Cracking Glass

[Ken Whelan <ken at ubh_com>]

> The more recent example is my car's windshield's gravel-hauler-induced
> crack gradually growing to big cross-windshield gotta-replace-it-now size
> over about three months. Slow, and "bursty".

> Glass cracking can be strange. I would bet your tank probably either isn't
> quite level or the stand isn't quite flat, or maybe there has always been
a
> small defect in the glass somewhere and it was only recently that you had
> the right forces acting on the pane to cause the fracture to propagate. 

>     -Bill

The physics of cracking glass is easy. 

Stress is defined as Force/Area.    At the end of the spreading crack,  the
area at the tip of the crack approaches zero.   This means that the stress
at that point of that fracture is approaching infinity.  Stress =
Force/.0001 (or whatever the area is).  This means the stress goes very
high.

This means that it takes very little force at around the tip of the crack to
make it spread.  That force can be induced by a unlevel aquarium,   the
weight of the water,  stresses induced at assembly or many other things.
Once that crack starts(or nick or whatever) it isn't going to stop by
itself.

Now my terminology is almost certainly incorrect,  (don't remember the
correct terms for stress and force) the theory is what I was taught in a
physics class long long ago.

Later
Ken