Papers by Author: Jean Yves Buffière

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Authors: Emilie Ferrié, Jean Yves Buffière, Wolfgang Ludwig
Abstract: In-situ fatigue tests monitored by Synchrotron Radiation X-ray microtomography were carried out in order to visualize the three dimensional (3D) shape and evolution of short cracks in the bulk of a cast Al alloy. After the in-situ fatigue test the sample has been infiltrated with liquid Gallium (Ga) in order to visualize the grain structure of the material. Irregularities of the crack advance along the crack front can clearly be correlated to the grain structure of the material.
Authors: Jean Yves Buffière, Emilie Ferrié, Wolfgang Ludwig, Anthony Gravouil
Abstract: This paper reports recent results on the characterisation and modelling of the three dimensional (3D) propagation of small fatigue cracks using high resolution synchrotron X ray micro-tomography. Three dimensional images of the growth of small fatigue cracks initiated in two Al alloys on natural or artificial defects are shown. Because of the small size of the investigated samples (millimetric size), fatigue cracks grown in conventional Al alloys with a grain size around 100 micrometers can be considered as microstructurally short cracks. A strong interaction of these cracks with the grain boundaries in the bulk of the material is shown, resulting in a tortuous crack path. In ultra fine grain alloys, the crack shapes tend to be more regular and the observed cracks tend to grow like ”microstructurally long cracks” despite having a small physical size. Finite Element meshes of the cracks can be generated from the reconstructed tomographic 3D images. Local values of the stress intensity factor K along the experimental crack fronts are computed using the Extended Finite Element method and correlated with the crack growth rate.
Authors: S. Savelli, Jean Yves Buffière, P.-H. Jouneau, Roger Fougères
Authors: Erembert Nizery, Jean Yves Buffière, Henry Proudhon, Armelle Daniélou, Samuel Forest
Abstract: The mechanisms of fatigue crack initiation due to second phase particles are studied in 2050-T8 and 7050-T74 plate material. The particles in the specimens gauge lengths are imaged using SEM at the initial state. In 7050-T74, Mg2Si particles are very often cracked before any loading, whereas Al7Cu2Fe particles are not. In 2050-T8, the fraction of (Al, Cu, Fe, Mn) particles initially cracked is larger than that of Al7Cu2Fe in 7050-alloy, but lower than that of Mg2Si particles for similar sizes. For (Al, Cu, Fe, Mn) particles, the proportion of cracked particles increases when the modified shape ratio (aspect ratio including orientation versus rolling direction) increases. This effect is present but less pronounced for Mg2Si particles in 7050-T74. Fatigue cracks initiate at cracked (Al, Cu, Fe, Mn) particles in 2050-T8 alloy, and at both Al7Cu2Fe (cracked during cycling) and Mg2Si in 7050-T74 alloy.
Authors: Éric Maire, Jean Yves Buffière, R. Mokso, P. Cloetens, Wolfgang Ludwig
Abstract: This paper generally presents different techniques available to image the microstructure of materials in three dimensions (3D) at different scales. It then focuses on the use of the more versatile of these techniques for aluminum alloys : X-ray tomography. The paper describes the recent improvements (spatial and the temporal resolution, grain imaging). Electron tomography is also presented as a promising technique to improve the spatial resolution.
Authors: Emilie Ferrié, Jean Yves Buffière, Wolfgang Ludwig, Anthony Gravouil
Abstract: In this paper we will present how it is possible to couple a 3D experimental technique with a 3D numerical method in order to calculate the stress intensity factors along the crack front taking into account the real shape of the crack. This approach is used to characterize microstructurally short fatigue cracks that exhibit a rather complicated 3D shape. The values of the stress intensity factors are calculated along the crack front at different stages of crack propagation and it can be seen that the crack shape irregularities introduce rather important fluctuations of the values of KI, KII and KIII along the crack front. The values of KI obtained taking into account the real shape of the crack are significantly different from the ones calculated using an approach based on a shape assumption
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