Tempered glass

Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the inner surfaces into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as plate glass (a.k.a. annealed glass) creates. The granular chunks are less likely to cause injury.

As a result of its safety and strength, toughened glass is used in a variety of demanding applications, including passenger vehicle windows, shower doors, architectural glass doors and tables, refrigerator trays, mobile screen protectors, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.

Toughened glass is physically and thermally stronger than regular glass. The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. For glass to be considered toughened, this compressive stress on the surface of the glass should be a minimum of 69 megapascals (10,000 psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi). The greater the surface stress, the smaller the glass particles will be when broken.

It is this compressive stress that gives the toughened glass increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin.

Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.

The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarizing sun glasses.

Toughened glass is used when strength, thermal resistance, and safety are important considerations.

The most commonly encountered tempered glass is that used for side and rear windows in automobiles.

It is used for its characteristic of shattering into small cubes rather than large shards and is sometimes referred to as safety glass in this context. (The windscreen or windshield is instead made of laminated glass, which will not shatter when broken.)

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Wikipedia