🚀 go-pugleaf

In article <1992Oct28.223610.203965@ns1.cc.lehigh.edu>,
ps02@ns1.cc.lehigh.edu (PENELOPE E. SMITH) wrote:

> Question:  Suppose I want to build glass bookcases anyway.  How fast will
> those cracks grow?  Can I make my shelves heavy enough, say, to last at least
> 100 years?  Does it help to smooth the stress concentrations at the supports?
> -- 


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.

Obviously the success of glass shelves is very dependant on the loading. 
Glass is extremely common in shopping malls, so its not that it shouldn't
be used for shelves.  "Smoothing" stress concentrations at the supports
will do no good.  If "smoothing" would work you would have to do it at the
greatest surface tensile concentration, which occures on the underside of
the shelf at the point farthest (in general) from any support (assuming
even loading), ie at the greatest bow or bend of the shelf.



kevin weber      weber@jedi.ssd.lmsc.lockheed.com
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