· For a material for which the longitudinal elastic response is linear in the applied uniaxial tensile or compressive stress, a constitutive equation, Hooke's law, relates the stress and strain: 
σ = E e 
where E is the elastic constant "Young's Modulus."
· In addition to the longitudinal strain, a sample tested in uniaxial tension or compression exhibits a lateral strain: 
Longitudinal strain: ε_z = (ΔL_z/L_z); Lateral Strain: ε_x,y = - (ΔL_x,y/L_x,y)
· The ratio of the lateral and longitudinal strains is an elastic constant known as Poisson's Ratio, ν. 
Poisson's ratio: ν = - (Lateral strain /Longitudinal Strain) 
ν = - {- (ΔL_x,y/L_x,y)/(ΔL_z/L_z)} = {ε_x,y/ε_z}