carbon fiber wing mast for my cat

Discussion in 'Multihulls' started by rallard, Dec 28, 2008.

  1. sailor2
    Joined: Jan 2009
    Posts: 110
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Europe

    sailor2 Senior Member

    If the non lengthwise fibres are not perfectly straight on multi, it reduces stifness and results more elongation to break. Therefore more impact strength. Isn't that a good thing on rtw multi ?
    The length wise fibers are straight and produce stiff hull in that direction ie. resist longitudinal bending in vertical plane for rigging loads in mainhull.
    An other good thing for upwind performance when keeping a headstay tight is most beneficial. Also risk of in column buckling of narrow bows are reduced due to that increase of stiffness. Remember team philips ? How about G3 ?
    Also lenght wise fibres span over much longer distance over bulkheads, so carry less load due to local panel pressure than fibres in other orientation, so this evens this effect out a little. If over pressure is applied on hull panel the fibers not aligned with fore&aft direction will still break first due to much shorter span.
     
  2. rob denney
    Joined: Feb 2005
    Posts: 749
    Likes: 110, Points: 43, Legacy Rep: 436
    Location: Australia

    rob denney Senior Member

    G'day,

    I assume just enough fibres in each direction to meet the strength and stiffness requirements, plus designated safety(fudge) factors. This will include both panel requirements and global ones.
    If you have enough fibres for the expected loads, kinking them (which is what will happen to fibres vacuumed onto a convex surface) will mean the fibres are pretty much useless.

    regards,

    Rob
     
  3. Boston

    Boston Previous Member

    Thanks for asking that one Rob
    as I was about to
    I thouht we went over fiber orientation, strength and Kinking considerations and Jim had explained why pre stressing the fibers was detrimental
    course Im still wondering if anyone has tried electrically aligning the epoxy matrix
     
  4. sailor2
    Joined: Jan 2009
    Posts: 110
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Europe

    sailor2 Senior Member

    So we may just have to agree we disagree then, as I don't see less stiffness allowing using less fibres for very same impact strenth as useless at all.
    Allthough we might assume very different amount of kink. I'm assuming just some smooth extra curvature, not sudden rapid change of direction the word kink usually references to, as that is what vacuuming over smooth convex surface results, not any sharp kinking, just extra curvature. Since I beleave we both have experience on that in practical level, I'm a little surprised on your conclusions as well. Don't you think all boats made by that method would have braken into peaces, should those fibres be useless ? A clear indication of not being useless at all.
     
  5. sailor2
    Joined: Jan 2009
    Posts: 110
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Europe

    sailor2 Senior Member

    As I already said, electrically aligning doesn't contribute anything, the chemical reaction already requires alignment, which happens already when curing takes place and all the molecules become a single big one.

    Which thread & post are you referencing to by that Jim statement above in bold ?
     
  6. Boston

    Boston Previous Member

    I believe the subject came up when I asked if pre stressing the fibers would result in a stronger end result

    it was explained to me that were the fibers cross they would become unevenly stressed as they curved around warp and weft and that the fiber was better off left in a relaxed state

    might also have something to do with the amount of epoxy able to penetrate the relaxed fibers and ones that are not relaxed

    but I think maybe I am just misunderstanding you

    are you saying that the longitudinal fibers are held in tension to keep them as straight as possible and the perpendicular fibers are not stressed but allowed to relax and gently roll around the straightened fibers
    I think that might eliminate this kinking thing we are considering if the bend in the perpendicular fibers isnt to severe and it allowed adequate epoxy penetration
    but
    it seems that is unlikely as one fiber has to do all the bending as the other is not doing any at all

    hey Eric you catching this new twist
    what do you think

    B
     
  7. Boston

    Boston Previous Member

    two posts to look at are # 48 concerning evidence to suggest aligning the epoxy matrix will result in aditional strength
    and post #37 on the prestressing question

