A question about 45/45 stitched glass

Discussion in 'Fiberglass and Composite Boat Building' started by Tungsten, Oct 26, 2014.

  1. Tungsten
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    Tungsten Senior Member

    Why is double bias glass stiffer in one direction compared to turned 90*?
    Thinking the fibers are 90* to each other the stiffness should be the same. But take a cured impregnated piece with epoxy and its easier to bend in one direction then the other,even a dry piece is easier one way then the other.

    Edit;; to be clear on direction , bending the stich it seams less stiff.
    What am I missing?
     
  2. rxcomposite
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    rxcomposite Senior Member

    Fibers are strongest/stiffest when oriented along the longitudinal axis (0 degrees). Any deviation from the axis will cause it to lose its strength/stiffness. Even with uni's, the fibers can be misaligned (off axis by a few degrees) when stitched. When woven, the fibers undulate up and down.

    Theoretically, a Bi Directional 0/90 WR should have equal number of fibers in the 0 and 90 degree direction but not always balanced. Most of the WR is not a balanced weave and its strength in the 0 and 90 degree is not always the same if you look at the published data.

    The weave also plays an important part. If the weave is one after the other as in plain weave, the undulations are the same in both direction. In crowfoot weave or harness satin weave, the number of undulations is less in the long(warp) direction. This makes it stiff in the long direction but less stiff in the warp(across) directions where it has more undulations. This undulations are like little "springs" that contribute to the flexibility.

    If the weave is bi axial (+-45) most likely the fibers quantity is the same and equal in strength/stiffness in both direction. Pull it at 0 or 90 degree and the fabric closes in. When laminated, it is easiest to bend a bi ax at 0/90 degree where the strength/stiffness is at lowest.

    Refer to for type of weave http://www.boatdesign.net/forums/materials/fiberglass-warp-weft-question-51748.html


    Conversely, a 0/90 fabric is stronger/stiffest in its direction but lowest at +-45 direction.

    These fiber direction change, minute as it seems, plays a major part in laminate property calculations and is defined as Ultimate strength and Modulus. In practical use, you see it as you have done when flexing in several directions the panels you have laminated.
     
  3. Tungsten
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    Tungsten Senior Member

    If the weave is bi axial (+-45) most likely the fibers quantity is the same and equal in strength/stiffness in both direction. Pull it at 0 or 90 degree and the fabric closes in. When laminated, it is easiest to bend a bi ax at 0/90 degree where the strength/stiffness is at lowest.
    --------------------------------------------------------------------------
    So maybe the piece I had was pulled a little changing the axis.Maybe so but all the left over pieces I have give the same result.Even a piece of 0-90 (400)laminated to 45/45(600) gives the same result.My piece is square and its easier to bend when bending the stitching at 90*on the 45 glass.Perhaps the glass isn't exactly 45/45,although the fibers appear to be 90* to each other they could be off a hair.

    Still confused.
     
  4. rxcomposite
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    rxcomposite Senior Member

    Try laminating a 0/90 glass and see how it bend in all direction. What bi ax are you using? It is commonly coarse glass (pretty stiff), stiched at +-45 to each other. Others have a light CSM on one side to enable good bonding.

    When I am short on biax, I cut the 0/90 at a 45 degree angle and use the cut side as the 0 reference angle. Wasteful, but in a bind, it works. Hard to handle as it closes in if you are not careful in handling.

    What you did was to laminate a 0/90 and +-45 together. Theoretically, it should give you the same stiffness in all direction. Maybe not so equal because you used different weight fabric.
     
  5. Tungsten
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    Tungsten Senior Member

    Yes using normal stitched 600 g 45/45 glass no mat. I also have 400 g same no mat all of it is easier to bend when you bend the stitching ,turn it 90* and it's noticeably stiffer.
     
  6. rxcomposite
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    rxcomposite Senior Member

    It seems to be the opposite to reason and I am also confused.:confused: If you say the biax is stitched and it is at 90 degree, it should be stiffer in the 90 degree side as the extra fibers (stitches) gives it more oomph. The 0 degree bending should be relatively easier.
     
  7. Tungsten
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    Tungsten Senior Member

    Well i thought this was normal and someone could explain why.
    Only thing I can think of is the fibers aren't a true 45/45.They appear to be 90 to each other but its hard to tell as you cant see the bottom fibers.

    I'll check next time I have the full roll out,I'll cut a square piece, the fibers should go exactly corner to corner.
     
  8. Tungsten
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    Tungsten Senior Member

    Have a look at Canracers photo in his thread,the fibers don't look 90 to each other.This must be why.My glass either was off a little at time of stitching or its been pulled a little.My stitches all look straight and parallel.

    [​IMG]
     
  9. Jim Caldwell
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    Jim Caldwell Senior Member

    When using biax it is helpful to out line the oversized square you want to cut out with masking tape to keep it from pulling out of shape. You then layup the over sized square by applying resin from the middle out toward the edge, let it tack up then cut off the tape and trim to size. Apply fresh resin to the loose edges carefully, the tacked up resin will keep the fibers aligned.
     
  10. Tungsten
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    Tungsten Senior Member

    I understand that Jim,what I'm getting at is why double bias (+- 45 that's what my supplier calls it) is stiffer in one direction then the other.

    Answer: the fibers aren't exactly 90* to each other right off the roll.This explains why its less stiff when you bend the stitching.My samples are clear so its hard to see both layers,in the photo above a hand layup you can see the difference.Both rolls I have 400 and 600 came from the same place.So I guess it depends on who made the glass I guess or my supplier really pulls on the glass when the y roll up my order?

    The only answer I can come up with.
     

  11. PAR
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    PAR Yacht Designer/Builder

    The direction of the (polyester?) stitching does add some slight stiffness, but not much, so laid with the stitching complimentary to the curve will allow it to lay down better, though this is a slight difference. If it's not laying well, the radius is too tight, which would be a pretty small curve, considering how well biax does drape. Sizing may also play a role, but again this also should be slight.

    Judging by the OP's experiences, I'd thing the fabric has been stretched in one direction, which is really easy to do with biax and coming off the roll, one axis has a closer fiber grouping/orientation than the other, making it denser in this direction. Since the stitching is perpendicular to the roll as it comes off, this would explain why it's stiffer in this direction.
     
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