Why does a toughened window crumble into thousands of small pieces instead of long blades?

The stored energy comes out at once. A crack only advances when something pulls its two sides apart, which is why a compressed layer traps a shallow one. But once an impact penetrates the shell, the tension energy in the core is released, too large for one crack to spread, so it splits again and again. The whole sheet comes apart into short grains.

Ordinary glass breaks into long blades, and toughened glass crumbles into thousands of grains. Exactly the same material, and a different thermal history.

Toughening heats the sheet and then cools its surfaces fast. The outside hardens first and stays compressed, the center cools after it and stays in tension, and those residual stresses balance each other.

The compressed layer is what strengthens it. A crack only advances when something pulls its two sides apart, so scratches and shallow cracks get trapped in it.

But if an impact penetrates the shell, the energy stored in the core is released all at once, too large for one crack to dissipate, so it splits again and again. The sheet comes apart into short grains, which is why this glass is used in cars and in doors. Small pieces can still cut, and a flaw at an edge or an inclusion can cause a sudden break.