Short Answer:

Thermal stress is the stress that develops in a material when it is subjected to a change in temperature. When the temperature of a body increases, it tends to expand, and when it decreases, it tends to contract. If this expansion or contraction is restricted, internal forces are generated within the material, resulting in thermal stress.

Thermal stress depends on the material’s coefficient of thermal expansion, temperature change, and modulus of elasticity. It is an important factor in engineering design because excessive thermal stress can cause materials to deform, crack, or even fail in structures and machines.

Detailed Explanation :

Thermal Stress

Thermal stress is a type of mechanical stress produced in materials due to temperature variations. When a body experiences a temperature change, its dimensions change due to expansion or contraction. If the body is free to expand or contract, no stress develops. However, when its movement is restricted or prevented, internal resistance is developed within the material. This internal resistance per unit area is known as thermal stress.

The development of thermal stress occurs because atoms or molecules of a material move farther apart when heated and come closer together when cooled. In many engineering applications, components are fixed or connected to other parts that may restrict this movement. As a result, compressive or tensile stresses are produced depending on whether the material is heated or cooled.

For example, in structures like bridges, rails, and pipelines, temperature changes throughout the day cause expansion and contraction. If there are no provisions like expansion joints, thermal stress can cause bending, cracking, or permanent deformation.

Formula for Thermal Stress

The magnitude of thermal stress can be expressed by the following formula:

Where,
σ = Thermal stress (in N/m² or Pa)
E = Young’s modulus of elasticity of the material (in N/m²)
α = Coefficient of linear expansion (per °C)
ΔT = Change in temperature (°C)

This formula shows that thermal stress increases directly with the material’s elasticity, expansion coefficient, and temperature change. Therefore, metals with higher α values (like aluminum) experience more thermal stress than those with lower α values (like steel) for the same temperature change.

Types of Thermal Stress

1. Tensile Thermal Stress:
   Occurs when the temperature decreases and the material tries to contract but is restrained. The material develops tension within itself.
2. Compressive Thermal Stress:
   Occurs when the temperature increases and the material tries to expand but is prevented. The material develops compression within itself.

Effects of Thermal Stress

1. Deformation:
   The shape or size of the component may change due to uneven expansion or contraction.
2. Cracking and Failure:
   High thermal stress may cause cracking, especially in brittle materials like ceramics or glass.
3. Loosening of Joints:
   In mechanical assemblies, bolts and joints may become loose due to repeated thermal cycles.
4. Loss of Strength:
   Continuous exposure to thermal stress may lead to material fatigue, reducing its strength and lifespan.
5. Buckling in Structures:
   Long members such as rails and bridges may buckle due to compressive thermal stress if not properly designed with expansion allowances.

Applications and Prevention

Thermal stress is a major consideration in the design of structures and machines exposed to temperature changes. To minimize its effects:

• Expansion joints are provided in bridges and railways.
• Insulating materials are used to maintain uniform temperature.
• Composite materials are selected to have similar coefficients of expansion.
• Controlled heating and cooling processes are followed in manufacturing to reduce sudden temperature variations.

In power plants, turbines, and engines, components face frequent heating and cooling cycles. Engineers use materials that can withstand these stresses without losing strength or shape.

Example

Consider a steel rod fixed at both ends. When the temperature rises, the rod tries to expand, but since both ends are fixed, it cannot move. This restriction generates compressive thermal stress in the rod. The magnitude of this stress can be calculated using the formula mentioned above.

Similarly, if the rod is cooled, it tends to contract but is unable to do so because of the fixed ends, creating tensile stress instead.

Conclusion

Thermal stress is the internal resistance developed in a material due to temperature changes when expansion or contraction is restricted. It plays a vital role in mechanical and structural design. Understanding and managing thermal stress helps prevent material failure, deformation, and mechanical faults. Proper material selection and design practices, such as using expansion joints and uniform heating, are essential to control the effects of thermal stress in engineering systems.