Still skeptical of glass as fluid

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2 total messages Started by Lew Mammel, Jr. Wed, 05 Nov 1986 19:05

#253

Author: Lew Mammel, Jr.
Date: Wed, 05 Nov 1986 19:05

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There have been a number of anecdotal reports of sagging glass windows.
It occurred to me that large telescopes would make a good test case for
this purported phenomenon. The Mount Palomar mirror is old, massive and
its shape is subject to exacting scrutiny.  If a window can develop a
"noticable" shape change in a few decades, I should think the same
phenomenon would be well observed at Mount Palomar. If noticable means
1 mm, then the Hale telescope (It's at Mount Palomar, right ?) is
sensitive to changes one ten thousandth as great. So if it sagged 1 mm
in 50 years, the optical quality would be affected in a few days.

I know that its elastic response to gravity induced stress has to be
dealt with, so what about its inelastic response?

Lew Mammel, Jr.

Re: Still skeptical of glass as fluid

#255

Author: Terry Morse
Date: Fri, 07 Nov 1986 17:29

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If you have any doubts that glass flows at room temperature, look at its
viscosity in the CRC Handbook.  The viscosity is very high at room
temperature, but it is *finite*.  Thus glass can flow a very slight amount
over a long time at room temperature.
-- 

Terry Morse  (408)743-1487
{ hplabs!cae780 } | { ihnp4!sun!sunncal } !leadsv!morse

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