Carbon Fiber-Kevlar Question

[northboundtrain]

Some canoe/kayak builders use a hybrid layup schedule with carbon fiber on the outside and Kevlar backing it up, so to speak, on the inside. This is done, as I understand it, to increase the impact (point load) resistance of the shell.

My question regards how these two materials work together to achieve greater impact resistance than a full carbon fiber layup of similar thickness and weight. The Kevlar has more "give" relative to the carbon, so why isn't the load taken entirely by the outer carbon portion of the hull with the Kevlar only absorbing the load after the carbon has failed?

Thanks for any info on this topic.

 

[David Cooper]

I can't give you information, but merely speculation. I suspect the hull will fracture more easily but that it will then hold together better in a fractured state than a broken hull with no kevlar where holes will open up and shards may stab into the fragile human.

 

[tom28571]

I would think that carbon on either inside or outside of a panel would be the poor choice for impact resistance. The tougher material should be inside or outside with the higher tensile material further toward the neutral axis with the resin matrix distributing the load to avoid point loading fractures. That is the way fiberglass structures as well as natural things like trees have always worked best. High tensile, more fragile fibers should not take the initial bending load or they will fail before the tougher fibers get near their failure load.

 

[PAR]

Kevlar is used on the inside, because it's simple physics, you want this material as far away from the impact point in the laminate as practical, to be most effective. I agree with Tom in that carbon directly on the outer face of the laminate isn't the best choice, though most usually apply gelcoat and mat and maybe some finish cloth, before the carbon goes in, helping to some degree.

 

[northboundtrain]

I guess, theoretically at least, at the point of impact the carbon fibers on the outside of the laminate are in compression, while the Kevlar fibers on the inside are in tension. Carbon has good compressive strength while Kevlar has relatively poor compressive strength but good tensile stregth–better than carbon even.

Could the theory behind this practice also be that the relatively thin carbon layer(s) on the outside can flex more than would a full-thickness carbon layup, thus allowing the Kevlar to do it's job, so to speak, and absorb the tensile load?

 

[northboundtrain]

I can't give you information, but merely speculation. I suspect the hull will fracture more easily but that it will then hold together better in a fractured state than a broken hull with no kevlar where holes will open up and shards may stab into the fragile human.

It's interesting to note that some of the new-generation football helmets have carbon-Kevlar shells. Clearly these are designed to sustain repeated impacts while maintaining full structural integrity as opposed to partially failing and going into a limp-mode, so to speak.

 

[tom28571]

As an electrical engineer, my formal schooling was a bit weak in mechanical attributes of materials. A friend gave me a copy of "Structures: Or Why Things Don't Fall Down" by J. E. Gordon about 40 years ago that provided a good basis for learning about this subject. Its mainly for the layman but was ideal for engineers from other disciplines. Written before the ready availability of carbon and Kevlar but this, or some similar text is extremely useful for anyone who wants to understand how stuff fails and how to keep it intact. Lots of marine architects, boat designers and builders of anything would benefit from a better exposure and understanding of such knowledge.

My post above was pretty simple and does not cover the subject of canoe skins all that well. The fact that carbon and Kevlar are wonderful materials does not mean that we can't screw up in their use.

 

[PAR]

I have that book Tom, got it many years ago and it is a good layman's text for the basics.

It's actually really easy to screw up composite laminates, if interested in the lightest and strongest elements. Anyone can make it tough enough or strong enough, but some math and understanding is involved, when you want to take full advantage of the materials employed, which (IMO) is the only reason to use these types of materials. In other words, you can use carbon to good use on a Tahiti ketch, but it's hard to justify why.