Thermal fracturing in glass occurs when a sufficient temperature differential is created within

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) o ...

. As a warmed area expands or a cooled area contracts, stress forces develop, potentially leading to

fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displ ...

. A temperature differential may be created in many ways, including solar heating, space heating devices, fire, or hot and cold liquids. Sloping glass surfaces are subject to greater

solar radiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/ ...

than vertical surfaces and so are more prone to solar thermal fracture. In framed window glass, the edges are relatively cooler than the exposed areas, so space heating devices in very close proximity may cause thermal fracture.

Factors affecting thermal stress

*Solar absorption: the temperature of glass depends on the amount of heat absorbed by the glass. So a high performance solar control glass will absorb more heat. so it will be more prone to thermal fracture *

Shadow A shadow is a dark area where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two- dimensional silhouette, ...

: the presence of shadows will result in relatively cooler areas in glass. Thus it results in temperature difference and may result in thermal fracture. *Edge strength: crack will form if the tensile strength of glass edge exceeds the critical point. Clean cut glass is the strongest and after that polished edge is strongest. *Artificial heating and cooling: if heating or cooling vents are present, the glass can heat or cool excessively and may result in thermal stress. *Frame type and colour: insulating materials keep the edges cool but conduction materials are influenced by their colour. dark colours are more absorptive so causes more heating. *glass type : edge strength of wired glass is less than of ordinary glasses due to weakening caused by cutting processes.

Different types of thermal fracture

*Low energy: Most common type of fractures. It is caused by damage to the edge of the glass. This weakens the edge, so less stress is required to cause the failure. The probability of this type of thermal fracture cannot be determined using thermal assessment processes. *High energy: These are rare and require high levels of thermal stress. The probability of this type of thermal fracture can be determined using thermal assessment processes.

Prevention of thermal fracture

*Low energy: edges of annealed laminated glasses are polished. Inspection is done to find damages in glass. *High energy: glasses are heat strengthened to avoid high energy thermal failure.

References

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