Why Does A Windscreen Crack In A Wobbly Line & Not Straight ?

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Offline neilep

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Dearest Academical Windscreenologists,

Windscreens are my all time favourite glassy thing that screens me from the wind when I am driving my car ....which is nice !

Please look at this windscreen crack !

[attachment=7748]

Nice eh ?..applied to my neighbours car in the name of science last Tuesday !

Notice how non straight the crack is ?..why's that then ?..why did the windscreen not crack in a straight line ?

If ewe tell me ..I will know....and that would be nice.

Thank Ewe



Neil
My Windscreen Has No Cracks !
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Men are the same as women, just inside out !

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Offline LeeE

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Why Does A Windscreen Crack In A Wobbly Line & Not Straight ?
« Reply #1 on: 26/03/2009 16:45:38 »
My guess would be because the material isn't perfect and that there are tiny variations through it; it's not crystalline, and also because it won't be supported perfectly evenly in it's frame, so different parts of it will be under slightly different amounts of stress, even before whatever hits the windscreen and adds its own localised stresses.

Or, like life, it's just a bit random and proper study of it would probably come under Chaos theory.
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Offline RD

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Why Does A Windscreen Crack In A Wobbly Line & Not Straight ?
« Reply #2 on: 26/03/2009 16:59:42 »
Windscreens are pre-stressed so they break into safe-ish pea-sized fragments rather than fatal dagger-size shards.
The stress in the windscreen screen will not be totally uniform, this could account for the crack not being in a straight line.
« Last Edit: 26/03/2009 17:11:28 by RD »

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Offline Karsten

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Why Does A Windscreen Crack In A Wobbly Line & Not Straight ?
« Reply #3 on: 26/03/2009 19:51:44 »
Why would it crack in a straight line? Would that not be much weirder? Is there much in nature that happens in a straight line or precise angles (other than crystal structures)?
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Offline graham.d

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Why Does A Windscreen Crack In A Wobbly Line & Not Straight ?
« Reply #4 on: 26/03/2009 20:20:29 »
I am not sure this is true RD. Are you thinking of toughened glass? Most windscreens now are made of laminated glass.

My guess is very similar to Lee's. There is also added stress in the area of a fracture, which is why fractures tend to spread. There will be a tendency to go in a particular direction, but the fracture will tend to random walk away from a straight line.

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Offline swansont

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Why Does A Windscreen Crack In A Wobbly Line & Not Straight ?
« Reply #5 on: 27/03/2009 18:35:23 »
Why would it crack in a straight line? Would that not be much weirder? Is there much in nature that happens in a straight line or precise angles (other than crystal structures)?

That's the crux of the issue, I think.  Glass isn't a crystal.  I've cleaved Silicon and gotten a nice straight line, because the crystal axis is the weak point.  There is no corresponding structure in glass.