Pure Wakeboard Engineering

[Lyle]

So as some of you know I build my own boards. 37 to be exact. I am currently working on a new one. One thing I am doing before I build it is making coupon samples to test different lay-ups. I am trying a few different lay-ups, carbon fiber, Kevlar, carbon fiber/glass, ect. My goal is to determine which lay-up produces the best strength to weight ratio. The coupons are 18"x4"x0.9", approximately the distance between binding centers, 1/4 the width of a wakeboard, and optimum thickness I have found. I am not varying the core material in this test. Later I might try some honeycomb core samples.

I am going to use a press hooked up to a computer that will record deflection vs. load. I did this once upon a time without the use of a computer, so the data isn't as precise as I would have liked. But one thing I do remember is the samples deflecting a lot before failure which from experience I know doesn't happen. Usually when one of my boards breaks it is quite an explosive snap, and I pretty confident they do not deflect nearly that much.

When thinking it through what I did the first time is I had two flat bars the coupon would rest on then apply a load right in the middle with a 1" bar across the width. I think the 1" bar was the flaw in the test system because the two flat bars would represent the load of the binding pretty accurately but the 1" bar across the coupon does not represent the load the water puts on the wakeboard. So for all of you who have actually read through this post w/o falling asleep any thoughts on a better way to represent the load the water applies the board as the rider comes down for a landing. . . .

Oh one other thing my last board was 4.5 lbs. I am hoping with this study I will be able to make a board under 4 lbs.

[IrideLITE]

wow thats sweet you make your own boards, but you got me kinda confused, whats the coupon and stuff?

Do you have any pics of your boards you have made? Good Luck!

[vette74]

Lyle, If all you are looking for is strength to weight ratio on each sample then the test you are conducting is fine. If you are trying to measure the actual deflection of the board under real world circumstances then the test you are conducting will be similar but not true. You need to find something that will go between the bar and the test piece to apply a uniform load on the test piece while it is deflecting. I cannot really think of anything right now.

[pooser]

I agree with vette. You just need something to make it a uniform load. At first I thought of just putting a piece of plywood down and then applying a load to that but a plastic material might work better. This just distributes the load over an area versus being confined to a point.

4 pounds??? really? how big was it. Hyperlite is making a big deal about a board being under 6 pounds. What type of materials are you using? Are they expensive? Are you studying engineering?

[Lyle]

I have a lay-up that I know works. Two years ago I made two boards that were exactly the same except one had 3 layers of uni-direction carbon on the top and the other had 4 layers. Both had 4 layers on the bottom. After 2 months of riding the 3/4 board broke when I did a huge out into the flats front flip. It was really loud when it snapped. The top skin pulled apart while the bottom skin only half broke. The other board, the 4/4 is still in one piece today. So I have made essentially a 4/4 coupon that I can use as a base line. So when I break them any coupon that breaks with less force than the 4/4 I know is not going to be strong enough.

The coupons have no rocker in them, it would take a lot more work to do this and I do not see a lot of value in it.

The load that water applies to the bottom of a board as the rider lands is pretty complex. It is definitely not a point load like my original test and the failure modes of the original samples represent this. As the load was applied you could see from the side the foam inside started to buckle they were deflecting so much. So the core was failing first, this is not the case with my wakeboards I make. I figured out the optimum foam density that is needed to keep the core from failing through trial and error.

I don't think the force of the water is an even load either, although it might be. I almost think it mirrors the rocker as someone has pointed out.

One guy on wakeworld suggested using sand to simulate water. Not a bad idea. Unfortunately I don't know if our press break will go that high, and it might be tricky to get consistent results. The limit on the press is approximately 700 lbs. I was toying around with making a silicone block to simulate the water approximately 8" by 4" and about 1/2" thick. That way the silicone would really deflect more towards the tips (where the bindings would be)at the outer edges to more closely simulate what the water is doing.

I don't really care about the deflection at failure of a wakeboard. Hell if it holds together that is all I care abou. However, this brings up another point. What if one of the samples fails at a lower load but deflects a lot more. That would indicate to me that that layup would be better at disipating the energy over a longer period of time and therefore could potentially be a better layup. For instance the Kevlar sample: I know Kevlar is weaker than carbon fiber but it also deflects a lot more before failure. . . does this mean it would be a better choice. . . more strain energy. Is it as simple as (load at failure) X (deflection at failure) = (bigger number better?)


