Propriétés mécaniques de milieux cellulaires soumis aux pressions de turgescence

Les tissus cellulaires végétaux sont caractérisés par une architecture cellulaire spécifique qui permet la croissance cellulaire par déformation irréversible. Cette déformation est notamment permise à l'aide la pression de turgescence qui s'exprime comme la différence entre les potentiels hydriques interne et externe. Afin de comprendre le comportement mécanique d'une architecture cellulaire sous pression osmotique et lors d'une sollicitation mécanique, un modèle 2D est développé. La génération d'un tissu cellulaire est explorée par un couplage Voronoi / Monte Carlo dans lequel, la forme, la taille et l'orientation des cellules sont décrites. Un calcul éléments finis est défini en 2D dans lequel la structure est maillée à l'aide d'éléments linéaires sous le code ANSYS. Les conditions de sollicitations sont modifiées pour tenir compte d'une pression homogène sur les parois cellulaires qui varie ou non avec la taille des cellules. Les paramètres élastiques sont discutés et reliés aux caractéristiques microstructurales

Approches de génération de structures alvéolaires et élasticité de produits céréaliers

La texture des aliments dépend intimement de la relation entre leur structure à différentes échelles et leurs propriétés mécaniques. Dans le cas de mousses solides à base de céréales, base de notre alimentation, la structure alvéolaire est fonction des variables du procédé mis en œuvre (extrusion, moulage ,fermentation, cuisson) et de la composition du produit. Afin de comprendre l'influence spécifique de la structure, indépendamment de celle du matériau intrinsèque, sur les paramètres d'élasticité, différentes approches de génération de structures alvéolaires sont décrites. En particulier, deux groupes de méthodes sont testés : générations sélectives (technique RSA) et générations coopératives (Monte Carlo, Voronoi). Des calculs de paramètres d'élasticité sont reliés aux caractéristiques des structures virtuelles pour chaque approche de génération. Les résultats calculés du module d'élasticité et du coefficient de Poisson sont discutés sur la base de la théorie des solides cellulaires ouverts

Damage Kinetics at the Sub-micrometric Scale in Bast Fibers Using Finite Element Simulation and High-Resolution X-Ray Micro-Tomography

This study combines experimental testing and computation analysis to reveal the role of defects and sub-micrometric microstructure in tensile behavior of hemp bast fibers. In particular, these structural defects represent the footprint of the processes to which the fibers elements are subject along the whole transformation chain from the plant to the end use product. Tensile experiments performed on elementary fibers and bundles in a wide diameter range (40–200 μm) are simultaneously conducted with X-ray micro-tomography observation. 3D images of ultra-fine resolution (voxel size of 280 nm) are achieved at different deformation magnitudes up to the complete failure thanks to the use of synchrotron radiation (ESRF, Grenoble, France). A Finite element (FE) model is implemented based on the conversion of the tomograms into 3D meshes. High performance computing is used to simulate the tensile response of the hemp bast fibers. In particular, the effects of notching and sub-micrometric structure of the fibers are explored. Results show the presence of different types of diffuse damage kinetics, which are related to the variability in the fiber size, surface defects and the presence of the lumen space. The damage behavior is found to be sensitive to the type of stress criterion implemented in the FE computation. The predictive analysis demonstrates the relevance of using embedded microstructure simulations to reveal the extent of stress localization and predict the failure properties in bast fibers for innovative composite manufacturing for instance

Etude de l'endommagement dans les composites à base d'amidon renforcés par des fibres naturelles

Cette étude aborde numériquement et expérimentalement l'endommagement interfacial dans les composites biopolymères. Le matériau est un amidon amorphe thermomoulé avec des fibres de chanvre. L'orientation des fibres est ajustée pour permettre une direction de traction perpendiculaire à l'interface de la fibre. L'essai couplé à une camera révèle un endommagement interfacial conduisant à la rupture. Un modèle en éléments finis est développé sur la base d'un critère d'endommagement de type Coulomb. Ce modèle explique l'endommagement observé par des calculs de sensibilité et d'identification

Modélisation de la propagation de fissures dans un biopolymère vitreux

La propagation de fissures dans un amidon vitreux est étudiée numériquement, analytiquement et expérimentalement. Des éprouvettes entaillées et en présence de trous sont testées en traction. La visualisation par caméra rapide révèle une déviation de la fissure et l'apparition de réseaux de fissures. Pour décrire la propagation locale en mode mixte, des simulations par éléments discrets et finis sont mises en uvre et comparées. Les deux méthodes sont capables de prédire le chemin observé de la fissure. La Méthode des Eléments Discrets est plus efficace pour la prédiction du réseau de fissures

