Sylvain Fréour

The hygroscopic behavior of plant fibers: a review

Environmental concern has resulted in a renewed interest in bio-based materials. Among them, plant fibers are perceived as an environmentally friendly substitute to glass fibers for the reinforcement of composites, particularly in automotive engineering. Due to their wide availability, low cost, low density, high-specific mechanical properties, and eco-friendly image, they are increasingly being employed as reinforcements in polymer matrix composites. Indeed, their complex microstructure as a composite material makes plant fiber a really interesting and challenging subject to study. Research subjects about such fibers are abundant because there are always some issues to prevent their use at large scale (poor adhesion, variability, low thermal resistance, hydrophilic behavior). The choice of natural fibers rather than glass fibers as filler yields a change of the final properties of the composite. One of the most relevant differences between the two kinds of fiber is their response to humidity. Actually, glass fibers are considered as hydrophobic whereas plant fibers have a pronounced hydrophilic behavior. Composite materials are often submitted to variable climatic conditions during their lifetime, including unsteady hygroscopic conditions. However, in humid conditions, strong hydrophilic behavior of such reinforcing fibers leads to high level of moisture absorption in wet environments. This results in the structural modification of the fibers and an evolution of their mechanical properties together with the composites in which they are fitted in. Thereby, the understanding of these moisture absorption mechanisms as well as the influence of water on the final properties of these fibers and their composites is of great interest to get a better control of such new biomaterials. This is the topic of this review paper.

A Hygroelastic Self-consistent Model for Fiber-reinforced Composites

Stress analyses are performed in unidirectional fiber-reinforced composites, exposed to ambient fluid, by extending a classical self-consistent model to hygroelastic solicitations. Constitutive laws are given for the macroscopic elastic properties and Coefficients of Moisture Expansion (CME) by considering a jump in moisture content between the fiber and the matrix. Inverse forms for the unknown CME of the constituent matrix are proposed. The macroscopic (ply) and local (fiber and matrix) internal stress states are evaluated for various moisture content ratios between the matrix and the ply. The macroscopic stresses are calculated by using continuum mechanics formalisms and the local stresses are deduced from the scale transition model.

Stress-dependent Moisture Diffusion in Composite Materials

Experiments have indicated that the diffusion properties of a penetrant organic matrix composite system may change with time due to evolution of the internal mechanical strain states experienced by the constituting matrix of the composite plies. A multi-scale approach coupling the internal mechanical states, predicted by continuum medium mechanics, and their localization at the ply-constituent scale to the traditional Ficks law governing the moisture diffusion process was used in order to achieve the modeling of the response of composite laminates submitted to environmental hygroscopic loads, from the transient part of the diffusion process to its permanent stage. Various numerical practical cases were considered: the effects of the internal swelling strains on the time- and space-dependent diffusion coefficient, maximum moisture absorption capacity, moisture content, and states of internal stresses are extensively studied and discussed.

On an analytical self-consistent model for internal stress prediction in fiber-reinforced composites submitted to hygroelastic load

The aim of this work is to demonstrate a fully explicit analytical micromechanical self-consistent approach dedicated to mechanical states prediction in both the fiber and the matrix of composite structures submitted to a transient hygroelastic load. The analytical forms obtained are applied to the case of carbon-epoxy composites. Rigorous continuum mechanics formalisms are used for the determination of the required time and space-dependent macroscopic stresses. The reliability of the new approach is checked through a comparison between the local stress states calculated in both the resin and the fiber according to the new closed-form solutions and the equivalent numerical model.

Determination of single-crystal elasticity constants in a cubic phase within a multiphase alloy: X-ray diffraction measurements and inverse-scale transition modelling

The scope of this work is the determination of single-crystal elastic properties from X-ray diffraction stress analysis performed on multiphase polycrystals. An explicit three-scale multiphase inverse self-consistent model is developed in order to express the single-crystal elasticity constants of a cubic phase as a function of its X-ray elasticity constants. The model is verified in the case of single-phase materials. Finally, it is applied to a two-phase (α+β) titanium-based alloy (Ti-17) and, as a result, the Ti-17 β-phase single-crystal elasticity tensor is estimated.

