The comparison to prestressed steel in concrete is a good one. In a simple case, when you bend a beam, the material on the concave side is under compression, and vice versa. If the beam is prestressed (using rods that run along the length of the beam), then you have to bend it more before the convex side is under tension. Tensile loads cause materials to yield. To be absolutely rigorous, it is shear stresses that cause materials to yield. We have to differentiate between stresses and loads. The stress that causes yielding is the same for tempered and untempered glass sheet. It's just that the sheet has the outer surface initially under compression. The way this is engineered is different for the glass and the concrete beam, but the results are similar. As a side note, tempering metals is a different process, but has the same name because it involves controlling the heat treatment. For the tempered glass, the outer surface is cooled faster than the inside, and because hotter material is less dense than cooler material, stresses result. Once the material is cooled to a certain point, the thermal energy of the atoms is insufficient to relax the stresses, and the tempered condition is frozen in. You could reverse the process by annealing - where you'd heat the glass up to a temperature where the stresses relax, and then cool slowly, not quickly.
Strength is not a simple term when you get into the engineering nitty gritty. The stress needed to propagate a crack in tempered and untempered glass is the same, it's just that for the same load, the stresses are less in the tempered material. ]
Hardness is defined as the resistance to plastic deformation, and is a difficult idea to quantify. Really, you indent the material and compare the results qualitatively. That's why there are lots of scales for hardness.
I'm sure there are standards, but I don't know what they are. They would depend on chemical composition, thicknesses, and cooling rates. I'd say that the difference between the max loads for tempered and untempered glass is significant. Again, though, as in my other post, there is a lot of scatter in the data for ceramic materials like glass. For this reason, glass, like other ceramics, are poor choices for most structural engineering applications. Another reason is that when they fail, they fail catastrophically.
G1
Strength is not a simple term when you get into the engineering nitty gritty. The stress needed to propagate a crack in tempered and untempered glass is the same, it's just that for the same load, the stresses are less in the tempered material. ]
Hardness is defined as the resistance to plastic deformation, and is a difficult idea to quantify. Really, you indent the material and compare the results qualitatively. That's why there are lots of scales for hardness.
I'm sure there are standards, but I don't know what they are. They would depend on chemical composition, thicknesses, and cooling rates. I'd say that the difference between the max loads for tempered and untempered glass is significant. Again, though, as in my other post, there is a lot of scatter in the data for ceramic materials like glass. For this reason, glass, like other ceramics, are poor choices for most structural engineering applications. Another reason is that when they fail, they fail catastrophically.
G1
