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Fracturing two-dimensional random fiber networks of different densities (porosities) were statistically analyzed using both high-resolution finite element models and image analysis algorithms. Under small strains, the finite element fracture models revealed that networks with high relative densities were able to localize evolving fractures to small cracks while surprisingly larger cracks were required to localize fractures in networks of lower density. Further, it is indicated that the pore size distribution in fiber networks is rather diverse and can be captured using two mixed Gamma distributions; one part describing the background size distribution containing the vast majority of pores, and a second part describing the size distribution of larger pores and open regions. The second part covers less than 5% of the total network area but seems to be of paramount importance for the network’s global fracture behavior. It seems as a fiber network’s crack sensitivity is related to the average pore size in the second part of the mixed Gamma distribution. The analysis is remarkably consistent with reported experiments.

Fine prediction of the elastic properties of paper materials can now be obtained using sophisticated fibre scale numerical approaches. However, there is still a need, in particular for low-density papers, for simple and compact analytical models that enable the elastic properties of these papers to be estimated from the knowledge of various structural information about their fibres and their fibrous networks. For that purpose, we pursued the analysis carried out in Marulier et al. (Cellulose 22:1517–1539, 2015. https://doi.org/10.1007/s10570-015-0610-6 ) with low-density papers that were fabricated with planar random and orientated fibrous microstructures and different fibre contents. The fibrous microstructures of these papers were imaged using X-ray synchrotron microtomography. The corresponding 3D images revealed highly connected fibrous networks with small fibre bond areas. Furthermore, the evolutions of their Young’s moduli were non-linear and evolved as power-laws with the fibre content. Current analytical models of the literature do not capture these trends. In light of these experimental data, we developed a fibre network model for the in-plane elasticity of papers in which the main deformation mechanisms of the micromechanical model is the shear of the numerous fibre bonds and their vicinity, whereas the fibre parts far from these zones were considered as rigid bodies. The stiffness tensor of papers was then estimated both numerically using a discrete element code and analytically using additional assumptions. Both approaches nicely fit the experimental trends by adjusting a unique unknown micromechanical parameter, which is the shear stiffness of bonding zones. The estimate of this parameter is relevant in light of several recently reported experimental results.

Foldable composite structures are bellows-like structures that can be stored in a flat state and deployed to a finite volume after folding. These composite structures are being increasingly demanded for numerous engineering applications including aerospace, aeronautics, robotics and medical. In this study, we report an original processing route for obtaining foldable composite laminates. These composite laminates were obtained by selectively curing pre-impregnated materials that were made of three layers of plain weave fabrics impregnated with an UV-curable epoxy-acrylate resin system. After selective curing, the composite laminates were made of rigid domains (cured zones) connected by flexible domains (uncured zones) that played the role of hinges upon folding. The selective curing of the photocurable resin system was performed using two different approaches. The first approach consisted in depositing UV-blocking masks designed numerically onto the surface of pre-impregnated materials before placing them inside the chamber of an UV-curing machine. The second one consisted in using a Digital Light Processing (DLP) 3D printing machine. The UV-curable epoxy acrylate resin exhibited fast curing kinetics characterized by a gelation time on the order of 1 s. Hence, both types of approaches enabled foldable composite laminates with very short time cycles (≈ 5 to 10 s) to be obtained. The as-obtained pre-impregnated sheets were folded into 3D composite structures and then irradiated with UV-light. This original fabrication method is versatile enough to provide a wide diversity of composite part geometries that are promising for many engineering applications. The observations of the microstructure of processed samples revealed that the reinforcement fabrics were rather well impregnated by the epoxy acrylate resin. These observations also tended to show that both types of approaches enabled a complete and efficient in-depth curing of the resin system.

Significance While most attention has so far been devoted to the tensile properties of crystalline cellulose, the main elementary building block of plants, we show here using atomistic simulations that their shear is also an important mode of deformation, occurring at stress levels lower than tension with much larger ductility. We also demonstrate how crystalline defects like dislocations drastically facilitate plasticity. This analysis can be used as a basis for the micromechanical modeling of cellulose microfibrils that are currently considered as promising eco-friendly alternatives to synthetic fibers for structural materials., Cellulose microfibrils are the principal structural building blocks of wood and plants. Their crystalline domains provide outstanding mechanical properties. Cellulose microfibrils have thus a remarkable potential as eco-friendly fibrous reinforcements for structural engineered materials. However, the elastoplastic properties of cellulose crystals remain poorly understood. Here, we use atomistic simulations to determine the plastic shear resistance of cellulose crystals and analyze the underpinning atomic deformation mechanisms. In particular, we demonstrate how the complex and adaptable atomic structure of crystalline cellulose controls its anisotropic elastoplastic behavior. For perfect crystals, we show that shear occurs through localized bands along with noticeable dilatancy. Depending on the shear direction, not only noncovalent interactions between cellulose chains but also local deformations, translations, and rotations of the cellulose macromolecules contribute to the response of the crystal. We also reveal the marked effect of crystalline defects like dislocations, which decrease both the yield strength and the dilatancy, in a way analogous to that of metallic crystals.

International audience; Flax fibre mats are promising and versatile biosourced reinforcements that can be used in composite parts obtained using various processing routes. To optimise their impregnation and the end-use properties of composites, it is crucial to better understand the process-induced evolution of their microstructure and their permeability. In this study, flax fibre mats were subjected to in situ X-ray microtomography compression experiments. The resulting 3D images enabled the evolution of several key descriptors of their microstructure under compression to be determined, and the evolution of their permeability to be quantified by direct fibre scale CFD simulations. The microstructural data were also used as input parameters of a modified directional Kozeny-Carman model, accounting for the anisotropy and heterogeneity of mats. Only one unknown directional parameter was identified by inverse method from permeability calculations performed on numerically generated 3D realistic fibre networks. The predictions of the proposed model were consistent with numerical simulations.

