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When natural fibres are exposed to humid environments, they absorb moisture and swell. Often, the natural fibre reinforced composites are produced in a combination with the widely used non-/low-swelling matrices, like maleic anhydride grafted polypropylene (MAPP) and epoxy. The water uptake resulting in fibre swelling causes the build-up of internal stresses when the fibres are surrounded, and thus constrained, by the typically used matrices. These internal stresses eventually lead to damage formation, resulting in loss of mechanical properties. The scope of this thesis is to investigate a potential strategy to decrease the aforementioned sensitivity of the natural fibre composites when exposed to hygroscopic cycling conditions. In this strategy the hydrophilic matrix polyamide-6 (PA6) will be used due to its ability to absorb water and swell, resulting in a composite with decreased stress build-up by the hygroscopic expansion of the fibres, due to a matrix that swells and shrinks together with the fibres. The hypothesis of having a good fibre-matrix interface when adding the hydrophilic matrix PA6 to the hydrophilic flax fibres, was evaluated with a wettability analysis. A wettability analysis implies using contact angle measurements to get an idea of the surface energy components calculated following the Van Oss model of both fibre and matrix. These surface energy components are then used to calculate the theoretical work of adhesion which shows the quality of the interface. Hygroscopic cycling is being applied on the traditional composites flax/MAPP and flax/epoxy together with flax/PA6 composites to analyze their behavior after a long durability cycling. The moisture content and sample thickness were monitored during the entire cycling process. After every cycle, a group of specimens was removed from the cycling protocol and mechanically tested in bending by using the three-point bending method. Therefore, mechanical degradation was monitored every cycle with a total of nine hygroscopic cycles performed. It is of great interest to take a closer look at the sample zone where failure has occurred and investigate the failure mode. By using scanning electron microscopy (SEM), different damage mechanisms can be detected for the composites between the different tested cycles showing degradation of the materials. This study revealed that the hydrophilic matrix PA6 and flax fibres result in a high work of adhesion value which shows a strong interfacial bonding and leads to a good impregnation of the thermoplastic. Furthermore, this was additionally supported by the high longitudinal and transverse strength of the flax/PA6 composites as by the investigation of the fractured surfaces of the reference-unaged samples. The flax/PA6 samples obtain higher strength values than the flax/MAPP composites at the end of the long durability study. These observations are supported by the analysis of the SEM pictures that indicate less damaged interfaces within the flax/PA6 specimens. In conclusion, the use of a hydrophilic matrix in natural fibre reinforced composites shows promising results for composite materials exposed to humid environments for a significant period.

Novel reactive thermoplastic resin systems such as Elium® have attracted significant research attention for composite applications because they combine the low-cost processibility of thermosets and the recyclability of TPs. In this work, the viscoelastic and impact behaviours of glass fibre-reinforced composites, containing Elium® and a commercial epoxy matrix were evaluated. To complement observations from both characterisation techniques, the fracture characteristics of both materials were also investigated by scanning electron microscopy.

In this work, a multi-scale model established from the concept of periodic homogenization is utilized to predict the cyclic and time-dependent response of thermoplastic-based woven composites. The macroscopic behaviour of the composite is determined from finite element simulations of the representative unit cell of the periodic microstructure, where the local non-linear constitutive laws of the components are directly integrated, namely, the matrix and the yarns. The thermoplastic matrix is described by a phenomenological multi-mechanisms constitutive model accounting for viscoelasticity, viscoplasticity and ductile damage. For the yarns, a hybrid micromechanical–phenomenological constitutive model accounting for anisotropic damage and anelasticity induced by the presence of a diffuse micro-crack network is utilized. The capabilities of the overall multi-scale model are validated by comparing the numerical predictions with experimental data. Further illustrative examples are also provided, where the composite undergoes time-dependent deformations under uni-axial and non-proportional multi-axial loading paths. The multi-scale model is also employed to analyze the influence of the local deformation processes on the macroscopic response of the composite. This paper is part of the COPERSIM project managed by IRT Jules Verne (French Institute in Research and Technology in Advanced Manufacturing Technologies for Composite, Metallic and Hybrid Structures). The authors wish to associate the industrial and academic partners of this project, respectively, Arts et Metiers Institute of Technology, Solvay, Plastic Omnium, PSA and Renault.

