Analysis of Strain-induced Peak Broadening in Titanium Alloys

X-ray diffraction measurement in the bulk of engineering components allows evaluation of lattice spacing variation by application of Bragg law and hence, by comparison with a strain-free reference specimen, computation of elastic strains and stresses. Diffraction peak position is related to average elastic strain within the scattering grains, whilst diffraction peak shape is related to crystallite size and so-called root mean square (RMS) strain, which refers to the spread of lattice parameter values around the average. This spread arises as a result of the anisotropic elastic and plastic properties of the individual crystallites, which, under macroscopic plastic deformation, lead to a non-uniform distribution of interand intra-granular elastic and plastic strains. It has been shown in the literature, that, based on monochromatic diffraction experiments, a correlation of peak width and equivalent plastic strain can be established for a given material. In this paper a correlation of plastic strain versus peakwidth is established based on energy dispersive Synchrotron Xray diffraction measurements of a Ti-6Al-4V 4 point bend specimen. The correlation is then used to map equivalent plastic strain within a plate specimen of the same material in which two semicircular notches act as stress concentrators during tensile loading, inducing a known, non-uniform equivalent plastic strain field. The resulting map is compared with measurements of equivalent plastic strain by digital image correlation. The extent of the plastic zone observed shows qualitative agreement, however the equivalent plastic strain values predicted by peakwidth analysis are lower than those deduced from digital image correlation.