One of the traditional methods used to improve the fatigue properties of metallic materials is shot peening. More recently, cavitation peening, in which the surface is treated using cavitation impact, has been developed, and the improvements this makes to the fatigue life and the strength of metallic materials have been reported. In order to clarify the difference between these two methods, stainless steel SUS316 L samples were treated by shot peening and cavitation peening, and the fatigue properties of the samples were evaluated utilizing a displacement controlled plane bending fatigue test. The residual stress and hardness before and after the fatigue test were measured, and the surface roughness of each specimen was also measured. It was concluded that the fatigue life of shot peened specimens at bending stress σa > 450 MPa was longer than that of cavitation peened specimens; however, the fatigue strength of the cavitation peened specimens was slightly larger than that of the shot peened specimens. The compressive residual stress introduced by both peening methods decreased during the fatigue test. The reduction in the compressive residual stress in the shot peened specimens was greater than in the cavitation peened specimens, and after the fatigue test, the compressive residual stress in the shot peened specimens was greater than that in the cavitation peened specimens. It was found that the fatigue strength corresponded well with the yield stress estimated from the Vickers hardness corrected by the residual stress obtained after the fatigue test.

Aluminum alloys used in monolithic parts for aerospace applications are subjected to distortion and residual stress (RS) generated by milling, affecting the product fatigue life. Particularly, the change in RS with depth (z) has a characteristic distribution with a maximum compressive RS at a z several tens of micrometers from the surface; however, the RS value depends on the measurement method used. In this study, the RS distribution with z from the surface after milling was measured for the AA7050-T7451 aluminum alloy by two-dimensional X-ray diffraction (2D method). The results were compared with those of four prior measurement methods, and the validity of 2D method was verified. The changes in subsurface RS with z showed similar distributions under all measurement conditions except when cos(α)-XRD was employed. The 2D method provides high repeatability. The in-plane RS distribution was also measured using 2D method to investigate the effect of milling conditions on this distribution. The RS values varied markedly depending on the measurement position, particularly at a small collimator diameter of 0.146 mm, allowing detection of localized extreme RS values. The maximum RS at z = 0 mm was − 85.6 MPa at a cutting speed of vc = 200 m/s and feed per tooth of fz = 0.05 mm, while it was − 16 MPa for vc = 450 m/s and 6.8 MPa for fz = 0.2 mm, revealing that the compressive RS changes to tensile RS as vc and fz increase.

As the fatigue strength of metallic components may be affected by residual stress variation at small length scales, an evaluation method for studying residual stress at sub-mm scale is needed. The sin2ψ method using X-ray diffraction (XRD) is a common method to measure residual stress. However, this method has a lower limit on length scale. In the present study, a method using at a 2D XRD detector with ω-oscillation is proposed, and the measured residual stress obtained by the 2D method is compared to results obtained from the sin2ψ method and the slitting method. The results show that the 2D method can evaluate residual stress in areas with a diameter of 0.2 mm or less in a stainless steel with average grain size of 7 μm. The 2D method was further applied to assess residual stress in the stainless steel after treatment by laser cavitation peening (LCP). The diameter of the laser spot used for LCP was about 0.5 mm, and the stainless steel was treated with evenly spaced laser spots at 4 pulses/mm2. The 2D method revealed fluctuations of LCP-induced residual stress at sub-mm scale that are consistent with fluctuations in the height of the peened surface.