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

Abstract

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.

Assessment of low cycle fatigue improvement of machined AISI 316 stainless steel by brush hammering

“…In this case, the effective stress intensity is reduced by the negative residual stress intensity factor. These observations are in good agreement with both experimental and modelling results reported in the literature for both ductile and hard materials . Indeed, the turning process produces tensile or very low compressive residual stresses in both loading and transverse directions ( σ 0 = −14 MPa in the loading direction and σ 90 = +340 MPa in the transverse direction), whereas subsequent wire brush hammering operation reverses those residual stresses to induce highly compressive ones ( σ 0 = −846 MPa and σ 90 = −750 MPa in the same directions, respectively).…”

“…This high surface hardness is expected to contribute significantly to the enhancement of fatigue crack initiation resistance near surface as reported in previous studies. 10,12 Ben Fredj et al 10 reported a 26% increase in the high cycle fatigue strength of a ground and brushed AISI 304 stainless steel sample, whereas Makhlouf et al 12 reported an increase in low cycle fatigue life by 307% at a strain amplitude Δε t /2 = 0.2% for a turned AISI 304 stainless steel. This last value is of the same order of magnitude (266%) found in this study for AISI 316.…”

“…In order to reduce the detrimental effect of excessive surface roughness induced by shot peening, less aggressive techniques were developed. Among these techniques, laser peening, ball and roller burnishing and more recently hammering by wire brush were used to replace the shot peening. The efficiency of these techniques has been demonstrated for high cycle fatigue strength of steels and aluminium alloys .…”

Role of machining defects and residual stress on the AISI 304 fatigue crack nucleation

“…It shows a fatigue lifetime decreasing with an increase in the rebuilt material density. This result is in accordance to the well‐known detrimental effect of tensile residual stress on the fatigue crack nucleation, which considered as damaging stress …”

“…Higher is the rebuilt material density, rapid is the fatigue crack coalescence and lower fatigue life is. This phenomenon is strongly associated with the high tensile residual stress field as supported by Makhlouf et al . observations for the AISI 304.…”