A computational approach to design new tests for viscoplasticity characterization at high strain-rates

International audienceRate-dependent behaviour characterization of metals at high strain rate remains challenging mainly because of the strong hypotheses when tests are processed with statically determinate approaches. As a non-standard methodology, Image-Based Inertial Impact (IBII) test has been proposed to take advantage of the dynamic Virtual Fields Method (VFM) which enables the identification of constitutive parameters with strain and acceleration fields. However, most of the test parameters (e.g. projectile velocity, specimen geometry) are not constrained. Therefore, an FE-based approach is addressed to optimize the identification over a wide range of strain and strain-rate, according to two design criteria: (1)-the characterized viscoplastic spectra (2)-the identifiability of the parameters. Whereas the first criterion is assessed by processing the FEA simulations, the second is rated extracting material parameters using synthetic images to input the VFM. Finally, uncertainties regarding the identification of material constants are quantified for each IBII test configuration and different camera performances

Développement de compositions polymères biosourcées sur base PLA pour des applications automobiles

National audiencePLA is a bio-based and biodegradable polymer with high tensile strength and rigidity. Nevertheless, its low impact toughness and its brittleness are obstacles for a use in highly loaded parts. To overcome these drawbacks, the influence of several additives is studied. First of all, PLA plasticization by TBC leads to a marked increase of ductility, however counterbalanced by a drop of tensile strength and rigidity. The formation of copolymers PLA-impact modifier (BS) allows to increase impact toughness but not ductility. Finally, quaternary compositions PLA-BS-TBC-clay nano-reinforcements have interesting tensile and impact properties compared to a mineral filled PP frequently used for automotive applications.Le PLA est un polymère biosourcé, biodégradable et à hautes rigidité et résistance en traction. Toutefois, sa faible résilience et sa fragilité sont des obstacles à son utilisation pour des pièces fortement sollicitées. Pour y remédier, cette étude s'intéresse à l'enrichissement progressif de compositions à base de PLA. Dans un premier temps, l'ajout de plastifiant (TBC) permet une nette augmentation de la ductilité du matériau, mais dégrade les autres propriétés en traction. La formation de copolymères PLA-modificateur d'impact (BS) permet un fort accroissement de la résilience, mais pas de la ductilité. Enfin, les compositions quaternaires PLA-BS-TBC-nanocharges d'argile constituent une piste viable pour une utilisation sous fortes sollicitations, grâce à des propriétés mécaniques en traction et à l'impact prometteuses, comparées à celles d'un PP chargé, classiquement utilisé dans l'automobile

A coupled viscoelastic-viscoplastic-damage model for short fibre reinforced composites

A constitutive model of viscous behaviour of short-fibre reinforced composites (SFRC) where complex distributions of fibre orientations are taken into account is proposed in this work. The approach considered for the computation of composite macroscopic behaviour is based on an additive decomposition of the state potential. The SFRC is assimilated to an assembly of several fibre media embedded in a polymeric matrix medium. One of the main assets of this approach is the possibility to model reinforcement with complex distributions of fibre orientations. Moreover, this decomposition allows the implementation of complex behaviour laws coupled with damage models. The polymeric matrix behaviour is typically strain-rate sensitive, i.e. viscoelastic-viscoplastic. This property has to be taken into account when the modelling of the composite behaviour over a large range of strain rate is intended. Therefore, a viscoelastic constitutive model, based on a generalised Maxwell model, and a viscoplastic correction scheme, based on an overstress approach, are implemented for matrix material. The developed constitutive model is then coupled to two damage laws. The first one is introduced in the framework of Continuum Damage Mechanics in order to model the ductile damage behaviour of the matrix material. The second one deals with fibre/matrix interfacial degradation through an interfacial debonding law. In order to identify the parameters involved in the present model, experimental tests are performed (case of polypropylene reinforced with short glass fibres). Micro-computed tomography is used for the characterisation of the fibres distribution of orientation. The efficiency of the proposed model is demonstrated by comparisons between numerical and experimental responses in different loading conditions, including dynamic loadings

Up-cycling of agave tequilana bagasse-fibres: A study on the effect of fibre-surface treatments on interfacial bonding and mechanical properties

