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


  • It is the change in length per unit length.
  • Strain = (Change in length, dl)/(Original length, l).
  • It is also a dimensionless quantity.

Elastic Limit

  • A 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

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