Measurement of microfibril angle in plant fibres : Comparison between X-ray diffraction, second harmonic generation and transmission ellipsometry microscopies

The orientation of cellulose microfibrils within plant fibres is one of the main factors influencing their mechanical properties. As plant fibres are more and more used as reinforcement for agro-composites, their mechanical properties have a strong influence on the final composite properties. It is, therefore, of interest to obtain reliable information about the microfibril angle (MFA) to better support the choice of fibres depending on the product requirements. In the present study, the reliability and specificities of three non-destructive methods that allow analysis on the same fibre glued on a holder; X-ray diffraction (XRD), second harmonic generation (SHG) and transmission ellipsometry (TE) microscopy; are investigated. Three types of plant fibres, with both low (nettle), and high (cotton, sisal) MFA values, are compared and their geometry and biochemical composition are characterised. The results obtained on the same fibre confirm that MFA analysis remains tedious and that despite their limitations, the methods are complementary depending on the information requested. Indeed, SHG is recommended for direct, qualitative and plane-selective mapping of heterogeneities in macrofibril orientations at various depths. However, reliable quantitative results with SHG depend on the initial image quality and could benefit from further image processing refinement. On the contrary, XRD and TE measure MFAs over the entire fibre thickness and provide variations along the fibres if a sufficient optical/spatial resolution is reached. Regarding the characterization of intrinsic defects in plant fibres, both SHG and TE suffer from uncertainties induced by the disorganization of the microfibril network and the lack of symmetry between the front and back fibre walls. Finally, all techniques prove to be dependant on the initial fibre alignment and geometry (i.e. twisting, double fibre configuration or form factor) which vary along the fibre length and should be carefully taken into account.publishedVersionPeer reviewe

Challenges of additive manufacturing technologies from an optimisation perspective

Three-dimensional printing offers varied possibilities of design that can be bridged to optimisation tools. In this review paper, a critical opinion on optimal design is delivered to show limits, benefits and ways of improvement in additive manufacturing. This review emphasises on design constrains related to additive manufacturing and differences that may appear between virtual and real design. These differences are explored based on 3D imaging techniques that are intended to show defect related processing. Guidelines of safe use of the term “optimal design” are derived based on 3D structural information

Compression performance of hollow structures: From topology optimisation to design 3D printing

International audienceIn this work, we experimentally evaluate the rendering of topology optimisation through the design of hollow structures manufactured using a 3D printing technique. The moving asymptote method is used as a mathematical optimisation strategy to virtually minimise the volume of 2D designs subject to hydrostatic pressure by half. Designs are converted to 3D models by extrusion in the building direction and printed using the Fused Deposition Modelling technique. Compression testing up to densification is performed and designs are evaluated. The results show that extrusion of the design in the building direction provides the best option to avoid mechanical anisotropy induced by processing. Depending on the type and extent of excluded regions, mechanical performance proves to be adapted to a wide range of designs and different types of mechanical anisotropies can be derived. Comparison with finite element results shows differences in behaviour related to mechanical instabilities that occur as a result of the lack of inter-filament cohesion and external frame unsoldering

The Influence of Microstructural Arrangement on the Failure Characteristics of 3D-Printed Polymers: Exploring Damage Behaviour in Acrylonitrile Butadiene Styrene

This study investigated how printing conditions influence the fracture behaviour of 3D-printed acrylonitrile butadiene styrene (ABS) under tensile loading. Dog-bone-shaped ABS specimens were produced using the fusion filament fabrication technique, with varying printing angles. Tensile tests were conducted on pre-notched specimens with consistent pre-notch lengths but different orientations. Optical and scanning electron microscopies were employed to analyse crack propagation in the pre-notched specimens. In order to support experimental evidence, finite element computation was implemented to study the damage induced by the microstructural rearrangement of the filaments when subject to tensile loading. The findings revealed the simple linear correlation between the failure properties including elongation at break and maximum stress in relation to the printing angle for different pre-notch lengths. A more progressive damage was found to support the ultimate performance of the studied material. This experiment evidence was used to build a damage model of 3D-printed ABS that accounts for the onset, growth, and damage saturation. This damage modelling is able to capture the failure properties as a function of the printing angle using a sigmoid-like damage function and a modulation of the stiffness within the raster. The numerical results demonstrated that damage pattern develops as a result of the filament arrangement and weak adhesion between adjacent filaments and explains the diffuse damage kinetics observed experimentally. This study concludes with a topological law relating the notch size and orientation to the rupture properties of 3D-printed ABS. This study supports the idea of tailoring the microstructural arrangement to control and mitigate the mechanical instabilities that lead to the failure of 3D-printed polymers