Biaxial Glass Cloth strength

Discussion in 'Materials' started by mrdebian, Apr 27, 2021.

  1. mrdebian
    Joined: Apr 2021
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    mrdebian Junior Member

    Hi all,

    I'm using in a kayak hull/deck among others 1 x 320g Biaxial Glass Cloth and 1 x 195 Twill Woven Glass.
    Twill is been a real pain to adapt to the difficult surfaces of the deck without making bubbles and I'm looking to replace it with something different.
    If I replace the Twill and use 2 x biaxial instead would it be a problem in terms of strength? Between those layers will be chopped mat.

    Thanks
     
  2. Blueknarr
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    Blueknarr Senior Member

    Welcome
    Odd,
    I've found twill to be easier to lay down than unwoven bi-axial.
    Unwoven is usually stiffer than woven of the same weight.
    Shouldn't be a problem replacing twill with bi-axial if the weights are the same.

    Are you using a bubble buster roller? A back and forth motion will actually force air under the cloth.
     
  3. mrdebian
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    mrdebian Junior Member

    @Blueknarr yes I do use bubble buster roller but the shapes are very difficult to use it.
    The problem with the twill I got is that the "space" between each fabric is very tight and resin can't be applied easy. I hope you understand what I'm trying to explain as I'm not a native English speaker.

    Thanks
     
  4. Dejay
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    Dejay Senior Newbie

    Maybe the fiberglass you've been using doesn't have the right sizing for the resin you're using?

    I also heard the tip to brush epoxy on the wood first and letting it get tacky before adding fiberglass. Because the wood otherwise can "soak up" the resin and leave the fiberglass dry.
     
  5. mrdebian
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    mrdebian Junior Member

    It is not epoxy nor wood. I am using a mold and polyester resin.
     
  6. rxcomposite
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    rxcomposite Senior Member

    Twill is better than plain weave or basket weave variety as it drapes well compared to the two I have mentioned. It is usually calendered to make it flatter, hence making it more uniform and porous free. If you have trouble with bubbles, use metal rollers instead of short nap hair for laying. Persistent bubbles can be burst with a sharpened nail attached to a wooden stick.

    Biax would be a different problem. It is usually large diameter fibers stitched together at +45-45 degree angle. It is stiff. Light mat is usually added to promote adhesion in between layers. There are other type of biax that is woven and does not need CSM but is more expensive.

    Never rely on biax alone. Its strength diminishes by about 90% on the 0 and 90 degree axis with the stitched variety. Woven, you get about 45% remaining strength. Biax is used to control torsion, not longitudinal strength.
     
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  7. mrdebian
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    mrdebian Junior Member

    UPDATE: I've corrected the links and image

    @rxcomposite thank you for the detailed answer.
    I'm using the following twill cloth:

    [​IMG]

    It's specs can be found in this link.

    To take the shape of the deck where the fittings are is impossible without bubbles unless you put a ton of resin. Using the stick you suggested might do damage to the gelcoat?

    Most of the people are saying that twill is easier to put but this is not the case with the above so I'm thinking that a different size of it might be easier?

    Any help is very much appreciated.
     
    Last edited: May 1, 2021
  8. rxcomposite
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    rxcomposite Senior Member

    Looks like plain weave to me. The specs says plain weave.
    Twill skips two to three tows. It is loosely woven so it is easy to drape.
     
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  9. fallguy
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    fallguy Senior Member

    can you explain this a bit more? I don't understand how this can be...for example, if we take a 6 oz woven tape and double it and apply it to a seam, there are 6 ounces of tows crossing the margin, for a biax tape, if we used a 12 oz biax tape, all 12 oz of tows cross the margin, which is twice as strong, unless there is some lost through the angles

    now, if you mean on a flat plane, I might understand that better, but I have still always understood wovens to be weaker by nature of how they are laid and the direction changes of each tow crossing the 90s

    for sure I have a bit to learn and reason to get away from the 45/45 love affair here; thanks for any reply


    ps-that is a plain woven based on what I know
     
  10. fallguy
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    fallguy Senior Member

    The reason the fabric is bubbling is you are demanding too much dimensional change from it.

