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RE: glass

About the Glass.  Glass is not a crystal structure.  Some people define it
as a liquid in suspension.  What that means is the atoms no not share the
same orientations as a mineral (quartz) with the same basic chemical
composition (SiO2).  So while the physics of force per area apply to all
substances, the outcomes depend on the nature of that substance and its
malleability (SP?) Also all glass is not created equal.  Tempered glass is
made by rapidly cooling the outside of the glass.  This creates a surface
tension effect as serves to strengthen the glass.  While this same tension
does indeed add strength it also increase forces on the glass itself.  So
when it does crack it is likely to shatter.  Or the crack will propagate due
to the glasses own weight and tension.  Wind shields of cars are tempered,
however they are coated so when they do fail the glass doesn't fly around
and hurt people. So this is why some glass acts differently than others and
why some things bend and others don't....chemical make-up and physical

Date: Thu, 18 Apr 2002 03:23:53 -0700 (PDT)
From: "S. Hieber" <shieber at yahoo_com>
Subject: Re: Cracking the case of the Cracking Glass -- after thoughts

Ken noted that the area at the tip of a crack approaches zero so stress
applied to a crack approaches inifinity at the tip, which propogates the
spread of the crack.  I'm paraphrasing Ken and probably doing an awful job
-- sorry, Ken.

I said there must be more too it because some materials don't just break
apart when they have a scratch.  In fact the same is true for ordinary
(non-tempered) glass under certain conditions.  When you polish out
scratches in glass, you are abrading the surface with fine grit.  You use
finer and finer grit, usually suspended in a liquid medium, until the minute
scratches are rubbed away and replaced with
subscopic scratches.   You repeat these steps until you produce the
desired sheen.  BTW, some kinds of high quality furniture finishes are
produce just this way -- by polishing the "varnish," urethane, lacquer, etc.
Polishing presents millions of places per square inch for the the stresses
to approach infinity, but polished glass can be used for years without
special handling after polishing despite those apparently tremedous odds of
emminent cracking! :-O.

Scott H.

Jim DeAngelo, CFM 
Dewberry and Davis 
METS Division 

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