russell89 said:
why not just stick to metal? composites are nasty to work with, cost way too much, are more apt to fail entirely when hit, and they delaminate. they are lighter but is saving a few pounds worth the money and effort? but if i had to pick i'd go with kevlar. its still light, fairly flexible, and not as much of a pain in the ass to work compared to carbon fiber
I'm not sure what you mean. There are tons of advantages to composites, but you need to understand what you are doing to get those advantages. Plastics can be shaped much more precisely than metal, and with less effort. You construct a mold, and then can bang out parts. With metal you need a press and fancy die to push against to get sheet metal with the appropriate shape. Even then you to attach some sort of skeleton which you have to build into it. If a metal panel is broken, fixing it can be a real pain in the butt, but much simpler when repairing composites. Properly made composites are impervious to chemicals, oxidation, water, fuel, oil, etc. The main thing that kills them (at least epoxy based composites) are heat and UV, which is completely different from metal.
You can also get unidirectional rovings for specific applications, say a driveshaft, and this drastically decreases the weight. A lot of production driveshafts are composite these days.
As for picking kevlar... Fiberglass, carbon fiber, kevlar, each have their own unique properties and are useful for different things. They have different modulus, tensile strength, shear strength, impact resistance, abrasion resistance etc. That said, these are usually difference per unit weight or volume. The tensile strength of fiberglass for instance is lower than carbon fiber given the same cross-section of glass, but if you make the cross-sectional area higher, glass can be significantly higher. You should pick based on a specific application.
Generally speaking, Kevlar has better abrasion resistance, but otherwise fiberglass does pretty darn well compared to Kevlar and carbon fiber. It isn't as good per unit weight or volume, but its almost as good and MUCH cheaper.
flatline16 said:
to let you know frp is heavier than an ester based resin with matte.
Say what? FRP = Fiber Reinforced Plastic. Chopped strand mat in ester based resin is a form of FRP as is woven glass, which is what I assume you are calling FRP. That said, chopped strand mat is about as bad as you can get in terms of strength/weight ratio. It is only used in cheap applications where weight is a non-issue (like people building "carbon fiber hoods" for sale to morons on eBay who want the CF look and don't mind the fact that is 2-3x heavier than a straight glass hood that was made with proper techniques and materials).
The reason that multiple layers of woven glass are used is because you may need more strength than one layer can provide. You could use "heavier" glass, but heavier glass is often harder to work with, especially if there is any significant curvature or you have to work against gravity. It is used to improve off-axis performance, but you don't really need more than 2 layers to do that. With two pieces of glass oriented 45 degrees apart from one another, the maximum error angle is 22.5 degrees. That means your worst case scenario is you are 7% off axis (shear). Three layers brings it down to 2%. I've seen aircraft wings made with 2 layers of glass on top of a layer of foam on top of a single layer. And that was it.