Re: Glass viewport at vacuum chamber
- From: Olin Perry Norton <norton@xxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 06 Apr 2006 11:16:28 -0500
ron wrote:
What should be the glass thickness at high vacuum chamber viewport.Uncle Al gave very good advice.
How should I calculate it?
I would like to have a view diameter of 20".
Thank you.
Please consider a much smaller window, or no window at all.
A dome will be stronger than a flat window.
(Domed to put the glass in compression, NOT IN TENSION.)
You ask a question with serious consequences,
as failure of this window could cause serious injury,
or even death.
Do not rely blindly on the information you get here.
Your design should be reviewed by someone at your institution
(a licensed engineer, or a safety review committee, preferably both)
before you try it. Also try contacting vendors. You're
talking about an expensive window, so they should be willing
to provide assistance.
I remember years ago that a manufacturer of optical windows
(can't remember name, sorry) had a product brochure with a
table showing required window thickness versus diameter and
pressure difference. Later versions of this brochure omitted the table -- I suspect for liability reasons.
[Along similar lines, all I'm trying to do here is point you
in the right direction. I'm not designing this window for you,
and I assume no liability. Your final design should be
reviewed and blessed by someone qualified to do so.]
Normally, the way you would design something like this is that
you would get a formula for the stress in the window from
a source like Roark's Formulas for Stress and Strain, and then
compare the calculated stress to the allowable stress in the
material you're using.
But, for glass, manufacturers seem reluctant to tell you what
value to use for a safe working stress.
This may be partially for liability reasons, but also because
the strength of glass is a complicated subject:
1. It's brittle, so failure is sudden and catastrophic.
Be very generous with factors of safety.
2. Failure is a statistical process, as pre-existing
cracks grow with time under the applied load. The
stress required to produce failure is a function of
how long the stress is applied, and the initial surface
finish.
3. Glass is subject to "static fatigue". Many people
believe that glass will eventually fail under any load,
no matter how small, if you just wait long enough.
Reading material:
http://www.schott.com/optics_devices/english/download/tie-33_design_strength_of_optical_glass_and_zerodur.pdf
http://www.sigmadyne.com/sigweb/downloads/SPIE-5176-3.pdf
http://icom.epfl.ch/publications/pubinfo.php?pubid=519.000
According to http://www.pegasus-glass.com/pyrex.asp :
"The mechanical properties of glass differ from those of metals.
The lack of ductility of glass prevents the equalization of stresses
at local irregularities or flaws and the breaking strength varies
considerably about a mean value. This latter is commonly found
to occur at a tensile strength of about 70 kg/cm2 (1000 psi)."
According to http://www.roymech.co.uk/Useful_Tables/Matter/Glass.html
"The practical tensile strength of glass is about 27 MPa to 62 MPa."
(This converts to 3900 to 9000 psia)
Doyle and Kahan (http://www.sigmadyne.com/sigweb/downloads/SPIE-5176-3.pdf)
state:
"Brittle materials such as glass do not possess a single characteristic strength.
The strength of the material is dependent on the distribution of cracks or
surface flaws. These factors, coupled with the inherent brittleness (cause
of catastrophic or rapid failure) mean that extremely conservative design
approaches are typically used for optical elements made of glass. Determining
a design allowable for glass elements is critical for optical systems using
relatively brittle glass types or for optical elements subject to relatively high
stress levels. Rule-of-thumb tensile design strengths are typically at 1000 -
1500 psi for nominal glass materials."
The CRC Handbook of Mechanical Engineering (pg. 12-97) states:
"For most types of glass a nominal strength of 70 MPa and
a design strength of 7 MPa are typical."
(70 MPa = 10,000 psi and 7 MPa = 1,000 psi.)
Additional considerations:
1. If your window glass is scratched, replace it.
Do not use it. A surface scratch is the beginning
of a crack.
2. Compression is much better than tension.
3. Pay attention to how you support your window at the edges.
Once we built a pressure vessel with a window. To
withstand the pressure we had bought a window made of
sapphire, which has an incredibly high strength. But,
we broke the window as we were tightening the flange
to hold it in place. Apparently we had some direct
sapphire-metal contact, and, due either to a small
burr or pinching as we tightened the flange bolts,
we broke the window. You'd like to clamp this window
uniformly around the edge, with some sort of
compliant gasket or o-ring between the "glass" and
the metal.
MOST IMPORTANT: I apologize if I'm wrong, but your question
causes me to think that you've never had any training or experience
in stress analysis. Particularly in a situation where failure
could cause serious injury or death, I can't think of a worse
situation than having someone who doesn't know what they're
doing relying on advice from a newsgroup. Get help from
a qualified professional. Then have another qualified professional
review the design.
Olin Perry Norton
.
- References:
- Glass viewport at vacuum chamber
- From: ron
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