Tempered glass is made by treating glass with heat or chemical processes. These alter the structure of the glass so that it is much stronger than ordinary glass. Tempered glass also shatters differently. Instead of large, knifelike shards, it forms small pieces, making it safer. It is used when strength, heat resistance and a safe breakage pattern are important: in bullet-resistant glass, vehicle glass, glass-topped tables and other furniture, dishes and cookware, for example.
Mechanical strength is the general ability of a material to withstand stress and strain. The mechanical strength of tempered glass can be four times as much as ordinary glass.
The tensile strength of a material is the measure of how far it can be stretched or pulled before its cross-section begins to deform. The tensile strength of tempered glass is about 65 mega-pascals, or 4276 Kilogram per square inch.
The process of tempering the glass sets up stresses within the material which give it a different shatter pattern compared to ordinary glass. In heat-based tempering, the glass is heated up to a uniform temperature and then the outside surfaces are cooled quickly with blasts of air. This means that the inner layer and the outer layers contract at different rates. The inner layer is in a state of high tension; it can be seen as "trying" to break, while the tough outer surface holds it together. Chemical tempering duplicates this effect using chemicals instead of heat.
When tempered glass is broken, all of this stored energy is released. The outer surfaces are no longer holding the inner layer together, and it springs apart into numerous small chunks. Because of their shape and small size, these pieces are much less hazardous than the large, dagger-like shards produced by normal glass when it breaks.
Thermal Shock Resistance
When subjected to sudden changes in temperature, some materials can crack or shatter. This is due to different areas of the material expanding or contracting at different rates. The phenomenon is called thermal shock. Tempered glass is very resistant to thermal shock, making it a good choice for items that are subjected to extreme temperatures or repeated heating and cooling.
Once it has undergone the tempering process, a piece of glass cannot be cut. Cutting the glass releases the stored energy inside the pane in the same way as other forms of damage, shattering the entire pane. This means that the glass must be cut and shaped before it is tempered.
The bending strength of a material refers to how much it can be bent before it breaks. Glass in general has a low bending strength, and this is true even of tempered glass.
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