Anisotropy and crystal plasticity study of fcc polycrystalline Ni by nanoindentation

National audienceL'objectif de l'étude est de mettre en lumière l'anisotropie des matériaux cristallins de type cfc aux échelles micro et nanométriques. Des résultats numériques et expérimentaux de nanoindentation sont présentés. Les essais expérimentaux ont été réalisés sur un échantillon de nickel polycristallin, avec un indenteur de type Berkovich. Les simulations ont été menées sous le code éléments finis ZEBULON, en y intégrant un modèle de plasticité cristalline en grande déformation. Trois directions cristallographiques principales, correspondant à trois grains présentant ces mêmes directions , ont été choisies comme axe d'indentation, à savoir [001], [101] et [111]. Les empreintes ont été analysées au microscope à force atomique (AFM). La topographie de la surface autour des empreintes a révélé des lignes de glissement associées aux différents systèmes activés, ainsi que des remontées de matière (bourrelets) fortement anisotropes et non-symétriques, dépendantes de l'orientation du cristal par rapport à l'indenteur. Ces observations sont en accord avec les résultats des simulations numériques. L'effet de l'orientation de l'indenteur dans chacun des plans d'indentation a également été étudié expérimentalement et numériquement

Anisotropy and crystal plasticity analysis of a FCC nickel polycrystal by nanoindentation and numerical modeling

International audienceAnisotropy and crystal plasticity analysis of a FCC nickel polycrystal by nanoindentation and numerical modelin

Étude de l'anisotropie et de la plasticité cristalline sur monocristaux de type cfc via la nano-indentation

L’objectif de l’étude est de mettre en lumière l’anisotropie des matériaux cristallins de type cfc aux échelles micro et nanométriques. Pour cela, un nanoindenteur (NHT) et un ultra-nanoindenteur (UNHT) sont utilisés pour les essais expérimentaux, et se révèlent être des outils d’analyse efficaces à ces échelles. Les campagnes expérimentales ont été menées sur deux types d’échantillons polycristallin: du Nickel recuit et écroui, tous les deux à gros grains (>100µm). Un travail d’imagerie EBSD a permis de rendre compte de l’orientation des grains des zones d’étude. Les indentations ont été réalisées selon différentes directions cristallographiques avec un indenteur de type Berkovich (pyramidal à base triangulaire). L’effet de l’orientation de l’indenteur dans chacun de ces plans d’indentation a également été étudié. La topographie de la surface autour des indents, réalisée avec un microscope à force atomique, a révélé des lignes de glissement associées aux différents systèmes activés sous indentation, ainsi que des remontées de matière le long de l’indenteur (bourrelets) fortement dépendants de l’état d’écrouissage du matériau, de l’orientation du grain étudié mais également de l’orientation de l’indenteur. Ces observations donnent donc de précieuses informations sur la symétrie et l’orientation des grains étudiés. Il s’agit d’une véritable signature du grain considéré. Un travail de modélisation de l’essai de nano-indentation a également été réalisé sous le code éléments finis Zebulon dans lequel est utilisé le modèle de plasticité cristalline de Méric-Cailletaud (1991), permettant de prendre en compte la cristallographie du matériau. Différentes études numériques ont ainsi pu être menées sur monocristaux de type cfc: effet de l’anisotropie dans le cadre de l’élasticité, effet de la plasticité monocristalline et phénomène de pile-up. Les résultats de simulations sont en bonne corrélation avec les observations expérimentales

The influence of obesity and gender on outcome after reversed L-shaped osteotomy for hallux valgus

