Glass strengthened by quenching a heated sheet with air so that compressive stress remains at the surface and tensile stress inside. When it breaks, it shatters into small granules.
Japanese name: 強化ガラス
Tempered glass appears in 3 practice questions across 1 topic, including Glasses and amorphous materials.
Fracture of glass starts when tensile stress acts on tiny flaws at the surface. Tempered glass has compressive stress left in its surface in advance, so that even when tension is applied from outside, the flaws do not open until that compression has been cancelled. As a result, its strength against bending and impact is about 3–5 times that of ordinary annealed glass. At the middle of the thickness, a tensile stress that balances the surface compression remains. The essence is not that the glass material itself has become stronger, but that residual stress keeps the surface flaws closed.
Sheet glass is heated to about 650 °C, above the glass transition temperature, and quenched by blowing air evenly onto both faces. The larger the temperature difference between surface and interior during cooling, the larger the stress that remains. It is therefore hard to create the temperature difference in thin sheets, and it is difficult to temper sheets thinner than about 3 mm adequately by air quenching. Also, the remaining stress is roughly proportional to the thermal expansion coefficient, so glasses with small thermal expansion, such as borosilicate glass and fused silica, can hardly be tempered by air quenching. Cutting and drilling are impossible after tempering, so all processing is finished before tempering.
When tempered glass breaks, the strain energy stored inside is released at once and the whole sheet shatters into blunt-edged granules a few mm across, so the risk of injury from large sharp fragments is small. This fracture pattern is checked by a fragmentation test that counts the number of fragments within a specified area. On the other hand, a spontaneous breakage is known in which tiny nickel sulfide particles in the glass change their crystal form and swell after some time, starting a crack from the central tensile layer. To reduce it, a heat soak treatment holds tempered glass at about 290 °C so that sheets containing such particles break in advance.
Glass cooled more gently so that the surface compressive stress is limited to about half that of tempered glass is called heat-strengthened glass. Its strength is about twice that of annealed glass, and when it breaks it forms large fragments similar to ordinary glass that tend to stay in the frame, so it is used for windows in high places and the like. Tempered glass is used for doors, partitions, furniture and the side and rear windows of cars. The windshield of a car is laminated glass with a resin interlayer so that it keeps visibility and shape even when cracked. In this way, the types are chosen with the difference in fracture pattern in mind, not strength alone.
Answer: B. Quenching makes the surfaces solidify first, and the interior, cooling and shrinking later, pulls them into compression and goes into tension itself
When air is blown on both faces of a glass sheet heated to about 650 °C, the surfaces cool and solidify first. The interior is still soft at this stage, so the shrinkage of the surfaces is absorbed by flow in the interior. When the interior later cools and tries to shrink, it pulls in the already rigid surfaces, so at room temperature compressive stress in the surfaces and tensile stress inside remain in balance. Glass fracture starts in tension from surface flaws, so the glass does not break until the applied tension exceeds the surface compression, and its strength becomes about 3–5 times that of annealed glass. However, when it breaks, the strain energy stored inside shatters the whole sheet into small granules, and it cannot be cut after tempering. The statement that tension remains at the surface has it backward, and no crystallization takes place.
Answer: B. About 60 MPa, about 2.1 mm
At the surface, y = t/2, σ = σc(1 − 3) = −2σc, so 2σc = 120 MPa gives σc = 60 MPa. The stress is zero where 12y²/t² = 1, that is, at y = t/√12 ≈ 2.89 mm, so the depth from the surface is 5.0 − 2.89 ≈ 2.1 mm (about 21% of the thickness). This distribution satisfies force balance: summed through the thickness, the stress is zero. About 2.9 mm takes the distance measured from the mid-plane as the depth from the surface. A pairing with the mid-plane tension equal to the 120 MPa surface compression does not satisfy force balance. A surface compression about twice the mid-plane tension and a compressive layer about one fifth of the thickness are the usual guidelines for thermal tempering.
In tempered glass for buildings, the surface compressive stress is roughly 70–150 MPa, often around 100 MPa. Standards set a lower limit; ASTM C1048, for example, requires at least 69 MPa. Heat-strengthened glass has about half of that. The stress is measured without breaking the glass by photoelastic methods using polarized light.
Tempered glass creates surface compression through the temperature difference produced by quenching, and chemically strengthened glass through ion exchange. In tempered glass the compressive layer is thick, about one fifth of the thickness, but it is unsuitable for thin sheets. Chemically strengthened glass has a thin compressive layer of a few tens of µm but a high stress, and can be used for sheets of 1 mm or less.
Because a large tensile stress is stored at the middle of the thickness, and once a crack enters that layer, the released strain energy drives the crack forward while it branches again and again. The larger the central tensile stress, the finer the fragments.
A surface compression about twice the mid-plane tension and a compressive layer about one fifth of the thickness are the usual guidelines for thermal tempering.
In thermally tempered glass of thickness t = 10 mm, let y b…Thermal tempering is difficult for thin sheets, in which it is hard to set up a temperature difference, whereas chemical strengthening can be used even on sheets 1 mm thick or less.
Which correctly describes chemical strengthening (strengthe…Tempering Chemically strengthened glass Amorphous silicon Metallic glass Network former Network modifier Non-bridging oxygen Reduced glass transition temperature