From: Jeff Sweeney Date: Tue, 27 Oct 92 08:59:05 CST Subject: Composites in the Garage!!?? The following discussion may be of interest to HPVers without access to rec.bicycles.tech. This construction technique seems ideal for the low-volume constructor. bsanders@mrcnext.cso.uiuc.edu (Barry Sanders) relates: Tue, 20 Oct 1992 21:32:04 GMT Yesterday, I had a conversation with a PhD candidate in the Materials Science dept. here at U of I, and also with a world-class racer. We were discussing the potential costs and complexities of actually engineering and building one's own super-radical monocoque frame from composite materials... jss5@ellis.uchicago.edu (jeffrey s sweeney) replies: Wed, 21 Oct 1992 13:49:53 GMT This may be even easier than you think. I have read details of a similar project in a book describing a human-powered plane, the Gossamer Albatross. The constructors made carbon-fiber tubes by wrapping epoxy-impregnated carbon-fiber cloth ('prepreg', available from suppliers serving home-built aircraft hobbyist) around thinwall aluminum tubing. The assemblies were cured by placing them inside a paperboard tube (from a carpet roll) and blowing hot air through it. After curing, the aluminum was etched away by a chemical bath (I don't remember what chemical). drela@athena.mit.edu (Mark Drela) tantalizes: Wed, 21 Oct 1992 22:41:44 GMT Making carbon fiber tubes this way is relatively easy. It is essential, however, to compact the tube during cure with a spiral wrap of heat-shrink tape. The tubes can also be usually simply pulled off the aluminum mandrel if it is prepared properly and enough torsion (+/-45 degree) plies are put into the tube layup. This avoids the etching process, which uses common pool acid (HCl). It's messy, nasty, and the cost of the dissolved thinwall aluminum tubes is very significant. Such carbon tubes are tremendously better than any metal tube in stiffness and/or strength per weight, but they don't tolerate dinging very well. Using prepreg woven cloth for the outside layer like on some of the production frames would help a lot. Making good joints for carbon tubes is critical. On HP aircraft and hydrofoil boats we've had very good success with carbon cloth "lugs" laid up wet around the joint, and lashing the whole thing together with carbon fiber tows to eliminate peeling. drela@athena.mit.edu (Mark Drela) gives more detailed info: Sat, 24 Oct 1992 05:58:52 GMT |> How are the aluminum mandrels properly prepared? Are you using fabric |> or fiber for the torsion plies? Is it prepreg? What tube diameters |> have you constructed with this method? Can you tell us what cloth |> and how many plies you use for your applications? To prepare the aluminum tube mandrel you need to * fill all gouges (any good slow epoxy is OK) * remove all burrs (with fine file and/or emery paper) * polish the surface (car polishing compound works very well) * put on several coats of mold release wax (same technique as car waxing) The layup sequence is * carbon fiber plies * peel-ply * shrink tape (wind on evenly with half-tape-width overlap to avoid bumps in the finished tube) * cook at appropriate temperature Wear clean vinyl gloves during layup. Skin oil is a big no-no. Carefully measure the perimeter of the mandrel to detect any taper. Obviously, the tube must be pulled off towards the smaller end, if any. The tube will pull off relatively easily if the fiber angle exceeds +/-30 degrees but will require hard pulling if it's less than 30 deg. It will practically fall off if it's +/-45 degrees. Typically, we use +/- 40 degree plies together with 0 degree plies in various proportions, depending whether the tube has to take mainly bending or torsion. We've only used uni-directional prepreg for all plies. It has better specifics than cloth prepreg, but is more damage-prone. As I stated, using cloth for the outer layer only is probably a good idea. We've made tubes from 0.25" to 3.5" in diameter, with 3 to ~12 plies. The number and orientation of plies is chosen to match the loads. The necessary calculations are a bit much to post! A really crude approach which is still better than pure guessing is to size the tube thickness like it was going to be 2024 aluminum. The tube will come out lighter, stronger, and much stiffer than the Al version. |> Do you use removable aluminum mandrels to form the lugs? Can you use |> ordinary machine tools to reem the lugs for bearing races? I don't |> understand what you mean by 'tows'. Could you explain this term? To make a joint, the end of the tube is first mitered. This is done by first rough-cutting the miter shape with a dremel cut-off wheel, and then sanding it to a perfect fit with a round cylindrical sanding tube of the appropriate diameter (80-150 grit). The carbon fiber sands quite well. Wear a dust mask. The joint is first jigged and epoxied at the mitered tube edge. After the epoxy sets, carbon cloth fillet patches are then laid up wet all around the joint seam, thereby forming a lug. To answer your question, a separate lug mold or madrel is not used. Enough layers are put on so the finished lug roughly matches the tube walls in thickness. The patch sizes are also varied to get nicely stepped lug edges. While still wet, the joint is tightly wound in a sort of a double-figure-8 pattern (exact pattern depends on the joint angle) with a carbon tow. The idea is for the tow to hold the cloth lug down against the joint seam and prevent peeling at the inside corner. Quite a few tow windings are put on -- maybe 10-20 or so. The tow ends are tacked down away from the joint with super glue and trimmed later. Such joints add very little weight can easily be made stronger than the tube itself. Notes: * A "tow" is a continuous unbraided bundle of carbon fibers. * Hardware like bearing races has to be bonded into the tube. * Do not bond any aluminum against carbon! In the presence of moisture a battery is set up and the aluminum rapidly disappears. Steel is more resistant to this, but it's always a good idea to put a thin fiberglass cloth layer into the bond line to keep the carbon and metal as far apart as possible. Mark Drela _______________________________ o/LO .' O .' Gravity-Powered Technologies Lab .' MIT Aero-Astro Department 33-214 Jeff Sweeney sweeney@dionheinz.uchicago.edu