This is a very basic question but what are the pros/cons of the following layups using either biax or multiple layers of plain weave and which is the most suitable for the charicteristics of kiteboards?
My first board which you can see in the 'projects' thread was done like pic A and has a great deal of flex, would i be right in assumming that B would be a lot stiffer in the 'y' axis (as you look at the diagram) but would have greater tortional/twist flex?
That is a really interesting question and one that for me the jury is still out on. Its just thought experiments for me so far so i will definitely defer to anyone whose done it.
I think it is right to say that the bi-axial will have increased torsional stiffness and some reduction in the stiffness on the zero and 90 degree axis but I'd go out on a limb and say that the loss would be c. 20%. Here's my thinking: as I understand from googling around, when you change the angle the fibres run relative to the direction whose stiffness you're interested in the stiffness changes by a factor of cosine squared. At 45 degrees cos squared is 1/2. This mean in biax the contribution to the stiffness along the long axis from the bundle of fibres running at 45 degrees is 2 times 1/2 =1. So in theory you get the same stiffness on the long axis with biax as you do plain weave. Same goes for healside toeside direction.
If you apply this same thinking to the plain weave, its contribution to the stiffness along the 45 degree line across the board will be the same as that of the biax.
Where I think there is some loss in the stiffness due to changing the orientation of the fibres is in the weave pattern distorting as it loads up. So if you think about the squares that the biax makes, with fibres running at 90 degrees to each other, bending a tip up will cause these squares to become elongated diamond shapes effectively delaying the time at which the fibres start to bear the load.There is a 'rotation' of the fibre directions. I think this rotation will cause a reduction in the stiffness in the same way that the under and over pattern in FG reduces the effective stiffness of the FG because the fibres have to stretch straight before they bears load.
In biax the diagonal line on which the board under a torsional load is much closer to the direction the fibres run and you get less of this 'rotational' loss in stiffness due to weave distorting. Boffins tell me that that the weave pattern (under and over) reduces the stiffness by about 30%. I think the reduction due to the distortion would be much less than this so that is why i'm guessing up to about 20% reduction in zero and ninety degree direction stiffness for biax. It is a guess so would love to hear Downunders results it you run the experiments.
An alternative might be to use plain weave but put carbon strips on the diagonals.
I guess the other practical issue is that you can get heavy weights in biax so you can do you layup in one pass rather than mess around with multiple layers of glass to wet out and de-bubble.
Depending on the material Plummet. I would not say 90 are useless at all if you using a foam as core for example.
If you look at this structure, this is stable structure (must be at 45 deg, but u get the idea:):
Now, remove ie red lines and you get unstable structure, not rigid at all. There will be so much tension on a horizontal lines they will eventually brake. And vertically nothing is holding your foam board and it is relaying only on the epoxy strength.
Also, unless you are shaping a concave in, only FG at 90 degrees is keeping your concave in a place. It is my opinion that triax can't keep the concave in. Once again, epoxy does that for you but should be a FG!
I've had on my to-do list to have a go at triaxial or uni + biax but at my local suppliers they tend to have only quite heavy weights. The issue that this brings up for vacuum bagging seems to be that it it more challenging to wet it out properly. In a pnuematic press + heat the issue seemed to be largely resolved.
I saw a video for Lightwave boards and they used 22 oz ( which I think is around 650gsm) piece of triaxial.
However, after my last board where I experiment with using a heat gun to drive air bubbles out and resin I have plucked up the courage to try in on on a single layer of triaxial. I really like the idea of a using a single layer of glass - less cutting out, less time to lay it up generally less stuffing around. If the heat gun is able to help wet it out thoroughly I think it could be a goer.
A performance question for everyone though. What is the performance impact of more or less torsional stiffness??