It is possible for windows to break due to uneven heating in a phenomenon known as thermal fracture. It can occur in ordinary window glass and in laminated glass, but not in tempered glass.
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It's most common on a clear sunny day after a cold night.
The centre of the glass is heated by the sun and the glass expands while the edges remain cool. The heated glass pulls on the cool glass at the edges, and cracks in a characteristic pattern.
The glass is stretched at the edges which causes small cracks to grow and when it reaches a critical length it fractures. The cracks are driven by stretching of the glass from the edge and turn away as they approach the compression zone at the centre. Their distinguishing feature is that they come out perpendicularly from the edge and do not travel into the centre.
Thermal fracture is more likely to happen if:
■ There are sun shades that keep one edge of the window in shade while the rest of the glass is heated;
■ Solar glass traps heat and gets hotter than ordinary glass.
■ Internal blinds that are too close to the window can trap heat in the airspace
■ When glass is cut, the edges are polished to remove any nicks or scratches. If the cracks are too big the glass will break easily.
Investigation of thermal fracture in a Brisbane building found that the centre of the glass could reach 65 degrees while it was 25 degrees cooler at the edge. In Brisbane the worst times for thermal fracture are April-May and August-September. Summer is the least problematic time because the sun is high in the sky for most of the day and does not shine directly at the windows.
It's unlikely that the actions of the occupants will cause thermal fracture. However, tinted adhesive film added after the window is installed has been known to cause it.
The proposition that post-it notes can cause thermal fracture seems unlikely, and is even harder to imagine in double-glazed panels because the inner pane is insulated. It is conceivable that a large poster on a single pane window might cause thermal fracture, but ideally installations should be designed with a large enough safety margin.
Response by: Professor John C Barry, faculty of science and technology, QUT
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