Several low-density crumpled paper-based materials were fabricated by varying both the volume fraction and the geometry and mechanical properties of paper sheets. Their 3D architecture was investigated using X-ray microtomography. Microstructure descriptors such as the pore size distribution, the mean curvature distribution and the volume fraction of ordered domains were finely analyzed. Their mechanical properties were also assessed using uniaxial compression tests. Our results showed that crumpled materials exhibited a particular porous microstructure with a reproducible mechanical behavior between foams and entangled fibrous materials. Their compressive behavior was characterized by successive elastic, strain-hardening and densification regimes. The effects of the geometry, microstructure and mechanical properties of the sheets on the process-induced microstructures and mechanical performances were discussed. In particular, a simple micromechanical approach was used to estimate analytically and from the 3D images the role of ridges and ordered domains on the mechanical properties of crumpled papers. The evolution of their Young's moduli and yield stresses were studied as a function of their relative densities and compared with experimental data available in the literature for other cellular materials, showing that crumpled papers are promising renewable alternatives to standard polymer foams for several engineering applications, due to the proper combination of mechanical properties, porosity, cost, and easy fabrication. Keywords: Crumpled sheets, Paper, Self-locked architecture, Core materials, Microstructure, Compression behavior, Structural applications, Sandwich panel

During compression moulding, Sheet Moulding Compounds (SMCs) are subjected to tensile strains that yield detrimental tears. To understand these mechanisms, tensile experiments were performed with two uncured industrial SMC formulations with low and high pore and fibre contents. These experiments were coupled with Digital Image Correlation to estimate mesoscale strain fields on the sample surface. X-ray microtomography was used to obtain 3D ex situ evolutions of pores and fibre-bundle orientation. Both formulations behaved as porous, elastoviscoplastic, anisotropic and shear thinning fluids, showing strain hardening followed by softening and sample breakage. During stretching, SMCs dilated with anisotropic pore growth, whereas fibre bundles aligned along the tensile direction following the prediction of the modified Jeffery’s equation. In addition, the ductility of SMCs was largely altered both by the initial pore contents and fibre-bundle flocs/aggregates induced during the prepreg fabrication, the latter leading to undesirable strain localisation bands enhancing sample breakage.

The properties of Sheet Moulding Compounds (SMCs) are altered by their porosity induced during the manufacturing or moulding of these prepregs. To characterise the pore scale mechanisms occurring during SMC flow, 3D synchrotron X-ray images were acquired during the compression of two uncured SMCs. For the high fibre content SMC ( 50 wt % ), with a high porosity, pores mainly decreased in size and disappeared during SMC consolidation. These mechanisms were related to the elasticity of the fibrous networks and to the permeability of the porous phase, estimated using pore scale CFD simulation. For the standard fibre content SMC ( 29 wt % ), with a lower porosity, the compressibility was limited and closed pores were transported towards the external surface of samples with non-affine motion, i.e., faster than the SMC flow and with tortuous trajectories. Besides, pores coalesced and decreased in size during compression. The size decrease was mainly related to the dissolution of pore gases.

Nanocelluloses (cellulose nanocrystals, CNCs, or cellulose nanofibrils, CNFs) are the elementary reinforcing constituents of plant cell walls. Because of their pronounced slenderness and outstanding intrinsic mechanical properties, nanocelluloses constitute promising building blocks for the design of future biobased high-performance materials such as nanocomposites, dense and transparent films, continuous filaments, and aerogels and foams. The research interest in nanocellulose-based aerogels and foams is recent but growing rapidly. These materials have great potential in many engineering fields, including construction, transportation, energy, and biomedical sectors. Among the various processing routes used to obtain these materials, ice-templating is one of the most regarded, owing to its simplicity and versatility and the wide variety of porous materials that this technique can provide. The focus of this review is to discuss the current state of the art and understanding of ice-templated porous nanocellulose-based materials. We provide a review of the main forming processes that use the principle of ice-templating to produce porous nanocellulose-based materials and discuss the effect of processing conditions and suspension formulation on the resulting microstructures of the materials.

An experimental study of the ultrasonic compression moulding (UCM) to manufacture biobased composites made of semicrystalline starch powders and softwood fibres is described. The main objective was to assess the potential of using this fast and economical processing technique to elaborate a 100% biobased composite which might substitute more usual polymer materials for structural applications. The starch powder was chosen as raw material for the matrix while the reinforcement was made of softwood fibres. Tablets made of starch only and composite beams were processed under different conditions and characterised by several techniques. Three types of starch powders and two types of fibres were used as raw materials. A morphological and crystalline analysis was carried out by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The native semicrystalline structure of starch granules was not totally preserved so as to obtain a homogeneous material. Diametral compression tests on tablets were performed to improve the processing route and obtain the materials with the best properties. Bending tests were used on composite beams to quantify the mechanical properties and study the effects of the processing parameters. The optimum processing parameters were defined and allowed obtaining a matrix for which the flexural strength reached 21 MPa. Mechanical properties were improved when fibres were added into the matrix: three-points bending tests showed a Young's modulus of 6 GPa, a flexural strength of 75 MPa and a flexural strain at break of 6% for a bulk density of 1.25. Considering these results, UCM appears to be a promising process to design a 100% biobased composite with mechanical properties comparable to those of classical discontinuous fibre composites., Ultrasonic moulding; Biobased composites; Starch granules; Wood fibres

