Surfboards consists of a foam (lightweight) core and a fiberglass (stiffness) skin. Wet hand-layup is the most commonly used manufacturing method, several layers of fibre glass fabric are laid up on the foam core and laminated using epoxy resin.
My question is about the fiberglass layup sequence for the skin. A surfboard is mainly subjected to compressive stresses (primary one being weight of the surfer). The compressive strength comes from the epoxy resin and the foam core, whereas the fiberglass adds stiffness to the skin.
A typical layup sequence used is 2-3 layers of woven fiber glass fabric (plain weave 6 oz each). My question is if the requirement is stiffness, wouldn’t Uni direction (UD) provide higher stiffness (2 plies of UD (0,90) vs one woven ply )? UD is non crimp hence stiffer and it will also have less resin rich pockets (more resin = brittle). Almost every surfboard I have seen uses woven plies, there must be a reason for it..?
Secondly, lit. reports "Placing a 45 degree ply on the surface of a laminate increases its buckling resistance". Now fibres mainly carry tensile loads whereas compressive loads are carried by the matrix, so I dont quite understand why specifically a 45 degree ply can help in preventing buckling (which is a result of compressive load).
Not an expert on surfboard design but here are some points that I can think of. You are correct that UD 0/90 would be stiffer as compared to a woven. Surfboards I believe would experience dynamic loading (from water) and would do better with less stiffer material. Since the loading is out-of-plane, weaves are generally better for these type of loading as compared to a uni-stack. Another thing might be the easy processability of woven fiber as compared to laying up 0/90.
Thanks
Aniruddh
Thanks Anirudh. Skin stiffness is a key requirement in a surfboard to reduce the flex...
Are u aware of u why specifically 45 degree plies are used on the surface of laminates to increase their buckling resistance?
I would suggest populating the [ABD] matrix and maybe try some of the formulations suggested in Section 4.7 here: (http://www-eng.lbl.gov/~ecanderssen/Composite_Design/Laminate_Design.pdf). I am not sure what literature reference you saw, but the answer would depend on end condition and aspect ratio.
Hope this helps
Aniruddh
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