Elastic Constants:

As there are various kinds of stresses possible, such as linear (normal) stress, shear stress, volumetric (bulk) stress, and correspondingly various modulii of elasticity namely Young's modulus, shear modulus and bulk modulus are described as elastic constants of a material. The term E is used in Eqs. (1) And (2) in the context of general stresses and corresponding strains is called as Young's Modulus.

Further to the three modulii of elasticity a significant elastic constant, used in defining the mechanical properties of a solid, is known as Poisson's ratio. Although testing the material for stress-strain relationship, you observe in which strains are generates not only in the direction of the applied stress, but also in direction perpendicular (lateral) to it. On additional investigations, you will search that the lateral strain is always proportional to the longitudinal strain and the proportionality constant is negative. This proportionality constant is defined as Poisson's Ratio and implies through υ.

∴          Poisson's Ratio, υ = - Lateral Strain/Longitudinal Strain

Although applying tensile force on a rod we found in which the deformations produced included change of volume of the solid as well as its shape. Such deformations might be found in several other cases of loading. Deformations including change of volume and shape are more common. Therefore, we may identify two special cases of deformations, distortion and dilatation.