Ski Builders Forum

Normally i post at Graf, but I build skis...so needless to say I was pretty excited when i found your site. So here is a "puzzle" that i found that has been bothering me, hopefully you have the answer:

I was recently reading my new composite construction book (my christmas gift to myself) and I came across a design warning that seems to be widely used in skis and snowboards. the analogy they gave was to hang a 1000lbs weight from a chain and a bungee cord. Both the chain and bungee have a 700lbs yield. the question was how do you hang the weight and have the two work together to support the weight. You couldnt just hang them side by side and latch the weight on because the chain is stiffer and would break before the bungee "kicks in". the solution is to hang the weight from the bungee and gradually let it down until the bungee is in tension, then hook up the chain.
NOW, translate this to carbon (chain) and glass or kevlar (bungee) because carbon has a much higher modulus of elasticity than glass or kevlar, so is stiffer. so the book was stating not to layup two different stiffness fabrics parallel to each other, because one will support all the weight till failure, then drop all the weight on the other. Almost universally skis and boards use triax glass and a strip of uni carbon. The carbon is running parallel to one of the layers of the triax. Doesnt this mean that we arent letting the glass get a chance to help out? And encouraging the carbon to fail? Do we need to put all glass in tension somehow while pressing? Thoughts, comments?

Check out this thread:
http://www.skibuilders.com/phpBB2/viewtopic.php?t=188

Basically, Henrik of Hendryx skis says exactly what you mentioned. Adding CF leads to premature failure.

We don't have too much experience with carbon fiber and have only used it in our latest ski. We used a pretty heavy (13oz) uni directional CF strip on the top of our ski. We'll see how it holds up.

-Kelvin

mattman wrote:You couldnt just hang them side by side and latch the weight on because the chain is stiffer and would break before the bungee "kicks in". the solution is to hang the weight from the bungee and gradually let it down until the bungee is in tension, then hook up the chain.

I'm not an ME and it's been a long time since I've been in a physics class so this could all be wrong, but I'll give it a shot. The important thing in this situation, I think, is the spring coefficent of the bungee cord. If it's 100 lbs/inch [yeah imperial units] and you want to load them equally, assuming that unloaded the chain and bungee are the same length, the top of the bungee should be 5 inches above the top of the chain. If it's 500 lbs/inch, it would be one inch.

I think this should be equivalent to preloading the bungee at 500 lbs of tension. But if you vary the load, either increasing or decreasing it, you're not going to load the chain and the bungee equally. So if you pretension the bungee at 500 lbs and load the chain/bungee with 500 lbs the chain won't be loaded. And all this is assuming that the bungee/glass is a newtonian spring, which they're not but I think you can assume they are.

So assuming this is all correct, the question becomes how elastic triax glass is. [???] Now, I guess you could take a strip, hang it, weight the end, and measure how much it streches. But if the glass is un-epoxied I don't think you'd get a very acurate measurement. And if the glass were epoxied I don't think you'd be able to measure it easily, since it would be small. Though there should be some data about this some where, but I haven't seen any.

Ok, I'm over thinking this and I've confused myself, so I'm going to give up while I'm behind...

Basically this theory is absolutely correct but it it depends greatly on what you're trying to achieve with the carbon fiber.

The simplest thing is just creating breaking strength. Carbon fiber has a much higher tensile strength than E-Glass so you can reduce the amount of fiber getting a strong and ligthweigth structure. In this case you definetly have to use 100% Carbon in the respective load axis.

With using 100% Carbon you also get a stiff snappy caracteristic. The disadvantage is the high price of the structure and the dangerous behavior in case of overloading. A Carbon fiber structure brakes suddenly and unexpectedly creating very sharp waste edges (high potential of getting severly injured). The structure is than completely distroyed and unuseable. A Glass fiber structure fails softly mostly just delaminating.


If you just want to add a little stiffness at small bending angles it absolutely makes sense to add a little carbon fiber to a glass structure. It is not true that the carbon fiber takes all the load. It takes load first but as it is able to stretch the glass beginns to take a good amount of load. When it comes to failure of the structure it is true that the Carbon part fails first. But then the Glass is still able to take the forces and keeping the structure useable and preventing injury.

Here is a nice summary of mechanical properties:

http://www.skibuilders.com/phpBB2/viewtopic.php?t=144