Stress Strain

Stress Definition

  • It is the resistive force per unit area.
  • Stress = (Applied Load, P)/(Cross-sectional area, A)
  • Unit-  Newton per Sq. meter

Strain Definition

  • It is the ratio of change in the configuration, within the elastic limit, of a body to its original configuration is called Strain.
  • It is a dimensionless quantity.

Strain Types

  • Longitudinal/ Tensile/ Linear Strain
  • Shear Strain
  • Volumetric Strain

Longitudinal Strain

  • The ratio of the change in length to the original length (within elastic limit).

L. Strain- ΔL/L

Where ‘ΔL’ is the change in length

& ‘L’ is the original length

Shear Strain

  • It is a relative displacement between two layers.

Shear Strain= tanθ≈ θ

θ= ΔL/L

Bulk or Volumetric Strain

  • The ratio of change in volume to the original volume (within elastic limit).


Where ‘ΔV’ is the change in volume

& ‘V’ is the original volume

Read More: Hydrostatics Definition and Laws

Elastic Limit

  • Maximum unit stress to which a material can be subjected and is able to return to its original form after the removal of strength.

Hooke’s Law

Within the elastic limit, stress is proportional to Strain.

E = (Stress)/(Strain), E is Proportionality Constant called Young’s Modulus of Elasticity.


Within the elastic limit, the extension of an elastic body is directly proportional to the force producing it.

Ultimate Stress

  • The Ratio of maximum load which a specimen sustains to its original area of cross-section.

Yield Point

  • Unit stress at which deformation first increases, without any increase in the applied load; and it is always above the proportional limit.

Ultimate Strength

  • The highest unit stress it can sustain before rupturing.


  • Ability to resist deformation under stress.

The factor of Safety (FOS)

  • The ratio of failure stress to the allowable stress (of the material).

For ductile Materials

  • FOS = Ratio of Syt  to σ; where Syt is yield strength of a material
  • For brittle Materials
  • FOS = Ratio of Sut  to σ; where Sut is yield strength of a material

Read More: Fluid Mechanics: Objective Questions

Elasticity and Elasticity Types

Modulus of Elasticity Definition

  • The ratio of the Stress to the Strain, within the elastic limit (Hooke’s Law), is a constant, called Modulus of Elasticity or Coefficient of Elasticity.

Modulus of Elasticity Types

  • Young’s Modulus of Elasticity (E)
  • Shear Modulus or Modulus of Rigidity (G)
  • Bulk Modulus of Elasticity (K)
Young’s Modulus of Elasticity (E)
  • It is the ratio of Longitudinal Stress to the Longitudinal Strain, within the elastic limit.

E= L. Stress/L. Strain

Modulus of Rigidity (G)
  • The ratio of Shear Stress to the Shear Strain, within the elastic limit.

G= Shear Stress/Shear Strain

Bulk Modulus (K)

The ratio of Volumetric Stress to Volumetric Strain, within the elastic limit.

K= Vol. Stress/Vol. Strain

Relationships between E, G, and K

  • E= 2G(1+μ)
  • E= 3K(1-2μ)                             Here ‘μ’ is Poisson’s Ratio
  • E= (9KG) / (3K+G)

Read More: Surface Tension & Capillary Action

Read More: Strength of Materials Objective Questions

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