Fiberglass/Epoxy Sandwich Properties in Bending

Discussion in 'Boat Design' started by fredrosse, Apr 12, 2018.

  1. fredrosse
    Joined: Jan 2005
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    Location: Philadelphia PA

    fredrosse USACE Steam

    I would like some help with conventional wisdom here, with respect to reasonable assumptions for composite sheet material in typical hull construction. Say 1/2 inch thick plywood, with 10 oz cloth and epoxy making up a coating about 1/20 inch thick on both sides.

    Considering this sheet of plywood (or other core materials capable of enduring the shear flow), with fiberglass cloth/epoxy coating each side, and treating this as a conventional composite beam, is it reasonable to assume the thin epoxy has such a low modulus of elasticity that only the glass fibers play here? I know to be rigorous in the analysis here that both the glass and epoxy could be considered, but if the glass fibers are contributing the lions share of tensile forces resisting a bending moment before significant stress forces are developed in the epoxy, then the epoxy can be neglected with only moderately conservative results?

    I assume that the cross section of the glass fibers would credit only the actual glass strand sectional area multiplied by the cosine of the fiber orientation angle with respect to the bending stress direction. Is that the way this issue is usually considered?

    For this type of configuration, what should the effective elastic modulus and allowable working stress values for the glass fibers in pure tension?

    PS: I am not a student doing homework, just a 71 year old engineer who wants to dust off some structural knowledge. Thanks in advance for some information here.
  2. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    You're on the right track.

    It is not the Young;s modulus, but the tensile strength and the shear strength that is important.

    The beam you describe, under the application of a load, the outer fibres (whether composite/steel/ally) will take the tensile and compressive loads.
    The generic tensile value of epoxy (varies but close enough) is roughly 85MPa.
    Whereas if you look at the tensile strength of Glass ranges from 100 - 1600MPa.
    These are in laminates....since pure fibres are in the 2-3GPa range!

    Thus the fibres do the work. The epoxy is basically just there to hold it all together!

    A typical WR will be circa 250MPa and bulky GSM will be low down, circa 100MPa.

    In compression typically epoxy is circa 130MPa and WR 200MPa, CF can be, in the right layup circa 1500MPa!

    Thus in compression the epoxy helps some, glass is not known for its high compression strength. But of course these values vary greatly depending on the type of glass and epoxy. Hence just 'generic'.
  3. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Um, no, it isn't. This is where things get really complicated. For tension along the fibers, you basically want the highest glass content you can manage for the lightest and cheapest laminate. But for compression or any stress at an angle to the glass fibers, there are shear stresses that have to be born by the resin. And for the most part, the toughest laminates won't be ones with the highest glass content. In some composites, even a 2 degree compression misalignment can cause resin related failure modes and premature FRP laminate damage.

    For a skin as you described, the plywood substrate affects the shear deformation. Basically, the laminate can transfer the shear load to the plywood depending on the plywood species and grain orientation. This works better (a lot better) if the glass fabric is at a bias to the surface veneers. So treat the plywood as part of the laminate and run successive layers at a bias to the previous ones, and use a high modulus plywood such as meranti.

    Reputable suppliers of composite fabrics will have test data using an assortment of resin systems, manufacturing techniques (hand layup, vacuum bag) and laminate schedules (although probably not for a single layer of fabric over plywood). A more difficult issue is getting the relevant data for the plywood properties.pdf. Personally, for 10 oz weave over 1/2 marine ply, I use the laminate skin to toughen the panel and improve the weakest aspect of the plywood to try to create a more isotropic plywood panel. So I run the laminate fabric at a bias to the 0, 90 ply veneer geometry.

    Plywood tends to have a stiff bend direction and a weak bend direction. If it's really well made, or if you tell them to build it this way for you, you can get the plywood to be nearly the same in bending longitudinally and cross panel. This is done by using thinner longitudinal veneers on the outer skin. The point is, if the bend is significant and oil-canning isn't a concern, to not try to bend the plywood in the hard direction when building the boat. The panels will try to relax by bending in the easy direction instead. You can use the laminate fiber orientation to ensure you are bending each panel the easy way when you put it on the boat. This lets you nest the plywood pieces efficiently and then tune them with a layer of glass so they want to bend the way they will be bent on the boat. This is particularly handy for frameless stitch-and-glue builds. Now if it's just a gentle bend such as 3mm ply over some straight deck beams arranged to form a cambered deck, you need to maximize the stiffness across the spans to prevent hard spots and oil-canning. In this case, you can afford to exaggerate the anisotropic properties of the plywood.
    Last edited: Apr 14, 2018
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