Chapter 30 : Expansion of Solids and Thermal Stresses

 

Chapter: Expansion of Solids and Thermal Stresses

Introduction:

The expansion of solids with increasing temperature is a fundamental aspect of thermodynamics and materials science. This phenomenon has significant implications in various engineering applications, influencing the design and performance of structures and devices. In this chapter, we explore the expansion of solids, focusing on the coefficients of linear, surface, and cubical expansions, as well as the thermal stresses generated due to temperature changes.

 

Expansion of Solids:

1. Coefficient of Linear Expansion:

When a solid is heated, its dimensions change in response to the increase in temperature. The coefficient of linear expansion (αα) is a measure of how much a material expands per unit length for a one-degree Celsius increase in temperature. It is expressed as:

ΔL=L₀⋅α⋅ΔT

where:

• ΔL is the change in length,
• L₀​ is the initial length,
• α is the coefficient of linear expansion, and
• ΔT is the change in temperature.

 

2. Coefficient of Surface Expansion:

For two-dimensional expansions, such as the expansion of a sheet or a plane surface, the coefficient of surface expansion (ββ) is defined. It is related to the coefficient of linear expansion by the equation:

ΔA=A0⋅β⋅ΔT

where:

• ΔA is the change in area,
• A₀​ is the initial area, and
• Β is the coefficient of surface expansion.

 

3. Coefficient of Cubical Expansion:

In three dimensions, the coefficient of cubical expansion (γ) describes the volumetric expansion of a solid. The relationship is given by:

ΔV=V₀⋅γ⋅ΔT

where:

• ΔV is the change in volume,
• V₀​ is the initial volume, and
• γ is the coefficient of cubical expansion.

 

Relations Among Coefficients:

The coefficients of linear, surface, and cubical expansions are related mathematically:

β=2αβ=2α

γ=3αγ=3α

These relationships highlight the interdependence of the expansion coefficients for different dimensional changes.

 

Thermal Stresses:

When a material undergoes thermal expansion or contraction, internal stresses are induced. These thermal stresses can have significant consequences on the structural integrity of materials and are a critical consideration in engineering design.

 

Qualitative Aspects of Thermal Stresses:

1. Constrained Expansion:

• When a material is constrained from expanding freely, internal stresses build up.
• For example, if a rod is fixed at one end and heated, it will attempt to expand, but the fixed end prevents it, leading to the development of compressive stresses.

2. Constrained Contraction:

• Similarly, if a material is prevented from contracting freely upon cooling, tensile stresses develop.
• For instance, if a bar is cooled and its ends are restrained from contracting, tensile stresses will arise.

3. Applications of Thermal Stresses:

• Bi-metallic Strips: Bi-metallic strips consist of two different metals with different coefficients of expansion bonded together. When subjected to temperature changes, the strip bends due to the uneven expansion of the metals. This principle is employed in devices like thermostats.
• Railway Tracks: Gaps are intentionally left between railway tracks to accommodate thermal expansion. Without these gaps, the tracks could buckle due to the thermal stresses generated during temperature variations.
• Pipelines: In the design of pipelines, considerations of thermal expansion and contraction are essential. Expansion joints are incorporated to allow for the free movement of the pipeline without inducing excessive stresses.
• Bridge Construction: Thermal stresses are considered in the construction of bridges. Expansion joints and other design features are incorporated to allow for the expansion and contraction of the bridge materials with temperature changes.

Conclusion:

The expansion of solids with temperature is a critical phenomenon with widespread applications in engineering and materials science. Understanding the coefficients of linear, surface, and cubical expansions, as well as the associated thermal stresses, is essential for designing structures and devices that can withstand temperature variations without compromising their integrity. The application of these principles is evident in various everyday objects and structures, demonstrating the practical importance of a thorough understanding of thermal expansion and its effects.

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