropic strength is required, such as composite canoe/kayak construction/repair.
  • aracer
    aracer Posts: 1,649
    aracer wrote:
    since the carbon does all the work, the epoxy just keeps it in place (and will crack before you break any fibres).

    I'd say that sounds about right although they have to work hand in hand, the CF absorbs the impact so yes thats doing most of the work but without the epoxy it has absolutely no rigidity.
    True - even if it's the CF that's doing more of the work, both are needed. and in a way the epoxy is more important as you could make a frame from just epoxy, but not from just CF.
  • What you say is true, you're a lot better at putting it into words than I am.

    Yes it's very strong in one direction and has very little or no stretch, I have no experience in bike frames so i couldn't comment on how UD is used in that application.

    Woven cloth gives you good drapability too so it can take on intricate shapes, the UD i've seen makes good material to machine out for brackets (i'm talking about mounting the rear wing (VTP) onto commercial jets) which need superior weight/strength properties.

    I can imagine that solid CF forks would be a little on the stiff side, I was under the impression that CF forks were used to take some of the harshness out of the ride.

    Just got my first bike with CF forks (RIbble) and I think it rides very nice but I come from a leisure MTB background so i'm new to all this.
  • aracer wrote:
    True - even if it's the CF that's doing more of the work, both are needed. and in a way the epoxy is more important as you could make a frame from just epoxy, but not from just CF.

    Spot on :)

    I recently did a training day where we did basic chemistry on different matrices and like you say, epoxy could survive on it's own but CF would be next to useless (apart from making an itchy blanket!)