Science Scene - Shattered

Have you ever wondered why some forms of glass shatter into small pieces that are less likely to be jagged and sharp?  I know that as an engineer, these types of thoughts occasionally cross my mind.  Below is the explanation (plagiarized from the July 2008 Popular Science) of how tempered glass is made.  Thanks for keeping us safe scientists and engineers of the world :o)

 

The PopSci article describes the making of extremely strong glass droplets, with a tail.  You can not crush the droplet, but if you break the tail, the whole droplet shatters as the pent up energy is released.  The description is that when molten glass hits cold water, its outer surface cools rapidly and shrinks as it solidifies.  Since the center is still fluid, it can flow to adjust to the outer shell’s smaller size.  As that center eventually cools and solidifies, it also shrinks, but now the outer shell is already solid and can’t change its shape to accommodate the smaller core.  This creates internal stress, as the center pulls the outside in from all sides.

 

Tempered glass, common in cars and glass doors, works the same way.  Jets of cold air are used to rapidly cool the surface of hot sheets of glass, creating a milder internal tension that keeps the surface compressed at all times.  Tempered glass is extremely strong, but the pent up tension is released when the yield strength is reached, resulting in the shattering effect that prevents large shards of glass from being formed.

Tags: Popular Science, blogplugs, Tempered Glass, Science