    B
     
  8. sailor2
    Joined: Jan 2009
    Posts: 110
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Europe

    sailor2 Senior Member

    1) that might be mutual problem here.
    2a) when panel is loaded up after curing by surface pressure bending it, all fibres are taking up load. If material properties are the same in both directions, the fibres with longer span will take substantially less load than the other ones. That's the case with long hulls with small cross section. Therefore different material properties in different orientations in such cases are beneficial as that make those fibres take up the load more evenly. Alternative would be to use a lot of bulkheads or ring frames for same or beter results, but that adds extra weight.
    2b) if you'r talking about fibres bending before curing, then that's the target in hull cases to produce the different material properties, namely different elongation to break or different stiffness.

    None of this has anything to do with masts anymore as those are not loaded by external pressure (in impact nature by waves ) perpendicular to panel like boat hulls are. A very significant & important difference here.
     
  9. Boston

    Boston Previous Member

    going to go with option B
    I was talking about fibers being pre stressed to curing
    guess I should have specified that

    well I just drop in with the occasional question or idea to try and gain a better understanding as I am experienced in wood construction only

    once I get a better grip on this hole CF thing in general then Ill start thinkin masts exclusively
    but
    isnt a mast a shining example of a long *** hull with a small cross section
    might not have impact loads on it but its got more stress than my kid on prom night
     
  10. sailor2
    Joined: Jan 2009
    Posts: 110
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Europe

    sailor2 Senior Member

    I agree with all that above, pre stressing fibres in composites have nothing to do with prestressing steel in concrete matrix. Possible reasons for pre stressing are completely different !
    In conrete case the targit is to avoid cracking of matrix not capable of taking tension unlike in plastics.
    In composite case one possible reason I can think of would be targiting different stiffnes properties in cases where panels are far away from square ie. of high aspect ratio, and impact strength is the limiting factor for dimensioning. Another would be to get max stiffness due to straightness, but those benefits would be marginal over what is available from common practise methods. Different grade of carbon is very far more important on that issue.
     
  11. rob denney
    Joined: Feb 2005
    Posts: 749
    Likes: 110, Points: 43, Legacy Rep: 436
    Location: Australia

    rob denney Senior Member

    G'day,
    I suspect we are talking at cross purposes. Using masts explains my point better. Assume a mast male mould of 70mm diameter with 3mm thick of wet fibre on it, with the last layer being at 90 degrees. The circumference of that last layer is (70+6) *pi = 238.8mms. If you then apply pressure (vacuum, shrink wrap, autoclave) to consolidate the laminate, the finished wall thickness may drop to 2mm, giving a circumference for the outer layer of (70+4)*pi =232.5mm. The ends of the outer layer do not move, so there is a 6mm kink, or series of kinks in this layer.

    Where ever that kink or kinks are, the layer is adding very little strength or stiffness. If it's main purpose is to prevent the lengthwise fibres from popping out of column, it may not be a problem. But if it is a +/-45 layer and the mast exerts some torque, it is a big problem.

    This is one of the fudge factors that are politely referred to as safety factors by engineers and builders. If you have enough of this type of production problem, or don't allow enough material to compensate, things have no choice but to break.

    regards,

    Rob
     
  12. Bruce Woods
    Joined: Sep 2007
    Posts: 129
    Likes: 10, Points: 18, Legacy Rep: 90
    Location: perth

    Bruce Woods Senior Member

    Is it worth it.

    I suspect a 7 meter cat is trail-able, so a few kilos saved in the rig can make a huge difference when stepping and unstepping the rig.

    We went from a spun tapered alloy section on our sports boat to a carbon CST section and rigging/derigging became a breeze.



    I
     
  13. sailor2
    Joined: Jan 2009
    Posts: 110
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Europe

    sailor2 Senior Member

    Sure, for 76mm diam into 74mm diam is 2.63% reduction. That's a major one compared to elongation to brake and means major deformations before those fibres in hoop take up any significant load other than very locally.
    Very bad on masts needing increase on amount of hoop fibres needed.

