4.1 When uniaxial force is applied to a solid, it deforms in the direction of the applied force, but also expands or contracts laterally depending on whether the force is tensile or compressive. If the solid is homogeneous and isotropic, and the material remains elastic under the action of the applied force, the lateral strain bears a constant relationship to the axial strain. This constant, called Poisson's ratio, is an intrinsic material property just like Young's modulus and Shear modulus.

4.2 Poisson's ratio is used for design of structures where all dimensional changes resulting from application of force need to be taken into account, and in the application of the generalized theory of elasticity to structural analysis.

4.3 In this test method, the value of Poisson's ratio is obtained from strains resulting from uniaxial stress only.

4.4 Above the proportional limit, the ratio of transverse strain to axial strain will depend on the average stress and on the stress range for which it is measured and, hence, should not be regarded as Poisson's ratio. If this ratio is reported, nevertheless, as a value of "Poisson's ratio" for stresses below the proportional limit, the range of stress should be reported.

4.5 Deviations from isotropy should be suspected if the Poisson's ratio, μ, determined by the method described below differs significantly from that determined when the ratio E/G of Young's modulus, E, to shear modulus, G, is substituted in the following equation:

where E and G must be measured with greater precision than the precision desired in the measurement of μ.

4.6 The accuracy of the determination of Poisson's ratio is usually limited by the accuracy of the transverse strain measurements because the percentage errors in these measurements are usually greater than in the axial strain measurements. Since a ratio rather than an absolute quantity is measured, it is only necessary to know accurately the relative value of the calibration factors of the extensometers. Also, in general, the values of the applied forces need not be accurately known. It is frequently expedient to make the determination of Poisson's ratio concurrently with determinations of Young's modulus and the proportional limit.