Short Answer:

Tensile stress is the type of stress that occurs when a material is subjected to a pulling or stretching force. It tries to increase the length of the material. The internal resistance developed per unit area against this pulling force is known as tensile stress.

Compressive stress is the type of stress that occurs when a material is subjected to a pushing or squeezing force. It tries to decrease the length of the material. The internal resistance developed per unit area against this compressive force is called compressive stress. Both stresses are important in the design of structural and mechanical components.

Detailed Explanation:

Tensile Stress and Compressive Stress

Meaning of Stress:
When an external force acts on a body, it tries to deform it by either stretching, compressing, bending, or twisting. The internal resistance offered by the material to oppose this deformation is called stress. Stress is measured as the force acting per unit area of the material and is expressed as:

Depending on the direction and nature of the external load, different types of stresses are developed in a material. The two most common types are tensile stress and compressive stress.

Tensile Stress

Definition:
Tensile stress is the stress induced in a material when it is subjected to an external pulling or stretching force that tries to increase its length. The material resists this deformation by developing internal forces that act in the opposite direction to the applied load. The intensity of these internal resisting forces per unit area is known as tensile stress.

Mathematically,

Explanation:
When a load is applied on a material in opposite directions (away from each other), it tends to elongate. The internal molecular bonds stretch to resist the applied load, and this resistance creates tensile stress inside the material.

If the material is elastic, it will return to its original length after removing the load. However, if the applied load exceeds the elastic limit, the material will permanently deform or even break.

Example:

When a steel wire is pulled at both ends, it becomes longer due to tensile stress.
The tension in a rope or cable carrying a hanging weight is also due to tensile stress.
The bottom fibers of a bending beam experience tensile stress.

Units:
The SI unit of tensile stress is Pascal (Pa) or N/m². In engineering practice, it is often expressed in MPa (Mega Pascal).

Applications of Tensile Stress:

In designing tension members like tie rods, cables, and wires.
In testing the tensile strength of materials using a tensile testing machine.
In structural components like bridges and trusses which experience tension.
In springs and ropes used for lifting or pulling loads.

Compressive Stress

Definition:
Compressive stress is the stress developed in a material when it is subjected to an external pushing or squeezing force that tries to decrease its length. The material resists this deformation by developing internal forces that act in the opposite direction of the applied compressive force.

Mathematically,

Explanation:
When a load is applied on a material in such a way that it pushes its ends towards each other, the material tends to shorten. The internal atomic structure resists this compression and generates compressive stress. If the applied force is within the elastic limit, the material will return to its original length after removing the load. However, if the stress goes beyond the elastic limit, the material may permanently deform or buckle.

Example:

The column or pillar of a building under the weight of the structure experiences compressive stress.
A brick or concrete block under load undergoes compressive stress.
The upper fibers of a beam under bending are subjected to compressive stress.

Units:
The SI unit of compressive stress is also Pascal (Pa) or N/m², similar to tensile stress.

Applications of Compressive Stress:

Used in designing structural elements like columns, pillars, and foundations.
Important in construction materials such as concrete and stone which have high compressive strength.
Used in testing machines to determine the compressive strength of materials.
Applied in metal forming operations like forging and extrusion.

Difference between Tensile and Compressive Stress

Nature of Force:
- Tensile stress occurs due to a pulling force.
- Compressive stress occurs due to a pushing force.
Effect on Material:
- Tensile stress increases the length of a body.
- Compressive stress decreases the length of a body.
Molecular Action:
- In tensile stress, the molecules move apart.
- In compressive stress, the molecules come closer together.
Direction of Action:
- Tensile stress acts away from the material’s cross-section.
- Compressive stress acts towards the material’s cross-section.
Examples:
- A stretched wire or rope experiences tensile stress.
- A column or beam under a heavy load experiences compressive stress.

Importance in Engineering Design

Understanding tensile and compressive stresses is essential for designing safe and efficient structures and machines. Different materials are chosen based on their ability to handle these stresses:

Steel and aluminum are commonly used where tensile strength is important (e.g., bridges, cables).
Concrete and stone are used where compressive strength is required (e.g., buildings, pillars).

Engineers must ensure that both types of stresses remain within the safe working limits of materials to prevent failure, cracking, or buckling.

Conclusion:

Tensile stress and compressive stress are two fundamental types of mechanical stress that occur due to pulling and pushing forces, respectively. Tensile stress stretches the material and increases its length, while compressive stress squeezes the material and reduces its length. Both are expressed as force per unit area and play a key role in determining material strength and performance. Understanding these stresses helps engineers design strong, stable, and durable structures and machine components that can withstand various loading conditions safely.