A multi-scale FE2 approach based on the periodic homogenization theory is developed to predict the overall response of nonlinear mechanical and fully coupled thermomechanical 3D composite structures. The computational strategy integrates the periodic microstructure effects by introducing the architecture of the reinforcement and the local constitutive laws.The considered constituents' constitutive laws obey generalized standard materials laws and are formulated within the framework of thermodynamics of irreversible processes. The characteristic equations (equilibrium and thermodynamics laws) are formulated under the assumption of small strains and rotations. They are solved simultaneously at both scales (microscopic and macroscopic) using an incremental scheme. For the numerical implementation, an advanced Meta-UMAT subroutine is developed and combined with a parallelization technique in the finite element commercial software Abaqus/Standard. The multi-scale computational strategy is applied to simulate the overall response of 3D composite structures under complex thermomechanical loading paths. The composite structures consist of thermoplastic polymer matrix with viscoelastic-viscoplastic behavior and ductile damage, reinforced by different types of reinforcements (short fibers or woven fabrics). The anisotropic damage within the yarns is modeled through a micromechanical approach to follow the transverse micro-cracks density evolution. This computational strategy is deployed on composite structures having periodic microstructure, whose phases exhibit different types of nonlinear behavior laws (rheology, damage mechanisms and thermomechanical coupling). The capabilities of the multi-scale approach are demonstrated (i) by comparing numerical predictions with experimental results in terms of global response, macroscopic and microscopic strain fields, and (ii) through the access to spatio-temporal distributions of internal variables at the microstructure scale as well as the intrinsic dissipation in the constitutive phases.; Une approche de simulation numérique multi-échelles EF2 fondée sur la théorie de l'homogénéisation périodique a été développée pour prédire la réponse globale couplée mécanique et thermomécanique fortement non linéaire des structures composites 3D. La stratégie de calcul intègre les effets de la microstructure périodique en introduisant l'architecture des renforts et les lois constitutives locales. Les lois de comportement des constituants utilisées obéissent aux lois de matériaux standards généralisées et sont formulées dans un cadre de la thermodynamique des processus irréversibles (TPI). Les équations caractéristiques (équilibre et lois de la thermodynamique) sont formulées sous l'hypothèse des petites déformations et rotations, et résolues simultanément de façon incrémentale aux deux échelles (microscopique et macroscopique). Sur le plan numérique, une implémentation au moyen de routines UMAT imbriquées (Méta-UMAT) a été développée et combinée à une technique de parallélisation dans le code de calcul Abaqus/Standard. La stratégie de calcul multi-échelles est appliquée pour simuler la réponse globale de structures composites 3D soumises à des trajets de chargement thermomécaniques complexes. Les structures composites sont constituées d’une matrice polymère thermoplastique viscoélastique-viscoplastic avec endommagement ductile et renforcées par différents types de renforcements (fibres courtes ou tissus). L’endommagement anisotrope dans les torons de tissu est modélisé à travers une approche micromécanique permettant de suivre l’évolution de la densité de micro-fissures transverses. Cette stratégie de calcul peut être déployée sur les structures en matériaux composites ayant une microstructure périodique et dont les phases présentent différents types des lois de comportement non linéaires (rhéologie, mécanismes d'endommagement et couplage thermomécanique). Les capacités de l'approche multi-échelles sont démontrées en comparant les prédictions numériques aux résultats expérimentaux en termes de réponse globale et de champs de déformation macroscopiques et microscopiques. Les performances de l'approche sont également illustrées à travers l'accès aux répartitions spatio-temporelles des variables internes à l'échelle de la microstructure ainsi que la dissipation intrinsèque dans les phases constitutives.

