Thermal Stress, Part Two

My previous entry ended with the promise of further discussions about thermally induced stress in glass–and we shall have those discussions. However, before we continue down that road, let’s spend a little time reviewing the three types of glass that are used in the fabrication of products for use in buildings. It will help us when we continue discussing thermal stress breakage in glass. I’m sure that you know what those glass types are: annealed, heat strengthened, and tempered. This is not intended to be an in-depth tutorial but rather a bulleted list of things to keep in mind.

Annealed Glass: The last step in the production of float glass, before it is cut into stock sheets or cut-sizes, is a long trip down the annealing lehr, where the hot glass ribbon undergoes slow controlled cooling to ensure that it has minimal residual stress.

Heat Strengthened Glass: Glass that has been cut-to-size is heated to 1200 – 1300° F. and then subjected to controlled cooling (quenching) with air to create compression layers on both surfaces and the edges, with a balancing center tension zone. To comply with ASTM C 1048, heat strengthened glass must have a surface compression of 3,500 psi minimum and 7,500 psi maximum. It is generally accepted that heat strengthened glass is approximately twice as strong as annealed glass.

Tempered Glass: Glass that has been cut-to-size is heated to 1200 – 1300° F. and then subjected to controlled cooling (quenching) with air to create compression layers on both surfaces and the edges, with a balancing center tension zone. To comply with ASTM C 1048, tempered glass must either have a minimum surface compression of 10,000 psi, or a minimum edge compression of 9,700 psi. It is generally accepted that tempered glass is approximately four times as strong as annealed glass.

Note that heat strengthened and tempered glass are produced on the same equipment. Whether the glass is heat strengthened or tempered is dependent on how quickly it is cooled. Tempered glass is cooled more quickly by using higher volumes of air in the quench process. It is this more rapid cooling that generates the higher compression stresses and corresponding additional strength.

Some details about annealed, heat strengthened and tempered glass

Annealed Glass
· Very flat and reflected objects will be relatively undistorted if properly glazed.
· Can be cut into shapes, drilled, notched, edged, etc.
· When subjected to a 3 s uniform wind load, a stress of approximately 3,380 psi will yield a probability of glass breakage of 8 per 1000.
· For clean-cut glass edges, an edge stress of approximately 2,400 psi will yield a probability of glass breakage of 8 per 1000.
· If it breaks, the break pattern will be such that the glass will typically remain in the opening.
· It is not acceptable for safety glazing applications.

Heat Strengthened Glass
· Will cause reflected objects to have some level of distortion due to roller-wave and bow/warp resulting from the manufacturing process.
· Can not be cut, drilled, notched and edgework should be done before strengthening.
· Under some lighting conditions, the strain pattern induced by the air quench may be visible.
· When subjected to a 3 s uniform wind load, a stress of approximately 6,750 psi will yield a probability of glass breakage of 8 per 1000.
· An edge stress (based on seamed edges) of approximately 5,300 psi will yield a probability of glass breakage of 8 per 1000.
· If it breaks, the break pattern of properly heat strengthened glass will be similar to that of annealed glass and will typically remain in the opening.
· It is not acceptable for safety glazing applications.

Tempered Glass
· Will cause reflected objects to have some level of distortion due to roller-wave and bow/warp resulting from the manufacturing process.
· Can not be cut, drilled, notched and edgework should be done before strengthening.
· Under some lighting conditions, the strain pattern induced by the air quench may be visible.
· When subjected to a 3 s uniform wind load, a stress of approximately 13,500 psi will yield a probability of glass breakage of 8 per 1000.
· An edge stress (based on seamed edges) of approximately 10,600 psi will yield a probability of glass breakage of 8 per 1000.
· If it breaks, tempered glass will fragment into numerous roughly cubicle pieces and may evacuate the opening.
· Is acceptable for safety glazing applications, provided that it complies with applicable particle size requirements.

In my next entry, we will get back to our discussion on thermal stress.


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