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Regular versus Tempered Glass
Name: Cheyenne
Status: student
Grade: 6-8
Location: OH
Question:
Does regular glass have a stressed
layer/weak layer like tempered glass?
Replies:
Hi Cheyenne,
Regular glass does not have a stressed layer, but tempered glass is
made in such a way that its outer surfaces are in compressive stress.
Neither regular glass nor tempered glass have an intentionally-made
"weak" layer.
Perhaps it would help to understand the way both types are made.
First, you must remember that glass is very weak in tension, and very
strong in compression. When you bend a piece of glass, the surface
that is on the inside of the bend is in compression, but the surface
on the outside of the bend is stretched and therefore in tension.
Cracks start at the outside surface, and once the crack starts, the
glass breaks.
Tempered glass is made by taking a very hot, nearly molten sheet of
glass and cooling it very rapidly (usually by exposing it to a blast
of cold air). This instantly cools and freezes the outer surfaces,
while the inner part is still nearly molten. Then, as the glass sheet
fully cools, the inner part shrinks, but since the outer surfaces were
already cold and hard, the shrinking inner part causes the outer
surfaces to be stressed in compression. So once totally cool, the
inner part is "normal" glass, but the outer surfaces are highly
stressed in compression. When you bend a piece of tempered glass, the
outer surface that normally "sees" tension, is already in compression,
and so resists the tensile stresses, and therefore resists cracking
and breaking.
Normal glass is cooled very slowly, and therefore has no stresses. As
a result, as soon as it is bent, tensile stresses occur and breakage
results.
Regards,
Bob Wilson
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Update: June 2012
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