    If it won't lay down before you apply resins, it won't lay down with resins.

    You ought to offer a picture of the mould or shapes and people here will help you better source.

    Also, darting may be needed. If you are using multiple layers of the same glass, darting the same way is probably a problem, but another way to deal with it is a full cut and overlaps which should not be an issue. Except lots of overlaps is another type of problem; perhaps. And overlaps cannot be same places..etc.

    I am making a logical leap here, but you are demanding too much of the cloth.

    also, darting in glasswork does not require removal of the dart, generally, it is just sliced and laid over
     
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  11. rxcomposite
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    rxcomposite Senior Member

    Correct. If it won't drape due to complexity of the curve, slit it, then patch. Same technique with CSM. If it won't drape, tear it then patch.
     
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  12. rxcomposite
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    rxcomposite Senior Member

    It is standard calculation procedure based on Classic Lamination Theory or use of the standard Engineering Constant. It's use is sometimes covered by most class societies and even ISO. That is, as you rotate the angle of load from the direction of the fiber, you reduce the strength. Unis are the most notorious.

    Easy to experiment. Take a uni and pull the fibers on both ends. It is very strong. Now just clamp half of the sides and pull the opposite sides at an angle. It separates easily.

    Woven will exhibit higher strength at off axis load. The weave will tend to catch the the fibers being pulled due to entanglement.

    Unis are very strong, sometimes up to 3-4X the strength of the WR but will not tolerate off axis load. WR with a lot of undulations in the weave of the fibers are weak initially but more forgiving in the off axis load due to entanglement.

    To complicate matters, there are many types of weave. Random, Plain, Basket, Twill, Stitched, 3D Woven, and Pure Uni but glued together. There are seven classification I know off according to Schoefel.
     
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  13. rxcomposite
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    rxcomposite Senior Member

    No. Strength will be the same as it is a unit of measure. Doubling it would double the load capacity. Strength remains the same.
     
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  14. fallguy
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    fallguy Senior Member

    ps...bubbling is actually bunching the way you are describing things and there is too much fabric in the areas where you are experiencing trouble

    a shortage of fabric results in bridging and air behind the bridges

    an excess results in bunching which cannot lay down and appears as bubbles

    when you tear or slit the fabric, make sure you understand which is which as you don't slit a shortage and you wouldn't slit bunching at each bunch, but across the bunches and perhaps then in the middle, etc.

    I am sure rx has much more knowledge as to the hows. I understand, but guess at the hows.

    Changing the glass will not do enough if the fabric is not laid properly to deal with the shape. Conceptually, laying the shape out flat in your mind may point to the obvious problems where darting or reliefs are needed, or even removal of a section and replacement with a tad smaller piece as rx suggests. Edit(a tad smaller would not include required overlaps...I simply mean a part that will not bunch)

    I don't believe the fabric change will help as wovens and biax are relatively decent to shape.
     
    Last edited: Apr 29, 2021

  15. rxcomposite
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    rxcomposite Senior Member

    The rule of thumb is 1/4" radius for 90 degree bend for lightweight cloth and 1/2" radius for heavy cloth for it to fold properly or bridging will occur. Fill up the corners with pre wetted rovings or just use plain old bog, If all else fails, we employ a bubble buster or chaser. He does not get a break if bridges keeps showing up as the resin runs while it cures.

    If the mold is complex or has mounds with soft edges, fit the fabric first then cut and paste the patch. WR is workable, it just looks ugly with lots of patches.

    If the mold is highly compounded, the fabric will not drape well as it does not stretch. Here narrow widths and plenty of overlaps will work. Fit it dry. Just don't expect a uniform overlap. It could be 1" at start and be 2" at certain areas. Time to use scissors.
     
    Last edited: Apr 29, 2021
    fallguy likes this.
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