[ATM] Re: Second Thoughts on Mirror Support

[stuart452 at earthlink.net (Stuart Hutchins)]

Yes, testing is essential to have any quantitative idea what we are talking
about, even for ballpark numbers. I will measure compression and shear
modulus on my single sample tomorrow, but I don't know the state of cure,
and I don't want to destroy it yet.  Data point of one.

----- Original Message -----
From: "Good, Donald" <dgood@aha.org>
To: <atm@atmlist.net>
Sent: Sunday, February 08, 2004 9:42 AM
Subject: RE: [ATM] Re: Second Thoughts on Mirror Support


> It does not appear that the Modulus of Elasticity is given in the MatWeb
> data for specific silicone adhesives nor in manufacturer web sites.  And
> while the tensile strength is not the value used to calculate the forces
and
> flexibility, it should not be ignored since it is the point at which it
> breaks.

Yield and ultimate strengths are in the 300 to 500 psi range.  This will not
be approached in use. It is flexibility that determines how much it bends
the mirror.

 > It should not be too difficult to build a test stand that can load a
sample
> at a constant rate and measure the deformation.  Most important would be
the
> Modulus of Elasticity for tension (Young's modulus) and shear (modulus of
> rigidity)
> Testing should be for various thicknesses of adhesive, at various
> temperatures, for various surface conditions (polished and rough), and for
> various adhesives and base materials.

The unknown is the compliance of an RTV mounting pad. The base materials are
all so much stiffer, they will have little influence, and can be modeled
separately.  Failure is not the question. Surface bond is not the question
either, though it might be interesting.  Measuring various thickness and
diameters of pads is important, because the cylinders are all squat and fat
and glued top and bottom.  This restrains the expansion/contraction that
occurs when loading in compression/tension (Poisson ration), and increases
apparent spring constant of the assembly.  I don't think shear modulus is
affected the same way.

Stuart Hutchins