This study describes the properties of thermoplastic polymer composites based on polyethylene (of low and high density) and ethylene-propylene copolymers using various types of conifer needles (pine, spruce, fir, and cedar) as fillers. For the needles, thermogravimetric analysis (TGA) and TGA/Fourier transform infrared spectroscopy (TGA/FTIR) were performed to investigate their structures and thermal resistance, as required for the composite processing methods. Moreover, structural differences were studied for the analyzed fillers and composite materials (FTIR). The results were compared with the values obtained for composites with conifer wood flour. Composites with conifer needles (pine) had increased water absorption and similar strength properties. However, irrespective of the degree of filling, composites with pine needles were positively characterized by the highest melt mass flow rate (MFR) values and showed a slightly better impact resistance than composites filled with other flours. Thus, shredded coniferous needles with sufficient thermal resistance could be successfully used as fillers in composites. This conclusion was based on thermoplastic polymers as an alternative and/or supplement to the wood flour used in the manufacture of wood-polymer composites.

In this thesis, a multi-scale model established from the concept of periodic homogenization is utilized to study the cyclic and time-dependent response of thermoplastic-based woven composites. With the proposed approach, the macroscopic behaviour of the composite is determined from a finite element simulation of the representative unit cell of the periodic microstructure, where the local constitutive behaviours of the components are directly integrated, namely: the matrix and the yarns. The local response of the thermoplastic matrix is described by a phenomenological multi-mechanisms constitutive model accounting for viscoelasticity, viscoplasticity and ductile damage. For the yarns, a hybrid micromechanical-phenomenological constitutive model is considered. The latter accounts for anisotropic damage and anelasticity induced by the presence of a diffuse micro-crack network through the micromechanical description of a micro-cracked representative volume element. The capabilities of the multi-scale model are validated by comparing the numerical prediction with experimental data. The capabilities of the model are also illustrated through several examples where the composite undergoes time-dependent deformations under monotonic loading, constant or cyclic stress levels and non-proportional multi-axial loading. Furthermore, the multi-scale model is also employed to analyse the influence of the local deformation processes on the macroscopic response of the composite.; Dans ce travail de thèse, une modélisation multi-échelle est mise en place à partir du concept d’homogénéisation périodique pour étudier le comportement cyclique et dépendant du temps des composites tissés à matrice thermoplastique. Avec l’approche proposée, le comportement macroscopique du composite est déterminé à partir d’une simulation éléments finis effectuée sur une cellule unitaire représentative de la microstructure périodique, où les lois de comportement des constituants sont directement intégrées, à savoir: la matrice et les torons. La réponse locale de la matrice est décrite par une loi de comportement phénoménologique multi-mécanismes intégrant viscoélasticité, viscoplasticité et endommagement ductile. Pour les torons, une loi de comportement hybride micromécanique-phénoménologique est considérée. Cette dernière prend en compte l’endommagement anisotrope et l’anélasticité induite par la présence d’un réseau diffus de microfissures à travers une description micromécanique d’un volume élémentaire représentatif contenant des microfissures. Les capacités du modèle multi-échelles sont validées en comparant les prédictions numériques aux essais expérimentaux. Les capacités du modèle sont également illustrées à travers plusieurs exemples où le composite subit des déformations dépendantes du temps lors de chargements monotones, de chargements à amplitude constante ou cyclique et encore lors de chargement multiaxiaux non proportionnels. En outre, le modèle multi-échelle est aussi utilisé pour analyser l’influence des mécanismes de déformation locaux sur la réponse macroscopique du composite.

