From: jrtyner@eos.ncsu.edu (JOHN R TYNER)
Newsgroups: sci.materials
Subject: Re: carbon fiber bows
Keywords: composites, bows, etc...
Date: 8 Apr 92 21:14:05 GMT
Article-I.D.: ncsu.1992Apr8.211405.22598
Reply-To: jrtyner@eos.ncsu.edu (JOHN R TYNER)
Organization: Project EOS - North Carolina State University

>>>What is the difference in composition and mechanical properties
>>>between carbon and graphite fibre?
>>>Why carbon fibre materials are generally considered better than
>>>fiberglass?
>>>What is carbon foam (used in construction of bow limbs)
>>>Where to get carbon composites in small quantities?
>>
>>The way we knew it at Hercules was that carbon fibers only had to go 
>>through one 'low temp' furnace while the graphite fibers had to go
>>through both the 'high and low temp' furnaces. The majority of the 

>The difference is in the purity of the carbon content.  There is a
>higher concentration of "real" graphite in the graphite fibers.  Fibers
>are now being made from pitch bases and have high carbon contents, as
>can be seen by the densities (2.1 g/cc for pitch based, versus 1.8 for
>polyarcylonitrile (PAN))

>>Carbon fiber, in general, has a higher stiffness than glass fibers
>>which would allow you to use less fibers for the same stiff bow. The
>>only problem is that the strength is not necessarily as good as the
>>fiberglass. 

>Much higher stiffness!  35Msi compared to 5-10Msi.

>>If I was to design a bow, I would determine what a user wants from
>>the device (a certain shape, an energy distribution, etc...). Then
>>I'd sit down with a flat plate analysis program (a fairly close
>>analogy to the bow) and get the curvatures or stresses that you want
>>for your ideal bow. Materials, thicknesses, and ply orientations can
>>be chosen at this point... Kinda vague?  I know, but I really don't 
>>know what makes a bow good?!

>This approach is acceptable, but not ideal.  If you start with plate
>analysis, you are assuming homogeneous isotropic material.  Consequently
>you will probably end up with a relatively isotropic lay-up.  The actual
>design process (currently) is going to iterative, and not necessarily
>coming up with a true optimum, just a local one.

>Becareful about your starting point for design iteration.

>Also, strongly consider braiding processes for tubular like structures
>(such as the bow.)

I tried designing a recurve bow using composites as a class project in our 
Mechanics of Composite Materials class last semester.  Intent was to develop
a non-symmetric layup of suitable material and ply orientation so that drawing
the bow would create out of plane coupling strains acting to deform the bow 
further.  The real problem was in trying to understand just how the string 
tension and draw force act on the bow limb, and in the required non-linear 
analysis.  A bow is definitely non-linear.  Never finished completely, and 
I'm still looking for info on "what makes a bow good".  If I remember
correctly,
the material I selected had a high E1/E2 ratio, and the preliminary
layup was
mostly 0 and 90 degree plies.  Also, relatively high G.  These helped to
reduce
the undesired coupling strains and curvatures, leaving the ones I wanted
dominating.  If any of you can clue me in to some good information on
bow design
I'm at

jrtyner@eos.ncsu.edu

Posted to alt.archery, but got no response there.

John
Newsgroups: sci.crypt