MadSci Network: Engineering |
Since you asked two questions, I have
divided my answer into two parts. Safety Glass Wire mesh reinforced glass
is a form of safety glass. Safety glass is used in applications where you
want to contain any glass shards that may be created when something strikes
the glass and breaks it. One of the most common applications is automobile
windshields. If you were to look at a cross-section of your windshield you
would find two layers of glass with a thin layer of transparent plastic
sandwiched between them. Safety glass was originally patented by French
poet-chemist Edouard Benedictus who observed
that a test tube lined with a thin film of a nitrocellulose mixture broke
but did not shatter. He patented this discovery in 1910. In 1926 the
Triplex Safety Glass Co. of North America licensed the rights in America
and began producing safety glass. Among their first customers were Henry
Ford and the Ford Motor Co. The initial price of the glass was $8.80 per
square foot. Wire reinforcement is used where there is a desire for
security as well as containing the glass if it breaks. The wire mesh is
embedded in the glass during manufacturing. When the glass is broken the
bond between the glass and wire keeps pieces for falling out. The wire mesh
also stops someone from forcing their way trough the broken glass as would
be possible if there was just a thin layer of plastic. Currently there are several new products coming onto the
market that use advanced plastics in place of the wire mesh. ShatterGard for example has a
plastic film 0.014" thick that can be applied to
glass. These new plastics can withstand the force of a hurricane such as
Andrew which decimated southern Florida as well as stopping criminals. Tempered Glass Glass will only
fail under a tensile (pulling) load. A crack will start at a defect in the
surface and
propagate through the thickness of the glass. In normal glass the cracks
tend to go long distances quickly resulting in large pieces. Because of
this, the strength of soda-lime glass has typically been around 4,000
pounds per square inch. Tempered glass is made by heating ordinary glass in a
furnace to the point where it almost sags under its own weight and then
rapidly chilling it. The outer surfaces cool quicker than the interior.
Therefore compressive stresses are formed at the surface. The rapid
cooling essentially locks those stresses into place. There is a
corresponding tensile stress in the center of the glass, but it is far away
from flaws that will start a crack. Now when the glass is subjected to a
tensile load the tensile load must overcome the compressive stresses and
the inherent strength of the glass before the glass breaks. Because of
this the glass is much stronger, often as much as five times stronger or
20,000 pounds per square inch. The tempering also makes the cracks branch
rather than continue in a long line. So when the glass breaks it will
break into small pieces rather than large pieces. A more detailed explanation of the stresses and the
processing including illustrations can be found in Introduction to
Ceramics, 2nd Ed., W.D. Kingery, H.K. Bowen and D.R.
Uhlmann, John Wiley & Sons, (1976), pp. 830-833
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