IGMA - 'Tis the season for thermal stress

‘Tis the season for thermal stress
Margaret Webb
February 05, 2018
By Margaret Webb
In our northern climates, where temperatures can plummet to -30 C and colder, consumers can see an increase in glass breakage in their windows.  This year in the Ottawa area, an unprecedented number of glass breakages have been reported with temperatures in the -39 C range with intense sun for days on end. It’s a chilly time.

One of the common factors that can lead to breakage of annealed glass used in insulating glass units is thermal stress. How do you identify a thermal stress break from other types of glass breakage? Thermal stress breaks start at 90 degrees (perpendicular) to the glass edge. Depending on the degree of thermal stress, the crack may be a single break or a branch off in multiple directions. It all depends on how much stress needs to be relieved.

Breakage due to thermal stress involves the occurrence of a thermal gradient induced by uneven heating of glass by either solar irradiance or other heat sources. When sunlight impinges on a glass lite, some energy is reflected from the surface of the glass, some energy is absorbed by the glass and some energy is transmitted through the glass. Energy absorbed by the glass increases the temperature of the glass above the previously existing equilibrium condition. When glass is uniformly heated and if the support system can accommodate the thermally induced expansion of glass, no major stresses will be induced by the uniform temperature increase. However, the part of the glass surface that is shielded from direct sunlight by the edge supporting system or by shadow conditions will be heated unevenly.  The uneven heating of glass will give rise to thermally induced in-plane tensile and compressive stresses.  When the thermally induced tensile stresses become high enough and interact with certain edge conditions, a thermal break can result. This can be especially true on these cold days with the sun beating down on windows.

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.

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