Toughened glass is processed by controlled thermal treatments to increase its strength compared with normal glass. Tempering, by design, creates balanced internal stresses which causes the glass sheet, when broken, to crumble into small granular chunks of similar size and shape instead of splintering into random, jagged shards. The granular chunks are less likely to cause injury. As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle windows.
When glass is under load or tension it will bend and accommodate stress to a certain level and then suddenly fail once its threshold is met. This threshold for a windshield lite is approximately 9,400 psi. The failure can be sudden and spectacular. Once a crack starts there is little within its structure to stop it from propagating except with a windshield under normal intended use. The 3,000 psi pvb bond to the glass stops edge cracks and stone-breaks from cracking out further which makes repair possible. The inner structure is not mobile glass and does not suffer from dynamic fatigue. Once stress is removed the glass returns unchanged by the experience. A plate of glass in a window exposed to wind over years does not fail because it is constantly being stressed by the wind.
There are processes that increase the strength of glass. Toughening glass takes the glass to a temperature where it becomes flexible again and then it is chilled evenly and rapidly. The outer surface is chilled quicker than the inner glass. Because the core is trying to contract and the outer surfaces are already chilled the outer surface is being compressed while the core is under tension. The compression at the surface is pulling the glass together so any flaws are being closed rather than being free to open. The surface compression is the strength of the glass. It is only when the compressive strength is overcome that the glass will fail. The energy stored in the glass is released and the glass breaks in the characteristic form of toughened glass. The normal compression depth of windshield glass is 21% of the thickness on both surfaces with 58% tension stress in the center.