Re: ATM Source for Fiberglass, Carbon tubes

[Kurt Hillig <khillig@Chem.LSA.UMich.Edu>]

On Mon, 9 Jun 1997, John D. Gwinner wrote:

> > > Here's the mechanical properties of flat 'bar stock' like stuff:
> > > 
> > > Tensile Strength 390 ksi 
> > > Tensile Modulus 25 msi 
> > > Flexural Strength 272 ksi 
> > > Flexural Modulus 22.5 msi
> 
> [snip] ...

> ok, but is this a good stiff rod?  (that's what I meant by 'interpret',
> sorry for not being clear earlier)

Sorry for misinterpreting the question; unfortunately, ther estill isn't
enough information to give the answer you want.

Stiffness of an element depends on the modulus of the material, but far
more importantly it varies as the cube (I hope I remember that right) of
the moment of inertia of the element.  (Moment of inertia here refers to
the moment of a cross-section of the element, around the "neutral axis".)
So the shape of the element is much more important than the intrinsic
stiffness.  

4" diameter x 1/16" wall aluminum tubing has the same weight per foot of
length as 1/2" aluminum rod, but it's many orders of magnitude stiffer. 
Since stiffness varies with the cube of the moment, and the moment goes as
the square of the dimension, a factor of 10 increase in modulus allows
only a 30% reduction in size.

>
>  [snip]
> 
> My hunch was that it might be possible to use a reasonable cost carbon
> fiber tube by shifting to a smaller size than the Al as we may have a
> stronger material.  Apparently not ... 

>From what I've seen, composite (fiberglass, graphite, etc.) materials are
significantly more expensive than aluminum of the same overall strength
and stiffness; where they are best used are in areas where weight really
needs to be minimized - airplanes, bicycles, etc.  

                            Dr. Kurt Hillig
  UMNet Administration     I always tell the    fax (313)763-2180
 University of Michigan     absolute truth     phone (313)647-8778
Ann Arbor, MI  48109-3297    as I see it.    X.500 khillig@umich.edu

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