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Comportement en fatigue de pièces de Ti-6Al-4V obtenues par SLM et EBM : effet de la rugosité

Abstract : Ti-6Al-4V parts obtained by SLMand EBM, often post-treated by stress relieving andHIP, have a very rough surface finish that reduces their fatigue strength. The use of such parts forstructural applications therefore requires to understand and to quantify the effect of roughness onfatigue crack initiation mechanisms. This study presents both an experimental and numerical methodologyfor taking into account the effect of roughness on high cycle fatigue strength of parts obtainedby additive manufacturing. The lamellar microstructure of SLMandEBMspecimens treated bystress relief and HIP was characterized by optical microscope and EBSD observations. The "as-built"surfaces have a specific roughness. The comparison of different methods of roughness measurement(2D cross-section observations and profilometry) made it possible to characterize it. The role of thissurface state on fatigue strength was determined by uniaxial andmultiaxial fatigue tests on either "asbuilt",machined or chemically polished surfaces. Systematic SEManalysis of the fracture surfaces foreach batch has made it possible to identify the surface defects that caused the initiation. Additionalfatigue tests conducted on SLM "as-built" surface specimens, coupled with SEM observations andintra-grain misorientation (KAM) measurements at the notch tip, revealed the presence of microscopiccracks and confined plastic areas. The last part of this work proposes a numerical approach,based on F.E. calculations, to quantify the effect of the surface on fatigue strength. The meshes wereconstructed from the acquisition of the surface (2D cross-sections; profilometry; tomography) andthe calculations were carried out in elasticity and elastoplasticity. The criticality of the stress field regardingfatigue was evaluated through a parameter integrating the high near-surface stress gradientsthrough volume averaging and the many potential initiation sites through extreme value statistics.
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  • HAL Id : tel-02533650, version 1

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Bastien Vayssette. Comportement en fatigue de pièces de Ti-6Al-4V obtenues par SLM et EBM : effet de la rugosité. Autre [cond-mat.other]. Ecole nationale supérieure d'arts et métiers - ENSAM, 2019. Français. ⟨NNT : 2019ENAM0045⟩. ⟨tel-02533650⟩

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