Wood cells constitute the main building block in engineered wood-based materials, whose delimiting property frequently is moisture induced swelling. The hygroexpansion properties of wood cells, technically known as fibers, are used as input in predictive micromechanical models aimed for materials design. Values presented in the literature largely depend on the microfibrillar angle, the geometry of the fiber and limiting modelling assumptions. Synchrotron X-ray micro-computed tomography has recently prompted means for detailed measurements of the geometry of unconstrained individual fibers undergoing moisture-induced swelling, which makes it possible to directly quantify the hygroexpansion properties of the cell wall. In addition to a well-defined three-dimensional geometry, the present approach also accounts for large deformations and the fact that cell-wall stiffness depends on the presence of moisture. A mixed numerical-experimental approach is adopted where a finite-element updating scheme is used to simulate the swelling of an earlywood spruce fiber going from the experimental fiber geometry at 47 % relative humidity to the predicted geometry of the fiber in the wet state at 80 % relative humidity at equilibrium conditions. The hygroexpansion coefficients are identified by comparing the predicted and the experimental three-dimensional fiber geometry in the wet state. The obtained values are 0.17 strain per change in relative humidity transverse to the microfibrils in the cell wall, and 0.014 along the microfibrils.

The structural changes that are induced by the initiation and the propagation of a crack in a low-density paper (LDP) were studied using single edge-notched fracture tests that were imaged under an optical microscope or in laboratory or synchrotron X-ray microtomographs. The two-dimensional optical images were used to analyse the links between the mesoscale structural variations of LDP and the crack path. Medium-resolution X-ray three-dimensional images were used to analyse the variations in the thickness and local porosity of samples as well as their displacement field that were induced by the LDP fracture. High-resolution three-dimensional images showed that these mesostructural variations were accompanied by complex fibre and bond deformation mechanisms that were, for the first time, in situ imaged. These mechanisms occurred in the fracture process zone that developed ahead of the crack tip before the crack path became distinct and visible. They were at the origin of the aforementioned thickness variations that developed more particularly along the crack path. They eventually led to fibre–fibre bond detachment phenomena and crack propagation through the fibrous network. These results can be used to enhance the current structural and mechanical models for the prediction of the fracture behaviour of papers.

The evolution of paper microstructure parameters, such as porosity and fibre orientation, as a function of papermaking conditions is most often studied at a macroscopic scale. However, modelling the physical and mechanical properties of papers using upscaling approaches requires understanding the deformation micro-mechanisms that are induced by papermaking operations within the structure of paper fibrous networks for individual fibres and fibre-to-fibre bonds. We addressed this issue by analysing three-dimensional images of model papers. These images were obtained using X-ray microtomography. The model papers were fabricated by varying forming, pressing, and drying conditions. For each image, this analysis enabled an unprecedented large set of geometrical parameters to be measured for individual fibres (centreline, shape and inclination of the fibre cross sections) and fibre-to-fibre bonds (inter-bond distance, number of bonds per unit length of fibre, bond surface area) within the fibrous networks. The evolution of the as-obtained microstructure parameters was analysed as a function of papermaking conditions. All results were in accordance with the data available in the literature. A key result was obtained for the evolution of the number of fibre-to-fibre contacts per fibre as a function of the network density. A representative number of contacts was obtained using relatively small imaged volumes. These volumes must only contain enough fibre segments the cumulated length of which is of the same order as the mean fibre length. These results were also used to validate microstructure models for the prediction of the number of fibre-to-fibre contacts within fibrous networks.

International audience; Thermoforming allows the manufacture of structural parts for the automotive and aeronautical domains using long fiber thermoplastic prepregs with short cycle times. During this operation, several sheets of molten prepregs are stacked and subjected to large macroscale strains, mainly via in-plane shear, out-of-plane consolidation or dilatation, and bending of the fibrous reinforcement. These deformation modes and the related meso and microstructure evolutions are still poorly understood. However, they can drastically alter the end-use macroscale properties of fabricated parts. To better understand these phenomena, bias extension tests were performed using specimens made of several stacked layers of glass woven fabrics and polyamide matrix. The macroscale shear behavior of these prepregs was investigated at various temperatures. A multiscale analysis of deformed samples was performed using X-ray microtomography images of the deformed specimens acquired at two different spatial resolutions. The low-resolution images were used to analyze the deformation mechanisms and the structural characteristics of prepregs at the macroscale and bundle scales. It was possible to analyze the 3D shapes of deformed samples and, in particular, the spatial variations of their thickness so as to quantify the out-of-plane dilatancy or consolidation phenomena induced by the in-plane shear of prepregs. At a lower scale, the analysis of the high-resolution images showed that these mechanisms were accompanied by the growth of pores and the deformation of fiber bundles. The orientation of the fiber bundles and its through-thickness evolution were measured along the weft and warp directions in the deformed samples, allowing the relevance of geometrical models currently used to analyze bias extension tests to be discussed. Results can be used to enhance the current rheological models for the prediction of thermoforming of thermoplastic prepregs.

This study is an evaluation of the effectiveness of the flexography printing process for manufacturing catalyst-coated membranes (CCMs) for use in proton exchange membrane fuel cells (PEMFCs). Flexography is a maskless and continuous process that is used in large-scale production with water-based inks to reduce the cost of production of PEMFC components. Unfortunately, water has undesirable effects on the Nafion® membrane: water wets the membrane surface poorly and causes the membrane to bulge outwards significantly. Membrane printability was improved by pre-treating membrane samples by water immersion for short periods (

Gradient theories are capable of describing deformation of heterogeneous elastic materials better than classical elasticity theory since they are able to capture internal length effects. Here, crack-tip displacement fields at the tip of a mode I crack in gradient enhanced elastic materials are derived in closed form and contrasted with experiments. Heterogeneous materials, represented by discrete fiber networks, are analyzed in finite element models to judge the theory. It is shown that using a classical continuum approach to describe macroscopic singular-dominated deformation fields in heterogeneous materials lead to erroneous results because a structural effect that alters the displacement field becomes pronounced and results in severe blunting of crack-tips. A key conclusion is that the average segment length in the material gives the internal length scale parameter, used in the gradient enhanced continuum theory, hence allows for bridging between scales.

