Broken Window without a reason

Meteorite! Or atomic bird poop.

 

[Cliff Clavin]

...and that, Norm, is why you always feel smarter after a few beers."

[/Cliff Clavin]


Fantastic Clavinism! The best!

 

Tempered glass is under extreme stress. It is made by heating the glass very hot, then quenching it with air in a controlled environment. Most tempered glass manufacturers will discuss 'spontaneous breakage" of tempered glass. In over 40 years in the glass business, I have never witnessed it. Most unexplained breakage is caused by physical stress or thermal stress.

 

^^^ Almost certainly there was a very small spot under a lot of pressure such as a stop whose cushion had fallen off or a foreign object wedged in the channel.
Another possibility is vandalism. An old aquaintance who was in the auto glass business once demonstrated the effect of a regular glass cutter on tempered glass. The score would be "hot" with the glass doing a bit of crackling and tiny flecks off glass being thrown off, indicative of the stress Glassman mentions. Anyway, the slightest knock to the scored piece would then explode it.

 

I too believed that "spontaneous breakage " of glass was something car-guys talk about trying to one-up each other with horror stories, until spring of this year. I had worked on my '70 911 on Saturday, preparing it for a Sunday morning drive up the coast with the wife. Put the car away in the garage in perfect, undamaged condition, normal temperatures all night and certainly no signs of visible impact on the glass, and this is what I saw when I went to start the car on Sunday morning. ( The glass was OEM supplied Securit, professionally installed 6 months earlier during restoration).

Interesting response from my insurance agent, when I submitted the bill to him: " Oh yeah, ......we get that all the time, but it's usually the newer cars that have glass which is glued in place".

JZG
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I think it was caused by the "If its rockin don't bother knockin" sticker on the inside.

 

Very informative, Thanks Rob

 

glass is a fairly elastic material in that it bends reasonably well but its brittle and no piece of glass is perfect nor does it fit exactly into the frame nor is the frame perfect. Though the glass manufacturer tries hard to keep out flaws, all glass has micro-bubbles, micro scratches and internal flaws in the grain structure. Glass is also prone to rapid fracture once a crack reaches a critical length and then will grow rapidly.

When glass is installed, it frequently is deformed slightly to mate with the frame and the adhesive sealant will lock it into this pre-stressed state. Much like when a glass cutter scores a surface, if the micro flaws line up and the stress levels are high enough for that short bit of time... pop off the crack goes and the whole thing goes up. The more rigidly its supported... the higher the risk of stresses in the glass pane.

No tempering process is perfect. Pre-stresses exist in the glass from fabrication and installation. In the end, you are just unlucky and it loses it. Thankfully its rare. Some things make the risks higher, tempering it to create "safety" glass, internal tint or shade layers that introduce flaws and stresses (sun roofs), track mounted glass that gets bent and stressed during movement especially when mounted flat like in a sunroof.

We have airplane cockpit windshields that are 1" thick, contain heating element layers, crack stop layers, must be optically close to perfect and must resist speeds in excess of 700 mph and be good to 15psi at 45,000 feet... and they will still occasionally go kablewy... (but thankfully the acrylic layer will keep it intact and in place even at altitude)

nothings perfect.