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Old     (sunsport)      Join Date: Sep 2002       02-15-2008, 9:56 AM Reply   
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.

Skiing made me board,
Chrome Dome
Old     (joe_crawley)      Join Date: Jan 2007       02-15-2008, 10:50 AM Reply   
I didn't know and now I want pictures, it sounds awesome!
Old     (inty399)      Join Date: May 2007       02-15-2008, 11:17 AM Reply   
i have no idea. but it's tight your trying.
Old     (wakerider111)      Join Date: Jul 2006       02-15-2008, 11:25 AM Reply   
i don't know much at all about this stuff, but i know at times my mom has come up with some random thoughts as to what i could do to improve my riding (she has never wakeboarded even once) and they have worked sooo... i wont underestimate an opinion or random insite

A bigger/ wider center bar could be used, at least to see if it produces significantly different results...

maybe if the board was sitting on a center bar and then the two side bars came down... I supose this wouldn't change anything though... or would it? but i'll put it out there if it sparks something in you :-)

water gives, so it would be hard to simulate perfectly. I don't know either. Good luck

p.s. i agree, pictures sounds fun ;)
Old     (wakerider111)      Join Date: Jul 2006       02-15-2008, 11:42 AM Reply   
on second thought i think the reversing of the bar(s) that move with the bar(s) that don't would possibly make a difference. Is there not less and less surface area contact between the coupon/board and the bars as more pressure is applied, considering the fact that the coupons are curved like a board. doing the tests the way that you are presently doing them? I realize the whole simple rule of forces giving an equal force in the opposite direction which is why i originally didn't think it would make much of a difference.

If you are testing flat pieces with no curve than where i am going with this idea holds no ground. But if you are testing curved pieces, then i think it does make a difference if the moving bar is the center one or the two side bars... ok, now i am starting to make it more difficult than it is. The two side ones don't have to move. Just flip the piece over to simulate the effect. maybe you are already doing it like this. i don't know

starting over:
IF indeed the pieces being tested are curved are you testing force applied to the convex side or the concave side. Force applied from the CONVEX side would be best to simulate the strain of a board. this is the best idea i think i can come up with

if you have difficulty understanding what i am picturing, and you think it is worth while, let me know.
Old     (mxflyer281)      Join Date: Oct 2007       02-15-2008, 12:04 PM Reply   
Couldn't you use some kind of sand to act the same as water?
Old     (bird_dog0347)      Join Date: Feb 2006       02-15-2008, 12:28 PM Reply   
Ok, I will take a stab at it...

Since the board has a rocker, and obviously a center point on the bottom and lets assume the water is "butter" flat. The more you push down on the area the bindings are mounted to, the more surface area will be in contact with the water, thus more support.

Now, since water is not solid, you cannot test it on a solid flat surface, but perhaps on a surface with equal rocker. Like this...

Old     (wakebrdr38)      Join Date: Sep 2006       02-15-2008, 12:45 PM Reply   
i wanna see pics of these boards
Old     (socalwakepunk)      Join Date: Dec 2002       02-15-2008, 12:46 PM Reply   
All this talk about bars is making me thirsty.
Old     (snowslider76)      Join Date: Mar 2002       02-15-2008, 1:21 PM Reply   
I have no idea if this would work or not but couldn't you find some foam that has the same density as water as press down on that instead of the bars. Also why do you need to know what the maximum breaking point is anyway? If you took an average 200 lbs rider and figured out how much pressure they put on the board landing from 10' high wouldn't that be enough? If you can get past like 10% of that force before the board breaks I would say you have a pretty strong board. Maybe I'm way off here.
Old     (daniel_harmon)      Join Date: Feb 2008       02-15-2008, 1:29 PM Reply   
The main problem you are probably having is strain rate. The boards are deflecting more in your controlled tests because you are applying your loads in a must slower manner that a wakeboarder coming down would.

Using 1" flat stock is not really going to make a huge difference for your water reaction force loading. You are probably not going to be able to fully simulate the water anyways. By increasing the width of the bottom flat stock, you are going to require more force to snap the boards (P=F/A) The load will be distributed over a larger area as the board begins to deflect so, the narrower the bar the more easily it will snap. If you want, you can read up on fracture toughness and possibly come up with a new method that will involve a much more rapid strain rate and a more distributed support mechanism. (i.e. brace the board against two rod supports then use a pendulum to drop a sledge hammer with a small board attached (to increase contact area) to it.)

I imagine that this approach will give you a more accurate idea of how the coupons will behave in the sort of failures you are seeing.
Old     (team_o)      Join Date: Jun 2002       02-15-2008, 2:25 PM Reply   
As someone who's broken 100's of boards over the years trying to find the ideal layup, I feel your pain. The test machines, even at top speed, are nowhere near the explosive forces seen when a wakeboard snaps in use. We almost constructed a drop tank a few years back - the plan was to load the board at the bindings, and then drop it from 15 - 20 ft into the tank...

That being said, I have found there is a pretty decent correlation between relative strengths on the test machine, and what will or will not break on the water.

