not sure if the high temps attributed to the small crack in my windshield (about 3") grow to about 10"!! My car is a toyota avalon '04. Someone told me that living in South (or where it’s gen hot), you should always keep one of your windows cracked (no pun intended), or the heat inside car builds up causing any existing cracks in windshield to grow… is this right??? I’ve had other toyota cars that never had this problem… yep, flying rocks on highway that hit my windshield in past causing minimal damage (just the impact crack, 1/4") with no spreading after effects.
It’s got nothing to do with temperature. Windscreens flex when the vehicle moves, and as a result the crack grows. It’s that simple. Cracks in recent car windscreens grow faster than in 40 years old vehicles because the windscreen is actually an integral part of the structural strength of the vehicle. When the chassis flexes so does the windsreen.
I’ve lived and worked in some amazingly warmplaces, and temperature has no effect. Cracks grow just as fats winter or summer. The only way to stop them growing is not to move the vehicle, while rough roads make them grow literally before your eyes.
You can try your own experiment. Put a mark on each end of the crack when you start a jouney and then again when you get home. You’ll notice it has grown. Leave the vehicle parked in full sunlight with all windows up and nothing will change.
Crack propagation was one of the areas in Material Science class that I really did not get. The math is extremely hairy. But any time you have a crack and a source of vibration, the crack will grow.
However, preventing cracks from propagating in your windshield is very easy. For about $40, you can have the crack sealed with an epoxy compound. Most standard cracks simply disappear.
Many insurance policies will pay for this, as it saves them the expense of replacing the windshield.
Wow this is all fascinating to me! I really did think it might have something to do with temperature rather than vibration/ or flexing… I’d like to get that epoxy mentioned, does it have a name and where might I find it?
When a crack appears in something, it creates a ‘stress riser’. Think about the exact point where the crack stops - at this point, any flexing, expansion, or other force on the windshield creates a huge amount of force per unit area on the point where the crack begins. This force causes the crack to split apart even more.
When cracks appear in metal or plastic, a common field repair is to ‘stop drill’ them. This means putting a drill hole right at the start of the crack. Now the stress from flexing is transferred through a nice big circle instead of a tiny point, and this prevents the crack from continuing.
Actually, they can be purchased at most auto parts stores. They use a suction cup bracket and a turn screw device to squeeze the epoxy into the crack. A professional windshield guy might do a better and longer lasting job, but the do-it-yourself deals from the store are cheaper and certainly work.
All the DIY or professionally-installed repairs I’ve ever seen are for impact starbursts that are still small, say < 1/2" in diameter. Once a linear crack has propagated away from the star, it’s time for a new windshield.
My commuter car has a 10" crack because I ignored the starburst and it grew to be unrepairable on about day 3.
If anyone knows of a repair for long linear cracks, please post a link.
Metals have a “threshold” stress intensity below which they essentially don’t grow. When I say essentially, I mean it grows less than a ten-billionth of an inch with each flex cycle. So technically you’re right, but the growth rate falls off a cliff for low stress intensities. I suspect the same thing holds for windshield glass, because I have a couple of tiny spiderweb cracks on my windshield that haven’t visibly moved in at least five years.
This pro windshield supply company claims to be able to “repair stone damage or stone “chips” up to the size of a 50 cent piece, as well as a single long crack up to a length of 24 inches” http://www.windshieldrepair.com/
There are lots more; just google windshield crack repair.
This pro windshield supply company claims to be able to “repair stone damage or stone “chips” up to the size of a 50 cent piece, as well as a single long crack up to a length of 24 inches” http://www.windshieldrepair.com/
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Monkeyfist, huge thanx for the 2 links! My crack is about 10" so I’ll need someone professionally to repair it, which is cheaper than a new windshield… I hope they do a good job!
Anyone who’s interested in this kind of thing might enjoy “Why Things Break”, by Mark Eberhart. I certainly did.
Anyway, since the subject is up, temperature fluctuations can definitely make a difference. Many cracks can stabilize and outlive the vehicle. Others run right away. Still others seem stable but aren’t. Not long ago I had a crack from a stone that was stable from day one for nearly two years. But one winter morning I had to clear ice from my windshield, and when I got in my car, I saw I’d been too rough, and had to replace the windshield.
BTW, many insurance policies include replacing broken windshields at no charge or with a small deductible.
Above, someone mentioned drilling a tiny hole at the end of a crack. This does work well, but the problem is finding the end of the crack, which is usually quite a ways past where you’d guess, from looking at it.
