I grew up in the South (Virginia and Georgia) where, as we all know, it gets really darn hot. We were always told to crack the windows on the cars so they didn't crack. Is that real? Is is even possible? I mean, I'm not sure how much extra pressure would result from heating a car from 70 to 100 degrees F. Also, do you happen to know any real-life cases? Thanks!
This sounds like a question for Mythbusters. I actually submitted a question to those guys once about exploding bicycle tires. My question generated no buzz. So I'll work on your explosive question.
"If you Google 'exploding car window' or 'car window exploded' you find a lot of similar reports, even blog posts and Flickr sets, from drivers of a variety of vehicles. It appears at least at first glance that a lot of people did get it covered by insurance. I would personally let the dealer know because if it was a flaw caused during manufacturing, others could be affected by it as well.
There are a bunch of reasons people give for why the glass could have spontaneously "exploded," such as a defect in the glass, uneven stresses on the window from the frame, a defect in a rear defogger and/or a defogger inadvertently left on. I don't know much about this so I can't vouch for the accuracy."
The majority of these posts are anecdotal in nature and don't really go into the forces required for glass to shatter for no apparent reason.
Modern car glass is actually a laminate. A thin layer of flexible plastic is layered between two pieces of high-strength safety glass. Safety glass is made so that it breaks into small, square pieces of glass. Engineers found that these small pieces were much less likely to cause severe cuts and lacerations that the older forms of auto glass. The plastic inter-layer was added to keep the shattered pieces of safety glass from flying all over the place once it did break. The combination of safety glass with the plastic inter-layer has saved thousands of lives over the years.
Safety glass is manufactured to have built-in stresses. When broken, safety glass shatters into small, square-shaped pieces. Normally, safety glass breaks only when sudden outside forces deform the glass, such as when a rock or a passenger's face strike the glass. Ouch! But is possible to introduce the same kinds of stresses in a gradual way. For example, if a car has been driven around town for an hour or two with the air conditioner on full blast, the car interior (including the window glass) will have cooled down. Accidentally turn on the defroster and the window glass will be cooled even more (with the AC on, the defroster blows cold air onto the windshield). If that car is then parked in full noonday sun (lets say 100 degrees plus), the windshield will begin to heat from the outside first. Now let's assume that one of the two layers of glass was manufactured with a little too much stress. Keep in mind, this glass is in two layers with a plastic laminate in between acting as a tiny insulator. Differential heating will cause the two layers of glass to change dimensions at different rates. The outside, hotter layer will be slightly larger than the cooler, inside layer. And since they are attached to each other by the plastic inter-layer, stresses will build up. The windshield will actually start to bulge out slightly. If the glass is placed under too much stress, it will shatter.
In the vast majority of cases, auto glass is capable of withstanding all of the stresses placed on it by normal daily wear and tear. But occasionally, unusual situation do occur and broken glass will be the result. The fact that many insurance companies pay to replace glass that breaks for unexplained reasons suggests that they also understand that manufacturing defects can cause this kind of breakage.