International audience; The properties of short fibre-reinforced polymer composites depend on the distribution and the orientation of fibres which drastically changes during the forming of composites. During this stage, these materials behave as fibre suspensions and usually flow in confined geometries. To analyse their flow-induced fibrous microstructures, we previously conducted 3D real-time in situ observations of the compression of non-Newtonian dilute fibre suspensions using fast X-ray microtomography [T. Laurencin et al., Compos Sci Technol 134 (2016)]. Here, we successfully simulated these experiments with a multi-domain Finite Element code and compared them with the predictions of Jeffery's model. Often, the Jeffery's equations agree with the experimental and numerical data. However, for fibres closed to compression platens, important deviations were observed with faster simulated and experimental fibre rotation. Adopting the dumbbell approach and revisiting the recent work of Perez et al. [J non-Newtonian Fluid Mech 233 (2016)], an extension of the Jeffery's model is proposed to account both for the non-Newtonian rheology of the suspending fluid and confinement effects. Despite its simplicity, the new model allows a good description of simulation and experimental results.

International audience; Novel biocomposites were fabricated using preforms of unmodified starch powder and wood pulp fibers. Stacks of preforms were consolidated using thermo-compression (TCM) and ultrasonic compression moldings (UCM). The characterization of the microstructure of the biocomposites showed that TCM enabled a better preservation of the crystallinity of starch granules during their welding than UCM. However, UCM allowed a significant gain in processing time. For the best set of forming and material parameters, the composites exhibited an elastoplastic response with strain hardening. Their Young's modulus, flexural strength and strain at ultimate stress reached up to approx. 6 GPa, 70 MPa, and 8%, respectively. The best properties were associated to the partial preservation of the native crystallinity of starch and lowered porosity. Bleached and fibrillated fibers with a large aspect ratio also contributed to the enhancement of composite properties. These effects were explained by a better starch-fiber interface and the presence of a network of connected fibers within the composites.

Modelling physical and mechanical properties of fibrous materials requires a relevant description of their microstructures, e.g. the descriptors of fibres and fibre–fibre contacts. In this work, a method is proposed to identify fibres with complex cross sections and their contacts from 3D images of disordered fibrous media, obtained from X-ray microtomography. The image analysis procedure first consists in obtaining a map of the local fibre orientation. Fibres are then detected by deleting regions of high local orientation gradients. Therewith, dilatation operations using slender orientated structuring elements are performed to retrieve deleted fibre parts. The procedure thus provides 3D images with labelled fibres and fibre–fibre contacts. Its relevance is assessed by analysing the microstructures of three typical fibrous media used in short fibre reinforced polymer composites or as paper materials: mats of (i) mono-disperse copper fibres with circular cross sections, (ii) glass fibres bundles, (iii) wood fibres.

The optimisation of board packages often rely on their load bearing capacity. Then it seems attractive to measure how such thin-walled structures deform using for instance kinematic field measurement techniques, and to incorporate, at least partially, the gained kinematic information within mechanical models. Digital Image Correlation (DIC) can provide a vivid description of the buckling of box panels, e.g. during box compression tests. Therefore, we propose an analytical plate model to predict the elastic post-buckling behaviour of corrugated board box panels where the kinematic boundary conditions emanate from DIC measurements. Comparing experimental and calculated strain fields on the outer liner of board panels as well as box compression force lend some confidence to the model. Further results reveal the heterogeneity of in-plane forces, bending and twisting moments the box panels have to withstand as well as strain fields in usually inaccessible regions of the panels such as the inner liner. Thereby an improvement of the structure of box panels can be envisaged.

International audience; The physical and mechanical properties of short fibre-reinforced polymer composites depend on the geometry, content, distribution and orientation of fibres within the polymer matrix. These microstructure features are mainly induced during the forming stage, i.e., when composites usually flow in moulds and behave as non-Newtonian fibre suspensions. Their flow-induced microstructures still cannot be well predicted by current rheological models. To better understand them, non-Newtonian dilute fibre suspensions were prepared and subjected to lubricated compression experiments using a micro-rheometer mounted in a synchrotron X-ray microtomograph. These experiments enabled, for the first time, fast and in situ 3D imaging of the translation and rotation of fibres in the suspending fluid. Fibre motions were compared with the prediction of the Jeffery's model. Despite the use of a non-Newtonian suspending fluid and confined flow conditions, i.e., with a gap between compression platens of the same order of magnitude than the fibre length, we showed that Jeffery's prediction was satisfactory if the fibres were sufficiently far from the compression platens (approximately at a distance of once to twice their diameter). Otherwise, the experimental average orientation rates were higher than the Jeffery's prediction.

