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Tempered Glass Breakage
 In North America virtually all tempered glass for automotive, architectural and
other applications is made by the thermal tempering process. In this process
the glass is heated to about 1050º F and then very quickly cooled. This
produces glass with a different molecular arrangement in the middle than on the
surfaces. It puts the outside surfaces into high compression (about 25% of the
thickness for each side is in compression) and the middle (50% of the thickness)
into tension. Tempered glass has very high surface compression and thus is very
strong.
Photo courtesy of
Siegfried Herliczek, safety glass consultant of Glassig, Inc., Petersburg, Michigan
Email:
sig49270@worldnet.att.net
To break any piece of glass (tempered or otherwise) one needs two requirements,
a flaw in the glass such as a crack or chip and tension at the flaw.
Tempered safety glass is about five times as strong as normal glass and
difficult to break by impact with a blunt object and normally is very stable,
but there are several reasons why a piece of tempered glass may, however, break
spontaneously.
If the glass has a thin compression layer on the surface (due to the production
process), usually near the edge, a small crack (that gets bigger with time and
vibration) may eventually cause the glass to break.
If your tempered boat windshield had a poor edge finish or the glass was in
contact with a metal object then the vibrations from your boat could produced a
deeper and deeper flaw. Finally, when the rub or scratch gets deep enough and
enters the tension area of the glass, it breaks.
Finally, if the glass contains a nickel sulfide pellet in the tension area of
the glass it could lead to spontaneous failure. The nickel sulfide crystal
changes size with time and produces a crack in the tension area which causes the
tempered glass to fail. Nickel sulfide pellets are formed in the glass
manufacturing process from nickel found in many things including stainless
steel. Glass manufactures take great care to avoid nickel contamination of
their glass.
It is of interest to note that when tempered glass breaks it forms a break
pattern with many radial cracks all leading away from the original break point.
Thus, it is relatively easy to find the original break point, which can lead to
a possible explanation for the breakage.
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