Finite element analysis of fretting crack propagation

In this work, the finite elements method (FEM) is used to analyse the growth of fretting cracks. FEM can be favourably used to extract the stress intensity factors in mixed mode, a typical situation for cracks growing in the vicinity of a fretting contact. The present study is limited to straight cracks which is a simple system chosen to develop and validate the FEM analysis. The FEM model is tested and validated against popular weight functions for straight cracks perpendicular to the surface. The model is then used to study fretting crack growth and understand the effect of key parameters such as the crack angle and the friction between crack faces. Predictions achieved by this analysis match the essential features of former experimental fretting results, in particular the average crack arrest length can be predicted accurately

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

Large scale finite element simulations of polycrystalline aggregates: applications to X-ray diffraction and imaging for fatigue metal behaviour

International audienceLarge scale finite element simulations of the elastoviscoplastic behaviour of polycrystalline aggregates have become a standard technique to study the stress-strain heterogeneities that develop in grains during deformation. For a long time, comparison between continuum crystal plasticity and experimental field measurements was confined to the observation of surface behaviour. As for example the study of the development of intense deformation bands at the free surface of a polycrystal. Recent 3D experimental techniques open new perspectives in computational crystal plasticity. After reviewing how to define a representative volume element for polycrystal properties and showing that actual 3D computations, including grain shapes and orientations, are really needed to accurately determine the stress and strains distributions, two examples of applications of large scale simulations are described in this paper. First the simulation of 3D coherent X-ray diffraction in a polycrystalline gold sample is detailed. Based on the real geometry of the grains and their columnar nature, a 3D avatar is reconstructed. FE computations are then carried out to evaluate the effect of mechanical and thermal strain of the diffraction pattern resolved in the reciprocal space by complex FFT. Qualitative comparison with the experimental diffraction patterns shows that such computations can help understand the true nature of strain heterogeneities within the material. The second example of application deals with short fatigue crack propagation in polycrystals. One fundamental problem caused by short fatigue cracks is that despite decades of research, so far no reliable prediction of the crack propagation rates, comparable to the well-known Paris law in the long crack regime, could be established. This ``anomalous'' behaviour of short cracks is commonly attributed to factors like their complex three dimensional shapes and the influence of the local crystallographic environment affecting their propagation behaviour via a combination of physical mechanisms. Crystal plasticity computations based on the real grain shapes and orientations obtained thanks to diffraction contrast tomography are carried out using an ideal crack shape. The stress concentration at the crack tip is analysed with respect to possible crack growth directions

Nanox : a miniature mechanical stress rig designed for near-field X-ray diffraction imaging techniques

International audienceMulti-modal characterization of polycrystalline materials by combined use of three-dimensional (3D) X-ray diffraction and imaging techniques may be considered as the 3D equivalent of surface studies in the electron microscope combining diffraction and other imaging modalities. Since acquisition times at synchrotron sources are nowadays compatible with four-dimensional (time lapse) studies, suitable mechanical testing devices are needed which enable switching between these different imaging modalities over the course of a mechanical test. Here a specifically designed tensile device, fulfilling severe space constraints and permitting to switch between X-ray (holo)tomography, diffraction contrast tomography and topotomography, is presented. As a proof of concept the 3D characterization of an Al–Li alloy multicrystal by means of diffraction contrast tomography is presented, followed by repeated topotomo-graphy characterization of one selected grain at increasing levels of deformation. Signatures of slip bands and sudden lattice rotations inside the grain have been shown by means of in situ topography carried out during the load ramps, and diffraction spot peak broadening has been monitored throughout the experiment

Study of the flat to slant crack transition in ductile thin sheet material : simulations and experiments

International audienceFlat to slant crack transition can typically be observed in ductile thin sheet materials. The crack initiates perpendicularly to the loading direction from the notch and then turns to 45° with respect to the loading direction during crack propagation. This phenomenon is, however, still not well understood and, so far, attempts to simulate the transition in three dimensions often fail to predict macroscopic loads correctly. In this study an initial attempt has been made to reproduce the flat to slant transition performing an implicit 3D Finite Element simulation via adapting a Gurson-type model. A second void nucleation term for deformation under shear was introduced.The Lode parameter was used here to identify shear deformation. Using this modification the flat to slant transition has been reproduced successfully at loads similar to the experimental results. Further experimental investigations of void growth in the flat and slant crack propagation regime have been carried out. Cracks in Kahn tear test specimens have been arrested in the three regimes and subsequently been observed via Synchrotron Radiation Tomography of the crack tips 3D quantitative void growth analyses ahead of the crack tip in the flat and slant regimes have confirmed the change in fracture mechanisms: void growth in the flat region is substantially higher as compared to the slant crack propagation region

Analyse du contact entre surfaces rugueuses par la méthode des éléments finis et par un nouveau modèle numérique

Titre du résumé joint : Approches numériques du contact entre une surface rugueuse et un plan rigideInternational audienceDeux approches du contact normal sans frottement entre la surface rugueuse d'un matériau élastoplastique et un plan rigide sont proposées. Un nouveau modèle numérique est développé et comparé à une approche éléments finis. Il se nourrit d'une série de modèles mathématiques calibrés par des calculs éléments finis modélisant une seule aspérité. Les interactions entre aspérités sont prises en compte. Ce modèle permet de prédire la courbe charge-déplacement, l'aire réelle de contact et les champs d'ouverture de la fracture rugueuse tout en assurant un coût de calcul très réduit

Simulation de la diffraction cohérente d'un film polycristallin

http://hdl.handle.net/2042/38756International audienceLa diffraction cohérente permet de caractériser les hétérogénéités de déformation dans un polycristal. La figure de diffraction dans l'espace réciproque dépend de la forme du grain illuminé et de son champ de déformation. Dans ce travail, La déformation d'un film mince polycristallin est calculée par élément finis et les figures de diffraction de certains grains en sont déduites en fonction de la déformation imposée. L'influence de la densité de maillage, de la taille et de la forme du grain illuminé est étudiée

In situ multi‐axial loading frame to probe elastomers using X‐ray scattering

International audienceAn in situ tensile-shear loading device has been designed to study elastomer crystallization using synchrotron X‐ray scattering at the Synchrotron Soleil on the DiffAbs beamline. Elastomer tape specimens of thickness 2 mm can be elongated by up to 500% in the longitudinal direction and sheared by up to 200% in the transverse direction. The device is fully automated and plugged into the TANGO control system of the beamline allowing synchronization between acquisition and loading sequences. Experimental results revealing the evolution of crystallization peaks under load are presented for several tension/shear loading sequences

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

Experimental and computational analysis of toughness anisotropy in an AA2139 Al-alloy for aerospace applications

International audienceFracture toughness anisotropy of AA2139 (Al-Cu-Mg) in T351 and T8 conditions has been investigated via mechanical testing of smooth and notched specimens of different geometries, loaded in the rolling direction (L) or in the transverse direction (T). Fracture mechanisms were investigated via SEM and synchrotron radiation computed tomography (SRCT). Contributions to failure anisotropy are identified with: (i) anisotropic initial void shape and growth, (ii) plastic behaviour including isotropic/kinematic hardening and plastic anisotropy, and (iii) nucleation at a 2nd population of 2nd phase particles leading to coalescence via narrow crack regions. A model based in part on the Gurson-Tvergaard-Needleman approach is constructed to describe and predict deformation behaviour, crack propagation and, in particular, toughness anisotropy. Model parameters are fitted using microstructural data and data on deformation and crack propagation for a range of small test samples. Its transferability has been shown by simulating tests of large M(T) samples