Glaze Stresses

These are the stresses that develop in glazes. This happens because of the unequal expansion coefficients of the glaze and the substrate. If the Young s modulus (E) of the glaze and the substrate is the same, then the stresses developed in a thin glaze and in an infinite slab substrate are given by Equations 16.22 and 16.23, respectively. 

Effect of polymorphic transformation on linear thermal expansion coefficient. A silica containing crystobalite phase. B silica containing crystobalite and quartz phases. 

Formation of hysteresis loop in TiOj because of the existence of microfissures. 

These equations give the stresses developed by heating the glazed material from a stress-free state at Tq to a temperature T. The letter j is the ratio of the thickness of the glaze to that of the substrate. 

Even if the glazing is under compressive stress, crazing can occur in service. The reason is that glazes can absorb moisture and expand. Expansion brings about the tensile stress. The solutions for this problem are as follows  

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Polycarbonate It is a tough, transparent plastic that offers resistance to bullets and thrown projectiles in glazing for vehicles, buildings, and security installations. It with stands boiling water, but is less resistant to weather and scratching than acrylics. It is notch-sensitive and has poor solvent resistance in stressed molded products. Use includes coffee makers, food blenders, automobile lenses, safety helmets, lenses, and many nonburning electrical applications. 

The polycarbonate glazing is modeled as a simply supported plate subjected to nonlinear center deflections up to 15 times the pane thickness. Using the finite element solution of Moore (Reference 4), the resistance function is generated for each pane under consideration. Typically, the resistance is concave up, as illustrated for typical pane sizes in Figure 1. This occurs because membrane stresses induced by the stretching of the neutral axis of the pane become more pronounced as the ratio of the center pane deflection to the pane... 

Fig. 4.5.11. Stress curves—elastic recovery for glazes with a high modulus of elasticity (1) and with a low modulus of elasticity (2).

DIN 52 455, part 2 1982 Draft testing of sealing and glazing compounds in building constructions adhesion and extension test stress by water-heat cycling. 

Ceramic glazes are used in ampuls to introduce a controlled-break site. A band of paint is applied at the constriction of the ampule where controlled breakage is desired, so the contents can be withdrawn. The band is used to make Colorbreak ampuls, and its function is to act as a stress concentrator when bending stress is applied to the ampul to break off the stem for product withdrawal. Very consistent ampul break forces are achieved by the use of ceramic bands. 

FIG. 222. St ress development on cooling glazed bodies (a) expansion of glaze and porcelain bodies A,B ib) stress developed on cooling ffroni Klngery, 1960). 

The surface of most porcelain ware is glazed. The main requirement is suitable adjustment of the thermal expansion coefficients of body and glaze, in order to produce a low compression stress in the glaze after cooling. Adhesion of the glaze to the body is very satisfactory as a result of perfect wetting which follows from the similarity of the two materials. Penetration of glaze into the body pores also has positive effects. 

Compressive layers can also be formed by coating a high thermal expansion glass ceramic (e.g., nepheline type) with a low thermal expansion glass, a process similar to glazing clay-based ceramics. The compressive stresses are less than those produced by ion exchange, but modulus of rupture values are increased two to three times over that of the base glass ceramic. 

Polymers have many potential applications In solar technologies that can help achieve total system cost-effectiveness. For this potential to be realized, three major parameters must be optimized cost, performance, and durability. Optimization must be achieved despite operational stresses, some of which are unique to solar technologies. This paper Identifies performance of optical elements as critical to solar system performance and summarizes the status of several optical elements flat-plate collector glazings, mirror glazings, dome enclosures, photovoltaic encapsulation, luminescent solar concentrators, and Fresnel lenses. Research and development efforts are needed to realize the full potential of polymers to reduce life-cycle solar energy conversion costs. 

Materials cost reduction has been achieved through the use of thln-fllm polymeric materials In both the absorber and glazing portions of the collector. The films, attached to a lightweight bent-metal frame, form a set of stressed membranes that contribute to the overall strength of the panel. 

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