Digital image correlation is a photomechanical technique utilized to measure displacement fields. Although the method has been used successfully in a variety of applications, there is a need for guidelines to choose correlation parameters and for an understanding of their link with the analyzed texture and their effect on the uncertainty of measured quantities. In this paper, two criteria are proposed from a texture study to evaluate the best compromise between measurement uncertainty and spatial resolution prior to any mechanical test. These criteria are then validated on a priori uncertainty evaluations with different textures, and on a real experiment when comparing image correlation results with strain gauge data.

L’objectif est l’obtention d’une loi de comportement des composites BMC dont les coefficients sont fonction des proprietes des constituants (la fraction volumique, la distribution d’orientation, la forme des renforts). La demarche sera fondee sur une approche multi echelle d’homogeneisation des milieux aleatoires. Les problemes nouveaux qui se posent pour le BMC concernent la prise en compte du comportement non lineaire de la matrice.

The requirements of passive security, notably in the transport industry, impose to maximize the dissipation of the energy and to minimize the decelerations undergone by a vehicle and thus passengers due to violent shocks (crash). This paper aims at establishing efficient expected answers towards the preoccupations mainly emanating from transport industry. Currently, the behaviour laws implemented in the dynamic explicit schemes (RADIOSS, PAM-CRASH and LS-DYNA) do not integrate sufficiently the physical aspects in the material degradation, mainly the damage process, their kinetics, the variability and especially the heterogeneity of the composite materials microstructure. This paper deals with the development of a multi-scale predictive model coupling specific experimental methodologies and the micromechanical formulation of damage mechanisms in order to build constitutive laws for discontinuous fibre reinforced composites materials. The developed micromechanical modelling is based on an experimental methodology conducted over a range of strain rates from quasi static to 250 s−1. The latter has enabled identifying local probabilistic damage criterion formulated through the Weibull’s statistical integrating the strain rate effect and describing the progressive interfacial debonding under rapid loading. The developed model has been validated to predict the stiffness reduction and the overall elastic visco-damage behaviour for SMC composite material. The model simulations agree well with high speed tensile tests and confirm that the damage threshold and kinetic in the SMC are mainly strain rate sensitive.

Schapery-type constitutive theories for nonlinearly viscoelastic materials have been used extensively within the literature. Most of the applications are 1D but some 3D applications can be found. Most of the 3D applications are thermodynamically inconsistent extensions of the 1D constitutive theory. This paper shows and illustrates how thermodynamically admissible Schapery-type constitutive theories can be generated. In addition, a new 3D constitutive theory is introduced. A new data reduction procedure for obtaining the material parameters, which does not rely on creep–recovery tests, is introduced. The procedure leads to material parameters that are thermodynamically admissible and considers the whole mechanical response rather than particular values, as in most data reduction procedures. This procedure is tested on a thermoplastic material and the constitutive theory thus obtained is compared with data from other load histories.

This paper presents an experimental procedure, which enables us to assess the shear strain field in an adhesive joint between composite and aluminium. In practice, this strain field is representative of the progressive stress transfer between a loaded structure and a composite patch used for reinforcement purposes. Digital image correlation (DIC) is used to measure the displacement field through the thickness of a patched specimen subjected to a tensile test. The shear strain field derives from the measured displacement field. The shear strain clearly decreases when the distance from the free edge of the adhesive increases, as predicted by numerical and analytical models of the joint. These measurements are used to estimate the in situ shear modulus of the adhesive. It is observed that the shear modulus decreases when the shear stress increases, thereby illustrating the non-linear response of the adhesive.

The context of the study is the preventive reinforcement of aluminium structures with bonded composite patches. The paper deals with stress relief in the substrate under tensile loading, as well as crack propagation in the adhesive during fatigue tests. For this purpose, temperature measurements are performed through the thickness of the specimen using an infrared camera. The procedure provides two types of information. First, thermoelasticity is used to study the relief in the aluminium substrate during cyclic loading: stress distribution, global relief, and progressive load transfer zone. Second, the study allows the tracking of patch disbond and characterization of mode-II crack propagation in the adhesive. A Paris law is determined for the design of bonded composite patches subjected to tensile fatigue loading.

