What is stress and strain?

Short Answer

Stress is the internal force developed per unit area inside a material when an external force is applied to it. It tells us how strongly the particles inside the object resist the applied force.

Strain is the measure of deformation caused by stress. It shows how much a material changes in length, shape, or volume when force is applied. Stress causes strain, and both help us understand how materials behave under external forces.

Detailed Explanation :

Stress and Strain

Stress and strain are two closely related concepts in the study of elasticity and material strength. They help explain how materials react when forces act on them. Engineers and scientists use these concepts to design strong buildings, bridges, vehicles, and machines. When any material is stretched, compressed, or twisted, it experiences internal forces and deformation. These internal forces are called stress, and the resulting deformation is called strain.

Understanding stress and strain is important for ensuring the safety, durability, and performance of structures. If materials cannot handle the stress applied to them, they may bend, deform, or even break. Therefore, knowing how much stress a material can bear and how much strain it undergoes helps in building safe systems.

Stress

Definition of Stress

Stress is defined as:

“The internal force acting per unit area within a material when an external force is applied.”

Mathematically,

Unit of stress:

  • SI unit → Pascal (Pa) or N/m²

How Stress Works

When a material is subjected to a force:

  • The particles inside the material resist the force
  • Internal restoring forces develop
  • These restoring forces per unit area are called stress

Stress may be produced by stretching, compressing, bending, twisting, or shearing.

Types of Stress

  1. Tensile Stress – due to stretching
  2. Compressive Stress – due to squeezing
  3. Shear Stress – due to parallel or tangential forces

Each type affects materials differently.

Strain

Definition of Strain

Strain is defined as:

“The ratio of change in dimension to the original dimension of the material.”

Mathematically,

Unit of strain:

  • Strain has no unit because it is a ratio.

How Strain Works

When stress is applied:

  • The material stretches, compresses, or changes shape
  • The amount of this deformation is measured as strain

Strain tells us how much a material deforms under stress.

Types of Strain

  1. Tensile Strain – increase in length
  2. Compressive Strain – decrease in length
  3. Shear Strain – change in shape

Strain depends on the material’s elasticity.

Relationship Between Stress and Strain

Stress and strain are directly related.
When stress increases, strain also increases—until the material reaches its elastic limit.

Hooke’s law explains this relationship:

This means that within the elastic limit, materials return to their original shape when stress is removed.

Examples to Understand Stress and Strain

  1. Stretching a Rubber Band
  • Pulling creates tensile stress
  • The rubber band becomes longer → tensile strain
  • Releasing it returns it to normal
  1. Sitting on a Cushion
  • Your weight creates compressive stress
  • The cushion is pressed down → compressive strain
  1. Cutting Paper with Scissors
  • The blades apply shear stress
  • The paper experiences shear strain and splits
  1. Metal Wire
  • Hanging a weight stretches the wire
  • The wire experiences both stress and strain

Importance of Stress and Strain

Stress and strain are extremely important in engineering and design:

  1. Construction Safety

Buildings, bridges, and dams must withstand stress from loads, wind, and earthquakes.

  1. Material Selection

Engineers choose materials based on their stress–strain behaviour.

  1. Manufacturing

Forming metals into shapes requires knowledge of stress and strain.

  1. Mechanical Design

Springs, machine parts, and tools must handle specific stresses.

  1. Transportation

Vehicles must be able to bear loads and vibrations safely.

If these values are not understood or controlled, structures may fail.

Stress–Strain Curve

When materials are tested, a graph is created to show how they behave under increasing stress.
Key points on this graph include:

  • Elastic region
  • Elastic limit
  • Yield point
  • Breaking point

This curve helps determine the strength and flexibility of materials.

Everyday Life Examples

  • A book placed on a table exerts stress on the surface
  • A balloon stretches when air is filled
  • A guitar string stretches and produces sound when pulled
  • Floors experience stress when people walk on them

Stress and strain influence almost all mechanical actions around us.

Conclusion

Stress is the internal force per unit area that develops inside a material when an external force is applied, while strain is the deformation that occurs because of that stress. Stress tries to resist the applied force, and strain measures how much the material changes in response. These concepts are essential for understanding material strength, designing safe structures, and predicting how objects behave under loads. Together, stress and strain form the foundation of elasticity and material science.