Preventing the scatter of fragments is relatively easy. The windshield is actually a sandwich, with glass “bread” and an elastic plastic “ham” that can be indented without cracking.
When the bowling ball hits the windshield, most of the pieces of glass remain bonded to the plastic instead of flying around loose. But why it breaks into a million small fragments instead of the small number of pieces that you’d get by smashing a sheet of ordinary glass is another question. It has to do with how the glass is tempered, pretreated, to make it stronger.
Windshields, of course, do have to be stronger than ordinary glass. To make a material stronger, engineers often resort to pre-stressing it, subjecting it to certain forces. And that’s what they do to the windshield glass.
While the glass is still at a high temperature after being formed into shape, the surfaces, and only the surfaces, are instantaneously chilled. This locks in the molecular structure of high-temperature glass, which has a more expanded structure than room temperature glass.
When the whole sheet is then allowed to cool slowly down to room temperature, it retains the high-temperature structure frozen into its skin, while the interior shrinks down to the tighter room-temperature structure. Thus, a combination of opposing tension (pulling) and compression (pressing) forces has been locked into the glass, a sort of pent-up push-me-pull-you contest that strengthens the entire structure.
This pent-up energy is released the instant the glass becomes flawed or cracked anywhere. Utilizing this energy, the fracture quickly spreads like a chain reaction over the stressed surface. Because every part of the surface is stressed, the cracks and breaks erupt equally all over it, resulting in the familiar gravel-like pattern of a million pieces.
