Thank you for allowing me an opportunity to answer your interesting
question. Look no more...What you are referring to is widely
understood in the scientific world and it is known as ?thermal
stress?. That?s when liquid of an opposing temperature enters into an
already weakened area of the surface (glass, in this case) and causes
it to expand or fluctuate to such a degree that an additional or more
profound defect occurs along the defect's most vulnerable edges. Here
are some excerpts from articles about thermal stress on glass
?If you have any small dings or cracks get them filled. As the weather
drops your windshield must deal with the "thermal stress" of a warm
interior and a cold exterior. A small ding can turn into a very
expensive, spreading crack.?
SMART LIVING NEWS
?Heat also can damage windshields. Cold water on a hot windshield with
a small crack in it can make that crack much worse.?
?Some sweat when the heat is on?
According to this article in FLEET OWNER a company that produces a
washer water heater for automobiles says that its product does not
crack cold windshields because of it?s carefully ?measured and
controlled fluid flow?. Obviously the implication here is that
unmeasured and uncontrolled hot fluid on a cold windshield CAN, or
perhaps in some cases, DOES crack cold windshields:
?According to Microheat, HotShot produces hot washer fluid in about 30
sec. and sprays it on the windshield in intermittent spurts every six
to ten sec., for a total automatic washing cycle time of two and a
half minutes, or until the driver shuts it off. Thanks to the measured
and controlled fluid flow, it won't crack cold windshields, either,
the company says, even though it uses a burst of steam early in the
cycle process to unblock any frozen nozzles in the system and free
frozen wiper blades.?
?In hot water?
Here is a very revealing discussion from an extremely reliable source
? The Cluster Physics Research Laboratory at the University of
?One morning you wake up and there is a thick layer of frost on your
car. Rather than scrape the frost off of your windshield you decide to
pour a bucket of hot water on it to melt the frost. If your windshield
has a small crack in it this may cause the crack to run (become
Answer: When you pour hot water on the cold windshield it will expand
where the hot water is being poured. This will cause stress in the
windshield, which may cause the crack to run.
Why: Again, whether or not the crack runs will depend on where you
pour the water relative to the location of the crack.?
UNIVERSITY OF VIRGINIA
THE CLUSTER PHYSICS RESEARCH LABORATORY
PHYSICS 105 - HOW THINGS WORK - FALL, 1999
PROBLEM SET #9 - THERMODYNAMICS - SOLUTIONS
So you see, it?s not some strange phenomenon about hot and cold and
the fact that it?s glass, this is the same priciple that creates
potholes in the road. When hot and cold cause cracks or defects on the
asphalt to expand or contract the damage becomes stressed and
eventually more profound. Because asphalt is a bit more ?rubbery?
(oil, tar, etc) it can withstand quite a bit more stress that
something much more brittle and unforgiving ? like glass, which tends
to simply crack like an egg shell. As a matter of fact, if you want to
use an analogy with your customers just ask them if they ever taken a
cold egg from the refrigerator and dropped in it hot water. If there
are any weak places or barely visible cracks, they will almost
inevitably become more visible and more profound. Again, ask them if
they?ve ever poured warm tea or soda over cold ice. The cracking and
popping sound is thermal stress shattering the ice cubes. This is
US DEPARTMENT OF ENERGY ? NEWTON BBS
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Tutuzdad ? Google Answers Researcher
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