Thermal Stress in glass--Temporary Heat
The so-called temporary stress is to heat the glass product below the strain point temperature. For soda lime silica glass, this temperature is generally about 500 ℃, and then the glass product is quickly cooled, although there is a certain degree of cooling of the glass product at this time The temperature difference is subject to uneven temperature changes, but the thermal stress generated at this time exists with the existence of the temperature gradient, and disappears with the disappearance of the temperature gradient. This thermal stress is generally called temporary stress. When the glass product is below the strain point temperature, the glass is basically in the elastic deformation temperature range, which is a brittle state, and has the characteristics of a solid object with elastic and brittle characteristics. The structure of the glass is basically fixed, and the movement of ions is blocked. The structure and performance of the glass have little effect. Therefore, the glass product is quickly cooled below this temperature. Although there is a temperature difference, permanent stress will not be generated, and the thermal stress generated is a temporary stress. This is the reason why glass products can allow rapid cooling below the lower annealing temperature.
Although the temporary stress disappears with the disappearance of the temperature gradient of the glass product, its value must also be controlled. If the temporary stress exceeds the tensile strength limit of the glass product itself during the cooling process of the glass, the glass will also break. This principle can be used to cut tubulars and hollow glass products by quenching.
