... and as the surface area under this higher stress is smaller, these bonds break too, extending the crack deeper into the glass until it has broken all the bonds through the glass. This breaks the glass with a visible crack, examples of which can sometimes be seen in doors where they have been flung open and hit a wall and caused a crack in the glass. There is a visible line because there are no longer any bonds between those molecules, and new surfaces have formed at the edges of the broken pieces of glass. How Can Toughened Glass Resist This? Any cracks in the surface of the glass serve to raise the stress in a particular part of the glass. But, cracks can only grow in areas of tensile stress, where adjacent molecules are already being pulled apart. This makes it easy for the cracks to spread. Because ...

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