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Old 07-16-2004, 12:15 PM
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Carbon fiber mast

I would guess that most people that tinker with frpís have considered building a carbon fiber mast. My question is, how would having a pvc foam core effect the mast. I would think that a foam core could be made much cheaper than a mandrel for people planning to build a single mast and a foam core would provide flotation for dingies and beach cats so that they donít tend to turtle when knocked down. How would the core affect the mast structurally? Would it be rigid enough to be of value structurally? The round or nearly round shape of the mast makes it a much more complex problem than with flat areas like a deck. Since the failure mode would be in compression and there would be significant deformation before failure I think we would need a nonlinear finite element analysis to determine how much a core would affect the rigidity and strength of the mast. Has anyone done this kind of analysis or is there anyone on this forum that has the software to do this kind of analysis? Anyone have any thoughts on this?
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Old 07-16-2004, 06:12 PM
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SailDesign SailDesign is offline
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Your problem may be in fitting the core to the tube after it is built
The reason aluminum mandrels are used is that they are stiff enough and strng enough that the uncured carbon does not go out of shape. if you use a PVC foam mandrel, your stick is likely to look like a pretzel when done.

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Old 07-21-2004, 06:32 AM
Karsten Karsten is offline
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The foam core would stabilise the carbon skin and therefore increase the wrinkling strength of the mast. Wrinkling is something like buckling but only locally and with a much shorter wave length. For this effect the carbon skins must be actually critical in wrinkling which depends mainly on the mast diameter and wall thickness. Guess with dinghy masts you wouldn't have a wrinkling problem because the diameter is small and the wall thickness relatively large. Therefore the core wouldn't make it better in this case.
There shuld be formulas around for the wrinkling strength of tubes. You only have to adopt them for fibreplastics and put in the properties of your mast. If it's not critical you can stop there.
FE is possible but quite complicated. You need the stiffness properties for the laminate and a geometrically non- linear analysis. To get some results it usually helps to put tiny deformations with a wave length equal to the wrinkling wave length onto your mesh. Otherwise the computer gets confused when wrinkling starts and everything is perfectly straight up to this point.
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Old 07-21-2004, 09:05 AM
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grob grob is offline
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Originally Posted by Karsten
There shuld be formulas around for the wrinkling strength of tubes.

When the compression stress in the skin reaches 0.25*E*(t/r) the tube will suffer from local buckling (wrinkling).

E=stiffness t=thickness of tube r=radius of tube

This is a very simplistic formulae but is a good starting point.

Taken from a brilliant book for non engineers called "Structures or why things don't fall down" by JE Gordon see chapter "the various ways of failing in compression" (Penguin Books). I cannot reccomend this simple book enough.
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