Low cycle fatigue life improvement of AISI 304 by initial and intermittent wire brush hammering

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 superfi-cial 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 r0 = 749 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 r0 = 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 mate-rial. 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 Ni/Nr lower than 0.5

Potential fatigue strength improvement of AA 5083-H111 notched parts by wire brush hammering: Experimental analysis and numerical simulation

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 R0.1 and R0.5. It is found that the bending fatigue limit at R0.1 and 107 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 work-hardening 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

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 machining such as grinding and turning on the microstructural and mechanical changes of the machined surfaces of AISI 316L stainless steel (SS) have been studied. Surface aspects and surface defects have been examined by scanning electron microscopy (SEM). Machining-induced nanocrystallization has been investigated by transmission electron microscopy (TEM). Surface and subsurface residual stress distribution and plastic deformation induced by the machining processes have been assessed by X-ray diffraction (XRD) and micro-hardness measurements, respectively. The susceptibility to stress corrosion cracking (SCC) has been assessed by SEM examination of micro-crack networks which are characteristics of a machined surface immersed in boiling (140 ± 2 °C) solution of MgCl2 (40%) during a 48 h-period. The machined surface properties have been correlated to severe plastic deformation (SPD) resulting from specific cutting state of each process. High cutting temperature and plastic rate are considered to be at the origin of near-surface austenitic grain refinement that leads to equiaxed nanograins with a size ranging from 50 to 200 nm. Ground surface residual stress distribution heterogeneities at the micrometric scale are attributed to the random distribution of the density and the geometry of abrasive grains that represent micro-cutting tools in the grinding process. The relationship between residual stress distribution and susceptibility of the AISI 316L SS to SCC has been demonstrated, and an experimental criterion for crack initiation has been established

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

International audienceThe effects of machining such as grinding and turning on the microstructural and mechanical changes of the machined surfaces of AISI 316L stainless steel (SS) have been studied. Surface aspects and surface defects have been examined by scanning electron microscopy (SEM). Machining-induced nanocrystallization has been investigated by transmission electron microscopy (TEM). Surface and subsurface residual stress distribution and plastic deformation induced by the machining processes have been assessed by X-ray diffraction (XRD) and micro-hardness measurements, respectively. The susceptibility to stress corrosion cracking (SCC) has been assessed by SEM examination of micro-crack networks which are characteristics of a machined surface immersed in boiling (140 ± 2 °C) solution of MgCl2 (40%) during a 48 h-period. The machined surface properties have been correlated to severe plastic deformation (SPD) resulting from specific cutting state of each process. High cutting temperature and plastic rate are considered to be at the origin of near-surface austenitic grain refinement that leads to equiaxed nanograins with a size ranging from 50 to 200 nm. Ground surface residual stress distribution heterogeneities at the micrometric scale are attributed to the random distribution of the density and the geometry of abrasive grains that represent micro-cutting tools in the grinding process. The relationship between residual stress distribution and susceptibility of the AISI 316L SS to SCC has been demonstrated, and an experimental criterion for crack initiation has been established