Short Answer:

Thermal stress is the stress developed in a material due to change in temperature. When a material is heated or cooled, it tries to expand or contract, and if this movement is restricted, stress is produced.

If the material is free to expand or contract, no stress develops. Thermal stress becomes important when expansion or contraction is prevented in structures.

Detailed Explanation:

Thermal Stress

Thermal stress is an important concept in Strength of Materials that occurs due to temperature changes in a material. When temperature increases, most materials expand, and when temperature decreases, they contract. This natural change in size is called thermal expansion or contraction. If the material is free to move, no stress is developed. However, when this movement is restricted or prevented, internal forces develop within the material, and these forces produce thermal stress.

The amount of thermal stress depends on factors such as the temperature change, the material property called coefficient of thermal expansion, and the stiffness of the material. The stress developed can be calculated using the following relation:

where σ is thermal stress, E is Young’s Modulus, α is the coefficient of thermal expansion, and ΔT is the change in temperature.

Nature of Thermal Stress

Thermal stress can be either tensile or compressive depending on the condition. If a material is heated and expansion is prevented, compressive stress develops because the material tries to expand but is restricted. If a material is cooled and contraction is prevented, tensile stress develops because the material tries to shrink but is not allowed to do so.

If the material is partially restrained, only partial thermal stress develops. The amount of stress depends on the degree of restraint. Full restraint leads to maximum thermal stress, while no restraint leads to zero stress.

Importance of Thermal Stress in Civil Engineering

Structural Design

Thermal stress plays a very important role in the design of structures such as bridges, buildings, and pipelines. These structures are exposed to temperature changes due to weather conditions. Engineers must consider thermal stress to prevent cracks and damage.

Expansion Joints

To reduce thermal stress, engineers provide expansion joints in structures like bridges and railway tracks. These joints allow materials to expand and contract freely without developing stress. This increases the life of the structure.

Material Behavior

Different materials expand and contract differently when temperature changes. Materials with high coefficient of thermal expansion show more deformation. Understanding thermal stress helps in selecting suitable materials for construction.

Prevention of Cracks and Failure

If thermal stress is not properly considered, it can lead to cracks, deformation, or even failure of structures. For example, concrete roads may crack in hot weather if expansion is restricted. By considering thermal stress, engineers can prevent such problems.

Real Life Examples

Thermal stress can be seen in daily life. Railway tracks are provided with gaps to allow expansion in summer. Bridges have expansion joints to avoid damage. Metal rods expand when heated and contract when cooled. These examples show the practical importance of thermal stress.

Conclusion:

Thermal stress is the stress developed due to temperature change when expansion or contraction is restricted. It is an important factor in civil engineering design to ensure safety and durability of structures.