Reverse bending fatigue of shot peened 7075-T651 aluminium alloy: The role of residual stress relaxation

The effect of different shot peening treatments on the reverse bending fatigue behaviour of Al-7075-T651 was studied. The fatigue improvements with respect to the unpeened condition and the influence of the peening intensity on fatigue were discussed accounting for the effects of surface modifications (roughness and strain hardening) and of residual stresses. In particular the extent of the residual stress redistribution during loading was investigated by means of X-ray diffraction (XRD) measurements. No significant residual stress relaxation was observed in samples tested to a load level corresponding to the fatigue endurance at 5 · 106 cycles. Residual stress relaxation was observed only when the material plastic flow stress was achieved during the compressive part of the fatigue load cycle. Accordingly, shot peened samples with deep sub-superficial compressive residual stress peak showed a fatigue endurance level corresponding to the condition of incipient plastic flow. This phenomenon was also accompanied by sub-superficial fatigue crack initiation. On the contrary, samples tested at shorter fatigue lives showed crack initiation close to the surface. The initial and the stabilized residual stress profiles were considered for discussing the improvement in the fatigue behaviour due to peening. For this purpose, a multiaxial fatigue criterion was adopted to account for the biaxial residual stress field. The fatigue life was quite accurately predicted as long as fatigue initiation occurs on the surface.