1.1 ,This test method covers a procedure for experimentally determining the x-ray elastic constants (XEC) for the evaluation of residual and applied stresses by x-ray diffraction techniques. The XEC relate macroscopic stress to the strain measured in a particular crystallographic direction in polycrystalline samples. The XEC are a function of the elastic modulus, Poisson’,s ratio of the material and the hkl plane selected for the measurement. There are two XEC that are referred to as ¹/₂ ,S₂^hkl and S₁ ^hkl.