CF/FG/CF layup?

Discussion in 'Boatbuilding' started by BMVozar, Jun 8, 2016.

  1. BMVozar
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    BMVozar Junior Member

    Hello all, I'm new to join but have been reading this site everday for a year but maybe I skipped something.

    Can someone explain why fiberglass, lets say s-fiberglass triaxial cloth or what have ya (something strong), isn't found between two carbon fiber layers? I'm thinking more along the lines of as almost a coring material to separate the layers.

    I understand materials flex differently, but so do soft foam cores between fiberglass? Would this not result in an incredibly light but hard boat?
     
  2. TANSL
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    TANSL Senior Member

    I'm not sure I understand well what you propose.
    The inner layers of a laminate, near the neutral axis, have to withstand a tension / compression very low compared with that support by the outer fibers. Therefore, put a material very resistant to bending stresses in the center, as core material, makes no sense. The core is a material, from the point of view of the flexural strength, useless. The core only needs to support shear and get resistant fibers are the farthest possible from the neutral axis.
    If I misunderstood what you propose, I beg you to forgive me.
     
  3. BMVozar
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    BMVozar Junior Member

    TANSL, no you responded to it perfectly!

    I understand that distance of the outer resistant layers increases the stiffness, but I guess I'm asking, why are there pure carbon layups without a coring? Or if one were to use carbon in three layers, from an economical stand point wouldnt the best middle layer be fiberglass to replace the middle carbon layer? Or then by the same token, would a fiberglass/carbon/fiberglass layup be better?
     
  4. TANSL
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    TANSL Senior Member

    If one were to do that, and I see no reason to have to do it, I would prefer: carbon / glass / carbon. As I said, the central layer need not be as strong as the other two.
     
  5. vkstratis
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    vkstratis Marine Designer

    Dont confuse single skin laminates with sandwich ones. The reason to use a sandwich (cored) laminate is to increase stiffness of plates (increase unsupported span) with less weight and (probably) cost of materials/labor. The most common use of sandwich plates is decks. Imagine having to achieve a required thickness of 10,15 or 20mm with a single skin laminate. It will be very heavy opposed to a sandwich composed of 3-4mm skins and 12-15mm of a lighweight core material. The skins must be able to resist to compression and tension while the core must be able to resist shear. For example foams of adequate density or honeycomb panels can fit the job.

    The use of non e-glass reinforcements is another story. They can be used in single skin or sandwich laminates depending the requirements.
     
  6. BMVozar
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    BMVozar Junior Member

    Thank you for your response vkstratis!

    Can a carbon fiber/glass/carbon fiber layup be seen as one of the layers in a sandwich? Say two of those layers separated by foam? What are the effects of using a sandwich composite as the skins for a foam composite sandwich?
     
  7. vkstratis
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    vkstratis Marine Designer


    I am not sure I fully understand your question but nevertheless I will try to comment.

    Typically any laminate that consists of thin (relatively to the core) skins and a core is a sandwich laminate. You can have any resin with any reinforcement for your skins and any material for your core, depending on your requirements. Remember though that the benefit of using a sandwich laminate over a single skin one is that you can achieve great stiffness with less weight. So, as TANSL mentioned earlier there is no point using a material with great resistance in compression/tension as a core. You only need core shear strength, plus good interlaminar shear to transfer skin compression/tension through your laminate.

    Having said that, a e-glass/carbon/e-glass laminate will not be a "sandwich composite". It will be a "single skin" laminate of alternate layers of different reinforcement materials. Typically you don't mix those in a laminate for several reasons. They are different materials for different applications used with different resins for optimum results.

    Using a sandwich composite as skins for a sandwich (foam cored) composite will not make any sense. You can have cores up to 30mm thick to use with appropriate e-glass/polyester skins that can be used. I dont see why you could need more thickness. Bear in mind that plates dont stand alone in any structure. They can be supported by stiffeners, that you have to put into your design.

    hope this helps.
     
  8. Stumble
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    Stumble Senior Member

    You absolutely could, but it would not be an effective use of materials. If you had a few different sandwiches all the same thickness

    1) CF-fg-CF-foam core-cf-fg-cf
    2) Cf-cf-fg-foam core-fg-cf-cf
    3) cf-cf- thicker foam core-cf-cf

    1 and 2 are the heaviest
    2 and 3 are the stiffest

    So since 3 is both the lightest and the stiffest. Why would you choose to use either 1 or 2?


