Stress and Strain

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

τ = F/A

Where,

τ = 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]