Skiing made me board,
Lyle
Chrome Dome

[Lyle]

I have a lay-up that I know works. Two years ago I made two boards that were exactly the same except one had 3 layers of uni-direction carbon on the top and the other had 4 layers. Both had 4 layers on the bottom. After 2 months of riding the 3/4 board broke when I did a huge out into the flats front flip. It was really loud when it snapped. The top skin pulled apart while the bottom skin only half broke. The other board, the 4/4 is still in one piece today. So I have made essentially a 4/4 coupon that I can use as a base line. So when I break them any coupon that breaks with less force than the 4/4 I know is not going to be strong enough.

The coupons have no rocker in them, it would take a lot more work to do this and I do not see a lot of value in it.

The load that water applies to the bottom of a board as the rider lands is pretty complex. It is definitely not a point load like my original test and the failure modes of the original samples represent this. As the load was applied you could see from the side the foam inside started to buckle they were deflecting so much. So the core was failing first, this is not the case with my wakeboards I make. I figured out the optimum foam density that is needed to keep the core from failing through trial and error.

I don't think the force of the water is an even load either, although it might be. I almost think it mirrors the rocker as someone has pointed out.

One guy on wakeworld suggested using sand to simulate water. Not a bad idea. Unfortunately I don't know if our press break will go that high, and it might be tricky to get consistent results. The limit on the press is approximately 700 lbs. I was toying around with making a silicone block to simulate the water approximately 8" by 4" and about 1/2" thick. That way the silicone would really deflect more towards the tips (where the bindings would be)at the outer edges to more closely simulate what the water is doing.

I don't really care about the deflection at failure of a wakeboard. Hell if it holds together that is all I care about. However, this brings up another point. What if one of the samples fails at a lower load but deflects a lot more. That would indicate to me that that lay-up would be better at dissipating the energy over a longer period of time and therefore could potentially be a better layup. For instance the Kevlar sample: I know Kevlar is weaker than carbon fiber but it also deflects a lot more before failure. . . does this mean it would be a better choice. . . more strain energy. Is it as simple as (load at failure) X (deflection at failure) = (bigger number better?)


Skiing made me board,
Lyle
Chrome Dome

[LakeIolaLuke]

have you tried blending kevlar and carbon? sounds like together they share the properties you are looking for

[Lyle]

Sorry for the double post.

No I haven't at least not in the same axis. The reason being is my theory is that by mixing the types of composites the stiffer of the two will fail first and the other one will directly follow. A way to visualize this would be to say if you had a piece of wood that say it takes 10 lbs to break, and a strip of rubber that also takes 10 lbs to break but stretches a lot more. So if you line them up side by side and attach them to your mechanism to pull on them both at the same time, are they together going to withstand a force of 20 lbs? Most likely not. The wood will stretch to its limits and break then the rubber will break immediately afterwards.

[LakeIolaLuke]

Lyle, well trying can't hurt. what about bi-axially? say keep it rigid rail to rail, and flexy tip to tail?

An I would think if you can resin them both together in the same layup, they would share their better qualities, just like the resin and glass work together. The resin keeps is stiff, the glass keeps is from being brittle.

The kevlar would disperse the load across the layup, and the carbon would lend it's greater strength to keep the whole layup together

[senorbueno]

Lyle, if they both break at 10 lbs, in theory they would both fail at the same time, the rubber would deflect more with a lighter load, but they would still fail at the same load...

How does the press work? Is the material going to be cantilevered or are both ends supported and where are you going to apply the load? Don't quote me on this, but I think the equation you would want would be more like (without complicating it too much) 1/6(load at failure)^3 + (deflection at failure) = (bigger number would be better) That is assuming you are putting some type of distributed load on there. Wow, I'm really making this technical and I'm pretty sure my equation isn't even really right.

I'm actually confusing myself b/c I have a test over loads and failure and what not on Monday in my design of Mechanical Components class.

