295 Stress Evaluation and Common Causes of Breakages

The failure of glass is assumed when the principal tensile stress is equal to or greater than the characteristic strength calculated in Equation 29.5. The bending stress is assumed to vary linearly across the thickness of the plate, and the membrane stress is constant across the thickness of the plate. The total stress is obtained by superimposing the bending and membrane stresses. The stress components at each of the three nodes of the element are then used to calculate the principal stresses within the element. The nodal stresses are averaged at nodes that are attached to more than one element.

29.5.1 Common Causes of Glass Breakage

The causes of breakage for glass can be due to (not in order of importance)

• Excessive stress from wind pressure or other loads.

• Thermal stress due to differential temperature on different parts of the pane (for 33° C, the thermal stress is 20.7 N/mm2).

• Buckling due to large compression (e.g., glass rod and glass fins).

• Surface or edge damage.

• Deep scratches or gouges.

• Severe weld splatter.

• Direct contact with metal (e.g., window aluminum frame).

• Impurities like nickel sulfide (NiS).

• Excessive deflection bringing glass in contact with other hard objects.

29.5.2 Impurities

One big disadvantage in using tempered glass is the problem of spontaneous breakage due to impurities like NiS. NiS is formed when nickel-rich contaminants like nichrome wire and stainless steel are unavoidably introduced into the glass melting furnace, and when they are mixed with sulfur, NiS is formed. They are harmless in annealed or heat-strengthened glass since the induced stress cannot break the tensile failure stress of glass but causes instantaneous breakage when they are located at the tension zone of tempered glass and expand with temperature and time. For surface stress less than 52 N/mm2, NiS is not a problem since its expansion, together with the tensile prestress, cannot generate a breaking stress higher than the failure tension stress of the glass. Therefore, using heat-strengthened glass is a means of solving the problem of NiS. Heat-soaking test is a procedure to break the glass panels

FIGURE 29.12 Glass breakage due to nickel sulfide.

containing NiS in the factory rather than after installation. The time and temperature are important in heat-soaking, and their requirement varies from one country to another. Figure 29.12 shows a picture of glass breakage due to NiS, which is signified by the origin as a pair of butterfly wings.

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