Short Answer:
The types of stresses are classified based on the direction and nature of the force acting on a material. The main types include tensile stress, compressive stress, and shear stress. These stresses develop when external forces are applied on a body, causing internal resistance within the material.
In simple words, stress is the internal force per unit area that resists deformation. Depending on how the force acts—either pulling, pushing, or sliding—the material experiences different types of stresses. Understanding these stresses helps engineers design safe and efficient mechanical and structural components.
Detailed Explanation :
Types of Stresses
When a force acts on a body, it produces internal resistance within the material to oppose deformation. This internal resistance is called stress. The distribution and type of stress depend on the direction, magnitude, and nature of the applied external force.
In mechanics of materials, stress is defined as:
Where,
= Stress (N/m² or Pa),
= Applied force (N),
= Cross-sectional area (m²).
There are mainly three basic types of stresses:
- Tensile stress
- Compressive stress
- Shear stress
Additionally, in certain conditions, other forms like bending stress, torsional stress, and thermal stress also occur as combinations of these main types.
- Tensile Stress
Definition:
Tensile stress is the type of stress induced in a body when it is subjected to equal and opposite forces that tend to stretch or elongate it.
When a bar or rod is pulled by two forces acting away from each other, the material experiences an increase in length due to tension.
Mathematically:
Where,
= Tensile stress,
= Pulling (tensile) force,
= Original cross-sectional area.
Example:
- A steel wire under load when hanging a weight.
- A tie rod in a bridge subjected to pulling forces.
Effect:
Tensile stress causes elongation and may lead to necking or fracture if the applied force exceeds the material’s ultimate tensile strength.
- Compressive Stress
Definition:
Compressive stress is the stress induced in a body when it is subjected to equal and opposite forces that tend to compress or shorten it.
When a block or column is pressed by two forces acting towards each other, the material is under compression, and its length decreases.
Mathematically:
Where,
= Compressive stress,
= Compressive force,
= Cross-sectional area.
Example:
- Columns or pillars in buildings that support loads.
- Concrete cubes under compression testing.
Effect:
Compressive stress causes shortening or deformation of materials. Brittle materials like concrete and cast iron are stronger in compression but weak in tension.
- Shear Stress
Definition:
Shear stress is developed when the applied force acts tangentially or parallel to the surface of a material, causing one layer to slide over another.
When two equal and opposite forces act tangentially to a surface, the internal resistance developed is called shear stress.
Mathematically:
Where,
= Shear stress,
= Tangential or shear force,
= Area resisting the shear.
Example:
- Punching or shearing a metal plate.
- Scissors cutting paper.
- Riveted and bolted joints under load.
Effect:
Shear stress causes angular distortion and may lead to shear failure along the plane of applied force.
- Bending Stress (Combination Stress)
Definition:
Bending stress occurs when a beam or structural member is subjected to a bending moment. It is a combination of tensile and compressive stresses acting on different sides of the neutral axis.
Explanation:
- The upper surface of the beam experiences compression, while
- The lower surface experiences tension.
Mathematically:
Where,
= Bending moment,
= Distance from neutral axis,
= Moment of inertia.
Example:
A simply supported beam carrying a load at its center.
- Torsional Stress
Definition:
Torsional stress is produced when a shaft or circular member is subjected to a twisting moment or torque. The particles of the material are under shear stress due to twisting action.
Mathematically:
Where,
= Shear stress due to torsion,
= Applied torque,
= Radius of the shaft,
= Polar moment of inertia.
Example:
- Drive shafts in automobiles.
- Propeller shafts in ships.
Effect:
Torsional stress causes angular deformation known as twist.
- Thermal Stress
Definition:
Thermal stress develops when a material is subjected to temperature changes and is prevented from expanding or contracting freely.
Explanation:
When a metal bar is heated, it tends to expand. If this expansion is restricted, compressive stresses develop. Similarly, if cooling is restricted, tensile stresses are produced.
Mathematically:
Where,
= Modulus of elasticity,
= Coefficient of thermal expansion,
= Temperature change.
Example:
- Railway tracks expanding in hot weather.
- Metal rods fixed between supports and subjected to temperature variation.
Importance of Knowing Different Types of Stresses
Understanding different types of stresses is very important for mechanical and structural engineers because:
- It helps design safe and reliable components.
- It determines the maximum load a material can withstand.
- It helps prevent mechanical failure due to overloading.
- It guides in selecting proper materials based on working conditions.
- It ensures cost-effective and efficient design solutions.
Conclusion
The types of stresses represent how materials react internally under different types of external forces. The three primary stresses — tensile, compressive, and shear — are the foundation of stress analysis, while other derived stresses like bending, torsional, and thermal stresses are combinations of these basic types. By analyzing stresses, engineers ensure that structures and machine parts can safely carry loads without deformation or failure. Thus, understanding stress types is essential for designing durable and efficient mechanical systems.