The determination of a stress-free lattice parameter within a stressed material using elastic anisotropy

A method for the calculation of a stress-free lattice parameter from the analysis of diffraction data from stressed material is discussed, utilizing the elastic anisotropy of the material. The technique is demonstrated using data obtained during a uniaxial tension test on untextured austenitic (face-centred cubic) steel. The uncertainty in the calculated lattice parameter for various choices of number of diffraction peaks and different number of stress levels available for the calculation is considered. It is shown that when all the data are within the elastic regime, an accurate evaluation of the reference lattice parameter can be made. When some data are in the plastic regime, a more limited evaluation is possible. The use of plots of lattice parameter against Γ^hkl [= (h²k² + h²l² +k²l²)/(h² + k² + l²)²] as a method for monitoring plasticity as well as freedom from deviatoric stress is demonstrated.