This study aimed at investigating how ice templating conditions affected the shrinkage, the microstructure, and the mechanical properties of cellulose nanofibril (NFC) foams. Enzymatic and TEMPO-oxidized NFC foams were fabricated using two solidification techniques, i.e., quenching NFC suspensions in temperature-controlled baths, and mechanical stirring of NFC suspensions during solidification followed by quenching, prior to freeze-drying. Foams prepared by direct quenching using stabilized TEMPO-oxidized NFC suspensions exhibited higher specific mechanical properties and more regular anisotropic cells with unimodal size than enzymatic NFC foams. In addition, NFC concentration and NFC morphology severely affected the foam shrinkage and the geometry of foam cells. Controlling the solidification had also a drastic effect on the foam microstructure, e.g. foams prepared by mechanical stirring and quenching exhibited bimodal cell size and enhanced mechanical properties. The compressive behavior of foams showed successive elastic, strain-hardening and densification regimes with auxetic effects and strain localization. Both the elastic modulus and the yield stress were power-law functions of the foam relative density whose exponents reached unusual high values for enzymatic NFC foams, potentially because of their chaotic microstructures. The evolution of a typical TEMPO-oxidized NFC foam was studied under compression using X-ray microtomography, unveiling deformation mechanisms at the cell scale. Keywords: Cellulose nanofibrils, Foam, Ice templating, Mechanical behavior, X-ray microtomography, Deformation micromechanisms

International audience; In this study, thermo-compression molding was used to fabricate bulk materials from native starch powder. The objective was to determine the process parameters that enabled the welding of starch granules while preserving their crystallinity. The effects of forming parameters such as temperature, compression stress, and water content on the microstructural and mechanical properties of the starch samples were studied. For the optimal forming conditions, good compaction and cohesion of starch granules were obtained, while successfully preserving their native crystalline structure. The flexural Young’s modulus of the best samples reached an average value of 3 GPa and the flexural strength reached 10 MPa. However, these materials also exhibited unwanted cracks. The potential origin of these defects was associated to the heterogeneous distribution of water during processing as well as thermal shrinkage, moisture uptake, and viscoelastic recovery that occurred after the mold ejection.

In this study, we characterized and modeled the rheology of TEMPO-oxidized cellulose nanofibril (NFC) aqueous suspensions with electrostatically stabilized and unflocculated nanofibrous structures. These colloidal suspensions of slender and wavy nanofibers exhibited a yield stress and a shear thinning behavior at low and high shear rates, respectively. Both the shear yield stress and the consistency of these suspensions were power-law functions of the NFC volume fraction. We developed an original multiscale model for the prediction of the rheology of these suspensions. At the nanoscale, the suspensions were described as concentrated systems where NFCs interacted with the Newtonian suspending fluid through Brownian motion and long range fluid–NFC hydrodynamic interactions, as well as with each other through short range hydrodynamic and repulsive colloidal interaction forces. These forces were estimated using both the experimental results and 3D networks of NFCs that were numerically generated to mimic the nanostructures of NFC suspensions under shear flow. They were in good agreement with theoretical and measured forces for model colloidal systems. The model showed the primary role played by short range hydrodynamic and colloidal interactions on the rheology of NFC suspensions. At low shear rates, the origin of the yield stress of NFC suspensions was attributed to the combined contribution of repulsive colloidal interactions and the topology of the entangled NFC networks in the suspensions. At high shear rates, both concurrent colloidal and short (in some cases long) range hydrodynamic interactions could be at the origin of the shear thinning behavior of NFC suspensions.

External wall insulation (EWI) usually comprises a porous cement mortar used as protective external render into which short fibers are added to enhance its mechanical properties. The rheology of these porous, fibrous, and granular suspensions was investigated using lubricated compression tests in the fresh state, whereas flow-induced porous microstructures were studied using X-ray microtomography. We show that these suspensions exhibit a homogeneous isovolume flow regime and two heterogeneous flow regimes, i.e., a consolidating regime, and a consolidating and segregating regime. A decrease in the compression strain rate and/or an increase in the number of fiber contacts in the entangled fibrous network induced flow heterogeneity accompanied by heterogeneous modifications of density, porosity, and pore size distribution of render. These undesirable microstructure changes are prone to occur during mortar processing and placement. They drastically affect the properties of renders such as the permeability that was calculated using X-ray microtomography images and pore scale numerical simulation.

Cellulose nanofibrils (NFCs) are slender nanoparticles with outstanding mechanical properties that are used to enhance the mechanical properties of polymer nanocomposites and to fabricate dense and transparent nanopapers. However, the reinforcing effect of these nanoparticles is not as efficient for the elastic properties as expected from the classical models for polymer composites or fibrous materials. In this study, nanocomposite films made of TEMPO-oxidized NFCs and polyethylene oxide (PEO) were prepared by varying the NFC content over a wide range. The structural properties of these materials were characterized using AFM, TEM, SEM-FEG and XRD. These techniques showed that NFC nanocomposite films and nanopapers consisted of dense networks of tortuous NFCs with planar and random orientations. DMA experiments also revealed that the cohesive bonds between NFCs greatly contributed to the overall elastic response of these nanocomposites, even for low NFC contents. Based on these observations, we report an original multiscale network model that describes the elastic properties of NFC nanocomposites and nanopapers where the governing deformation mechanisms occurred at the numerous bonds between NFCs and in the NFC kinked regions, whereas the crystalline NFC regions were considered to be rigid bodies. This approach led to the formulation of an analytical expression for the stiffness tensors of NFC nanocomposites and nanopapers and revealed the effects of the network structure on their mechanical responses. The model predictions were satisfactory over the range of investigated NFC contents. In particular, this model showed the predominant role of the amorphous regions on the elastic response, whereas the contribution of the NFC–NFC bonds became significant at high NFC contents.

Compression moulded preimpregnated polymer-matrix composites are often porous materials. Rheological studies usually neglect their subsequent compressible complex flow behaviour, together with mouldcomposite friction effects. Therefore the proposed method, including a newly developed compression rheometer, allows the rheology and the compressibility of such materials together with the mould–composite friction phenomena to be characterised without taking into account a priori assumptions on both the rheology of the composite and the form of the friction law. Its validity and usefulness for improving the interpretation of rheological results is demonstrated using a modelling plastic paste and an industrial Sheet Moulding Compound (SMC) during lubricated (or not) compression tests performed at low or high temperatures.