    There are other good reasons not to bond CF to FG, they have such different stiffness the carbon will break before the fiberglass will absorb any of the flex. Basically it's like using a dyneema halyard and a bungie cord to hold up a sail. If you add any load to the sail the dyneema prevents it from moving, the bungie cord is just there doing not much.


    So why would you ever use a solid carbon piece instead of a sandwich? Primarily when you need as small a profile as possible. Instead of weight being the controlling factor, when you need something as thin as possible as a given stiffness then you switch to monolithic carbon. Something light rudder blades on high speed catamarans for instance. You may want them as light as possible, but far more important is making them as thin as you can at the requires stiffness.
     
  9. BMVozar
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    BMVozar Junior Member

    Wow! I was not anticipating such abundant and quick responses thank you!

    Now, does "stiffness" mean the same as "puncture resistant? I guess that's why numer 1 in STUMBLE's response would be selected? A sandwich composite seems a bit of an "eggshell" problem. Those layer can be thin and take a "bending" impact but what about a sharp rock or ,hopefully never encountered, another boat? I realize some companies incorporate kevlar whether that be to hold together the layer that is in tension in the sandwich or to prevent the object from puncturing by directly "catching" the object, I'm not clear on.

    I appreciate the patience and information! I really love the engineering that goes into boats and am just trying to grasp all avenues and recent developments with composites.
     
  10. daiquiri
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    daiquiri Engineering and Design

    So, to expand a bit more what Stumble has correctly said, from a more technical (mathematical) point of view. You have to pardon me if you already familiar with these concepts - in that case consider it as a short refresh of what you already know. :)

    Every time you have two materials which are arranged in such way to work in parallel, as in this case, they have to elongate by the same amount.
    Both glass-fibre and carbon-fibres behave according to the so-called Hooke's Law, which means that the relationship between stress (Sigma) and strain (Epsilon) is linear. In one-dimension it is expressed as:
    σ = E ε (eq.1)
    or
    ε = σ / E (eq.2)​
    where σ is the force per unit area of the material, ε is the elongation expressed as a percentage of the original length. The multiplier E is called Young's modulus (or just Modulus) and is a measure of material's stiffness.

    So, when you make two layers of different materials work together, the elongation of the two has to be the same. Layers are glued together and ideally there is no slippage between them (in practice there is some, due to elasticity of the bonding resin, but I shall ignore it here for simplicity of the math), hence the elongations of the two layers are identical.
    By using the suffix "g" for glass fiber (GF) and "c" for carbon fibers (CF), the same-strain condition is expressed as:
    εg = εc​
    hence, from the eq.2:
    σg / Eg = σc / Ec​
    or
    σc / σg = Ec / Eg (eq.3)​
    Now let's plug in some numbers. From this table:
    Fibers.gif
    we see that modulus of S-glass and high-strength carbon fibres are:
    Eg = 86 GPa
    Ec = 230 GPa
    hence, the eq.3 gives:
    σc / σg = 2.7​

    In other words, in a CF-GF laminate, CF will carry the 2.7 times the stress of GF. It means that GF will carry just 27% of the overall stress acting on the laminate.

    But that's just a part of a bigger picture.
    Again, from the table, the breaking (ultimate) stress of GF and CF are, respectively:
    σu,g = 4.58 GPa
    σu,c = 3.54 GPa​
    When the stress of the CF layer reaches its tensile stress (3.54 GPa), the laminate will reach an elongation of just 1.5%. GF will elongate by the same amount, for the reasons seen at the beginning of this explanation.
    But how much stress will the glass layer carry at 1.5% elongation, when the carbon layer breaks? The answer is given by the eq.1:
    σg = Eg ε = 86*0.015 = 1.29 GPa​
    In other words, just below the breaking point of the CF, the GF will carry 1.29 GPa, which is just 36% of the ultimate stress of the glass.
    Hence, a moment before CF start failing, the GF shall still have a safe margin of 2.25 GPa.

    So, by including a glass layer, and by properly taking into account the thicknesses of two layers, a post CF-failure structural resistance can be included in the design, which can be useful for some applications. It is a sort of artificial "plasticity", similar to the plasticity region of the stress-strain curves of metals.

    The load-elongation curve of such simplified hybrid laminate becomes qualitatively similar to this:
    CF_GF Laminate.gif
    The failure of the CF layer occurs along the horizontal constant-load curve, until all the load has been taken over by the GF layer. From there on, the laminate behaves like a plain GF laminate (with thickness reduced due to the interrupted CF layer).

