is fiberglass legal to use on a first robot?

[ChrisH]

Originally Posted by Gdeaver

With composite structures the orientation of the fibers affect it's characteristics. One problem with pultrusions is the large majority of fibers are in the length direction. Because of this point impacts are a problem with Pultrusions. This can be partially addressed by filling the pultrusion with 2 lb, 2 part expanding foam. With pultrusions a good way to increase the point impact strength and increase the load bearing capacity is with tube in tube construction. We took a 2" 1/8 wall and a 1" 1/8 square tube pultrusion. The 1" was inserted in the 2". The space between the 2 on one end was filled with epoxy and chopped carbon fiber for 4" and bolts where set in the epoxy to mount the hinge hardware. The space between the 2 tubes was then filled with the 2 part foam with in 6" of the other end and then that end was filled with epoxy fiber mixture. The resulting structure Has point impact resistance and is stronger than going to heavy wall units and is still very light. With composite work I would strongly recommend using an amine laminating epoxy and not polyester resins. The surface hardness and strength is much better with epoxy.

For most commercially available pultrusions, the only transverse fibers are in a random mat finish ply on the outside. The finish ply has practically zero strength. So transverse properties come completely from the matrix resin. If you use a pultrusion the best way to transmit a large torque into it (like on one end of an arm) is to put it into a socket. Another method is to put a rod up the inside and wrap wet layup fiberglass around the outside. Be sure to use a matrix resin that is compatible with the resin in you pultrusion, epoxy with epoxy and polyester with polyester.

Drilling a couple of bolt holes in the side is a recipe for disaster. If you must do something like this, drill through the top and bottom rather than the side. Use big washers to spread the clamp loads and torque the heck out of it. The whole point is to minimize loads that tend to split the pultrusion open. Though if you do this you probably also want to fill the gaps between the rod and the pultrusion with epoxy or similar so that when you clamp down you don't break it in compression.

Composites can be great materials but the real trick is getting the load in and back out. Bond whenever you can because bolt is a four letter word. Bolts lead to stress concentrations and tend to cause failures in unpredictable ways. Unless you use the specialized fasteners we do here at the bird farm, but $50-$80 each is a little expensive for most teams.