Short Answer:

Strain is the measure of deformation or change in the shape or size of a material when an external force or load is applied to it. It shows how much a body stretches, compresses, or distorts under stress. Strain is expressed as the ratio of the change in dimension to the original dimension of the material.

In simple terms, strain represents the amount of deformation that occurs in a body due to applied stress. It has no unit because it is a ratio. The concept of strain helps engineers understand how materials behave when subjected to different kinds of loads and stresses.

Detailed Explanation:

Strain

Definition and Meaning:
In mechanical engineering, strain is defined as the ratio of the change in dimension (such as length, diameter, or shape) to the original dimension of a material when it is subjected to an external force. It is a measure of deformation that occurs inside a body due to applied stress.

Mathematically, strain is given by the formula:

Since both numerator and denominator have the same units, strain is a dimensionless quantity, meaning it has no unit.

In simple words, when a material is stretched, compressed, or twisted, it experiences a change in its shape or size. This change compared to its original size is known as strain. For example, if a steel rod of 1 meter length elongates by 1 mm under tension, then the strain is . This means the rod is deformed by 0.1%.

Concept of Strain

When a load is applied to a material, internal stresses are generated to resist that load. These stresses cause small changes in the material’s dimensions — it may either elongate (in tension) or shorten (in compression). The measure of this deformation is called strain.

Strain is an important concept because it helps engineers determine how flexible or stiff a material is. A material with high strain can deform more under a given load, while a material with low strain is stiffer and resists deformation.

Strain is always associated with stress. When stress increases, strain also increases proportionally up to the material’s elastic limit. Beyond that limit, the material may permanently deform or break.

Types of Strain

There are different types of strain depending on the kind of deformation that occurs in a body. The main types are:

1. Longitudinal Strain

Longitudinal strain is produced when a body changes its length under the action of an external force. It is the ratio of change in length (ΔL) to the original length (L).

If the material elongates due to a pulling force, it is called tensile strain.
If it shortens due to a compressive force, it is called compressive strain.

Example:
When a metal wire is pulled at both ends, it elongates — this produces tensile strain. When it is compressed, it shortens — this produces compressive strain.

2. Lateral Strain

When a body is stretched or compressed in one direction, it also contracts or expands in a direction perpendicular to the applied load. This change in dimension at right angles to the direction of applied force is called lateral strain.

Example:
If a rubber band is stretched, it becomes longer but thinner. The decrease in its thickness is due to lateral strain.

3. Volumetric Strain

Volumetric strain is the ratio of the change in volume of a body to its original volume when it is subjected to uniform pressure from all sides.

Example:
When a metal ball is immersed in deep water or placed under hydraulic pressure, its volume decreases slightly. This reduction in volume represents volumetric strain.

4. Shear Strain

Shear strain occurs when a material is subjected to tangential forces that cause one layer of the material to slide over another. It measures the angular distortion between two perpendicular faces of a material.
Shear strain is denoted by the Greek letter θ (theta) and is measured in radians.
Example:
A deck of cards when pushed sideways on the top while keeping the bottom fixed represents shear strain.

Poisson’s Ratio

When a material is stretched, its length increases and its diameter decreases. The ratio of lateral strain to longitudinal strain is called Poisson’s Ratio (μ).

It is also a dimensionless quantity. For most engineering materials, the value of Poisson’s ratio lies between 0.25 and 0.35.

Example:
For steel, Poisson’s ratio is approximately 0.3. This means when a steel rod elongates in length, its diameter decreases proportionally by 30% of that strain.

Importance of Strain in Engineering

1. Helps in material selection:
   Strain helps engineers understand how much deformation a material can undergo before breaking or failing.
2. Determines elasticity:
   The amount of strain developed under a given stress indicates whether a material is elastic or plastic.
3. Ensures structural safety:
   By calculating strain, engineers can ensure that the deformation of structures remains within safe limits.
4. Used in stress-strain analysis:
   The relationship between stress and strain gives valuable information about a material’s behavior under different loading conditions.
5. Design and testing:
   Strain is used in experimental methods such as strain gauge testing to measure real-time deformation in machines and structures.

Factors Affecting Strain

• Type of material: Elastic materials like steel show less strain, while rubber shows more strain.
• Magnitude of applied force: Higher force produces greater strain.
• Temperature: Heat can increase strain by softening the material.
• Shape and size of the body: Longer or thinner bodies deform more easily.
• Duration of loading: Continuous or repeated loads can increase strain over time.

Conclusion:

Strain is the ratio of deformation to the original dimension of a body when it is subjected to an external load. It indicates how much a material changes in length, shape, or volume under applied forces. Different types of strain include longitudinal, lateral, volumetric, and shear strain, each describing a specific kind of deformation. Strain is a key concept in mechanics because it helps engineers analyze material behavior, design safe components, and prevent structural failures. Though it is invisible to the eye, strain defines the limits of a material’s strength and flexibility.