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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.

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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 -

The effects of milling as machining process and a post-machining treatment by wire-brush hammering, on the near surface layer characteristics of AA 5083-H111 were investigated. Surface texture, work-hardening and residual stress profiles were determined by roughness measurement, scanning electron microscope (SEM) examinations, microhardness and X-ray diffraction (XRD) measurements. The effects of surface preparation on the fatigue strength were assessed by bending fatigue tests performed on notched samples for two loading stress ratios R 0.1 and R 0.5. It is found that the bending fatigue limit at R 0.1 and 10 7 cycles is 20% increased, with respect to the machined surface, by wire-brush hammering. This improvement was discussed on the basis of the role of surface topography, stabilized residual stress and work-hardening on the fatigue-crack network nucleation and growth. The effects biaxial residual stress field and surface workhardening were taken into account in the finite element model. A multi-axial fatigue criterion was proposed to predict the fatigue strength of aluminum alloy notched parts for both machined and treated states.

The effects of wire brush hammering on low cycle fatigue behaviour of AISI 316 austenitic stainless steel has been investigated on turned samples through an experimental study combining strain controlled fatigue tests, scanning electron microscope examination and X‐ray diffraction analysis. An increase in fatigue life by 266% was reported at an imposed strain amplitude of Δεt/2 = 0.2%. This improvement is limited to Δεt/2 ≤ 0.5%. It is found that wire brush hammering produces a surface texture that favours, under cyclic loading, nucleation of randomly dispersed short cracks of the order of 50 µm in length stabilized by a compressive residual stress field. In contrast, turned surface showed much longer unstable cracks of the order of 200 µm in length nucleated in the machining groves and propagated under the effect of a tensile residual stress field. It has also been established that wire brush hammering can be used as intermittent treatment to improve the residual fatigue life of components subjected to cyclic loading. The treatment is very efficient if it is performed at a fraction of service lifetime ni/Nr lower than 0.5.

The effects of machined and treated surface characteristics on the fatigue strength were analyzed on the basis of experimental results related to AISI D2 ground surface and AA 5083-H111 hammered surface. The fatigue strength improvement resulting from controlled grinding and mechanical surface treatment was discussed on the basis of the beneficial effect of the work hardening and the stabilized residual stress. A numerical procedure using F.E.M for calculating residual stress and work hardening evolution under cyclic loading has been developed. The validation of the numerical procedure was carried out by comparing the calculated residual stress profiles to those resulted from XRD measurements. The multi-axial criterion accounting for the work hardening and the residual stress was used to predict the fatigue life of notched samples. IntroductionIt is well established that residual stress and work hardening, generated by machining and surface treatment processes, influence the fatigue life of mechanical parts [1][2][3]. That is why they are often considered in fatigue strength predictive models using multi-axial fatigue criteria [3] and fatigue life assessment by local strain-life approaches [4,5]. Experimental and modeling results confirm the detrimental effect of tensile residual stress which is considered to promote fatigue crack nucleation and to accelerate their propagation [6]. In contrary, it has been demonstrated that the compressive residual stress, generated by controlled machining processes and surface treatments, improves the fatigue strength by delaying the crack nucleation and by reducing the crack propagation rate [2,7,8]. However, most of published data related to fatigue crack nucleation and growth involving residual stress were discussed on the basis of their initial distribution [9][10][11]. Moreover, the majority of fatigue predictive methods take into account the initial measured residual stress values and neglect their evolution under cyclic loading. This assumption is controversial by published data related to stabilized residual stress profiles showing relaxation and redistribution phenomena [2,8,12,13]. Moreover, it has been reported that the most fraction of residual stress relaxation is monotonic, since it occurs at first cycles. Relaxation could progress under cyclic loading if the plastic misfit itself continues to change until stability [13][14][15]. Since the stabilized residual stress state is an important parameter for fatigue life prediction, it is of paramount importance to determine, in this study, the parameters that govern their evolution during cyclic loading. These parameters will be taken into account by the developed numerical procedure to calculate stabilized residual stresses and therefore fatigue life time. Analysis of the residual stress cyclic stability is based on the experimental results of surface characterization and fatigue tests conducted on AISI D2 ground surface and AA 5083-H111 machined and wire brush hammered surfaces.