In this thesis, a multi-scale model established from the concept of periodic homogenization is utilized to study the cyclic and time-dependent response of thermoplastic-based woven composites. With the proposed approach, the macroscopic behaviour of the composite is determined from a finite element simulation of the representative unit cell of the periodic microstructure, where the local constitutive behaviours of the components are directly integrated, namely: the matrix and the yarns. The local response of the thermoplastic matrix is described by a phenomenological multi-mechanisms constitutive model accounting for viscoelasticity, viscoplasticity and ductile damage. For the yarns, a hybrid micromechanical-phenomenological constitutive model is considered. The latter accounts for anisotropic damage and anelasticity induced by the presence of a diffuse micro-crack network through the micromechanical description of a micro-cracked representative volume element. The capabilities of the multi-scale model are validated by comparing the numerical prediction with experimental data. The capabilities of the model are also illustrated through several examples where the composite undergoes time-dependent deformations under monotonic loading, constant or cyclic stress levels and non-proportional multi-axial loading. Furthermore, the multi-scale model is also employed to analyse the influence of the local deformation processes on the macroscopic response of the composite.; Dans ce travail de thèse, une modélisation multi-échelle est mise en place à partir du concept d’homogénéisation périodique pour étudier le comportement cyclique et dépendant du temps des composites tissés à matrice thermoplastique. Avec l’approche proposée, le comportement macroscopique du composite est déterminé à partir d’une simulation éléments finis effectuée sur une cellule unitaire représentative de la microstructure périodique, où les lois de comportement des constituants sont directement intégrées, à savoir: la matrice et les torons. La réponse locale de la matrice est décrite par une loi de comportement phénoménologique multi-mécanismes intégrant viscoélasticité, viscoplasticité et endommagement ductile. Pour les torons, une loi de comportement hybride micromécanique-phénoménologique est considérée. Cette dernière prend en compte l’endommagement anisotrope et l’anélasticité induite par la présence d’un réseau diffus de microfissures à travers une description micromécanique d’un volume élémentaire représentatif contenant des microfissures. Les capacités du modèle multi-échelles sont validées en comparant les prédictions numériques aux essais expérimentaux. Les capacités du modèle sont également illustrées à travers plusieurs exemples où le composite subit des déformations dépendantes du temps lors de chargements monotones, de chargements à amplitude constante ou cyclique et encore lors de chargement multiaxiaux non proportionnels. En outre, le modèle multi-échelle est aussi utilisé pour analyser l’influence des mécanismes de déformation locaux sur la réponse macroscopique du composite.

From its appearance in the scientific literature, graphene has received much interest, first from the scientific community and then, after the scale-up of the methods suitable for its production, from the industrial world. Nowadays, this nanofiller has to be considered as one of the most promising and performing materials ever discovered: in fact, it is deserving application in different fields, including nanoelectronics, quantum physics, catalysis, energy research and engineering of nanocomposites and biomaterials. The remarkable growth in science, graphene underwent during the last 15 years, is strictly connected to its peculiarities, such as high Young's modulus, high fracture strength, remarkable thermal and electrical conductivity, large specific surface area, high charge carrier mobility and also biocompatibility. At present, one of application fields where graphene is gaining a lot of success refers to the fabrication of polymer composites on a commercial scale and low/affordable cost, including both thermoplastic and thermosetting host matrices. These composites do exploit the homogeneous dispersion level of the nanofiller within the polymer matrix, mainly for enhancing the electrical and thermal conductivity of this latter, as well as its mechanical properties, in a remarkable way, even in the presence of very low graphene loadings. In addition, the obtained nanocomposites usually show higher thermal stability with respect to the unfilled polymer matrix. At present, there are about six main areas related to emerging applications for this nanofiller, including displays/screens, biomedical devices, memory chips, fuel cells/batteries, inks, coatings and, from an overall point of view, new smart materials. This paper aims at reviewing the current state of the art about the use of graphene as a nanofiller in both thermoplastics and thermosets, highlighting the current limitations and providing an overall point of view on its possible applications/further developments in the next future.

peer-reviewed The present study deals with the quasi-static and high strain rate characterization of Kevlar-129 based thermoplastic composites. Two different thermoplastic matrices, namely, Polypropylene (PP) and Polyetherimide (PEI) were used to manufacture composite laminates. Quasi-static compression tests were performed at strain rates of 0.041 s−1 and 0.045 s−1. High strain rate tests were performed using a split Hopkinson pressure bar apparatus within the strain rates ranging from 2548 s−1 to 4379 s−1. Stress-strain relations reveals the rate-sensitive behaviour of composites. Kevlar/PP (K-PP) showed higher peak stress under quasi-static loading as compared to the high strain rate test. Comparable peak stresses were revealed under quasi-static and high strain rate loading for Kevlar/PEI (K-PEI) composite. Also, high strain rate compression properties such as peak stress, peak strain and toughness of K-PP were 25%, 27% and 6% higher than that of the K-PEI composite. The failure mechanisms of both the composites were characterized through macroscopic and scanning electron microscopy. K-PP failed majorly due to matrix crush and fibre failure while K-PEI failed due to shear cracking. Damage study reveals that a single fibre based composite system can be tailored to act as an energy-absorbing or dissipating material system by varying the thermoplastic matrix materials