carbon fiber wing mast for my cat

These were my thoughts as well.

 

Have you really never seen process called filament winding ?
Or do you for some reason think prestressing is normally not used in filament winding process ?
"you would likely see a measureable improvement" I hope you only meant in stiffness in hoop (and in some less extent in +-45 orientation), not in strength properties, if not I disagree.
I think the differencies in mainload path ( 0deg direction) are rather small, and not necessarily in favour towards filament winding in comparison with using autoclaved female mould process. Some of the advantages in hoop properties come from the fact filament winding doesn't need overlaps, which save a little weight, but if joins are under trailing edge where some external track is to be bolted on, the advantages are really small if any at all.

 

Speaking of possible gains in strength or stiffness by pre-stressing the fibers in the layers of the laminate stack before curing process, there is one other process worth mentioning used in mast construction.

Using autoclaved prepregs on aluminium female mold, the process involves all vacuum & some extra pressure being applied before increasing temp for curing to begin. That means the alloy extending due to thermal expansion while the pressure difference prevents or at least reduces any movement of fibres related to the mold.
The result is all fibres being stretched in zero degrees oriention before curing begin. Kept under some constant tension while curing takes place and releaving that tension after curing process has taken place by cooling alu mold slowly down in controlled fashion.
That process clearly pretensions zero degree fibres the most, +-45 degs somewhat less, and the hoop fibres dont get stressed along their length, but instead perpendicular fashion trying to open up small gap between them ; but in practise hoop laminate will just reduce it's thickness a rather neglible amount instead and no gaps form at all.
Overall the result when done correctly is some increase in length wise stiffness, with very little (theoretical rather than practical ) increase in strength in same orientation. Since that process produce masts needing no fairing, it produces light masts for their strength & stiffness, which in small sizes is a more significant advantage than the prestretching advantage in zero degrees.

Advantages in filament winding are far more valuable when tubes under more significant twisting loads are produced ( shafts for underslug rudders if not balanced too much) rather than masts where most loads are in zero degrees. Netherless both methods are good examples in prestressing fibres in practical processes producing measurable advantages in stiffness properties.

 

Of course I am familiar with filament winding--most of my free-standing mast designs are built by Composite Engineering who has two filament winding machines. We use the bigger one to lay up over a male mandrel, either aluminum or composite depending on the design, and he uses an autoclave for curing. But we do not consider that this process "pre-stresses" the fibers. The fibers themselves are not under much stress at all when they are laid onto the mandrel or when they are curing. Their primary strength and stiffness in the resulting laminate comes from the autoclave's pressure as it squeezes the fibers into the resin to make a denser laminate with a high fiber content. Void content is lower. It does not due to the fact that the fibers are "pre-stressed."

The same is true with other spar manufacturers that use male mandrels and autoclaves, like Hall Spars. They use prepreg carbon, so the fiber-to-resin ratio is already determined. As the layers go on, they are periodically "consolidated" with vacuum pressure to squeeze the laminate together to get rid of large voids and possible kinks in the hoop directions that Rob Denny mentioned. When the mast is ready to cure, it goes into the autoclave. Yes, the mandrel expands on the underneath side of the laminate (inside the mast) and the autoclave pressure squeezes on the outside. Again, the fibers themselves are under little stresses, not what I would call, "pre-stressed". This phenomenon assists in keeping the fibers straight, but more importantly, the resulting laminate is denser than otherwise would be if the autoclave was not used. This results in higher mechanical properties in the laminate.

Eric

 

pre stressed fibres

Hi All,
If you use the rule of mixtures you will see that a unidirectional laminate can acheive a maximum stiffness(or modulus) in proportion to its volume fibre ratio. Ply testing of carbon and glass UD laminates over many years has shown that high quality laminates usually equal or exceed the theoretical modulus using the rules of mixtures calculation (in tension and compression). This means that the laminates are acheiving their theoretical stiffness for that process. So extra "stiffness" can't be obtained by pretensioning. In fact for example a pretensioned mast laminate would be a poor move as the laminate would have to contract further before the laminate could take up load than if the fibre was at a neutral starting tension. The usual winding, prepreg, infusion and hand laid laminates that are of good quality reach their maximum stiffness proportional to Vf in these processes. The only way to improve the stiffness further is to reduce the resin ratio ie have more fibres in the same space. This is also possible using various processes. So infusion and autoclaves can produce laminates with Vf=50-60% easily. Advanced infusion & autoclaves can go higher. The theoretical is about 98%=Vf at hexagonal close packed with fibres touching. In short the processes in use today produce very good laminates. Regards Peter Schwarzel carbon-works dot com dot au

 

Your position on pretentioning fibers is comforting as I built my entire cat "Jolie Julie" as well as the mast and the canoe boom(new this season) with unidirectional fibers.
My original intent when I stated this tread in 2008 was based on the fact that I already had a good rotating wing mast made of white spruce wood lattices , but that I found too heavy for my rather light 40' cat and I also wanted to get rid of the spreaders and their heavy stainless steel wires. I therefore built a smaller/stiffer and much lighter carbon wing mast and with a view to replace the spreaders by a different type of shrouds, I introduced this tread mainly to obtain information and opinions on my intention to use a set of double and continuous rope runners(see drawing on post #1)going from a higher position on the mast, down through a 4:1 adjustable vang block just abve deck and back to a 12' lower position on the mast.
The only answer that I received on my inquiry was from Eric Sponberg

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From there, the discussion turned mostly towards carbon mast construction, such as sandwich or single skin (I used a CoreCell core), which was extremely interesting although it was not my personal goal.
I abandon the discussion in 2009 and, with my wife, we went sailing(lived aboard) for over 16 months and added another 6000+nm on Jolie Julie and gave her a real good road test with the new mast and the double runners.
I am glad to report to Eric that he was right on the button with the above statement, it could not be better said, it is exactly how the mast behave, smooth, safe and easy to handle... and it greatly replace the need for spreaders and the gear that goes with it and all the weak points and unnecessary stresses that they create on a mast!
The new mast is much lighter, especially way up high, and very strong and stiff. The old mast offerred excellent performances, the new carbon mast does the same thing just a bit better on all aspects.
Last spring I added a carbon canoe type boom(sometimes called "pocket boom" in the USA) on Jolie Julie. It is basically a balsacore sandwich construction with layers of 9oz, unidirectional carbon at 0/90° and carbon /kevlar renforcements where needed. The old 17' aluminum boom weighted 83 lbs. and the much bigger new one is only 72lbs. The sail falls right in nicely and the look is terrific with the wing mast. I simply used the top 17' half sections of the mold that I made for the mast.