(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?

Residual thermal stresses are stresses that remain in a material, like glass, after it has been cooled down from a higher temperature. When a glass piece is heated, it expands, and when it cools down, it contracts. However, different parts of the glass cool at different rates, causing uneven contraction. This uneven contraction leads to residual thermal stresses in the glass piece.

(a) Residual thermal stresses are introduced into a glass piece during cooling because of the following reasons: 1. The difference in cooling rates between the surface and the interior of the glass: The surface cools faster, causing it to contract more quickly than the interior. This disparity in contraction rates causes stresses to develop within the material. 2. The inherent nature of glass: Glass is a brittle material, which means it does not easily deform or flow when subjected to stress. As a result, it cannot easily relieve the stresses generated during cooling.

(b) Thermal stresses can also be introduced upon heating a glass piece, but the reasons are slightly different: 1. Differential heating rates: Different parts of the glass could be heated at different rates, causing some areas to expand faster than others. This can lead to the formation of thermal stresses. 2. Constraint from external boundaries or supports: If a glass piece is held or supported during heating, it may not be free to expand uniformly. This constraint can introduce thermal stresses within the material. However, these thermal stresses often have a lesser impact on heating than they do on cooling. This is because glass becomes more viscous and capable of deforming at higher temperatures, allowing it to better handle temperature-induced stress.