BACKGROUND Hallux valgus deformity (HV) affects around every fourth individual, and surgical treatment is performed in every thousandth person. There is an ongoing quest for the best surgical management and reduction of undesirable outcomes. The aim was to explore associations of obesity and gender with radiological and clinical outcome after reversed L-shaped osteotomy (ReveL) for HV. MATERIALS AND METHODS This study was carried out in a retrospective cohort design at a single University Hospital in Switzerland between January 2004 and December 2013. It included adult patients treated with ReveL for HV. The primary exposure was body mass index (BMI) at the time of ReveL. The secondary exposure was gender. The primary outcome was radiological relapse of HV (HV angle [HVA] > 15 degrees [°]) at the last follow-up. Secondary outcomes were improvable patient satisfaction, complication, redo surgery, and optional hardware removal. Logistic regression analysis adjusted for confounders. RESULTS The median weight, height, and BMI were 66.0 (interquartile range [IQR] 57.0-76.0) kilograms (kg), 1.65 (IQR 1.60-1.71) metres (m), and 24.0 (IQR 21.3-27.8) kg/m$^{2}$. Logistic regression analysis did not show associations of relapse with BMI, independent of age, gender, additional technique, and preoperative HVA (adjusted odds ratio [OR$_{adjusted}$] = 1.10 [95% (%) confidence interval (CI) = 0.70-1.45], p = 0.675). Relapse was 91% more likely in males (OR$_{adjusted}$ = 1.91 [95% CI = 1.19-3.06], p = 0.007). Improvable satisfaction was 79% more likely in males (OR$_{adjusted}$ = 1.79 [CI = 1.04-3.06], p = 0.035). Hardware removal was 47% less likely in males (OR$_{adjusted}$ = 0.53 [95% CI 0.30-0.94], p = 0.029). CONCLUSIONS In this study, obesity was not associated with unsatisfactory outcomes after ReveL for HV. This challenges the previous recommendation that preoperative weight loss may be necessary for a successful surgical treatment outcome. Males may be informed about potentially higher associations with unfavourable outcomes. Due to the risk of selection bias and lack of causality, findings may need to be confirmed with clinical trials

Single crystal plasticity parameters identification from residual imprint topography after nano-indentation

International audienceFundamental deformation mechanisms of FCC materials under indentation have been probed at the grain scale. Experimental tests have been conducted on large-grained annealed and coldworked polycrystalline nickel samples with a Berkovich indenter. Indentation axes have been chosen to be close to the three main crystallographic directions [001], [101] and [111]. Pile-ups and slip traces have been revealed around the residual imprints by analyzing topographic measurements obtained by atomic force microscopy. It is shown that the indenter orientation in each indentation plane drives pileups and slip traces which in turn contain precious information about the crystallographic orientation and the hardening state of the studied grain. Imprint topographies after pile-up formation therefore carry information that one can exploit to assess some intrinsic material properties at the grain scale. A 3D finite element modeling of the nano-indentation test at the grain scale has been developed, making use of crystal plasticity constitutive laws. Six different virtual materials having the same macroscopic behavior have been built. The simulation results show a good agreement with experimental tests and also a great pile-up sensitivity to interaction matrix components. These results pave the way to the interaction matrix identification using an inverse finite element method

A viscoelastoplastic stiffening model for plant fibre unidirectional reinforced composite behaviour under monotonic and cyclic tensile loading

International audienceAt room conditions and standard strain rate (ε˙ ∼ 10−4s−1), unidirectional (UD) plant-based reinforced organic polymers often exhibit nonlinear mechanical behaviour in tension. A viscoelastoplastic model (VEP model) for the simulation of UD plant fibre composite mechanical behaviour in tension, previously validated from twisted flax yarn epoxy composite under room conditions and standard strain rate, is calibrated with new data obtained from flax fibre epoxy composite under repeated progressive loading and a wide range of strain rates (ε˙ ∼ 10−3 to 10−7 s−1). The VEP model does not reproduce well the experimental observations. There seems to be a lack of stiffening in this phenomenological model. We propose an improved VEP model, developed within the frameworks of thermodynamics and limited to uniaxial tension and infinitesimal strains. An internal variable s representing the stiffening is added to create a VEP-stiffening model. This internal variable represents the coupled effects of reorienting cellulose microfibrils in kink band areas, spiral spring-like extension of cellulose microfibrils, and shear-stress-induced crystallization of the amorphous cellulose of flax fibres. The stiffening phenomenon was considered viscous, without a threshold, and was related to the tension energy in the direction of the fibres. Three viscosity coefficients drive the three phenomena: η (elastic), K (plastic), and Ks (stiffening). In the chosen formalism, this leads to two thermodynamic potentials φVEPs and ΩVEPs in which the stiffening phenomenon is strongly coupled with all the others. This VEP-stiffening model of the UD flax fibre epoxy composite correlates well with experimental observations. The paper also explores the evolution of the three viscous phenomena (elastic, plastic, and stiffening) by simulation of different loading conditions: monotonic, cyclic, and creep. This VEP-stiffening model can easily enrich existing multiaxial models of UD behaviour in the fibre direction. Implemented in a finite element model, it could be used at different length scales to numerically explore the origin of the mechanical behaviour of plant-based reinforced polymers