Homogenization of linear viscoelastic materials is possible using the viscoelastic correspondence principle (VCP) and homogenization solutions obtained for linear elastic materials. The VCP involves a Laplace–Carson Transform (LCT) of the material phases constitutive theories and in most cases, the time domain solution must be obtained through numerical inversion of the LCT. The objective of this paper is to develop and test numerical algorithms to invert LCT which are encountered in the context of homogenization of linear viscoelastic materials. The homogenized properties, as well as the stress concentration and strain localization tensors, are considered. The algorithms suggested have the following two key features: (1) an acceptance criterion which allows to reject solutions of unacceptable accuracy and (2) some algorithms lead to solutions for the homogenized properties where the thermodynamics restrictions imposed on linear viscoelastic materials are encountered. These two features are an improvement over the previous algorithms. The algorithms are tested on many examples and the accuracy of the inversion is excellent in most cases.

The aim of the study is the in situ identification of the mechanical properties of the fibre -matrix interphase in a composite. Chemical reactions occur between the coating of fibre and the matrix during the elaboration of the composite. The mechanical properties of the interphase influence the properties of the composite. The knowledge of these mechanical properties is thus very significant. We propose a methodology to identify these properties in situ based on measurements of kinematics fields by digital images correlation. The virtual fields method based on the virtual works principle has been used for the local identification the elastic properties of a thick interphase.

Composite patches are often used to reinforce or to repair damaged structures, especially aeronautical components [1]. The mechanical properties of the bonded joint between metallic substrate and composite patch clearly influence the quality of the reinforcement. So the objective of this work is to study in detail the mechanical response of such a joint.

Elastic behavior of sheet molding compound (SMC) composites with a given orientational distribution of fibers under cyclic loading is investigated herein. Fatigue tests were carried out over various strain ranges. During each test, evolution of Young’s modulus was measured and the composite was analyzed using scanning electron microscopy. Observations revealed the principal form of degradation to be matrix fiber debonding. A constitutive model that takes into account the reduction of overall elastic properties, i.e., Young’s modulus, was developed. This model uses a Mori-Tanaka mean field approach coupled with a micromechanical damage law. The energetic failure criterion and the failure probability are functions of local shear and normal stresses calculated at each point of the interface of each fiber family. A procedure for identifying the most appropriate material parameters is described in detail. The proposed model agrees well with the experimental results.

The objective of the present study is to determine the influence of the loading rate on the critical energy release rate GIc of fibre-reinforced epoxy laminates. In order to perform pure mode I loading at higher opening velocities, a new test device is developed. The approach is based on a symmetrical opening displacement applied to a DCB specimen. In the data reduction, the influence of the kinetic energy has to be taken into account. The results obtained on the unidirectional carbon–epoxy laminate T300/914 at crack opening rates up to 1.6m/s show a slight effect of the loading rate on GIc.

Material overall mechanical behaviour varies significantly under rapid straining as compared to quasi-static loading. Analysing the damaged elastic behaviour of composite materials under dynamic loading requires theoretical tools and experimental approaches integrating the strain rate effects. This work is concerned with development and optimisation of an experimental methodology devoted to the micro and macroscopic characterisation of composites mechanical behaviour under high-speed loadings. The applied experimental procedure has been optimised in an attempt to isolate the inherent inertial disturbances attributed to the test system. The optimisation aims at minimizing the amplitude of measurements perturbation in order to give rise to homogeneous stress/strain fields within the tested specimen. Using a servo-hydraulic machine, monotonic and interrupted tensile tests were performed at different strain rates and coupled to scanning electronic microscope observations. The developed approach has been applied at strain rates up to 200s−1 for two composite materials: SMC-R26 and a woven carbon-epoxy laminate.

