Short Answer:

Strain is the measure of deformation in a material when an external force is applied to it. It shows how much a body changes in length, shape, or volume compared to its original dimensions. Strain has no unit because it is a ratio of two similar quantities — change in dimension to the original dimension.

There are mainly three types of strain: longitudinal strain, lateral strain, and volumetric strain. In some cases, shear strain is also considered as another type. Each type of strain represents a different kind of deformation depending on how the external force acts on the body.

Detailed Explanation:

Types of Strain

Meaning of Strain:
When a body is subjected to external forces, it undergoes deformation. This deformation may be in the form of elongation, compression, change in shape, or change in volume. The ratio of this change in dimension to the original dimension is known as strain.

Mathematically,

Strain is a dimensionless quantity because it is the ratio of two lengths or dimensions. It helps engineers measure how much a material deforms under different types of stresses such as tension, compression, or shear.

When a material experiences deformation, it may change its length, width, or volume. Based on the type of deformation, strain is divided into several types which are explained below.

Longitudinal Strain

Definition:
Longitudinal strain is the strain produced in a body when there is a change in its length due to the application of an external load along its length. It is the ratio of the change in length () to the original length () of the material.

Explanation:
If a wire or rod is stretched by applying a tensile force, its length increases. Similarly, if it is compressed, its length decreases. The amount of elongation or shortening per unit of original length represents the longitudinal strain.

When the material is stretched, it produces tensile strain.
When the material is compressed, it produces compressive strain.

Example:
When a steel wire is pulled at both ends, it elongates due to tensile strain. When a column is pressed from both ends, it shortens due to compressive strain.

Application:
Longitudinal strain is important in designing structural members such as rods, bars, and beams that carry tensile or compressive loads.

Lateral Strain

Definition:
Lateral strain is the strain developed in the direction perpendicular to the direction of the applied force. It is caused when a material is stretched or compressed, resulting in a change in its thickness or diameter.

Mathematically,

Explanation:
When a rod or wire is pulled in tension, its length increases, but its diameter decreases. This decrease in diameter is known as lateral strain. Similarly, when the rod is compressed, its length decreases, but its diameter increases.

Example:
If a rubber band is stretched, it becomes longer and thinner. The reduction in its width is due to lateral strain.

Relation with Longitudinal Strain (Poisson’s Ratio):
The ratio of lateral strain to longitudinal strain is known as Poisson’s Ratio (μ).

This relationship helps engineers understand how materials behave under axial loads.

Volumetric Strain

Definition:
Volumetric strain is the strain produced when the volume of a body changes due to applied forces or pressure. It is defined as the ratio of the change in volume () to the original volume () of the body.

Explanation:
When a body is subjected to uniform pressure in all directions (like in fluids or under hydraulic pressure), its volume either increases or decreases without changing its shape. The measure of this change is called volumetric strain.

Example:

When a metal sphere is placed under high pressure, its volume decreases slightly due to compressive forces.
When a balloon is inflated, its volume increases due to internal air pressure.

Application:
Volumetric strain is important in studying the behavior of materials under pressure, such as hydraulic systems, pipelines, pressure vessels, and containers.

Shear Strain

Definition:
Shear strain occurs when a body is subjected to tangential or shear forces that cause one layer of the material to slide over another. It measures the angular deformation or distortion between two perpendicular planes.

Formula:
Shear strain is usually represented by the Greek letter (theta), which is the angle of deformation in radians.

Explanation:
When a force acts parallel to the surface of a body, it changes the shape without changing the volume. The distortion angle between the sides of the body gives the measure of shear strain.

Example:

When a card deck is pushed sideways at the top while the bottom remains fixed, the shape of the deck changes — this represents shear strain.
Similarly, a bolt under torque or a glue joint under shear force also experiences shear strain.

Application:
Shear strain is important in designing components such as rivets, bolts, couplings, shafts, and joints that experience tangential forces.

Importance of Different Types of Strain

Understanding the different types of strain is very important for engineers because:

It helps predict how materials will deform under various loading conditions.
It assists in designing components that can resist failure due to deformation.
It helps in analyzing stress–strain relationships to understand material properties.
It aids in selecting suitable materials for applications such as tension, compression, or shear loading.
It ensures structural safety, reliability, and efficiency in mechanical systems.

Conclusion:

Strain is the ratio of deformation to the original dimension of a material when it is subjected to external forces. Depending on how the force acts and how the body deforms, strain is classified into longitudinal strain, lateral strain, volumetric strain, and shear strain. Each type represents a specific form of deformation — either in length, width, volume, or shape. Understanding these types of strain is essential in mechanical and structural design because it helps engineers ensure that materials perform safely within their elastic limits and do not fail under load.