Tempered glass thermal

The upper use temperature for annealed ware is below the temperature at which the glass begins to soften and flow (about Pa-s or 10 P). The maximum use temperature of tempered ware is even lower, because of the phenomenon of stress release through viscous flow. Glass used to its extreme limit is vulnerable to thermal shock, and tests should be made before adapting final designs to any use. Table 4 Hsts the normal and extreme temperature limits for annealed and tempered glass. These data ate approximate and assume that the product is not subject to stresses from thermal shock. 

Polycarbonate is available monolithically (in a single sheet) in thicknesses up to 1/2 inch. In this range of thickness, polycarbonate is twice as expensive as thermally tempered glass. In thicknesses over 1/2 inch where lamination is required, it is roughly three times as expensive as an equivalent thermally tempered laminated lite. 

Meyers, G.E. "Design Criteria and Acceptance Test Specification for Blast Resistant Thermally Tempered Glass Glazing," Department of Defense Explosive Safety Seminar, Anaheim, CA, Aug 1986. 

Tempered glasses are useful because failure normally occurs under an applied tensile stress, and failure in ceramics and glasses is almost always initiated at the surface. When a permanent compressive stress, called a residual compressive stress, is placed on a surface, either through thermal or chemical means, the applied stress must first overcome this residual compression before the surface is brought into tension under which failure cau occur (see Figure 5.53). Notice that the residual stress is compressive in nature at the surface of the plate and is tensile in the center (shaded areas). When... 

Figure 13.5 Thermal process that results in tempered glass, (a) Initial configuration, h) The glass is quenched to a temperature that is below Tg, which results in the rapid contraction of the exterior, (c) Resulting transient state of stress, (d) The relaxation of these stresses occurs by the flow and deformation of the interior. (< ) Second quenching step results in a more rapid cooling rate for the exterior than for the interior. This results in a glass with a smaller specific volume in the center than on the outside. (/) Final state of stress at room temperature.

If properly introduced, thermal residual stresses can be beneficial, as in the case of tempered glass. 

Qualitatively explain how the following parameters would affect the final value of the residual stresses in a tempered glass pane (a) thickness of glass, (b) thermal conductivity of glass, (c) quench temperature, (d) quench rate. 

Thermal stresses have been used for some time to temper glass and this places the surface in residual compression. Thermal tempering depends on the ability of materials to relax stress. In the cooling of the glass, the temperature of the... 

NOVEL METHOD FOR STRESS INSPECTION OF TEMPERED OR THERMALLY STRENGTHENED GLASS... 

Novel Method for Stress Inspection of Tempered or Thermally Strengthened Glass... 

Because of the lack of symmetry in achievable cooling, glass tubes, containers, and other complex-shaped products can not be meaningfully tempered by thermal means. 

Figure 6.25 (a) Schematic representation of tempering with nozzies injecting cooiing air (grey arrows) and (b) residual stress profile in a thermally tempered glass sheet. 

Figure 6.28 Schematic representation of residual stress generation in a thermally tempered glass sheet. Viscous flow is possible over Tg. Thermal gradient builds in. On passing the glass transition and obtaining uniform temperature, stress builds in.

Such a control of the residual stresses is destructive, so non-destructive methods are preferred (Chapter 11). Notably, the Akeyoshi et al. (1967) approach for thermally tempered glasses can be generalized to chemically tempered glasses (Bouyne and Gaume, 2001). 

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