Unfortunately the only way to get a baseline is to "break test" boards that have survived, then make test runs of weaker boards and have your team ride those. If they don't break that's your new standard, and so on...
Old     (trickyboarder08)      Join Date: Jul 2005       02-15-2008, 2:48 PM Reply   
wouldn't Two 1/2 in bars work. Just put them where you're feet would go.

(Message edited by trickyboarder08 on February 15, 2008)
Old     (jetpilotz)      Join Date: Mar 2007       02-15-2008, 3:22 PM Reply   
I don't think you can replace a water load with a point force, it is more of a distributed load. Depending on the angle your board meets the water, it could be an even distribution or an exponential one. I would use an impact force rather than a press. I would also use maybe a stress/strain gauge on the top and bottom of the sample, seeing as how the top would be in compression and the bottom in tension (assuming the force came from the top)

on an nonengineering sidenote, i would hit it with a sledgehammer because thats what the water feels like when i fall.
Old     (helix_rider)      Join Date: Mar 2003       02-15-2008, 3:49 PM Reply   
You guys who break multiple boards amaze me...I must not be riding very hard. If I ever actually break a board while riding it, I'm going to feel like a champion. The next second I'll be PO'd :-)
Old     (sunsport)      Join Date: Sep 2002       02-15-2008, 4:33 PM Reply   
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.

I like the idea of sand, that is probably going to be the closest thing to water. 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.

A picture of one of my boards is in my profile, but it is about 5 years old. I just like the paint job.

Skiing made me board,
Chrome Dome
Old     (sunsport)      Join Date: Sep 2002       02-15-2008, 4:47 PM Reply   
Ah one other thing to think about:

I don't really care about the deflection at failure of a wakeboard. Hell if it holds together that is all I care about. But wait a second does deflection make a difference. 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 lay-up. 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,
Chrome Dome
Old     (daniel_harmon)      Join Date: Feb 2008       02-15-2008, 5:41 PM Reply   
Not sure that the load at failure x deflection = solution will really give you a reliable number. However, the issue you are noticing is correct, the increase in ductile behavior in kevlar over carbon fiber or conversely, the increase in brittle fracture in carbon fiber over kevlar would allow the kevlar to survive and the CF to break. The more ductile material is going to be able to absorb more energy without fracturing by deflecting. Both materials are relatively brittle though and will fracture instead of moving into the short but present plastic range where any deformation is permanent (i.e. your rocker changes from factory shape)

If you can get a hold of a stress-strain curve for your materials and somehow come up with a number for stress during a landing, you may be able to better select materials and layering density. I would definitely use a fairly high factor of safety though, because your assumptions will never be 100% accurate.

I still think an impact test of some sort would be in order. To quote a materials expert, "Toughness is a measure of the amount of energy a material can absorb before fracturing. It becomes of engineering importance when the ability of a material to withstand impact load without fracturing is considered. One of the simplest methods for measuring toughness is to use an impact-testing apparatus.(Foundations of Material Science and Engineering by: Smith & Hashemi)"

By doing this with several sample layups, you could form a baseline for the strongest patterns, resins, materials, etc; regardless of the specific load that the board will see. From there, you could come up with the proper board design. Also, this may give you some incite into combinations of layups allowing for specific strengths in certain areas and more flexibility in others.}
Old     (sunsport)      Join Date: Sep 2002       02-15-2008, 6:16 PM Reply   
Hey Daniel,
I am still not convinced it is an impact loading situation. The foam I am using for the most part fails at approximately 90 psi. I know the skin composite is at least 10 times this probably more like 1000 or even higher. So if it truly was an impact situation I think upon failure the core would pretty much turn to dust because of the huge difference. Grant it the skin protects the core but not that much. For instance I have seen impact failures on wakeboards from rails or handle dings. Once the skin fails from impact the damage done to the core under the skin is pretty severe.

Skiing made me board,
Chrome Dome
Old     (daniel_harmon)      Join Date: Feb 2008       02-15-2008, 6:32 PM Reply   
I think we may have our definitions crossed. The impact you are describing is more of what I consider a compressive crush loading. The impact I am talking about is more like holding a toothpick between your thumb and middle finger and breaking it by flicking it with your other hand. It loads one side in a compressive manner and the other side of the layup in a tensile manner. A charpy impact test really gauges a materials stiffness (Modulus of Elasticity) as well as the ductility of the material (goes hand in hand with the materials ability to absorb impact/load without yielding). See links