    In order to obtain a curve like the above one, where ultimate load is just above the "yield" load (and hence can maintain the structural integrity after the CF layer has failed), the percentage of CF fibers was set to approxim. 0.1 . If the CF percentage is too high, the "yield" load of the laminate could become higher than the ultimate load, so the structure would fail immediately after the failure of the CF layer.
    Such situation would give a curve more similar to this one:
    CF_GF Laminate 2.gif
    - in which the role of the GF is nearly useless.

    It should be noted that the price to pay for this "plasticity" safety feature is a lower CF+GF laminate modulus (hence higher deformation under a given load) when compared to a pure CF laminate, lower maximum load and a higher weight/load ratio.

    The above explanation is simplistic and does not take into account for the stress-redistribution done by the resin matrix and for the fact that not all fibers will fail at the same time. A realistic stress-strain curve of a hybrid carbon-glass laminate will not have such sharp corners and will be non-linear. There will be a region of gradual transition between CF+GF to GF-only area of the curve. Check this paper for more info and some measured data: http://www.sciencedirect.com/science/article/pii/S1359835X15000664
    This is what an empirical (measured) stress-strain curve of a hybrid CF/GF looks like (taken from the above article):
    Test CF_GF laminate.gif

    Another reason to use the combination of CF and GF is the enhanced fatigue capabilities of the resulting hybrid laminate, when compared to a plain GF laminate. See this paper for more info and some test results:
    http://www.enggjournals.com/ijet/docs/IJET14-06-01-082.pdf
    http://www.montana.edu/composites/documents/AIAA 67056.pdf

    Cheers
     
  11. Stumble
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    Stumble Senior Member

    Nope, I tried to answer, but it's beyond my knowledge. Maybe one of the guys who really understands this stuff will chime in.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Resin systems and fabrics are best if modulus matched, otherwise something gives before something else and it's a weak link situation, which means one of the laminate elements (resin or fabric choices) will fail, before the other elements within the laminate give, which is self defeating. In other words, there's no sense in using a high elongation modulus fabric, if other fabrics or the resin choices aren't equally up to the tasks. It's just a waste of usually expensive materials.
     
  13. daiquiri
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    daiquiri Engineering and Design

    In general it is as you said, but sometimes it is desireable to have a material which gives gradually and has a degree of ductility. So CF/GF combinations are used in order to fit the stress-strain curve of the hybrid material to requirements of the design, when the design requires such feature.
     
  14. BMVozar
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    BMVozar Junior Member

    Excellent information all around thank you!!

    Daiquiri thankyou for diving in with engineering formulas! I'm in an engineering program now and we just discussed those formulas actually!

    It makes sense that different moduli will result in different layers taking certain percentage of the load.
    That really cleared up some random thoughts I had. I suppose I was wondering if fiberglass would offer a flexing component to absorb blows in a carbon shell as a "core". I understand this is considered a single skin, but separating two carbon layers by any distance even if just layer of FG sounds like a sandwich core to me.
    Forgive me if I'm droning, but different moduli of material limit our ability to combine layers because one will give before the other? But is that to describe a CF/GF scenario, or does that consider as well a CF/GF/CF. As I understand it the carbon will be taking the majority of the load, but what if the fiberglass wasnt intended for strength but more to increase the distance of the carbon layers to increase stiffness. Separating two layers by a distance increasesstiffness exponentially, no? So wouldn't it make sense two separate twocarbon layers by fiberglass?

    Please bear with me! I appreciate all the responses and just like bouncing ideas!
     

  15. TANSL
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    TANSL Senior Member

    Although what I will say is implicit in previous post, I'll try to give a different approach.
    When scantlings for PRF elements, plates or stiffeners, are calculated, it is required to study the stress distribution widthwise thickness. Ie calculate the stress that supports each layer (each layer is considered a different material with different mechanical properties) and find out the relationship "actual stress / maximum admisible stress" for each layer. That relationship will call "compliance factor". The closer to unity is this factor, the better we will be using the material.
    The total thicknesses are a secondary issue because, although the rules speak of minimum thickness, these are almost always met. But it may happen that the panel, for example, with a huge thickness not meet at all with adequate distribution of stresses.
    A good designer of the structure must therefore ensure that the compliance factor of the various layers is as similar as possible (and near to "1" if posible) and also very important, which is distributed symmetrically about the neutral axis.
    This try to get the outer layers and interior are loaded similarly and that the elongation of the adjacent layers are the most similar as possible.
    See a study in the accompanying figure. This is an initial study, unoptimized
     

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