Effects of fatigue and fretting on residual stresses introduced by laser shock peening

The effects of fatigue and fretting fatigue on the distribution of residual stresses in shot and laser shock peened Ti-6Al-4V samples have been investigated. Residual elastic strains have been determined using high-energy synchrotron X-ray diffraction. Laser shock peening introduces a considerable compressive residual stress, the compressive zone extending 1.5 mm below the surface. The effects of fatigue loading have been investigated using a notched three-point bend geometry. The residual stress field was found to be largely insensitive to fatigue cycling, at least for the applied stress range studied. For fretting fatigue, while the residual stresses at depth were little affected, within 0.5 mm of the surface significant stress relaxation was observed; the extent of relaxation being greatest in the direction parallel to the fretting direction. The states of residual stress have been quantified using the concept of eigenstrain, which quantifies the retained plastic misfit resulting from peening. Finite element modeling has been used to determine the eigenstrain profiles causing the measured elastic strain profiles, and the changes to these eigenstrain profiles due to fretting. Our results suggest laser shock peening confers much greater fretting fatigue resistance than traditional shot peening alone due to the much deeper compressive zone.