Thermal stress is the tension or compression that occurs in a material when it undergoes a change in temperature. Glass is a poor conductor of heat, which means that different parts of a glass pane can have different temperatures at the same time. For example, if one part of a glass pane is exposed to direct sunlight and another part is shaded by a curtain or a blind, there will be a temperature difference between them. This temperature difference causes the glass to expand or contract unevenly, creating thermal stress.

If the thermal stress exceeds the strength of the glass, it can cause the glass to fracture. The fracture usually starts from the edge of the glass, where the stress is highest, and propagates towards the center. The fracture may appear as a single crack or multiple cracks in a pattern. The plastic interlayer in laminated glass may hold the broken pieces together, preventing them from falling out of the frame. However, this does not mean that the laminated glass is safe to use after cracking. The cracked laminated glass may lose its structural integrity and performance, and pose a risk of injury or damage.

There are several factors that can increase the temperature difference and hence the thermal stress on laminated glass, such as:

• The type and thickness of the glass: thicker glasses and glasses that absorb more solar radiation (such as tinted or coated glasses) are more prone to thermal stress than thinner glasses and glasses that reflect more solar radiation (such as clear or low-e glasses).
• The quality and condition of the glass edges: poor quality or damaged edges can reduce the strength of the glass and make it more vulnerable to thermal stress. Shells, vents or shark teeth on the edges are common defects that can trigger thermal stress fractures.
• The type and position of shading devices: shading devices such as curtains, blinds, louvres or films can create uneven heating on laminated glass by blocking or reflecting part of the solar radiation. This can be especially problematic if the shading device covers only part of the glass pane or if it changes its angle or position during the day.
• The type and design of the framing system: framing materials such as metal or wood can have different thermal expansion coefficients than glass, which can create additional stress on laminated glass. The size and shape of the frame, as well as the presence of thermal breaks or spacers, can also affect the heat transfer and ventilation around the laminated glass.
• The location and orientation of the building: buildings that face south or west are more exposed to direct sunlight than buildings that face north or east. Buildings that are located in areas with high temperature variations between day and night or between seasons are also more likely to experience thermal stress on laminated glass.