    Do the same with assuming diam as 1000mm like on hulls, and deformations are much much smaller before fibres begin taking up load resulting just initial reduction of E-modulus on tests. That can be beneficial on impact tests with very small reduction on statical strength. In fact elongations are still so small with carbon that matrix does'n have any higher fatigue rate if epoxy is selected properly for the case.

    You can even use ballistic kevlar29 with high elongation epoxyes and intentionally make fibres not straight to increase impact strength against impacting solid objects for military purposes. In that case the deformation of plate takes place over much larger area as it would with straight fibres and as a result can absorb energy much greater amount as well. The result is similar than with metal armour when metal is exceeding yeild point but not ultimate tension limits. The impacting body does not go through, but goes past the initial surface quite a lot unlike with straight fibres. On the other hand the panel is damaged over much larger area due to single hit, and any further hits are more likely to hit already damaged areas. Not a good thing, but it increses the ultimate limit impact energy a single hit can have without penetration.
     
  14. sailor2
    Joined: Jan 2009
    Posts: 110
    Likes: 1, Points: 0, Legacy Rep: 25
    Location: Europe

    sailor2 Senior Member

    Some dinghys use carbon masts as well. Some of them stayed and others free standing. So yes, carbon masts can have benefits on small sizes, but in those cases the design costs are spread in large amount of boats. One offs might think a bit more if going CF is a good thing including financial considerations.

    Other possibility is if CF mast is made on site, but Alloy one have to be transported over long distances causing a lot of cost in some areas.
    Then CF can cost less than alloy even including both construction & design for oneoffs. It's not the same for everybody.
     

  15. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,003
    Likes: 206, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    A lot of traffic occurred here all at once. Here's my take: I have never known any builder to pre-stress the fibers of a laminate layer, either in building a boat or in building a mast. Rob Denny is right in preventing kinking fibers in masts, particularly in circumferential and off-axis orientations, although longitudinal fibers (parallel to the mast axis) rarely kink with good laminating techniques. As masts are laid up, usually over a male mandrel, the laminate has to be "consolidated" in small steps--thin layers--so that kinking does not happen.

    I have never known anyone to use electricity to align the molecules in a resin matrix. I don't know how you would do that. I don't think it is necessary, and maybe not desirable. Think of it: A laminate is already non-isotropic (or alternatively, orthotropic) by reason of the mix of fibers, particularly in mast laminates where up to 80% of the fiber can be running parallel to the mast axis. We rely on the resin to hold this mass of fibers together, and for that, we expect the resin cross-linking to accomplish that. If we could somehow "electrically align" the resin molecules, we would be aggravating the situation by making the laminate even more orthotropic. We would lose what little strength and stiffness we have in the resin to hold everything together. So I don't see the need for electically aligning the resin molecules.

    In most composites, glass or carbon, the best fabrics are knitted, not woven. In knitting, the fibers of one layer are held straight and knitted to adjacent layers. All layers have fibers that are laying straight and not weaving over and under each other. Are they perfectly straight--no, but they are straight enough. Woven broad goods have fibers that weave up and down over each other, and the angles of these fiber directions necessarily give the fabric a lower strength and stiffness than a knitted fabric of equal fiber type, orientation, and weight. But that is OK, because you engineer to the fabric at hand. You probably know what the fabric or laminate stack will do going into the laminate design, and if you are thorough, you will do a small test laminate and have a laboratory test for the final strength and stiffness properties before you build the actual part. In all these cases, the fibers are what they are, woven or knitted, and the resin, once cured, locks them in place so that they don't move under load.

    Would you gain anything in strength or stiffness if you pre-stressed the fibers in the layers of the laminate stack before you wet out with resin and cured--yes, you would likely see a measureable improvement. Is it worth it? it would be really cumbersome, I think, to accomplish that. As I said, I have never seen it done. It certainly can't be done in a female mold of complex curvature--it can only be done over a male mold, or in a female mold of simple curvature, and then only in one direction.

    Eric
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.