Sensitivity of the residual topography to single crystal plasticity parameters in Berkovich nanoindentation on FCC nickel

International audienceFundamental deformation mechanisms of FCC materials under indentation have been probed at the grain scale. Experimental tests have been conducted on large-grained annealed and cold-worked polycrystalline nickel samples with a Berkovich indenter. Indentation axes have been chosen to be close to the three main crystallographic directions [001], [101] and [111]. Pile-ups and slip traces have been revealed around the residual imprints by analysing topographic measurements obtained by atomic force microscopy. It is shown that the indenter orientation in each indentation plane drives pile-ups and slip traces which in turn contain precious information about the crystallographic orientation and the hardening state of the studied grain. Imprint topographies after pile-up formation therefore carry information that one can exploit to assess some intrinsic material properties at the grain scale. A 3D finite element modelling of the nanoindentation test at the grain scale has been developed, making use of crystal plasticity constitutive laws. Six different virtual materials having the same macroscopic behaviour have been built. The simulation results show a good agreement with experimental tests and also a great pile-up sensitivity to interaction matrix components. These results pave the way to the interaction matrix identification using an inverse finite element method

Sur la caractérisation des interactions entre dislocations dans un cristal CFC de Nickel par analyse de la topographie des empreintes Berkovich

International audienceUne étude a récemment montré expérimentalement et numériquement que la distribution et les dimensions des bourrelets plastiques autour d’une empreinte Berkovich sont très sensibles à l’orientation indenteur/cristal et aux paramètres d’écrouissage [1]. Dans la loi de plasticité cristalline utilisée [2], trois paramètres (r0 ,q,b) pilotent l’écrouissage sur chaque système de glissement et six (h1 ,h2 ,h3,h4 ,h5 ,h6 ) les interactions entre les dislocations présentes sur les différents systèmes. Ces six paramètres d’interaction, actuellement difficilement accessibles par des mesures expérimentales, définissent une matrice de dimensions 12x12

Identifiability of single crystal plasticity parameters from residual topographies in Berkovich nanoindentation on FCC nickel

International audienceThe information richness of imprints topographies obtained after Berkovich nanoindenta- tion tests at grain scale is assessed for identifying all or part of the parameters of a sin- gle crystal plasticity law. In a previous paper (Renner et al., 2016), the strong potential of imprints topographies has been shown through a large experimental campaign con- ducted on nickel samples. A 3D crystal plasticity finite element modelling (CPFEM) of the nanoindentation experiment using the Méric-Cailletaud has also showed a large sensitivity of residual topographies to the indenter/grain orientation and to the plastic parameters, including the interaction matrix coefficients specifying the interactions between disloca- tions on different slip systems. This makes imprints topographies very good candidates to provide information for the single crystal parameters identification. The present paper fo- cuses on the Méric-Cailletaud law parameters identifiability using residual topographies. A method is built to define the best well-posed inverse problem to ensure the parame- ters identification using a crystal plasticity finite element modelling updating (CPFEMU) method. An identifiability index proposed by Richard et al. (Richard et al., 2013) for mea- suring the information richness of the indentation curve is extended to the analysis of residual topographies. This index quantifies the possibility to achieve a stable/unstable so- lution using an inverse method. For the studied behaviour, the results show that eight of the nine Méric-Cailletaud law parameters can be identified using three topographies

Climate Risk Assessment for Ecosystem-based Adaptation: A guidebook for planners and practitioners

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