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The science of windshield cracks

Flipping through an issue of Nature once, I noticed that two scientists were spending hours pushing warm panes of glass into cold water to see how cracks form. Not everybody's idea of great time, but the researchers learned something about the complex processes involved with fracture; pretty useful stuff for engineers involved in bridge building.

For wisdom on Alaska's favorite glass crack, the windshield variety, I called Michael Marder, a physics professor at the University of Texas in Austin who wrote the Nature article. Marder said he wasn't a windshield expert, but he called me back the next day after curiosity had driven him into the university's library.

He told me a windshield isn't one solid piece of glass. As anyone can attest who has succumbed to the irresistible urge to heave a cinder block through a junkyard car window, a layer of clear plastic is embedded within car glass. This plastic, installed to keep glass from flying about in an accident, is sandwiched between two layers of glass. The entire windshield is about as thick as a pile of five dimes.

During windshield manufacture, glass is pressed into plastic from both sides with a pressure of about 800 atmospheres, which is about 800 times the force Earth's atmosphere exerts on you as you read this.

Auto glass formed under such stress shuns blows that would shatter house window glass. Car windshield glass would be as rock-resistant as granite if it were not for invisible flaws, Marder said.

Gravel given wings by cars in the opposite lane often finds a weak spot in the glass, leaving behind a pitted, round indentation. Flaws such as this are the seeds from which cracks germinate.

Water vapor in the air, even in tiny amounts, is often the fuel that keeps cracks running across the windshield. Water molecules act like scissors with edges no thicker than an atom, travelling to the tip of a crack and snipping glass apart.

Temperature differences also cause cracks to grow. If a windshield's inner surface is 70 degrees on a 40-below day, a war is being waged within the glass. The cold outside surface of the glass contracts as the hot surface expands. At the interface, cracks expand.

Both the water vapor and temperature scenarios need another element to lengthen cracks—bumpy motion, which pulls the glass apart. Although he hasn't visited the state, Marder guessed rough roads are pulling apart glass in Alaska cars and trucks.

Marder compares a crack to the tip of a lightning rod. But instead of sitting passively waiting for the air's electrical discharge, a crack rushes to meet the energy acting upon the windshield.

If caught early, that crack can be stopped. Windshield repair people recommend drivers get in quickly after a rock has dimpled the glass. That's so the indentation doesn't get filled with impurities such as dust or glass cleaner that hinder the repair process.