Stress and Strain

  • Stress is defined as the internal resistance set up by a body when it is deformed. It is measured as force per unit area.  
        unit of stress is N/m2, called Pascal (Pa).
  • Tensile stress: Stress that tends to stretch or lengthen the material – acts normal to the stressed area.
  • Compressive stress: Stress that tends to compress or shorten the material – acts normal to the stressed area.

    Tensile or compressive stress normal to the plane is usually denoted ‘Normal stress’ and can be         expressed as

σ = F/A

Where,

σ = Normal stress

F = Normal force

A = Area

  • Shearing stress: Stress that tends to shear the material – acts in plane to the stressed area at right angles to compressive or tensile stress.

Shear stress can be expressed as

Ï„ = F/A

Where,

Ï„ = Shear stress

F = Shear force

A = Area

  • Strain is a geometric measure of deformation representing the relative displacement between particles in a material body. Strain is caused by external constraints or loads. 

Tensile strain (ε) = Increase in length (dl)/Original length (l)

Compressive strain (ε) = decrease in length (dl)/Original length (l)


Shear strain = Transverse displacement (dl)/Distance between two faces (l)


∴ Shear strain = dl/l = tan(θ) = θ ,[As θ is very small]