Carbon and Composites
We see it everywhere, but how much do you know about carbon fiber and other composites?
It's commonly listed as a luxury commodity, but is it worth all the hype?
In my opinion, yes... and no...
Poor design work can not be made up for with excessive use of carbon or other composites.
But lets get into the technical details first.
Carbon fibers Kevlar and most other composites used in vehicles start out as threads. Each thread has its own strength and weight to it. Kevlars tend to be heavier and stronger, whereas carbon strings are lighter.
These strings are weaved together into sheets/shapes. These sheets and shapes are flexible and can form very easily. When you apply two part epoxy to the composites, the weaves become locked together and the strength of carbon fiber really becomes apparent. What is interesting however, is that a composite sheet is only strong in the directions of the threads. This means that a single sheet of thin show fiber can bend and flex easily, and can in fact be cut easily with a pair of scissors. If you double or triple up the sheets, the added thickness starts strengthening the sheet up, making you need to use tin snips or zip disks to be able to cut through them.
In forming composites into complex geometries, molds are traditionally used as it is easy to form a carbon sheet to fit a mold. However it is possible to use a loom to weave parts.
Next, other tips useful in creating the strongest/lightest carbon fiber.
1. Let the carbon set in a vacuum. Two part epoxies used in composite work don't need oxygen to harden, so they set well in vacuums. Vacuum's also help suck out excess epoxy and air bubbles. Having more epoxy then necessary only increases the weight of the sheet and has a very negligible effect on the strength of the sheet. Bubbles on the other hand create weak spots and should be avoided at all costs.
2. For extra strength honeycomb plastic/aluminum and other filler materials can be used in between composite sheets. The fillers themselves don't provide much in regards to strength. But the advantage is seen in thickening the sheet. When you separate the layers you allow that bending moment in the sheet to be turned into a tensile/compression forces that are in plane with the composite threads. When using honeycomb it's best to use only enough epoxy to adhere the sheets to the honeycomb, as any more only adds to the overall weight and hardly adds any strength.
3. Some epoxies provide extra strength, but to fully set need to be cooked in an oven. This is easy to do for small pieces, but a heat treating company may need to be used for larger body panels.
4. The lightest composites will NOT be shiny, extra gloss coats only add to the weight of the carbon sheets. If you look at some luxury cars they use very thick gloss coats, so much so that they are at least on par with the weight of other interior materials.
Finally, there are new composites on the verge of coming to the market. One of these is metal matrix composites. These are standard metals with various composite threads running through the material to increase strength in certain directions. These provide stronger and lighter machinable materials.
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