The Mystery of the Propagating Windshield Crack

Surely, any driver who has spent at least one winter in the north has had a windshield chipped by gravel thrown up by some passing vehicle. About the only thing a person can do in such a situation is to inspect the seemingly insignificant damage, shrug his shoulders, and muse (as did Jeeter, while viewing a crumpled fender in the 1941 movie, Tobacco Road), "Well, it don't hurt the runnin' of it none."

Although most of use have had this experience, the shocker usually comes the next day when we climb in to go to work, and discover that this "little nick" has spread halfway across the windshield.

What I would like to know is: Has anyone ever seen this phenomenon occur? All the cars that I have driven up here without exception have ended up with windshields looking like jigsaw puzzles, but somehow, it all seems to have happened when I had my back turned.

Just several days ago, I was leisurely driving along a freshly gravelled road on the University (Fairbanks) campus when this idiot came barreling at me from the opposite direction at a speed that must have been close to 15 mph. Then, crack! ... I got a little chip on the windshield. I was only a little annoyed at the time, because the guy had all day to get past that place.

But when I left for work the next morning, I was flabbergasted to find that "little chip" had turned into a crack about six inches long. Even worse, I found when I turned out the very next day, that this monster was creeping all over my windshield.

I am sure that many motorists have had the same experience. But why does it happen, and why doesn't it happen when we're around to watch?

Apparently, the answer lies in the varying rate of expansion and contraction of different materials (known in loftier circles as the thermal coefficient of expansion). We do not have a garage, so the truck that I was driving was subjected to a wide range of temperatures between the times that I came home and the following morning. Metal has a higher thermal coefficient of expansion than glass. The truck frame apparently warped enough during the night to place the stress on the windshield frame necessary to crack the glass further.

Also, once a blemish, such as a crack, has been made, stresses concentrate on the end of the crack, and further breakage is almost guaranteed to continue along that same line (faults producing earthquakes behave the same way).

If you get a chip on the windshield and it results in a conchoidal (dish-shaped) fracture that shows no obvious outward rays, you probably don't have to worry about it spreading across the whole scene. But if you get one which shows little cracks extending out from the center, it's an almost certain bet that it will spread.