MATERIAL when the material is stressed WITHIN the elastic limit. This ratio is known as Poisson’s ratio (μ ).\(μ = \frac{{Lateral\;strain}}{{Longitudinal\;strain}}\)Strain:The ratio of change in DIMENSION to the original dimension is called strain\({\rm{Strain}} = \frac{{{\rm{Change\;in\;dimension\;}}}}{{{\rm{Original\;dimension}}}}\)It is a dimensionless quantity.Linear strain:The ratio of axial deformation to the original length of the body is known as longitudinal or linear strain\({\rm{Linear\;strain}} = \frac{{{\rm{\delta L}}}}{{\rm{L}}}\)Lateral strain:The strain at right angles to the DIRECTION of the applied load is known as lateral strain. The length of the bar will increase while the breadth and depth will decrease.\({\rm{Lateral\;strain}} = \frac{{{\rm{\delta b}}}}{{\rm{b}}}{\rm{or}}\frac{{{\rm{\delta d}}}}{{\rm{d}}}\)