A fretting crack initiation prediction taking into account the surface roughness and the crack nucleation process volume

This paper presents an experimental study of the fretting crack nucleation threshold, expressed in terms of loading conditions, with a cylinder/plane contact. The studied material is a damage tolerant aluminium alloy widely used in the aerospace application. Since in industrial problems, the surface quality is often variable, the impact of a unidirectional roughness is investigated via varying the roughness of the counter body in the fretting experiments. As expected, experimental results show a large effect of the contact roughness on the crack nucleation conditions. Rationalisation of the crack nucleation boundary independently of the studied roughnesses was successfully obtained by introducing the concept of effective contact area. This does show that the fretting crack nucleation of the studied material can be efficiently described by the local effective loadings inside the contact. Analytical prediction of the crack nucleation is presented with the Smith-Watson-Topper (SWT) parameter and size effect is also studied and discussed.Comment: 21 figure

In situ 3D characterization of fatigue cracks displacement fields

The three dimensional growth of fatigue cracks in samples of nodular graphite cast iron ischaracterized using laboratory X-ray computed tomography. The cracks grow from laser machined artificialdefects, their development is monitored in situ using laboratory X-ray computed tomography (lab. CT) andDigital Volume Correlation (DVC). The combination of both techniques gives access to the 3D displacementfield at the tip of the crack (mainly mode I opening)

Simulation par éléments finis de la déformation de polycristaux à partir d'images de tomographie par contraste de diffraction

National audienceLa tomographie par contraste de diffraction donne accès à la forme, l'orientation et l'état de déformation élastique des grains dans des volumes polycristallins pouvant contenir à l'heure actuelle jusqu'à mille grains. La combinaison de cette technique avec la méthode des éléments finis est particuliè- rement prometteuse pour analyser le rôle de la cristallographie locale sur les mécanismes de déformation et de dégradation dans des matériaux polycristallins. Dans ce travail, un échantillon polycristallin de titane est imagé en 3D puis maillé et sa déformation de traction est calculée par éléments finis.See http://hal.archives-ouvertes.fr/docs/00/59/29/18/ANNEX/r_974AO844.pd

Wavelet-based texture segmentation of titanium based alloy lamellar microstructure: application to images from optical microscope and X-ray microtomography

A texture segmentation algorithm which combines grey level intensity from 2 images after discrete wavelet transform and variance has been applied on 2D images to segment lamellar colonies in (α+β) titanium alloy Ti6A14V. Images were acquired using both optical microscope and X-ray tomography. The results are satisfying for the former technique and encouraging for the latter one. Possible extension of the method to volumetric data is presented

Influence of forging conditions on the fatigue mechanisms of low alloy steels: a 3D study

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 ?m wide x 50 ?m deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface

Characterization of polycrystalline materials by X-ray diffraction contrast tomography

National audienceSynchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for characterizing the grain microstructure in a variety of polycrystalline mono-and multiphase materials. Direct visualization of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques is possible, but restricted to specific material systems (Ludwig 2009b). However, conventional attenuation or phase contrast imaging techniques do not give access to the crystallographic orientation of the grains and additional diffraction measurements are required. On the other hand, three-dimensional X-ray diffraction methods like Differential Aperture X-ray Microscopy (DAXM, (Larson, 2002) or 3D X-ray Diffraction Microscopy (3DXRD) (Poulsen, 2004) can analyse the 3D grain structure and/or elastic strain tensors of individual grains in polycrystalline materials, but are blind to the microstructural features (inclusions, cracks and porosity) visible in attenuation and/or phase contrast imaging techniques . A recent extension of the 3DXRD methodology, termed X-ray diffraction contrast tomography (DCT) (Ludwig 2009a), combines the principles of 3DXRD and X-ray absorption tomography. With a single scan, DCT can provide simultaneous access to the grain shape, crystallographic orientation, full elastic strain tensor and the local attenuation coefficient distribution in three dimensions. The technique applies to a range of plastically undeformed, polycrystalline mono-phase materials, fulfilling some conditions on grain size and texture. The straightforward combination with in-situ microtomographic observations opens interesting new possibilities for the characterization of microstructure related damage and deformation mechanisms in these materials

The grain microstructure of polycrystalline materials as revealed by the combined use of synchrotron X-ray imaging and diffraction techniques

National audienceCombining the principles of x-ray imaging and diffraction techniques, it has recently become possible to map the 3D grain microstructure in a range of polycrystalline materials. Associating this 3D orientation mapping with conventional attenuation and/or phase contrast tomography yields a non-destructive characterization technique, enabling time-lapse observation of dynamic processes in the bulk of structural materials. The capabilities and limitations., as well as future perspectives of this new characterization approach will be discussed and illustrated on selected application examples

Guest editorial: fatigue design and material defects

This issue of Fatigue and Fracture of Engineering Materials and Structures contains a collection of manuscripts presented at the Second International Symposium on Fatigue Design and Material Defects (FDMD II) held in Paris, France, on June 11 – 13, 2014 organized by the French Society for Metallurgy and Materials (SF2M) and the German Association for Materials Research and Testing (DVM)

3D X-ray Microtomography Volume Correlation to Study Fatigue Crack Growth

International audienceGlobal digital volume correlation is used to analyze a series of computed tomography images of a nodular graphite cast iron specimen subjected in situ to a fatigue test. From the obtained displacement field, a specific procedure is implemented to extract stress intensity factors all along the crack front. The proposed methodology allows one to measure key parameters in fatigue crack propagation directly from 3D images

Internal fatigue crack propagation in a Ti-6Al-4V alloy: An in situ study

Fatigue specimens of a Ti-6Al-4V alloy containing internal artificial defects with controlled and reproducible size and shape have been produced. These defects systematically led to the initiation of a fatigue crack which propagation has been monitored in situ by synchrotron X-ray tomography during R=0.1 uniaxial fatigue tests at 20 Hz. The crack growth curves of the internal cracks have been obtained for 6 samples. Ex situ fatigue tests have been performed on samples submitted to a supplementary heat treatment or containing a defect put into contact with air. The results obtained tend to support the fact that internal fatigue cracks grow from the notch in a vacuum environment. On the fracture surfaces of samples containing an artificial defect not connected to air, two regions have been observed. They correspond to the Rough Area and the Fish Eye regions observed for internal cracks initiated from natural defects. The transition between those two regions takes place when the plastic radius size is equivalent to the grain size