The aim of this study was to assess the feasibility of upcycling fibre residues from the harvesting and production of tequila to green composites. Specifically, four different surface-modified natural fibres were assessed as raw material for green composite production. Before any surface treatment, the morphology and tensile properties of agave bagasse fibres from the tequila production batches were determined by optical and environmental scanning electron microscopy (ESEM) and single fibre tensile test, respectively. Further to this, agave fibres were exposed by immersion to four surface treatments including alkali, acetylation, enzymatic and silane treatments, in order to improve their morphology and compatibility with polylactic acid (PLA). The effects of these treatments on fibres’ morphology, mechanical properties (i.e. Youngs modulus and ultimate tensile strength), interfacial shear strength (IFSS), and water absorption were assessed. Overall, surface treatments showed improvements in agave bagasse fibre properties with the best results for alkali treated fibres with an ultimate tensile strength of 119.10 ​MPa, Young modulus of 3.05 ​GPa, and an IFSS of up to ~60% higher (5.21 ​MPa) to that performed by untreated samples. These tests allowed to identify alkali treatment as the most suitable for agave bagasse fibres. These results shed light on the interfacial interaction between agave bagasse fibres and PLA and the potential to up-cycle these residue agave fibres to manufacture PLA-based green composites

Méthode des Champs Virtuels pour la caractérisation de comportements viscoplastiques et d'endommagement, à partir de mesures de champs mécaniques hétérogènes

The behaviour of materials subjected to high-energy dynamic loadings (impacts, blasts \dots) is usually sensitive to strain-rate (viscoplastic) and/or damage. Conventional procedures for the characterization of corresponding models of behaviour use statically determined tests requiring restrictive hypotheses. So, it is impossible to deal with heterogeneous mechanical fields and the exploitation of tests is limited to small levels of strain. Moreover, several tests have to be performed, at constant strain rate, to characterize viscoplasticity. However, these limitations do not allow to take advantage of the large amount of information available thanks to full-field measurements. One solution is to use statically undetermined tests to deal with heterogeneous fields. Among available tools, the Virtual Fields Method (VFM) has undeniable advantages compared to classic FEMU methods. This study focuses on the development of the VFM for the characterization of Johnson-Cook's viscoplastic model of behaviour. An asset of the VFM is that it makes possible the characterization of the viscoplastic part of the model with only one testing, under dynamic conditions, thanks to a statically undetermined exploitation of heterogeneous strain and strain-rate fields. A short-term prospect is to use the VFM to identify parameters of elastoplastic models of behaviour coupled with damage (e.g. Lemaitre). The feasibility was demonstrated for numerical data.Le comportement de matériaux subissant des chargements dynamiques à hautes énergies (impacts, explosions \dots) est généralement sensible à la vitesse de déformation (viscoplastique) et/ou à l'endommagement. Les procédures classiques de caractérisation des modèles de comportement associés utilisent des essais statiquement déterminés nécessitant le respect d'hypothèses limitatrices. Ainsi, le traitement de champs mécaniques hétérogènes est impossible, ce qui limite l'exploitation des essais à de faibles gammes de déformations et oblige à réaliser plusieurs tests à vitesse de déformation constante pour la caractérisation de la viscoplasticité. Enfin, ces restrictions empêchent de profiter pleinement de la quantité importante d'information accessible par mesures de champs. Une solution est alors de recourir à des essais statiquement indéterminés, permettant d'exploiter des champs hétérogènes. Parmi les outils disponibles, la Méthode des Champs Virtuels (MCV) présente des avantages certains par rapport aux méthodes classiques de recalage par éléments finis. L'accent est mis dans ces travaux sur le développement de la MCV pour la caractérisation du modèle viscoplastique de Johnson-Cook. Un des grands atouts de la MCV est qu'elle permet de caractériser la partie viscoplastique du modèle à partir d'un unique essai en conditions de chargement dynamiques, grâce à son exploitation statiquement indéterminée de champs de déformations et de vitesses de déformations hétérogènes. Une perspective à court terme, dont la faisabilité est vérifiée sur des tests numériques, est l'identification simultanée par la MCV de paramètres de modèles élastoplastiques endommageables (Lemaitre)

A viscoelastic-viscoplastic model for short-fibre reinforced polymers with complex fibre orientations

This paper presents an innovative approach for the modelling of viscous behaviour of short-fibre reinforced composites (SFRC) with complex distributions of fibre orientations and for a wide range of strain rates. As an alternative to more complex homogenisation methods, the model is based on an additive decomposition of the state potential for the computation of composite’s macroscopic behaviour. Thus, the composite material is seen as the assembly of a matrix medium and several linear elastic fibre media. The division of short fibres into several families means that complex distributions of orientation or random orientation can be easily modelled. The matrix behaviour is strain-rate sensitive, i.e. viscoelastic and/or viscoplastic. Viscoelastic constitutive laws are based on a generalised linear Maxwell model and the modelling of the viscoplasticity is based on an overstress approach. The model is tested for the case of a polypropylene reinforced with short-glass fibres with distributed orientations and subjected to uniaxial tensile tests, in different loading directions and under different strain rates. Results demonstrate the efficiency of the model over a wide range of strain rates