I think it's really cool that you build your own boards though, do you have any pics of the process and or the final product? I love DIY kinda stuff

[arctic_pitbull]

really cool project, but as for calculating how to set up the composites and stress testing the board, it is my opinion that it involves to much dynamics and mechanics of materials to give you a good answer.

Because even if you get a "ultimate tensile strength" from your testing, it's really hard to know what kind of loads it will get in practical use. You could always make a computer simulation, but it's probably not worth the time
I'd stick to guesstimates on this if I were you, and in that case your stress test is great It's hard to distribute the load 100% realistically, so just keep it simple and/or add a factor of safety.

Also if your into mechanics of materials I'd suggest www.matweb.com for information on the different composite materials.

senorbueno: No they would not fail at the same time in theory, because of different elasticity. Tension and elongation/elasticity is closely connected. Just think of a simple tensile test; you actually don't pull with a certain load that creates tension, but you pull the sample and measure elongation as a function of original length....(tired now, so sorry for bad explaination)

Good luck with the project,

TJ

[pooser]

I don't know if you're still checking this thread but i have a few points to add. I think you could probably model the force of the water as a point load or a distributed force. Since you don't know the actual forces involved in an impact the value you get when the coupon breaks doesn't really mean anything. This seems to be purely a comparative test. You have one coupon that you know works so if another coupon fails around the same level it is probably gonna work too as long as your testing method is consistent.

As far as deflection goes, I think a lower deflection would be better. Everyone is familar with the hype surrounding flex boards but I can't seem to understand how a board with more flex is gonna give you more pop. When a board deflects at the wake energy is lost. Some force is used to bend the board instead of being used to pop you upwards.

I wouldn't be to afraid to experiment with mixing materials. In your example, the rubber and wood wouldn't break at 20lbs but it wouldn't break at 10 lbs either. If one material is lighter or stiffer but slightly less strong it could add just enough strength without adding too much weight.

I'm pretty sure carbon is your best bet for weight, strength, stiffness. The only downside is cost.

[Roos]

I think the idea with silicon would work better than with just the point preassure. If you put a piece of sillicon or any other flexible material that would distribute the weight on the coupon and then a metal plate or something that wouldn't deflect at all.

My bet is on carbon fibre as well but you might try different thicknesses of the material and maybe different types of epoxy and techniques to glue it all together.
I know that you would use carbon fibre and epoxy to get the lightest possible race boat and it should apply to wakeboards as well. The best way to epoxy the carbon fibre is to use a vacume pump and suck all the exess epoxy out of the material. (I'm not sure of how it works)

Also you might want to consider how the material is shaped. When it comes to race boats (again) the best design is to make the surface of the pad (the area in connection with the water) flat. But most race boats has a rounded v-pad because it makes the material much stronger. If you can design som kind of perpendicular rocker that would probably make the board hold together much better.
An other thing that should matter is if the board builds up air pokets beneath it (both to make it quicker, but also to make the impact softer). Some boards are v-shaped, which make them land softer because the board doesn't hit the water as hard as it would with a flat surface.

If you could put a thread on here (maybe/probably you already did) with your project that would be great. (If you allready posted them, please post a link here)
I was thinking about doing this myself just to get a really light board. My idea was to create a plug out of a LF watson (or another nice board) and just mold a copy of it in carbon fibre.
How did you create your boards?

Pardon the less than perfect english, my excuse it that I'm from Sweden...

Mike

[Cyclonecj]

Don't know about the math,but what about pressing the coupon against an inflated car or motorcycle tire?

[Nordica]

If you went back to some old civil or mechanical engineering books you could easily figure out what the force would be distributed along the coupon (sorry i don't really feel like looking right now but maybe i'll check when i have some motivation). I seem to remember these equations being fairly easy and with a little more info (ie. materials, bonding, ect) you could make a fairly educated decision from a fairly simple process. I'm going to be on spring break with nothing to do for a week so sadly if you wanted me to try a few things i would be more than happy to (wtf i'm such a bitch these days) try a few equations for you and see if i can get anything to work out..... at least i am enjoying what i do i guess

[smokedog2]

I agree, you will get further with a good materials book than this test.

I'd like to see a picture of the original board failure. You may be way off of on the FM.

It sounds like it could have been a delamination failure.