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The effects of hammering by wire brush as a method of improving low cycle fatigue life of highly ductile austenitic stainless steel AISI 304 have been investigated through an experimental study combining imposed strain fatigue tests and assessment of surface characteristic changes under cyclic loading by SEM examinations and XRD analysis. It has been shown that the fatigue life of wire brush hammered surface was increased by 307% at an imposed strain rate of 0.2% and only 17% at an imposed strain rate of 0.5%, comparatively to the turned surface. This increase in fatigue life is explained in terms of fatigue damage that is related to crack networks characteristics and stability which are generated during fatigue on both turned and wire brush hammered surfaces. The improvement of brushed surface is attributed to the role of the surface topography, the near surface stabilized compressive residual stresses and superficial cold work hardening on the fatigue crack network nucleation and growth. It is found that wire brush hammering produces a surface texture that favors, under cyclic loading, nucleation of randomly dispersed short cracks of the order of 40 lm in length stabilized by the compressive residual stress field that reached a value of r 0 = À749 MPa. In contrast, turned surface showed much longer unstable cracks of the order of 200 lm in length nucleated in the machining groves with high tendency to propagate under the effect of tensile residual stress field that reached value of r 0 = 476 MPa. This improvement is limited to strain rates lower than 0.5%. At higher strain rates, a cyclic plastic deformation induced martensitic phase alters furthermore the fatigue behavior by producing high cyclic strengthening of the bulk material. This phenomenon lead to a reduction in strain imposed fatigue life. It has also been established that wire brush hammering can be used as an onsite surface treatment to improve the residual fatigue life of components subjected to cyclic loading. The efficiency of this treatment is demonstrated if it is performed at a fraction of service lifetime N i /N r lower than 0.5.

Mechanical brushing of machined AA5083H11 surface, performed in optimized conditions of brush wire hammering, generates distributions of compressive residual stresses. These distributions are controlled by the process parameters such as the pressure of the brush wires on the surface, the speed of rotation or the number of passes. X-ray diffraction (XRD) measurements relative to different surface preparations show that the stresses due to brushing are less deep and that the maximal values in compression are the same as the ones from shallow shot-peening treatment (without defects). Moreover, the bushing provides high quality surface at a microscopic scale (low roughness) and integrity state of the treated surfaces, better for cyclic stability of compressive stresses. The stabilized state of residual stresses resulting from brushing and shot-peening is determined by finite element analysis. These analyses are experimentally validated by XRD using the psi-tilt method on tested fatigue specimens. Fatigue results show that the good surface topography generated by the wire brushing process compensates the reduction of the amplitude and the depth of the compressive residual stress profiles compared to shot-peening so that it could provide an equivalent fatigue improvement rate (20 to 30% of σD2.106cycles) of the AA5083H11.

GONZÁLEZ -Effect of machining processes on the residual stress distribution heterogeneities and their consequences on the stress corrosion cracking resistance of AISI 316L SS in chloride medium -

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