Méthode des Champs Virtuels pour la caractérisation de comportements viscoplastiques et d'endommagement, à partir de mesures de champs mécaniques hétérogènes

Le comportement de matériaux subissant des chargements dynamiques à hautes énergies (impacts, explosions ) est généralement sensible à la vitesse de déformation (viscoplastique) et/ou à l'endommagement. Les procédures classiques de caractérisation des modèles de comportement associés utilisent des essais statiquement déterminés nécessitant le respect d'hypothèses limitatrices. Ainsi, le traitement de champs mécaniques hétérogènes est impossible, ce qui limite l'exploitation des essais à de faibles gammes de déformations et oblige à réaliser plusieurs tests à vitesse de déformation constante pour la caractérisation de la viscoplasticité. Enfin, ces restrictions empêchent de profiter pleinement de la quantité importante d'information accessible par mesures de champs. Une solution est alors de recourir à des essais statiquement indéterminés, permettant d'exploiter des champs hétérogènes. Parmi les outils disponibles, la Méthode des Champs Virtuels (MCV) présente des avantages certains par rapport aux méthodes classiques de recalage par éléments finis. L'accent est mis dans ces travaux sur le développement de la MCV pour la caractérisation du modèle viscoplastique de Johnson-Cook. Un des grands atouts de la MCV est qu'elle permet de caractériser la partie viscoplastique du modèle à partir d'un unique essai en conditions de chargement dynamiques, grâce à son exploitation statiquement indéterminée de champs de déformations et de vitesses de déformations hétérogènes. Une perspective à court terme, dont la faisabilité est vérifiée sur des tests numériques, est l'identification simultanée par la MCV de paramètres de modèles élastoplastiques endommageables (Lemaitre).The behaviour of materials subjected to high-energy dynamic loadings (impacts, blasts ) is usually sensitive to strain-rate (viscoplastic) and/or damage. Conventional procedures for the characterization of corresponding models of behaviour use statically determined tests requiring restrictive hypotheses. So, it is impossible to deal with heterogeneous mechanical fields and the exploitation of tests is limited to small levels of strain. Moreover, several tests have to be performed, at constant strain rate, to characterize viscoplasticity. However, these limitations do not allow to take advantage of the large amount of information available thanks to full-field measurements. One solution is to use statically undetermined tests to deal with heterogeneous fields. Among available tools, the Virtual Fields Method (VFM) has undeniable advantages compared to classic FEMU methods. This study focuses on the development of the VFM for the characterization of Johnson-Cook's viscoplastic model of behaviour. An asset of the VFM is that it makes possible the characterization of the viscoplastic part of the model with only one testing, under dynamic conditions, thanks to a statically undetermined exploitation of heterogeneous strain and strain-rate fields. A short-term prospect is to use the VFM to identify parameters of elastoplastic models of behaviour coupled with damage (e.g. Lemaitre). The feasibility was demonstrated for numerical data.VALENCIENNES-BU Sciences Lettres (596062101) / SudocSudocFranceF

Some Advantages of Advanced Inverse Methods to Identify Viscoplastic and Damage Material Model Parameters

International audienceThe chapter aims at providing an overview of the research activities performed in the two past decades in the field of materials characterisation under dynamic loadings (i.e. from 10-3/s to 10+3/s for structural crashworthiness and impact applications) and identification of parameters of constitutive behaviour and damage models. The different testing devices to load the material sample on the expected strain rate range are presented and discussed first, including the different experimental measurement techniques applied to analyse the stress-strain curves. From the normalised direct approach, two different numerical approaches, based on inverse problem resolution techniques, are introduced and discussed: the well-known Finite Element Model Updating method and the most advanced one based on the Virtual Fields Method, that enables to take the full advantages of full-field measurement techniques, such as the Digital Image Correlation method, into account. Applications for different materials or structures (including riveted and welded joints) and models, such as viscoplasticity and damage, are given to illustrate these advanced methods

An efficient modelling of inelastic composites with misaligned short fibres

AbstractThis paper deals with the modelling of the behaviour of short-fibre reinforced composites. The composite is seen as an assembly of a matrix medium and of several fibre media. Each fibre medium, characterised by its own orientation of fibres and volume fraction, is considered as a one-dimensional elastic medium. The matrix material has an elastoplastic behaviour. All types of hardening laws can be considered, thanks to a valuable adaptivity of the modelling. The use of the Drucker–Prager criterion for plasticity and non-associative plasticity rules allow to deal with compressible plastic flow. Moreover, all kind of orientation of fibres, in particular random orientations and imperfect alignments, can be modelled in a simple way. The influence of the fibres’ orientation on the mechanical response of a polymer matrix composite subjected to tensile/compression tests is analysed in detail. Finally, simulated behaviours of composites are compared to experimental data found in the literature