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Chapter 13: Problem 15

(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?

Short Answer

Expert verified
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.

Step by step solution

01

Part (a): Introduction to Residual Thermal Stresses in Glass when Cooling

Residual thermal stresses are created in a material as a result of the temperature change, which causes an uneven expansion or contraction of the material. In the case of a glass piece, when it is cooled, the outer layers contract more rapidly than the inner layers. This uneven contraction rate causes the internal and external layers to undergo stress, which results in residual thermal stresses.
02

Part (a): Process of Cooling

During the cooling process, the outer layers of the glass piece cool faster than the inner layers due to their direct contact with the cooling environment. As a result, the outer layers shrink and create tensile stress, while the inner layers are still hot and not contracted yet. This imbalance in contraction rates leads to the development of residual thermal stresses within the glass piece.
03

Part (a): Factors Influencing Residual Thermal Stresses

The magnitude of residual thermal stresses depends on several factors, including the temperature change, the cooling rate, and the glass's coefficient of thermal expansion. A higher temperature change or a faster cooling rate can lead to a greater difference between the contraction rates of the outer and inner layers, resulting in higher residual thermal stresses. Similarly, a glass with a higher coefficient of thermal expansion will experience more significant residual thermal stresses due to its greater sensitivity to temperature changes.
04

Part (b): Introduction to Thermal Stresses upon Heating

Upon heating a glass piece, there is the possibility of introducing thermal stresses, but it depends on the specific situation. When heating a glass piece, the outer layers will heat up and expand faster than the inner layers, which may lead to compressive stress. However, if the glass undergoes a uniform temperature increase, the stresses induced during heating can be minimal or non-existent.
05

Part (b): Factors Influencing Thermal Stresses upon Heating

The introduction of thermal stresses upon heating depends on factors like the heating rate, the glass's coefficient of thermal expansion, and the temperature change. If the heating rate is low and the temperature change is uniform, the thermal stresses can be minimal. However, if the temperature change is uneven, it can result in thermal stresses due to the differential expansion between the layers of glass.
06

Part (b): Final Thoughts on Thermal Stresses upon Heating

In conclusion, thermal stresses may be introduced upon heating if there is an uneven temperature change or expansion rate within the glass piece. A slow and controlled heating process, which ensures uniform temperature distribution, can mitigate the introduction of thermal stresses.

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Most popular questions from this chapter

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