Glass is a strong but brittle material that is sensitive to small defects on its surface. Where there are stresses on the glass, these defects can lead to cracking and failure. Stress can arise from direct mechanical events such as uniform loads or more localised impacts, or indirectly from different temperatures in adjacent regions of the glass.
In the UK, glazing that faces south or west will generally become warmer than glazing facing east or north. South or west facing glazing is at greater risk of thermal breakage, which occurs when an area of the glass becomes warm but is adjacent to an area that remains cooler. The warmer part of the pane expands while the cooler area does not. This restrains the expansion of the warmer area which is therefore subjected to tensile stresses. If the difference in temperature between the two areas is sufficiently large, a fracture in the glass will occur.
Thermal fracture is characterised by a crack that starts at the edge of a glass pane, initially perpendicular to the edge and face of the pane. It subsequently propagates in a meandering pattern across the glass. The initial crack may be concealed within the glazing rebate upstand or the glazing bead. Imperfections on the edge of the glass will act as regions of stress concentration and stresses will be raised at these locations.
The glass that is used in insulated glazing units is at greater risk of thermal fracture because it will heat up faster and become hotter than in single glazing. Annealed glass will fail at lower stresses than heat strengthened, toughened glass. The likelihood of a pane of annealed glass suffering from a thermally induced crack will be greatly influenced by the condition of the pane edge.
Source: BRE Group: Avoiding cracking in Insulated glazing units (IGUs)
