The change in length of the cylinder may be determined from the longitudinal strain.
Since whenever the cylinder will elongate in axial direction or longitudinal direction, this will also get decreased in diametre or the lateral strain will also take place. Therefore we will have to also take into consideration the lateral strain.as we know that the poisson's ratio (ν) is
where the -ve sign emphasized that the change is negative
Consider an element of cylinder wall which is subjected to two mutually ⊥r normal stresses σL and σH .
Let E = Young's modulus of elasticity