[ATM] Re: Second Thoughts on Mirror Support

[stuart452 at earthlink.net (Stuart Hutchins)]

Jeff,

Right material, still the wrong number, tensile strength is not the modulus
of elasticity. Check this link for some average values for silicone rubber:
http://www.matweb.com/search/SpecificMaterial.asp?bassnum=O5200

for modulus E, Cross section A, height H, compression (dh), the force on a
sample is
F = EA(dh)/H.  The tensile modulus listed above is 89 ksi or 89 E3psi.
Further, the squat cylinder pad is glued on top and bottom surfaces, so it's
expansion in Area is restrained.  This will boost the Force seen for a given
(dh) to a higher number than predicted by the bulk modulus.  I'm curious to
see what my test sample says, but it will be slow curing because top and
bottom are covered by the aluminum plates which sort of have to be there.

Regards

Stuart Hutchins


----- Original Message -----
From: "Jeff Anderson-Lee" <jonah@eecs.berkeley.edu>
To: "Mark Holm" <mdholm@telerama.com>
Cc: <atm@atmlist.net>
Sent: Thursday, February 05, 2004 1:07 AM
Subject: Re: [ATM] Re: Second Thoughts on Mirror Support


>
>
> Mark Holm wrote:
>
> > > Mike Lindner wrote:
> >
> >> Jean-Guy Moreau wrote:
> >>
> >>>>>> Does anyone know how compute how much compression a 25mm round by
> >>>>>> 3mm
> >>>>>> thick RTV pad would experience under a 0.306kg load?
> >>>>>
> >>>>
> >>
> >>> From http://www.matweb.com/SpecificMaterial.asp?bassnum=O5200
> >>
> >> Compressive Yield Strength of silicone is 4 MPa or 580 psi.
> >>
> >> Using english units (sorry, rest of the world), a 1"x1/8" cylinder
> >> holding a 2/3 pound mass would be compressed about 0.007". Empirical
> >> confirmation of this analysis is left as an exercise to the reader. ;-)
> >>
> > You need to use the compressive modulus not the compressive strength.
> > Compressive modulus is not listed at the web site referenced, but
> > tensile modulus is.  For small amounts of compression, compressive
> > modulus is often pretty close to tensile modulus, so it is probably
> > safe to  use that number, which is  89.9 ksi  Which means 89900psi.
>
>
> Wrong material.  Try one of the following:
>
> http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDOWC03
> Dow Corning 733 Glass & Metal Sealant  (335 psi)
> http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDOWC12
> Dow Corning 832 Multi-Surface Adhesive/Sealant  (350 psi)
>
> >
> > Also, don't forget to convert that 2/3 pound mass into pound force
> > units.  (Multiply by acceleration due to gravity, but in what units?
> > That is why I usuallly end up converting to metric.  It is a lot
> > easier to figure out the units that way, even though one often ends up
> > with a lot of zeros to deal with.)
>
> The 2/3 lb is in pound force units. ;-)
>
> Jeff
>
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