(a) Explain why residual thermal stresses are introduced into a glass piece when it is cooled. (b) Are thermal stresses introduced upon heating? Why or why not?

Question: Explain the cause of residual thermal stresses in a glass piece when it is cooled and whether thermal stresses can be introduced upon heating. Discuss the factors that influence the presence and magnitude of these stresses. Answer: Residual thermal stresses in a glass piece are caused by uneven contraction rates as the outer layers cool and contract faster than the inner layers, creating stress within the material. Thermal stresses can be introduced upon heating if there is an uneven temperature change or expansion rate. Factors influencing the presence and magnitude of these stresses include the temperature change, cooling or heating rate, and the glass's coefficient of thermal expansion. A slow and controlled heating or cooling process can help mitigate the introduction of these stresses.