The non-Newtonian rheology of concentrated fiber suspensions, such as short fiber reinforced polymer composites during their processing, depends on both the microstructure of their fibrous network and the deformation micromechanisms arising at fiber–fiber contacts. In this work, these two aspects are investigated using model concentrated fiber suspensions made up of short glass fiber bundles impregnated in a transparent polymer. For that purpose, multiresolution x-ray microtomography was used to analyze the fibrous microstructures, showing that the studied suspensions exhibit a planar fiber orientation with fairly straight fiber bundles, the connectivity of which can be modeled by the geometrical statistical tube model. Besides, bundle–bundle contact forces together with the interaction between the bundles and the suspending fluid are analyzed by using pull-out experiments. These tests allow the influence of the pull-out velocity, the confining stress and the volume fraction of fiber bundles on the pull-out force to be quantified. Combined with the microstructure analysis, such results are then used to propose a bundle–bundle contact model which can be implemented into multiscale rheological models dedicated to concentrated fiber suspensions.

The study of the rheology of injected or compression moulded compounds like SMC or BMC is made particularly difficult by the high content and the intricate arrangement of their fibrous reinforcement. For these two types of compounds, inappropriate rheological testing protocols and rheometers are often used, which leads to a very large scatter of the experimental data. This study describes specific sampling and specimen’s preparation methods, as well as dedicated rheometry devices to test their rheology. Following the proposed protocols, it is possible to obtain rheological measurements showing low scatter of the recorded stress values: about ±10% for SMC and about ±15% for BMC-like compounds.

Model composite plates composed of highly conductive slender copper fibres impregnated with a poorly conductive and transparent PMMA matrix were processed with different fibrous architectures, i.e. with various controlled fibre contents and orientations. Their microstructure was characterised using both optical observations and X-ray microtomography. Their in-plane thermal conductivity was measured by using a specific testing apparatus combined with an inverse modelling method. Results point out the strong link between the anisotropy of the in-plane conductivity and of the microstructure. The role of the fibre content on the conductivity is also emphasised. An analytical conduction model which accounts for the influence of the fibre content, the orientation, the aspect ratio and the thermal resistances at fibre-fibre contacts, was proposed and its predictions were compared with the experimental results. Using only one fitting parameter, namely the conductance at fibre-fibre contacts, this model shows a good prediction of all the experiments.

International audience; The present study provides original experimental data concerning the evolution of the microstructure of a bundle of fibres during its deformation. For that purpose, a model saturated fibre bundle was processed and was subjected to a compression loading by using a specially designed micro-compression rheometer which was mounted on a synchrotron X-ray microtomograph. Thus, 3D images of the evolving fibrous microstructure could be obtained. Results first show that the compression induced both the bundle consolidation and liquid phase migration. Secondly, (i) the position, the orientation, the displacement and the deformation of each fibre together with (ii) the position and the evolution of each fibre-fibre contact were followed during the compression. The tracking of these microstructure descriptors allows the consolidation micro-mechanisms to be analysed and provides useful information for theoretical or numerical models used to predict fibre bundle deformation during processing of fibre bundle reinforced polymer composites.

The interplay between residual stress state, cohesive and adhesive properties of coatings on substrates is reviewed in this article. Attention is paid to thin inorganic coatings on polymers, characterized by a very high hygro-thermo-mechanical contrast between the brittle and stiff coating and the compliant and soft substrate. An approach to determine the intrinsic, thermal and hygroscopic contributions to the coating residual stress is detailed. The critical strain for coating failure, coating toughness and coating/substrate interface shear strength are derived from the analysis of progressive coating cracking under strain. Electro-fragmentation and electro-fatigue tests in situ in a microscope are described. These methods enable reproducing the thermo-mechanical loads present during processing and service life, hence identifying and modeling the critical conditions for failure. Several case studies relevant to food and pharmaceutical packaging, flexible electronics and thin film photovoltaic devices are discussed to illustrate the benefits and limits of the present methods and models. © 2010 Elsevier B.V. All rights reserved.

The influence of internal stress anisotropy resulting from anisotropic loading in a roll-to-roll (R2R) process, and polymer substrate anisotropy on the crack onset strain (COS) of thin oxide coatings was analyzed. Experimental data obtained for R2R processed films were compared with data obtained using an isotropic sheet-to-sheet (S2S) process with the same anisotropic substrate. In the R2R case the COS was found to increase by 20% between the transverse direction and the machine direction. In the S2S case the COS was found to be independent of orientation, except at a 45° in-plane orientation with respect to the machine direction, where it was 15% higher. The internal stress in the machine direction could not be determined, presumably due to deposition-induced curvature changes of the polymer substrate, and was therefore fitted to the COS data. Fracture mechanics analysis and finite element modeling of the experimental data showed that the influence of substrate anisotropy was marginal, and that it was the process-induced internal strain in the coating which controlled the COS. © 2010 Elsevier B.V.

The load-bearing capacity of boxes made up of thin-walled G-flute corrugated boards was investigated by performing monotonic and cyclic compression tests on boxes of various geometries. Results provided a large experimental database on the influence of the box dimensions and compression velocities on buckling parameters such as critical load and the corresponding critical axial deformation. Cyclic loading tests permitted the detection of the onset of an irreversible strain state, which could be related to damage initiation and propagation in the microstructure of the panels of boxes. Moreover, main differences, which could be observed between G-flute corrugated boards and folding boards, were emphasized. Finally, the ability of some usual modelling approaches to predict the critical load of boxes was discussed. Copyright © 2009 John Wiley & Sons, Ltd.