The requirements of passive security, in the transport industry, impose to maximize the dissipation of the energy and to minimize the decelerations undergone by the passengers of a vehicle at the time of violent shocks (crash). The research works, developed currently in this thematic, appear in a strong current that mobilizes many laboratories of research in France and abroad. They aim to bring suitable answers to the preoccupations emanating the transport industry notably. These aspects become primordial in the case of composites with random reinforcement. However, currently, the behaviour laws implemented in the dynamic explicit schemes (RADIOSS, PAM-CRASH and LS-DYNA) do not integrate sufficiently the physics of the deterioration kinetics, the variability and especially the heterogeneity of the composite materials microstructure. Damage mechanism have been analysed at different scales. Performing interrupted tests at different high strain rates, the lost of stiffness and the number of cracks have been determined simultaneously. These data obtained for a SMC (Sheet Moulding Compound) have been used to develop a modelling of the anisotropic behaviour of damaged composite for strain rate up to 200 s(-1).This paper presents the building of a multi-scale predictive model coupling specific experimental methodologies and the formulation of damageable behaviour laws for discontinuous fibre reinforced composites materials.

Les exigences de sécurité passive, dans l'industrie du transport, imposent de maximiser la dissipation de l'énergie et de minimiser les décélérations subies par les passagers d'un véhicule lors de chocs violents (crash). Les travaux de recherche, développés actuellement dans cette thématique, s'inscrivent dans un courant fort qui mobilise de nombreux laboratoires de recherche en France et à l'étranger. Ils visent à apporter des réponses pertinentes aux préoccupations émanant notamment de l'industrie automobile. Cependant, à l'heure actuelle, les lois de comportement implémentées dans les codes de calculs dynamiques explicites (Radioss, Pam-Crash, LS-Dyna) n'intègrent pas encore suffisamment la physique de la cinétique de dégradation, la variabilité et surtout l'hétérogénéité de la microstructure des matériaux composites. Ces aspects deviennent primordiaux dans le cas de composites à renfort aléatoire. Cet article présente des résultats expérimentaux sur l'effet de la vitesse de déformation (jusqu'à 200 s-1) sur les propriétés mécaniques, en particulier celle relatives à l'endommagement. Cet effet est quantifié à l'échelle micro au travers de la densité de fissures et à l'échelle macro au travers de la mesure des pertes de raideur. Ces résultats servent de base à la construction d'un modèle prédictif multi-échelle de comportement. Une nouvelle loi de comportement dynamique anisotrope est proposée. Les données de cette loi sont fonction des paramètres de la microstructure comme l'orientation, la longueur, la fraction volumique des renforts et des propriétés mécaniques des renforts, de la matrice et de l'interface. La méthodologie expérimentale et théorique est appliquée à un composite de type SMC (Sheet Moulding Compound)

Analysing the damaged elastic behaviour of composite materials subjected to dynamic loading requires an investigation of the strain rate influence on the damage mechanisms initiation and evolution. Low and high-speed tensile tests (ε̇≤200s−1) coupled to SEM observations were conducted upon a sheet molding compound composite (SMC-R26). The results of the developed experimental approach contributed to analyse the strain rate effects, at microscopic and macroscopic material scales, on the overall behaviour and damage growth. It was shown that the strain rate mainly governs the damage threshold and accumulation. This yields to a mechanical behaviour accommodation corresponding to an increase of the ultimate strain and stress. Mainly, this work has established the visco-damaged nature of the non-linear behaviour under a moderate dynamic loading.

A theoretical model has been developed to predict the mechanical behaviour of reinforced polymers where the matrix is nonlinear viscoelastic. The homogenisation approach was used since this allowed the global behaviour of the composite to be predicted from knowledge of the behaviour of each constituent. The approach involved a proper linearisation of the nonlinear viscoelastic matrix, in conjunction with the Mori–Tanaka scheme and the viscoelastic correspondence principle. Theoretical predictions were compared with finite element simulations of the microstructure.