Glass windows are fitted within frames, and the thickness of the frame is larger in comparison to the glass pane. During a fire, there is an intense heat flux that is suddenly experienced on one side of a glass pane. As a result, there is a sharp thermal gradient created across the thickness of the pane.
The rapid heating caused by the fire is called ‘thermal shock’, while the deep gradient created is called ‘thermal stress.’ Thermal stress causes the pane to break.
The thickness of the frame is greater than that of the pane. During a fire, thermal stress is focused on the central region of the pane, and the fire transfers thermal heat as thermal radiation onto the glass pane. The edges/rims are shielded from radiation and convective heating. The edges lose energy/heat into the frame by conduction through the pane.
As a result, the heat focused on the central region of the glass causes it to expand. While the edges of the pane happen to be relatively cooler, this induces stress in the material. This shifting gradient is known to cause the breakage of glass.
Even in cases of uniform heating, or when the glass window experiences a uniform thermal load, the sides/panes of the glass remain relatively cooler. This is because the heat from the sides would dissipate into the thicker window frames, rendering the sides cool.
The cracking of glass happens before breakage. Cracks originate from the sides of the glass embedded in frames. The time between the glass first cracking and completely shattering is dependent on the type of glass and the nature of the fire.
Not all glass windows break at the same temperature. Different types of glass have different thermal resistance capabilities.
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