Notch fatigue behaviour of shot peened high-strength aluminium alloys: Experiments and predictions using a critical distance method

The present paper is aimed at investigating the effect of different shot peening treatments on the fatigue behaviour of Al-7075-T651 samples carrying different types of notches. The Wohler S-N curves were determined by reversed bending for the different experimental conditions. A different improvement of the fatigue strength was found, depending on the peening intensity, and, particularly, a different effectiveness of the treatments for different notch geometries: the more critical notches received the larger beneficial effect by the shot peening treatment in terms of reduction of the notch fatigue sensitivity and increment of the notch fatigue strength. The fatigue improvements with respect to the unpeened condition and the influence of the peening intensity on fatigue were discussed accounting for the residual stress effects. The extent of the residual stress redistribution during loading was investigated by means of X-ray diffraction (XRD) measurements on plain specimens. An evolution to a stabilized value of the surface stress, depending on the applied loads, was found. Residual stress fields produced by the shot peening on the notched specimens were evaluated by means of a numerical technique, making use of XRD measurements on the plain specimens. Moreover, the application of the fatigue load has been modelled in order to investigate the residual stress relaxation in the vicinity of the notch. The stabilized residual stress values have been incorporated into a multiaxial fatigue criterion to predict the fatigue resistance. Finally, a critical distance theory method has been used to predict the fatigue resistance of bodies carrying stress concentrators in the presence of a compressive residual stress field. (C) 2010 Elsevier Ltd. All rights reserved.