Double bias function in sandwich panel

Discussion in 'Boat Design' started by lyqqmxm, Mar 16, 2014.

  1. lyqqmxm
    Joined: Jan 2008
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    lyqqmxm Junior Member

    hi all

    i have a question, why the double bias is sometimes recommended for use in sandwich panels. in my point, it exhibit low values for σ and high values for τ compared with 0/90°. :confused:
     
  2. michael pierzga
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    michael pierzga Senior Member

    What is double bias . 0 x90 ....45 x 45 ....
     
  3. waikikin
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    waikikin Senior Member

    Generally Double Bias is 45/45 & Bi-Axial 0/90 in line with the usual axis of the weft & warp..... often interchanged though, creates some confusion:eek:

    Jeff
     
  4. Steve W
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    Steve W Senior Member

    One thing ive learned over the years is that almost anything works, and its amazing how little glass is needed to satisfy the structural requirements in a panel, my preference is to use all uni instead even though it is a little harder to handle, I like that there is no need for overlaps and you have continuity of fiber for the full length and with of the panel and you can orient at any angle you choose. With either DB or biax you need at least a 50mm overlap. Some folks will, if they are using more than one layer, will use both DB and biax, essentially ending up with a quadaxial with equal strength in all directions like metal but often you don't need this. I remember when the Gougeons were building Adrenalin, after testing they found a 0-90 orientation to be more efficient. I love DB for repair work but prefer Uni for new work, the bonus is its not expenesive.

    Steve.
     
  5. P Flados
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    P Flados Senior Member

    The most important considerations for a layup schedule is understanding the expected loading directions and understanding the amount of fiber needed to deal with the loading. For stiffness and strength, you get the most out of fibers that are in pure tension.

    If an end supported beam is only exposed to tension, compression and bending, having the bulk of the fibers oriented from one supported end to the other is best.

    Take the same panel and tell me that the primary loading is bidirectional torsion and I will that you need most of your fibers oriented in overlapping layers at 45 ° to the previous beam.

    If you tell me that this same beam is loaded in single direction torsion (think prop shaft for example), I will tell you that a spiral wind around a solid core with all fibers designed to be in tension is best.

    Now for each of the above, you may need some low percentage of fibers at 90° or 45° - 45° to the bulk of the fibers to avoid splitting concerns.

    If the loading is totally mixed (sometime primary stress is at a 45° to what it is at other times) and I will tell you that either a 3 axis cloth or layers of cloth that include 50% 0° -90° and 50% at 45° - 45° are probably best.

    If you intentionally want to reduce stiffness, you actually do the opposite of the above. Try to get the fibers all at 45° - 45° off of the axis that is intended to bend. This how they make steel wire mesh reinforced high pressure hose and the fiber orientation is done specifically to allow bending the hose.
     
  6. philSweet
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    philSweet Senior Member

    The purpose is to approximate an isotropic material, such as a sheet of aluminum, with a laminate .

    If buckling is a concern, specifically shell buckling, the best that you can do is produce a laminate that is isotropic - has the same properties in all directions. Balanced 0,+-45,90 buckles at about 97% of an equivalent isotropic plate assuming you have at least 6 layers of everything, so isotropic in all three dimensions. Triax is still pretty good if you can get enough layers. So you begin by taking care of the buckling requirement, then, if you still need additional strength along a particular direction, you can add some uni. That's six layers for each skin, by the way. The buckling strength vs an isotropic plate drops quickly with fewer layers.

    The biggest problem is usually trying to match the core properties to the skin properties. The thinner the skins, the denser (stiffer) the core needs to be to resist point loads and abuse that tend to initiate failures.

    The other point to note is that bucking is not the same thing as failure. Buckling is a divergence where the load continues to be supported, but the surface has suddenly deformed. It only depends on the modulus of elasticity, not on the strength of the material. If the laminate then fails due to the stresses encountered by that deformed shape supporting the load, that is an entirely separate matter that is governed by the material strength. The slenderness ratio generally governs whether or not a panel will collapse after it has buckled. Short columns have a high buckling resistance, and will tend to collapse after buckling. Slender ones hold a much smaller load, and are more likely not to collapse under the minimum load that causes buckling.

    <edit> Laminates can not be isotropic in all three directions. What I should have said is that if you have enough layers, you can at least represent the material with bulk properties in all three directions. At best, the laminate can only be isotropic in the plane of the sheet.
     

  7. lyqqmxm
    Joined: Jan 2008
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    lyqqmxm Junior Member

    Thanks for your answer. In ISO 12215-5 or other rule, the requirements for single-skin plating and sandwich plating are checked in the two principal panel directions corresponding to the long side and the short side plate. How check other directions such as +-45.
     
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