When struck, a car window doesn't just crack, it shatters into tiny, rounded, harmless pieces. The glass undergoes this dramatic failure because it's tempered with heat. Tempering makes glass very strong, but as soon as a crack starts, the glass breaks into smithereens.

Now, David J. Green of Pennsylvania State university in State College, Rajan Tandon of the Caterpillar Technical Center in Peoria, Ill., and Vincenzo M. Sglavo of the University of Tresnto in Italy have developed a way to Tempered Glass chemically so that it can withstand some cracking before ultimately shattering. "This is very useful," Green says. "It gives you a bit of warning before failing."

What's more, the strength of the new glass is much more predictable than that of ordinary Tempered Glass. The strength of conventionally treated glass can vary from one piece to the next by as much as 20 percent from the average. The strength of the new glass, however, deviates just 2 percent.

This precision opens up new applications for ceramics, including glasses, says S. Jill Glass of Sandia National Laboratories in Albuquerque, N.M. "Designers of different products have been reluctant to use ceramics because they can't predict exactly when [the materials] will break," she says. Engineers often overdesign a product for safety, making the components needlessly thick and heavy.

All Tempered Glass derives its high strength and its tendency to shatter explosively from the forces between its atoms. Those on the outer surface crowd together while those deeper in the glass remain free of stress. Defects cannot easily break through that outer layer, so only a very strong blow can initiate a crack. Release of the stress triggers the spectacular breakage.

Green, Tandon, and Sglavo figured that changing this internal stress profile could alter the way the glass splits apart. They compressed the atoms about 25 micrometers below the surface, where they could act as a barrier to block the propagation of cracks starting at the surface.

The news come from www.bossgoo.com

Tempered Glass