In this work, the microstructure and the rheology of a glass-fibre-reinforced fresh mortar were studied. Various fibre contents and aspect ratios and two types of fibrous reinforcement, i.e. slender fibre bundles and fibres, were tested. The microstructure was analysed by using X-ray microtomography. It is shown that the non-deformed mortar is a porous granular suspension, the porous microstructure of which is not influenced by the presence of fibres, which in turn display a 2D planar random fibre orientation. The rheology was investigated by subjecting samples to constant axial strain rate and lubricated compression. The roles of the actual strain, the mortar resting time, the fibre content and aspect ratio on recorded stress levels are emphasised. Besides, for the investigated strain rate and material parameters, the mortar flow is quasi-incompressible and does not affect significantly the porous microstructure nor the fibrous one. Lastly, the stress increase which is induced by the addition of fibre bundles is similar to that predicted by Newtonian models of semi-dilute fibre suspensions.

Bulk Moulding Compounds (BMC’s) are short fibre reinforced polymer composites that behave, during their forming, as concentrated fibre suspensions. Their suspending fluid is also a concentrated granular suspension made up of the polymer filled with mineral fillers. In this work, a method is proposed to model their flow. Firstly, the rheology of an industrial BMC was analysed by performing two types of mechanical tests, i.e. lubricated simple and plane strain compression experiments. Experimental results underline the roles of the current strain, the strain rate as well as the mechanical loading on the rheology of the BMC. Secondly, a 3D tensorial rheological model is proposed to reproduce the simple and plane strain compression experiments. Then this model is implemented into a Finite Element code dedicated to the simulation of compression moulding. Simulation results are finally compared with experiments achieved with rather complex flow situations.

International audience; X-ray microtomographs of samples cut from non-deformed and compression moulded Sheet Moulding Compounds (SMC) plates have been analysed using the phase contrast mode. Results emphasise a significant decrease of the porosity after moulding, this phenomenon being enhanced with increasing the initial SMC temperature. They also show that pronounced migration of the polymer-matrix occurs through the thickness of the plates during compression moulding. The substructures of compression moulded plates through their thickness exhibit a core zone sandwiched between upper and lower skins. Inside the skins, the fibre content and the porosity are weaker than in core zones, and fibre-bundles are highly broken up. On the contrary, fibre-bundles remain preserved in core zones. Therein, the significant flattening and widening of bundles, their weak bending as well as their orientation have been quantified.

Probabilistic model of coating fragmentation under uniaxial tensile loading is developed. Analytical expressions of the crack spacing evolution are obtained for small-strain and large-strain fragmentation regimes. The model is applied for coating and interface property identification of several thin brittle coating/polymer substrate systems. An estimate of the stress transfer length, derived from the fragmentation data, is found to correlate with the interfacial shear strength thus suggesting that both parameters reflect an intrinsic property related to the mechanical efficiency of coating/substrate interface.

Highly concentrated planar fibre-bundle suspensions with a transparent PMMA matrix were processed with various initial bundle contents and orientations. They were submitted to simple compression and plane strain compression deformation modes. First rheological measurements are presented. They highlight the role of the bundle content and orientation on recorded stress levels. The transparent matrix allows the observation of fibrous microstructures before and after compressions: The in-plane deformation of bundles (flattening and bending) as well as the evolution of their orientation are analysed and discussed.

Compression mouldings of commercial SMC were performed with an instrumented industrial press under various process conditions. Results underline the influence of process parameters such as the initial SMC temperature, the axial punch velocity and the geometry of the mould on local normal stress levels. They also show negligible fibre-bundle segregation in the principal plane of the moulded parts. Thereby, a one-phase plug flow shell model is proposed as a direct extension of the plug flow model proposed by M.R. Barone and D.A. Caulk [J Appl Mech 53(191):1986;361–70]. In the present approach, the SMC is considered as a power-law viscous medium exhibiting transverse isotropy. The shell model is implemented into a finite element code especially developed for the simulation of compression moulding of composite materials. Simulation and experimental results are compared, emphasizing the role of the SMC rheology on the overall recorded stress levels. Despite the simplicity of the model, rather good comparisons are obtained.

Because of their remarkable specific physical and mechanical properties, polymer composites that are reinforced with discontinuous fibers or fiber bundles are suitable materials for many aeronautic, automotive, shipbuilding, electrical, electronic, health and sport applications. Among these materials, sheet molding compounds (SMCs), bulk molding compounds (BMCs), glass mat thermoplastics (GMTs) and carbon mat thermoplastics (CMTs), i.e. polymer composites that are reinforced with centimeter length fiber or fiber bundles [ADV 94, ORG 12a] are the subject of several ongoing development and research programs. Other similar polymer composites that are reinforced with biosourced fiber or fiber bundles (e.g. flax, hemp, wood, bamboo, kenaf, sisal and jute) are also short-fiber promising polymer composites.

The rheology of NFC suspensions that exhibited different microstructures and colloidal stability, namely TEMPO and enzymatic NFC suspensions, was investigated at the macro and mesoscales using a transparent Couette rheometer combined with optical observations and ultrasonic speckle velocimetry (USV). Both NFC suspensions showed a complex rheology, which was typical of yield stress, non-linear and thixotropic fluids. Hysteresis loops and erratic evolutions of the macroscale shear stress were also observed, thereby suggesting important mesostructural changes and/or inhomogeneous flow conditions. The in situ optical observations revealed drastic mesostructural changes for the enzymatic NFC suspensions, whereas the TEMPO NFC suspensions did not exhibit mesoscale heterogeneities. However, for both suspensions, USV measurements showed that the flow was heterogeneous and exhibited complex situations with the coexistence of multiple flow bands, wall slippage and possibly multidimensional effects. Using USV measurements, we also showed that the fluidization of these suspensions could presumably be attributed to a progressive and spatially heterogeneous transition from a solid-like to a liquid-like behavior. As the shear rate was increased, the multiple coexisting shear bands progressively enlarged and nearly completely spanned over the rheometer gap, whereas the plug-like flow bands were eroded.

