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Unread 17-10-2006, 08:34
Tristan Lall's Avatar
Tristan Lall Tristan Lall is offline
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Cost Determination, Section 5.3.4.4

Quote:
Originally Posted by Peter Matteson
Also the cost of the material could in fact be crippling. Remember you still have to account for the material cost in your $3500 additional materials budget.
Quote:
Originally Posted by Al Skierkiwicz
As Peter stated, there are certain constraints that limit the materials. As a rule of thumb, exotic materials cannot be used. The test being are they available to everyone. If you make it will other teams be able to buy the same thing.
I'd be surprised if the cost of the resin and fabric was exceedingly high—fair market value for a couple of rolls of carbon fibre fabric and resin is only a few hundred dollars (maybe $800 USD for enough for a basic frame*). While not exactly cheap, it's a far cry from $3500. Most of the costs for custom composite work come from the labour and the tooling, both of which seem to be covered by the sponsor.

Quote:
Originally Posted by Peter Matteson
Also it may just be heavier than a welded tubing or kitbot frame. I believe it was the 767 where they designed a composite tail to save weight and when all was said and done it weight virtually the same as an all aluminum one.
This is an interesting point, related to the relatively unknown, and to some degree, unpredictable failure characteristics of composite materials. Carbon fibre tends to exhibit a non-uniform response to stress; the material breaks unevenly as the plies delaminate and snap. And once it's broken, the piece is virtually always trash, because of the cracking and splintering that takes place along the material grain (it's highly anisotropic). Also, fatigue of composites is rather unpredictable—it's certainly not characterized in the same manner as steel or even aluminum. Boeing probably didn't want to take a risk with the tail, and ended up building it more strongly to withstand the significant fatigue associated with the application.

Now, given that FIRST robots are regularly subject to some pretty jarring impacts, from strange angles, I think you'd be hard-pressed to design a composite frame that could resist all manner of stresses with a very low probability of catastrophic failure. You'd certainly want to consult with an expert, in order to go this route.

Quote:
Originally Posted by Gdeaver
Also beware that certain prepeg resins are toxic and I wouldn't want to use them around students. Amine allergies are enough to worry about. Putting some fiber in your robot can be a good thing.
Additionally, do not expose yourself to carbon fibre dust (e.g. from cutting). It can irritate mucous membranes (and to some degree, bare skin), and is rather worse when it gets in your lungs. Wear filter masks (and maybe coveralls) if you must cut it, and take measures to dispose of the dust.

*I haven't done the stress calculations to figure out if this "cheap" stuff is enough for your application. Maybe you'll need the aerospace-grade product, which will probably blow the bank.

Last edited by Tristan Lall : 17-10-2006 at 08:46.
 


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