Miata Mailing List: April 1994, Message #88
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   From: Ken Warren 
Subject: Re: windshields
   Date: Tue, 5 Apr 1994 21:45:33 -0400

Having had windsheilds replaced in every car I've ever owned, *including* 
the Miata, I suspect I'm becoming an expert. The cracks you see in glass 
*are* stress fractures. Where does the stress come from? In the case of 
tempered glass, it's part of the manufacturing process most likely. In 
the case of other glass that cracks (i.e. windshields), it's often due to 
having a piece knocked out. (can you say stone chip?) There are other 
failure modes, but these are the most common for auto glass.

Once you get a stone ship in your windshield, the process from there on 
depends on lots of factors: temperature, how much you drive your car, how 
*fast* you drive your car (figure out the total air pressure on your 
windshield at 70 mph; you might be surprised at the number), how big the 
chip is, and *where* the chip is. 

If it's cold, the glass is more "brittle"; it doesn't flex as easily and
is more likely to crack. If you drive your car a lot, the shaking will
cause the crack to grow. If you drive fast, air pressure will cause the
crack to grow. A big chip is more likely to grow a crack than a small one,
especially if there is a small amount of spiderwebbing around the chip.
(Get these epoxy-injected ASAP;  it often prevents cracks.) A chip near
the edge of a windshield is more likely to grow a crack than one in the
middle; the stress is already unbalanced at the edges of the glass.

If a crack starts, there are a couple of things you can try to keep it
from spreading. You can: apply a small dab of clear nail polish to the tip
of the crack, on the outside. This can help stabilize the crack, and will
also relieve wind pressure by spreading it out. You can take a glass
cutter, and make a tiny nick at the very tip of the crack. This helps
spread and relieve stress at the tip, where stress is highest. Beyond 
this, your only alternatieve is a windshield replacement, as far as I 
know. Once a crack starts to grow, there's no way epoxy injection will 
fix it. As a matter of fact, you can't even epoxy-inject the chips most 
likely to crack; the device they use to ream out the chip is a couple of 
inches across and needs to sit directly on the glass.

Hope this answers some questions.

--
Ken
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           Ken Warren               E-Mail: kenw@netaxs.com
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