Hello, I have been reading everything I can find about composites but I haven't found any type of equation/calculator for determining composite thickness requirements. I know different resins, etc. have a bearing on the strength but, there's got to be a "safe" relation between how much composite equals the strength of an object made of steel/aluminum. I am trying not to waste too much material and loose the weight saving of making parts out of composites. I'm also not looking to make parts that risk my life on (bike frames, control arms), just automotive body panels, intake manifold or a pickup tonneau cover.  I think a formula wouldn't be too hard that would make parts safely 15-20% strong than OEM and still have a substantial weight savings. For example: How thick would a fender need to be if it was made out of e-glass or carbon fiber vs. OEM sheet metal?  I know a wheel well would have to thicker to survive a rock from the tire tread at speed, but how thick? Would a layer or two of kevlar add to 2mm of e-glass be good or over-kill?  Everywhere I see PSI tensile strength,etc. but does it mean that I would need half the thickness of an aluminum bracket if the fiberglass is twice the metal's strength rating?

The tensile strengths can be used for comparison, you just need to understand what they mean and how composite materials work. As Warren said, the data given for materials is really intended for engineers, not the average hobbiest.