Why does the escape route window break in a bus or something when they are hit with those tiny orange hammers (emergency hammer?) with metal tips. I mean you can’t even get much force to the hit because the handle is so small you can’t use do a good swing.
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Tempered glass is made in a special way so that the outer layers of the glass are in compression, and the inner layer is in tension. That is, the outside is being squeezed really hard by the inside that is being stretched really hard.
Normally glass can’t take high tension (stretching) loads without cracking. But the outer layers in compression prevent a crack from starting in the inner tension layer. The sharp point of little hammer is just enough to put a small chip in the outer protective layer. This lets a crack start in the inner inner tension layer. Once the crack starts, its all over. The crack runs (**very** fast) throughout the entire piece of glass.
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