Spontaneous glass damage is a phenomenon in which hardened (or forged) glass can spontaneously rupture for no apparent reason. The most common causes are:
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- Small damage during installation such as a wedge or cracked edge then develops into a larger pause that normally radiates from a defective point.
- Binding the glass in the frame, causing the pressure to develop because the glass expands and contracts due to thermal changes or winding due to wind
- Internal defects inside glass such as nickel sulfide inclusions
- Thermal stress in a glass
- Insufficient glass thickness to withstand wind loads
Video Spontaneous glass breakage
Incorrect installation
When the glass is being moved and installed, it is easy for the glazier to trace or slicing the edges of the glass with various tools. It may also be for fasteners such as nails or screws that are used to install glass jars to nick the edge of the glass if the fastener is mounted at an improper angle. This incision or small chip may not result in direct damage. But over time, when the glass expands and contracts, stress concentrations may develop around the nick, causing damage. In the case of tempered glass, the entire unit is usually broken.
Maps Spontaneous glass breakage
Bind in a frame
The glass expands and contracts with changes in temperature and deflection due to wind, so almost all modern glass is set on a tough block at the bottom and with room for expansion on the sides and top. Gaskets that hold the glass in the frame are also usually resistant to protect the glass from the wind. If no space is provided around the unit, the glass will be attached to the frame, causing internal pressure to expand on the glass that may exceed the strength of the glass, resulting in damage.
Internal defects and inclusions in glass
The inclusion of nickel sulfide ("stone") can be present in the glass. The most common cause of this inclusion is the use of stainless steel machinery in glass manufacture and handling processes. Small shavings of stainless steel containing nickel change structure over time and grow, creating internal pressure in the glass. When this pressure exceeds the strength of the glass, the damage results. This type of damage is almost always found in tempered glass and is represented by a typical "picture eight" pattern, with each "loop" of the number eight approximately. 30mm in diameter.
Alternatively, small pieces of refractory brick can be eroded by a liquid glass from the internal wall of the furnace during processing and become embedded in the finished glass. These are also known as "stones", and can also break glass when the glass is heated, because they create thermal anomalies.
Thermal stress
Thermal stress damage is most common in large sealed insulating glass with a heavy (reflective) absorbent layer. This layer is usually applied to the "number two" surface (the inner face of the outer lite). This causes the outer glass to heat up more than the inside because the layer converts the radiant heat from the Sun into a reasonable heat. When the outside lite expands due to heating, the entire unit is curved outward. If the spacer bar or other edge conditions connect two glass lites in a very rigid manner, the bending stress may expand beyond the strength of the glass, causing damage. This is the cause of extensive glass damage at John Hancock Tower in Boston.
Inadequate glass thickness
A panel that is too large or thin, has not been well engineered for the wind load on the site, can be solved by the wind. See Bernoulli's principle of the wind.
Remedies
Any damage problem has more severe consequences where glass is installed above or in public places (such as in high-rise buildings). Safety film glass can be applied to tempered glass panels to protect from its fall. An ancient precaution is to install a metal screen under a skylight.
References
American Society for Testing and Materials (ASTM)
- ASTM E 2431 - Practice for Determining Flat Resistant Flat Resistance Single Rocky Rock for Thermal Burden.
- ASTM E1300 - Standard Design Practice for Determining Glass Load Resistance in Buildings.
Source of the article : Wikipedia
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