X-Ray Elastic Constants and Their Meaning for Al and Fe
NORTHWESTERN UNIV EVANSTON IL DEPT OF MATERIALS SCIENCE
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In the measurement of residual stresses via diffraction using x-rays or neutrons it is strains that are actually determined, by employing the interplanar spacing d subscript hkl of the hkl planes as an internal strain gauge. The change in this spacing is measured from the shift of diffraction peaks and Braggs law at several orientations of the sample to the incident beam, and the resultant strains are converted to stresses with the diffraction elastic constants , S subcript 1hkl and S subscript 2hkl2. While these take on the values -vE and 1 vE respectively for an isotropic solid, in anisotropic materials their values depend on many factors preferred orientation, shape and orientation of second phases, interaction between grains. In fact there are reports of variation these constants with plastic deformation 2 and theory predicts variations with morphology. It is demonstrated that the diffraction elastic constants for residual stress measurements can be determined even in the presence of oscillations in interplanar spacing vs sin squared psi. Furthermore, these values can be employed to ascertain whether the oscillations are due to local fluctuations in plastic deformation, or elastic anisotropy.
- Metallurgy and Metallography