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Name: Bud B.
Status: other
Age: 50s
Location: N/A
Country: N/A
Date: 2000


Question:
Glass can contain a defect that will cause it to spontaneously explode after a period of years, with no obvious cause. This is often seen in auto glass when for no obvious reason a rear window or windshield will explode as if it has been shot out. It seems like th epresence of nickel sulfate or some suce contamination is the cause. What is the cause and the failure mechanism? Here is the origin of my question. Our pastor was simply driving down the street when the rear window of his car simply exploded. No impact of any kind, it simply exploded. I know there is an explanation but cannot find it. I first heard that this was a relatively common condition when I was at a ballistics test lab in Aberdeen, Maryland. A man pulled in and thought his windshield had just been shot out as he drove along the road. He was in a real panic. There were two glass company technicians there and they said knowingly between themselves what I think I remember as "nickle sulfate" and nodded in agreement. The motorist was eventually assured that he had not been shot at and after calling the police he drove away. I never got the explanation from the glass guys and have wondered about it since. I have read your responses on glass as a liquid, (I remember vividly when someone gave me the "Glass is a river" speech) but see nothing that relates to this question. Things do not just happen as a rule, something causes them. I was just curious but will assume that you have not the faintest idea.


Replies:
More likely, the glass broke from "simple" harmonic oscillation. Every object has a natural vibrational frequency. If that frequency is supplied to the object from an external source, the object will vibrate sympathetically. If the external source supplies enough energy such that the elastic modulus of the object is exceeded, the object will break. You have probably seen opera singers break goblets by singing a tone. No chemical impurities in that glass! No defects either, structural or chemical!! (How do they do it? They vary their pitch until the glass starts to vibrate, then hold that pitch until the vibration exceeds the elastic modulus of the crystal.) Where did the vibration come from? Could be any one of a zillion possible sources...

Tim Spry



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