CARBON FIBER AND KEVLAR CLOTH
It seems that most people consider carbon fiber to be a super light and strong wonder material. But is it really a good material for wood kayak construction? I already knew that is was light, stiff, and exceedingly brittle. But the strength (e.g. tensile strength) and suitability was not clear to me. Below are two different tests whose results may surprise you.
Test One: Composite WeightThe first problem I had with carbon fiber fabric is that there are not many types available at a reasonable price. The most common seems to be 6 oz. plain weave fabric. And that fabric has a drawback. A laminate using this fabric will weigh significantly more than one made with 6 oz. glass fiber using typical hand lay-up methods. Since CF is much less dense than glass, CF fabric weighing the same per square yard will necessarily have more volume. This extra volume soaks up more resin which makes the completed structure heavier than a similar glass lay-up.
I reached this conclusion by measuring and weighing three sample types of 6 oz. woven CF, plain weave E-glass, and satin weave S-glass, both before and after applying resin. The samples were placed between two layers of plastic film where I squeezed out excess resin from the cloth with a roller. The final samples contained no core material such as wood, only cloth and resin.
The 2 inch square cloth dry samples of CF, E-glass, and S-glass weighed 8.2, 8.2, and 8.0 grains respectively, and were .16. .07, and .06 inches thick (using my cheap caliper). So they were all very close to being the same weight and all very close to the advertised 6 oz. cloth weight, regardless of thickness.
The wet-out samples weighed 23.5, 18.5, and 16.0 grains. The CF sample weighed 47% more and the E-glass sample 16% more than the S-glass sample. The S-glass sample yielded a 50:50 ratio by weight. I don't assume that these numbers represent the absolute numerical relationship between these fabrics due to possible errors in my methods, but the different weights among the samples is too large to ignore and also matches well with my experience. Carbon fiber is really stiff, and has many good applications, but it may not be as light as you expect without using advanced techniques like vacuum bagging. Satin cloth yields a lighter lay-up than open weave cloth, but not by a great deal.
Test Two: Composite StrengthBut is CF stronger? It depends on what is meant by "strong". CF has a much higher modulus so it will be stiffer, but its tensile strength falls between s-glass and e-glass (s-glass being higher). Where CF really falls down is lack of resiliency. It only takes a comparatively small deflection before it fails. Samples I prepared for destruction testing on Sam McFadden's testing jig seem to support this data. The two sample types had one layer of 6 oz. CF inside and two layers 6 oz. e-glass inside, with one layer e-glass outside on both. The e-glass samples were about 5% heavier but reached about 45% higher load before failure (fracture and collapse of the sample). The CF samples failed at less than half the deflection of the glass samples.
Based on these results I doubt I will use CF fabric as a hull laminate in the future. I think it still has some uses though. Stand alone objects that do not rely on a wood core as part of the structure may still work very well. Things like carbon tubes for toggle handles, coaming lips, hatch lips, and various brackets are what I have in mind.
For more information on this subject, find the 11/2002 issue of Sail Magazine. It has a fine article comparing CF, e-glass, and s-glass.
I have not used or tested Kevlar. With a little research I found that Kevlar is weak in compression, degrades in uv, impossible to sand, difficult to cut, expensive, difficult to get a good resin to fiber ratio, and difficult to repair. And it's not near as pretty as wood nor as techy looking as CF. On the plus side it is light in weight, tough (fibers do not fracture easily), and has high tensile strength. It's possible that it might be useful if applied to the inside of a light weight racing kayak. Considering all of the drawbacks, I'm not very motivated to use it. It would be interesting to test it though. I think Sam McFadden has some samples that he is going to test. Stand by for further information.