International audience; A numerical model for the behavior of flexible fibers under inertial flows was developed by coupling discrete element method and finite volume method. The fibers were discretized into several beam segments, and the equations of motion were integrated with a 2nd order accurate explicit scheme. The 3D Navier-Stokes equations were discretized by a 4th order accurate (space and time) unstructured finite volume scheme. Momentum exchange between the fluid and fibers was enforced by including a source term of the fiber hydrodynamic force in the Navier-Stokes equations. The choice of an appropriate model for the hydrodynamic force on a fiber in a fluid flow depending on the Reynolds number is discussed and covers a range of Reynolds number between 10−2 and 102. The current numerical model is validated against different experimental studies, including deflection of fiber in uniform flow, fibers in isotropic turbulent flow, and concentrated fiber suspension in channel flow. The numerical model was able to reproduce the damping/enhancement phenomena of turbulence in a channel flow as a consequence of the micro-structural evolution of the fibers.

International audience; The complex behaviour of corrugated board packages under compression loading is investigated in this work. Original experimental data are obtained by using a Digital Image Stereocorrelation technique for measuring the displacement and strain fields of the panels' outer liner of the tested boxes. The stress field is also estimated by accounting for the anisotropic mechanical behaviour of the outer liner, its residual stress state induced by the processing of the corrugated board and the effects of box manufacturing operations and compression. Results show that these fields are extremely heterogeneous on the panels' surface. Most stressed areas are located along the panels' edges. The elastic limit of the outer liner is reached quite soon during compression. Box geometry and panel flaps are of primary importance on the observed phenomena. This approach delivers useful information to improve kinematic and constitutive assumptions for buckling and post-buckling models of boxes or thin-walled sandwich structures.

International audience; This study provides original experimental data on the microstructural mechanisms of the hygroexpansion of a material made up of lignocellulosic fibres. A paperboard made up of several layers was chosen and subjected to relative humidity variations during X-ray microtomography scanning. The 3D images of the evolving media were analysed using a digital image correlation technique to measure the displacement field within the studied material. This technique allowed the hygroexpansion of the studied material and of each layer of this latter to be analysed in the in-plane and out-of-plane directions. Results show that the hygroexpansion is highly anisotropic. The microstructural hygroexpansive mechanisms for the pore and fibre phases could also be revealed. They have been shown to depend strongly on the fibre content of the fibrous layers. This analysis provides also useful information concerning the size of the Representative Elementary Volume (REV) for the hygroexpansion phenomenon of dense lignocellulosic fibrous networks. In view of the obtained results, the relevancy of common theoretical models used to predict the hygroexpansion of materials such as papers and boards is also discussed.

International audience; Corrugated boards with small flutes appear as good alternatives to replace packaging folding boards or plastic materials due their small thickness, possibility of easy recycling and biodegradability. Boxes made up of these materials have to withstand significant compressive loading conditions during transport and storage. In order to evaluate their structural performance, the box compression test is the most currently performed experiment. It consists in compressing an empty container between two parallel plates at constant velocity. Usually it is observed that buckling phenomena are localized in the box panels, which bulge out during compression [1]. At the maximum recorded compression force, the deformation localises around the box corners where creases nucleate and propagate. This maximum force is defined as the quasi-static compression strength of the box. The prediction of such strength is the main topic of interest of past and current research works. For example, the box compression behaviour of boxes was studied by Mc Kee et al. [2] and Urbanik [3], who defined semi-empirical formula to predict the box compression strength, as well as by Beldie et al. [4] and Biancolini et al. [5] by finite element simulations. But comparisons of these models with experimental results remain rather scarce and limited.

International audience; The rheology of Newtonian concentrated fibre-bundle suspensions with nearly planar orientation states is investigated within the framework of the homogenization method for periodic discrete structures. These suspensions are seen as forming a connected network of bundles. At their contact points, the flow induces local Newtonian interaction forces and moments. "Numerical rheometry experiments" are performed on representative elementary volumes of these suspensions to explore the influence of bundle volume fraction and orientation state on the rate of change of bundle orientation. Three bundle orientation distributions are particularly investigated: Gaussian-based, uniform and crenellated orientation distributions. Two types of numerical simulations are performed, i.e., instantaneous and time-evolution ones. The obtained numerical results permit to discuss the role of the shape of the orientation distribution, typically on the rate of change of the second-order orientation tensor. They are also compared to the well-known Jeffery theory and its subsequent modifications like the Folgar-Tucker theory. The accuracy of some well-known closure approximation functions of the fourth-order orientation tensor is examined. It appears that, in the case treated here where the suspending fluid and the fibre-fibre interactions are Newtonian, the Jeffery's equation gives a fairly good fit (the best fit of all examined theories) of the homogenization results, despite the completely different physics of both approaches. Some differences are observed in situations where fibre-bundles are highly aligned along the flow direction. It is also observed that the shape of the orientation distribution strongly affects the rate of the orientation change. At last, the tested closure approximations can lead to good description of some particular cases of orientation distributions. Nonetheless, in general cases, they are not relevant at all so that other strategies would have to be used in order to compute the evolution of fibre orientation.