🚀 go-pugleaf

Kevin Weber writes:
>I once saw a rather amazing display of the effect in question.  A professor
>of mine took two 1/2 inch glass rods.  One he slammed against a table top
>which caused the rod to obviously shatter into a million pieces (yes, he
>did it in a contained area behind a shield).  The next rod he immersed into
>an acid solution (concentrated sulfuric I believe) then rinsed the rod.  He
>slammed it agianst the table.  It did not break!  
>
>Thinking back, you could conclude that the presence of moisture is not the
>sole cause of crack propogation, but also crack geometry.  The acid would
>round the crack tips and prevent propogation.  Correct me if I'm wrong out
>there (and I'm sure you will) but this may be the one result of tempered
>glass, in addition to getting rid of crystalline defects.

The acid technique does work, for the reason listed, but once you
scratch (by handling in any way) such a treated piece of glass, you have
the same problems again, so it is not a general way to increase the
strength of glass.

Tempered glass is glass that has been cooled rapidly in a controlled
fashion so that it ends up with its surface in compression, which is
balanced by the interior being in tension.  This phenomenon is possible
because glass gets denser the more slowly you cool it (the structure has
more time to "relax").  The fact that the surface is in compression
greatly reduces crack growth due to purely mechanical reasons, because
the stress at the crack tip must overcome the surface compression in
order to grow.  I don't know about growth due to chemical reasons, such
as being attacked by those darned polar water molecules, but presumably
it would make that more difficult too.

One of the problems with tempered glass is that the stress contained in
the glass is easily sufficient to shatter it, if it can only get out.
This means that *one* crack that gets past the surface compression zone
will be sufficient for catastrophic failure of the entire part.  This is
why car windows break up into little itty bitty pieces instead of big
shards, but usually are extremely reliable.

I heard a story once (possibly Urban Folklore) about a bunch of car
windows spontaneously shattering in Southern California once.  It seems
it became a popular topic of discussion, because people couldn't figure
it out.  So this psychic said something about the area was cursed by
some dead person or other, and got a lot of press.  Some responsible
people investigated, and it turned out that a factory got its
information wrong about how to install windows, and there was a piece of
metal rubbing on the glass that was causing the catastrophic cracking.

This has been cross-posted to sci.materials and alt.folklore.urban.  If
there are any alt.folklore.urban follow-ups, please delete sci.materials
from the subject line.

				- Rob