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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.

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