[TowerTalk] Multiple-layer stress modulus???

[Mike Valentine - W8MM]

Chad Kurszewski WE9V wrote:
> 
> >That got me thinking that my assumtions might be out in left field.
> >
> >For 2"O.D./1.5"I.D. -> pi((2*4)-(1.5*4))/32(2) = 0.537 in*3
> >
> >
> >For 2"O.D./1.75"I.D. -> pi((2*4)-(1.75*4))/32(2) = 0.325 in*3
> >    1.75"O.D./1.5"I.D. -> pi((1.75*4)-(1.5*4))/32(1.75) = 0.242 in*3
> >
> >    Sum of modulii = 0.325 + 0.242 = 0.567
> >
> >0.567/0.537 -> 5.6% improvement from taking it in two layers???
> >
> >Weird, eh?  What am I doing wrong here??
> 
> Mike,
> 
> I don't think that you can add the modulii like that.
> The correct way (from QHS book) is the total OD and the total ID.
> 
> I'm not sure of the reason.
> 
> ---
> Chad Kurszewski, WE9V         e-mail:  Chad_Kurszewski@csg.mot.com
> The Official "Sultans of Shwing" Web Site:  http://www.QTH.com/sos

Hi Chad, thanks for the reply.  I looked thru Dave's book as closely
as I could muster and didn't see the reference; and I was looking there
for it!

I asked an M.E. (I'm an E.E.) at work who used to build hi-performance
bicycle frames, and he thought one just added them.  When I showed him
my example, he thought that it was approximately correct and the
differences were probably due to the formula being a short-cut from the
integrals that tell the real story and that 5% was maybe due to some
dropped polynomial terms that didn't show up in the formula for sake of
simplicity.

What page is your reference?

Thanks again for taking the time.

Mike - W8MM - EM79sd

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com