I checked out your website, pretty neat concept. Do you use investment casting, sand casting, or do you use something else to make your fins? May I ask what alloy you use? I'm finishing up my ME degree this semester and am taking/have taken several materials courses so this topic interests me.
Old     (alanp)      Join Date: Apr 2001       02-15-2008, 6:53 PM Reply   
as childish as some these posts can become the intelligence on this message board still amazes me. good luck
Old     (ccryder)      Join Date: Apr 2002       02-15-2008, 7:24 PM Reply   
What if you use a bladder which is between the board and a solid surface. Have the board clamped down to the same surface at the binding locations. I would suggest one side to be clamped rigid and the other side to be held down by a roller to allow lateral movement of the board while bending. Pressurize the bladder hydraulically and measure bladder pressure and board deflection. The bladder would give a distributed load similar to water, I think.. It would be hard to figure the force since it is hard to come up with the area, but you might learn some of the strength characteristics. The pressure would be uniform in the bladder to distribute an even force over the area of contact and it will change shape as the board bends. Something to think about.. You might be able to use a series of fire hoses hooked to a manifold kind of like a baffled water bed. How much deflection do you normally see before fracture occurs?
Old     (rs1071)      Join Date: Feb 2007       02-15-2008, 8:24 PM Reply   
Lyle, I am stymied with the same loading questions. How exactly does the force of the water on the board vary when landing/riding/whatever? I have modeled it as a distributed load for lack of a better understanding.

Your test sounds great, but I am not sure that you want to be testing the deflection vs. load. That might give you the flexural rigidity, which is important, but it's not the best way to get the data you are looking for.

Andy really hit it on the head. The board is a sandwich composite, which gains its strength from the separation of the face sheets. The top sheet is generally in compression, the bottom in tension. The core is in shear, with the highest shear at the neutral axis.

So what does this have to do with your testing? I think what you would benefit more from is a boring old tension test of the face sheet. This will give you the modulus of elasticity and tensile strength, along with some other important properties. The ASTM standard outlining a tensile test in composites is D3039/D 3039M.

The results of this testing while combined with a shear test of a core sample and the known thickness of glass can actually give you some pretty awesome predictions. I'll include some screenshots of the analysis I did of an aspen-core wakeboard. The geometry was simple, which allowed for a pretty simple model.


I would recommend checking out this book (if it is available at a local library): "An Introduction to Sandwich Composites" by Dan Zenkert. He does a really good job explaining bending in composites.

Good luck and I'm glad to see that there is someone else that's building their own stuff!

Andrew Brinks
Check out some of our projects at:
Old     (malibuboarder75)      Join Date: Jan 2004       02-15-2008, 8:26 PM Reply   
Where are the pictures?

Also, I think pictures of the testing device would help to. I think the best way to test would be to have the load at each end of the sample. Put a sand bag in the middle of the sample. The sample would deflect around the sand bag.
Old     (wakebrdr38)      Join Date: Sep 2006       02-15-2008, 9:53 PM Reply   
pictures please
Old     (jetpilotz)      Join Date: Mar 2007       02-16-2008, 10:03 AM Reply   
I agree with Daniel and Andrew, but I'm also finishing up my ME degree, so maybe we aren't being open minded enough. I would like to help though it sounds like fun. I do not think your board will break from deflection though, and I think if you could get your hands on some stress/strain gauges (the ones that you stick on the test material and they measure the forces on the surface, i can't think of the technical name right now, rosettes maybe?) the data might surprise you.
Old     (sunsport)      Join Date: Sep 2002       02-20-2008, 9:09 AM Reply   
So back in '95 when I made my first board, still in college, I had calculated all the theoretical data, and what it would take to make a board, theoretical strengths and what not. Then after 100 hours or so of work, I had a 3 lb board. It was a pretty good reality check when it broke on the third ride.

The more boards I made the more I learned about the true failure modes. I don't think the failures I have seen have been impact failures. I think the load applied to the board when the rider comes down from a landing is very rapid, a fraction of a second, but not true impact where the board is hitting a hard surface (similar to the izod impact test), which happens in milliseconds.

Anyway I think what I am going to do is build a silicone block approximately 1/2" thick and 6" long to simulate the water.

All the samples are complete. Here is the data. I will take some pictures of them when I get a chance. . . .

Skiing made me board,
Chrome Dome
Old     (allen)      Join Date: Apr 2005       02-20-2008, 9:59 AM Reply   
You want to use a 4 point bend, fatigue frame. Two bars on the bottom out wide and two in closer a bit narrower then your feet. The exact mesurments I couldn't tell you with out doing an FEA. We do hundreds of fatigue samples a month. It will still break in the middle but you will be "bending" the board with two. You are "creasing" the board with one. Thats why you are able to deflect the board more with one. You have less linear stress with one bar as opposed to two.

Old     (sunsport)      Join Date: Sep 2002       02-20-2008, 10:16 AM Reply   
Thanks Al, actually that is why I am thinking to use a 6" long silcone block right in the middle to break the samples over. My hope is to take out any point loading that would be seen using bars. I have a test sample we will break first to confirm that the failure mode is the same as a wakeboard. If we see any "creasing" then we will have to rething how we are breaking the coupons.

Skiing made me board,
Chrome Dome
Old     (allen)      Join Date: Apr 2005       02-20-2008, 10:49 AM Reply   
I think with a silicon block you still will not see as good of results as a 4 point. There is a reason why its commonly used in our industry. You also have to remember that on the water you are also going to get lattitudinal loading. The board will deflect with a side load accross the board which is going to change the stresses in the board.


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