Longitudinal strain : Whenever the bar is subjected to the axial load ,there will be increase in the length of the bar along the direction of loading. Therefore the longitudinal strain is defined as ratio of increase in the length of the bar in the direction of applied load to that of the original length (gauge length).

i.e, e = dL/L
where
e= longitudinal strain
dl= increase in length
L = gauge or original length


Lateral strain: Whenever the bar is subjected to the axial load ,there will be decrease in the dimensions of the bar in the perpendicular direction of loading. Therefore lateral strain is defined as ratio of decrease in the length of the bar in the perpendicular direction of applied load to that of the original length (gauge length).
i.e, e = dB/B or dD/D

where
e= lateral strain
dd= decrease in depth
D= gauge or original depth
db= decrease in breadth
B = gauge or original breadth

Poisson’s ratio:  The ratio of lateral strain to that of the longitudinal strain is termed as poisson’s ratio and it is represented by Ï» or 1/m.

i.e, Ï» or 1/m =  lateral strain/longitudinal strain

Value of the Poisson’s ratio for most materials lies between 0.25 and 0.33.