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

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

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

Anarchism, Utopianism and Hospitality: The Work of René Schérer

René Schérer (born 1922) is lamentably almost unknown to the Anglo-American world as his work has, as yet, not been translated . He is one of the main specialists of the French “utopian socialist”, Charles Fourier (1772-1837), and a major thinker in his own right. He is the author of more than twenty books and co-editor of the journal Chimères. Colleague and friend at Vincennes university (Paris 8) of Michel Foucault, Gilles Deleuze, Félix Guattari, Jacques Derrida, Jacques Rancière, Jean-François Lyotard, François Châletet, Alain Brossat, Georges Navet, Miguel Abensour, Pierre Macherey… he continues to host seminars at Paris 8 (now located at St. Denis). He is a living testimony to a radical past, and a continuing inspiration to a new generation of young thinkers. This article aims to convey the original specificity of his understanding of anarchism. By so doing, it will stress the importance of his work for any thinking concerned with a politicised resistance to social conformity and the supposed “state of things” today

Entangling voluntarism, leisure time and political work: the governmentalities of neighbourhood planning in England

Neighbourhood planning was the first volunteer-led statutory planning tool to be created in the UK. Whilst it has provoked debate and critique covering numerous practical and theoretical aspects (Wargent and Parker, 2018), little attention has been paid to the actual experience and motives of the volunteers who spend their leisure time by volunteering to prepare a plan. Given the range of leisure activities that have been shaped in the context of a neo-liberalised policy environment we add to longstanding debates concerning the political nature of leisure and how neo-liberal policies require, and exploit, volunteer time and input while claiming to offer forms of empowerment. Qualitative data derived from neighbourhood plan volunteers is presented here to highlight the political work of neighbourhood planning, thus responding to calls to extend the analysis of the political in and through leisure (Rose et al, 2018). It is argued that neighbourhood planning pushes the boundaries of what can be legitimately asked of volunteers and expected in terms of delivering policy outcomes

The Parental Non-Equivalence of Imprinting Control Regions during Mammalian Development and Evolution

In mammals, imprinted gene expression results from the sex-specific methylation of imprinted control regions (ICRs) in the parental germlines. Imprinting is linked to therian reproduction, that is, the placenta and imprinting emerged at roughly the same time and potentially co-evolved. We assessed the transcriptome-wide and ontology effect of maternally versus paternally methylated ICRs at the developmental stage of setting of the chorioallantoic placenta in the mouse (8.5dpc), using two models of imprinting deficiency including completely imprint-free embryos. Paternal and maternal imprints have a similar quantitative impact on the embryonic transcriptome. However, transcriptional effects of maternal ICRs are qualitatively focused on the fetal-maternal interface, while paternal ICRs weakly affect non-convergent biological processes, with little consequence for viability at 8.5dpc. Moreover, genes regulated by maternal ICRs indirectly influence genes regulated by paternal ICRs, while the reverse is not observed. The functional dominance of maternal imprints over early embryonic development is potentially linked to selection pressures favoring methylation-dependent control of maternal over paternal ICRs. We previously hypothesized that the different methylation histories of ICRs in the maternal versus the paternal germlines may have put paternal ICRs under higher mutational pressure to lose CpGs by deamination. Using comparative genomics of 17 extant mammalian species, we show here that, while ICRs in general have been constrained to maintain more CpGs than non-imprinted sequences, the rate of CpG loss at paternal ICRs has indeed been higher than at maternal ICRs during evolution. In fact, maternal ICRs, which have the characteristics of CpG-rich promoters, have gained CpGs compared to non-imprinted CpG-rich promoters. Thus, the numerical and, during early embryonic development, functional dominance of maternal ICRs can be explained as the consequence of two orthogonal evolutionary forces: pressure to tightly regulate genes affecting the fetal-maternal interface and pressure to avoid the mutagenic environment of the paternal germline

Human oocyte-derived methylation differences persist in the placenta revealing widespread transient imprinting

Thousands of regions in gametes have opposing methylation profiles that are largely resolved during the post-fertilization epigenetic reprogramming. However some specific sequences associated with imprinted loci survive this demethylation process. Here we present the data describing the fate of germline-derived methylation in humans. With the exception of a few known paternally methylated germline differentially methylated regions (DMRs) associated with known imprinted domains, we demonstrate that sperm-derived methylation is reprogrammed by the blastocyst stage of development. In contrast a large number of oocyte-derived methylation differences survive to the blastocyst stage and uniquely persist as transiently methylated DMRs only in the placenta. Furthermore, we demonstrate that this phenomenon is exclusive to primates, since no placenta-specific maternal methylation was observed in mouse. Utilizing single cell RNA-seq datasets from human preimplantation embryos we show that following embryonic genome activation the maternally methylated transient DMRs can orchestrate imprinted expression. However despite showing widespread imprinted expression of genes in placenta, allele-specific transcriptional profiling revealed that not all placenta-specific DMRs coordinate imprinted expression and that this maternal methylation may be absent in a minority of samples, suggestive of polymorphic imprinted methylation