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While browsing a local news site, I stumbled upon this picture of a broken bus door. The glass was fractured due to a rock impact. As far as i know there are no heating elements on the window section in the picture. Now, what interests me is the apparent periodicity and structure of the fracture pattern.

Are periodic fractures something commonly encountered in tempered glass? Is the mechanism that generates such fractures understood? Is it a consequence of the manufacturing process? Perhaps the way the glass is affixed to the door?

Thanks in advance

Glass fracture

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It's an artifact of production - in particular, the construction of the conveyor belt of the machine performing the quenching process.

The pattern can be observed through a polarizing filter in the undamaged glass:

enter image description here source

and it's a result of the structure of the conveyor belt through which the surface of the glass is cooled to generate the stress that gives it the special properties:

enter image description here source

The contact area of the glass with the conveyor is small enough that it doesn't negatively impact the process - all throughout the surface the stress is introduced in sufficient amount, but the amount of stress applied differs by the "openness" of given area, creating this pattern.

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  • $\begingroup$ great answer- I'll delete mine. your photos are very revealing! $\endgroup$ – niels nielsen Jun 5 '18 at 17:29
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    $\begingroup$ Thanks! I always wondered why I see those patterns while driving wearing my sunglasses. I thought it might have something to do with the heating elements... Mystery solved. $\endgroup$ – ChP Jun 5 '18 at 19:57
  • $\begingroup$ It's totally an artifact? Or, are different patterns considered for functional purposes? $\endgroup$ – zahbaz Jun 6 '18 at 6:53
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    $\begingroup$ @zahbaz: Check this video - there are rollers with helical contact area/support protrusions. If I understand correctly, this constantly changes points of contact throughout the process and results in glass free of these artifacts. (you'll also note this can only produce flat or only marginally non-flat panes of glass; nothing like car windshields or food bowls.) $\endgroup$ – SF. Jun 6 '18 at 9:17
  • $\begingroup$ @zahbaz: I also remember encountering glass that broke into fancy curvy (sinusoidal) strips. But I don't think it has any practical use/purpose, it's just an artifact of a different machine design. $\endgroup$ – SF. Jun 6 '18 at 11:11

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