Carbon Fiber Parts

Discussion in 'Materials' started by Fanie, Sep 27, 2014.

  1. groper
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    groper Senior Member

    You can lay a woven with the fibers in the direction you want aswell, it doesn't a dress the real point and that is the crimped nature woven fabrics. They are woven through each other and are crimped like an accordion bellows. The fibers need to be laid flat and straight to get a MUCH higher strength and stiffness. The difference is chalk and cheese if you make some samples and feel it yourself...
     
  2. Fanie
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    Fanie Fanie

    Agreed.
     
  3. Tungsten
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    Tungsten Senior Member

    Somewhere on here I remember some testing of carbon and s glass laminates but I cant find it.Maybe it was an Eric Greene article not sure but it was a very easy read.Hope someone can find it.
     
  4. SukiSolo
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    SukiSolo Senior Member

    Whilst this is correct, to my mind, I would not say that plain and twill cannot take reasonable loads. Not necessarily the main scantling forces, but 'skins' that are still able to outperform the glass equivalents.

    Theoretically carbon is 8X stiffer (as whiskers) - before it goes in a resin matrix, than wood/steel/aluminium, hence the 5X guide but under perfect conditions. My own rule of thumb for hand lamination is 3 to 3.5X but I haven't had to submit for classification or external verification. Equally I have not had a performance failure either in the relatively small number of bits/parts built. I know I'm sticking my head over the parapet....;)

    Completely second Groper and others about making a sample to get a feel for this material, Also as per Dougs post, it has razor sharp edges prior to trimming, take care of your fingers. It also dulls tools very fast, like bandsaw blades, planes etc - very abrasive.

    The biggest mistake people make with carbon IMHO is to think because it is stiff that a much smaller thickness of a component is required. If I have interpreted this right, you ask will 2mm of carbon be better than a 10mm woven roving one. Well the answer is no, but two 1mm skins of carbon over a lightweight 7mm or so core would be 'better' ie lighter and as stiff. Hard to fully say as the exact materials need speccing etc. The key thing to remember is that section size is still the most important part
     
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  5. Fanie
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    Fanie Fanie

    Hello Sukisolo,

    That was roughly what I expected, and of course nothing like structure features to improve stiffness. Failure usually is due to warping and bending and seldom because one exceeds the material capabilities.
     
  6. groper
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    groper Senior Member

    Sigh.... Here I go again with another lecture....

    Firstly, you will never find structural scantlings of any professionally designed boat larger than 7m, done with a woven carbon. You will also never find the same said, for a carbon laminate without a core material between the thin skins of carbon. Carbon boats are sandwich boats, period. You will never find a solid carbon laminate being used for structural hull scantlings, unlike a solid glass laminate you see on so many production boats.... Why? Carbon is extremely expensive, only a fool would use it anything but the most efficient manner possible. This is in a manner which it's high modulus and tensile/compressive strength can maximised, so the total quantity of it can be minimised ... Simply really... If you don't use it like this, you are simply wasting money, arnt you!

    Now if a fool comes along, and thinks just because he is using carbon, that he will automatically end up with aerospace standard strength to weight ratios, then he is an ignorant fool. There are no "rules of thumb". You can engineer a sandwich laminate with eglass and acheive a result. You can do the same with carbon, but it will be lighter and may have a different core material or core thickness to achieve the same result you required. Each time you are looking for a result, you need to recalculate the entire sandwich mathematically. Trying to wing it or guess, will result in you wasting money, or not meeting your required result, or delivering a result in carbon, which is inferior in weight and strength/stiffness to a well designed eglass structure.

    With carbon, you've gotta do the math or be a fool...

    image.jpg
     
  7. Fanie
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    Fanie Fanie

    Lets see some more of them.

    I'll give the first hint. If you do a 45o layup you cut material double the diameter to fit side by side as you wound the part. For a 100mm dia hence you cut material 200mm wide, for a 200mm you cut material 400mm wide etc.
     
  8. groper
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    groper Senior Member

    And here's a second hint;

    image.jpg
     
  9. groper
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    groper Senior Member

    Oops, you'll need this too;

    image.jpg
     
  10. Fanie
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    Fanie Fanie

    Layup ratio for a stayed mast... I got it from someone this morning -
    Uni 60%, 45o 30%, rest 10%
     
  11. SukiSolo
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    SukiSolo Senior Member

    Absolutely, if you read carefully what I said, nothing is contradicting your wisdom on the subject, or published data on tensile/compression etc. However those tables are mostly for 'perfect' specimens and in standard test procedures.

    Maybe there are tables (Gpa values etc) of less than perfect layups etc but so far I have not seen them, so prefer some caution. I prefer to test samples if necessary, or prototype. I do not expect to achieve aerospace standard layups by hand even if correct resins/ratios etc are applied. Difficulty achieving perfect post cure cycle for one thing at high enough temp. There are other ways to construct things too, like not using a monocoque, and this does open up to other modes where differing combinations of fibres may be more advantageous.

    Same hymn sheet really.
     
  12. groper
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    groper Senior Member

    If you read the data sheet, you will see that the tensile strength of eglass uni directional exceeds that of woven carbon. So this is a case in point, if you don't understand what fabrics you are using and how to use them, you'll be up the creek without a paddle and a much lighter wallet...
     
  13. SukiSolo
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    SukiSolo Senior Member

    Well said groper.

    Thank goodness there is such a wide range of cloths and tape types available to give the option(s) on construction. At least one of the UK mast manufacturers does use plain weave carbon in specific areas of spar construction, along with primarily uni directional and low angle bias uni. I'm going to assume the spar engineers got their sums (and test specimens) right, so a reasonable example of mixing cloth types to get specific load engineering at that point. Noted also are specific point or area doublers etc to spread loadings ie hounds, gooseneck, lowers etc.

    Carbon is as you say not cheap, and never likely to become so for manufacturing reasons. There has been and still is a lot of engineering going into very specific weaves for certain applications where ultra light weight and stiffness is important. Hence mixed cloth types and custom weaving offer a number of possible structural solutions.

    My own take on some carbon stuff is not dissimilar ie why do it in expensive carbon when another material let's say E Glass or whatever may be a better alternative?. Maybe I just prefer a reasonably 'honest' use of material in that the product ie boat or other is actually better in some way from it's use. I have some issues with a carbon fibre sink (household) for example. It has been done, but it does not appeal to me. There was a bath shown recently on a TV programme in clear finish carbon - £16k and the architect/designer of the house fell in love with it, but fortunately got the (nicer) white f/glass version instead. Don't even get me started on the fake vinyl versions etc :D

    Each material has its' own virtues and vices, so it gives us something to argue about....;)
     
  14. groper
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    groper Senior Member

    [​IMG]

    The reason for using woven, again, is for cosmetic purposes. The final layer often is a woven, but beneath it is all the non crimp fibers doing the lions share of the structural work. If you check the data sheet, you will also also see that the woven carbon is approx 2.5x less tensile and compressive strength at the same fiber volume fraction compared to non crimped fiber...
     

  15. dinoa
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    dinoa Senior Member

    The published values of CF are difficult to reproduce by homebuilders. To consistently approach 60% fiber ratio is difficult even with bagging or infusion. For wet hand layup say 40%. A safety factor of 2 would also be prudent. So, when using Hexcel published values for say CF uni 55% volume fraction tape of 1100 Mpa tensile you might find that even after vacuum bagging and post curing, test coupons will fracture at 500 and usually with a wide variance between samples. So before finalizing the design do some testing then apply the results with a safety factor of 2. The same would apply for glass but published values are usually easier reproduce.

    Dino
     
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