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What Makes Windshield Glass So Tough?

We tend to think of glass as fragile–but in fact, glass is pretty strong stuff.

A flawless fiber of glass pulled lengthwise is five times stronger than steel! Although glass is quite strong, it’s also very brittle–and the brittleness explains why glass breaks so easily.

But windshields in cars and airplanes are much more resistant to breaking than regular glass.

What Makes Windshield Glass So Tough?

Glass is commonly made more resistant to breaking in one of two ways: by tempering or by laminating it. Tempered glass is first heated just below melting, then exposed to bursts of cold air. The cold cools and shrinks the surface of the glass faster than the inside, compressing the surface inward.

This more dense, inwardly compressed surface makes the glass stronger, so it takes a lot more force to break it. Tempered glass shatters into tiny bits, rather than large, jagged shards like regular glass. If you’ve seen little squares of a car windshield scattered on the street, you’ve seen how tempered glass shatters.

Car windshields are also commonly made from laminated glass, which has a tough, thin layer of plastic sandwiched between two layers of glass. If laminated glass shatters, it remains sticking to the plastic rather than scattering in splinters or shards.

Airplane windshields and bullet-resistant glass take this concept to a whole new level, with several layers of plastic layered with, and bonded to, layers of glass. That makes a composite material strong enough to withstand high pressures, varying temperatures, and even the impact of a bullet or an unlucky bird.