Extension of Mori-Tanaka Approach to Hygroelastic Loading of Fiber-reinforced Composites - Comparison with Eshelby-Kroner Self-consistent Model

The scale transition model historically proposed by Mori and Tanaka to predict the average and local elastic behavior of heterogeneous structures is extended to hygro-elastic load for the first time. Explicit constitutive laws satisfying the fundamental assumptions of the model are given for the determination of the effective macroscopic coefficients of moisture expansion (CME) in composite structures by considering a jump in moisture content between the fiber and the matrix. Explicit forms are also given for the calculation of local (fiber and matrix scale) internal stress states from the localization of the macroscopic hygro-mechanical states (ply scale). Comparisons for several compositions of composite structures (volume fraction of the constituents, i.e., the epoxy matrix and the reinforcing fibers) are performed between the numerical predictions given by the Mori-Tanaka model extended to hygro-elastic load and the recently proposed Eshelby-Kroner self-consistent hygro-elastic model. Discrepancies in the calculations appearing with an increase of the reinforcing fiber volume fraction are extensively discussed to conclude on the limitations of the micro-mechanical approach developed in the present work.

Identification of moisture diffusion parameters in organic matrix composites

In this work, moisture penetration in glass fiber-reinforced polymers was systematically investigated through both experiments and theoretical approaches. The diffusivities of the neat resin and those of unidirectional composite plates containing various glass fiber volume fractions have been identified through the analysis of hygro-thermal aging tests. The main aim of the present paper consists in comparing the evolutions, as a function of the fiber volume fractions, of the moisture diffusion coefficients deduced from experiments to the corresponding values, predicted by the many traditional scale transition relations available in the literature.

Determining Ti-17 β-Phase Single-Crystal Elasticity Constants through X-Ray Diffraction and Inverse Scale Transition Model

The scope of this work is the determination of single-crystals elastic constants (SEC) from X-ray diffraction lattice strains measurements performed on multi-phase polycrystals submitted to mechanical load through a bending device. An explicit three scales inverse self-consistent model is developed in order to express the SEC of a cubic phase, embedded in a multi-phase polycrystal, as a function of its X-ray Elasticity Constants. Finally, it is applied to a two-phases (α+β) titanium based alloy (Ti-17), in order to estimate Ti-17 β-phase unknown SEC. The purpose of the present work is to account the proper microstructure of the material. In particular, the morphologic texture of Ti-17 a-phase, i.e. the relative disorientation of the needle-shaped grains constituting this phase, is considered owing to the so-called Generalized Self-Consistent model.

Determination of the Macroscopic Elastic Constants of a Phase Embedded in a Multiphase Polycrystal-Application to the β-Phase of a Ti-17 Titanium Based Alloy

A one-site elastic self-consistent model following the mathematical formalism introduced by Kroner and Eshelby (KE) has been developed in order to solve the case of multiphase materials. This model has been applied to duplex steels and aluminium - silicium carbide Metal Matrix Composites (MMC) in the aim to study the evolution of their stiffness at pseudomacroscopic scale. Simulations justify the usually implicit hypothesis of the identity of the elastic moduli of a given phase, at macroscopic and pseudomacroscopic scales. The implementation of KE model by this hypothesis yields a new implicit formulation for the stiffness of a given unknown phase embedded in a two-phases material. This original characterization method will be applied to the β-phase of Ti-17 alloy. The singular behaviour in terms of residual pseudomacrostress of each phase after uniaxial loadings will be deduced from these data.

Homogenization of Moisture Diffusing Behavior of Composite Materials with Impermeable or Permeable Fibers — Application to Porous Composite Materials

In order to predict the long-term durability of polymer matrix composite materials submitted to humid environments, the moisture diffusion behavior has to be investigated. The knowledge of the effective diffusivity is actually required, for estimating the moisture content of polymer-based fiber-reinforced materials, even when a basic behavior such as Ficks law is assumed to occur. The original contribution of the present work is to provide new analytical solutions for the effective diffusivities from the solving of unit cell problems on representative volume elements by means of several multi-scale approaches. Composite materials with impermeable or permeable fibers are extensively investigated. The proposed approaches are extended to the practical case of composite materials containing realistic voids volume fractions.