Moreover, they must withstand small impacts from road debris such as stones as well as road accidents.
Since they are stored outdoors, they are subject to constant heating and cooling as well as dramatic temperature changes throughout the year. Passenger vehicles, for example, have all three requirements. Tempered glass is used when strength, thermal resistance, and safety are important considerations. The strain pattern resulting from tempering can be observed by viewing through an optical polarizer, such as a pair of polarizing sunglasses. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture. The compressive stresses on the surface of tempered glass contain flaws, preventing their propagation or expansion.Īny cutting or grinding must be done prior to tempering. Consequently, annealed glass is fragile and breaks into irregular and sharp pieces.
#Panthershieldz tempered glass crack
Tension applied to the glass can drive crack propagation which, once begun, concentrates tension at the tip of the crack driving crack propagation at the speed of sound through the glass. Annealed glass has almost no internal stress and usually forms microscopic cracks on its surface. As a result of the increased surface stress, when broken the glass breaks into small rounded chunks as opposed to sharp jagged shards.Ĭompressive surface stresses give tempered glass increased strength. For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi). Fully tempered 6-mm thick glass must have either a minimum surface compression of 69 MPa (10 000 psi) or an edge compression of not less than 67 MPa (9 700 psi). The more rapid contraction of the outer layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass.
Tempered glass is about four times stronger than annealed glass. Variations in glass stress are clearly seen when the glass is photographed through a polarizing filter (bottom picture).
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