What is Strain in Mechanics – Strength of Material

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When a force is applied to a rigid body, It changes its size and shape due to internal mechanical stress acting on it. This change is length is represented by strain. In this article we will discuss what is strain in mechanics, it’s type and how it is calculated. We suggest you also read this article on stress in mechanics.

What is Strain in Mechanics

Strain inside a body measures the deformation in a body by the application of external forces. Mathematically strain is equal to the ratio of change in length to the original length.

Strain is a dimensionless quantity and It is denoted by “ε”. Lets try to understand strain in context to external force and stress applied.

When rubber band is pulled with an external force. It changes its length and cross section area. This change in length and diameter can be represented by Strain.

Value of strain produced in rubber band depends on:

  • External Force: Higher the applied external force, Higher will be the strain produced inside material.
  • Cross Sectional Area: Higher the cross section area, lower will be the  strain.
  • Resting Force Exhibited by the Body: Resting force exhibited by a body depends on its material. For example, Steel will exhibit higher resistance compared to rubber material.
Types of Strain in Mechanics Based on Direction

When an external force is applied to a body, there is a change part length. This change in length can be in longitudinal or lateral direction. Based on direction of change in length. strain can be classified in following types.

Based on direction of change in length. strain can be classified in four types.

Normal Strain

Normal Strain in mechanics is the strain produced inside a body when a normal force is applied to it. It results in normal stress and can be further classified in two types.

Lateral Strain

Ratio of change in length to the original length in the direction perpendicular to the applied force in known as Lateral Strain.

Longitudinal Strain

Ratio of change in length to the original length in the direction of applied force is known as Longitudinal Strain.

Shear Strain
Change in the tangential angle in the direction of shear force acting on a body is known as shear strain.

Change in the tangential angle in the direction of shear force acting on a body is known as shear strain. Shear strain value is measured in radians.

Shear Strain (γ) = Tan θ

Volumetric Strain

Volumetric strain is produced inside a body when external force is applied in all directions of a body. Mathematically volumetric strain is the ratio of change in volume to the original volume.

True Strain vs Engineering Strain

When an external tensile force is applied to a metal rod. It’s diameter keeps on decreasing with the application of force and starts breaking at some point.

Engineering Strain: It is the total change in length to the original length.

True Strain: It is the change in length to the original length when force is applied. Mathematically true strain is the sum of change in length to the original length from length Lo to L.

Conclusion

To sum up, strain in a body is used to calculate deformation inside a body when an external force is applied to it. It is used during product design for failure analysis.

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