FATIGUE-CRACK INITIATION AND PROPAGATION ON SHOT-PEENED SURFACES IN A316 STAINLESS-STEEL

Shot-peening was found to affect crack behaviour by delaying both crac k initiation and crack propagation. Cracks were formed preferentially at the specimen edges, and crack propagation was found to be higher in the depth direction than along the surface, giving initial quarter-el liptical crack fronts with the major axis along the thickness directio n. Shot-peeing significantly increases the ratio of initiation life to propagation life in comparison to unpeened specimens. Surface polishi ng was found to be detrimental to fatigue resistance. Indeed, it was o bserved that polishing had a similar effect for specimens of different peening intensities and thereby different depths of residual compress ing stress. For high stresses with low lifetimes, the propagation peri od dominates, whereas for low applied stresses with long lifetimes the initiation period dominates. At high applied stress, cracks were init iated at the lower surface rather than the upper surface of the specim en even though a stress ratio R of -0.8 was used. This suggests that r elaxation of residual stresses is more readily achieved by the action of the compressive stress in the fatigue cycle.