Residual elastic strain due to laser shock peening: Synchrotron diffraction measurement

Abstract

Residual elastic strain in a thin slice parted off from a laser shock peened plate of titanium alloy Ti–6Al–4V was measured using high-energy diffraction on station 16.3 at SRS Daresbury. Diffraction peaks were collected for reflections (00.2), (10.1), (10.2), and (11.0) from the hcp (hexagonal close-packed) α-phase of the titanium alloy. Reference values of the lattice spacing for each of the reflections were found from the diffraction pattern collected from a stress-free sampling volume. The residual elastic strain values calculated on the basis of each reflection were then computed and plotted as a function of distance from the treated surface. Residual strain profiles show significant differences, reflecting the variation in the elastic and plastic with orientation of the grain, i.e. anisotropy. The average macroscopic residual elastic strain was therefore computed using an appropriate average, taking into account multiplicity of each reflection. Since the greatest contrast in elastic and plastic properties exists between directions (00.2) and (11.0), the difference between residual elastic strains measured for these reflections was plotted, together with the ‘difference strain’ between (00.2) and (10.1). These showed a very good correlation with the plastic strain profile introduced by laser shock peening.