IGMA – ‘Tis the season for thermal stress
In a thermal breakage situation, the edges of the glass are subjected to higher tensile stresses than the surface of the glass away from the edges. The thermally induced edge stress will be of a tensile condition, which occurs when the centre of the glass is heated substantially and the outer edges of the glass, protected by the glazing system, remain much cooler and create tension as the centre portion expands.
Because thermally induced stresses are membrane stresses, both the top and bottom surfaces of the glass are exposed to the same stress. Because of the orientation of the edge stress, breakage at the edge of a thermally loaded glass plate usually results in a crack normal to both vertical and horizontal projections of the edge.
Other factors that may increase the potential for thermal stress breakage are altitude (the greater the altitude, the higher the risk factor), building design (shadows created as a result of the design, such as overhangs and the colour of outdoor glazing stops), orientation (south- and southwest-facing windows are at increased risk), glass edge conditions (any imperfections at the glass edge), retrofit installation of films, the window frame (if the glass is embedded deep in the frame, dark coloured frames can be a risk factor), the kind and type of glass (fully tempered glass has a much lower risk factor for thermal stress breakage and dark tinted glass has an increased risk), the number of glazing lites (tripled-glazed units should be evaluated to determine if heat treating the middle lite will reduce the risk factor), solar absorption and radiation. There are other factors that can increase the risk of thermal stress breakage and a combination of any of these factors can greatly increase the risk of breakage.
So what can a manufacturer do? IG fabricators should advise their customers who can in turn advise consumers of the risk factors. An ounce of prevention is worth a pound of cure and will greatly reduce consumer dissatisfaction with their windows.
IGMA’s publication, TM-1500-14, Guidelines to Reduce Instances of Thermal Stress Breakage, includes detailed descriptions of each condition and recommendations on how to reduce risk factors for thermal stress breakage. Copies of this publication and others are available from the IGMA website at igmaonline.org.