Frédéric Jacquemin

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 closed-form solution for the internal stresses in thick composite cylinders induced by cyclical environmental conditions

The paper presents a new semi-analytical model to assess the internal stress field in cylindrical pipes due to cyclical temperature and humidity conditions. The solution applies to both thin or thick laminated pipes composed of orthotropic plies following Ficks diffusion laws. The only restriction put on those diffusion laws is that the Arrhenius activation energy should be ply independent. Firstly, the resulting time and space dependent moisture concentration field within the interior of the pipe wall is calculated. It is the sum of the permanent solution, due to the mean relative humidity conditions imposed on the two lateral surfaces of the pipe, and a cyclical vanishing solution. Secondly, the mechanical problem is solved leading to the full stress field solution over the pipe interior and close to the lateral surfaces. The final solution can be implemented in a friendly-user software to assess the combined effects of the pipe thickness, ply stacking sequence and transient service conditions over the internal stress field.

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.

Chemical shrinkage characterization techniques for thermoset resins and associated composites

Control and optimization of curing process is very important for the production of high quality composite parts. Crosslinking of molecules of thermoset resin occurs in this phase, which involves exothermy of reaction, chemical shrinkage (Sh) and development of thermo-physical and thermo-mechanical properties. Exact knowledge of the evolution of all these parameters is required for the better understanding and improvement of the fabrication process. Sh is one such property of thermoset matrix, which is difficult to characterize due to its coupling with thermal expansion/contraction. A number of techniques have been used to determine volume Sh of thermoset matrix, which later on has been used to find tensor of Sh for the simulation of residual stresses and shape distortion of composite part, etc. Direct characterization of volume Sh of composites has also been made by some authors. Though not much, but some work has also been reported to determine the Sh of composite part in a specific direction. In this article, all the techniques used in the literature for the characterization of Sh of resin and composite are reported briefly with their respective advantages, disadvantage and important results.

Qualitative and quantitative assessment of water sorption in natural fibres using ATR-FTIR spectroscopy

In the field of composite materials, natural fibres appear to be a viable replacement for glass fibres. However, in humid conditions, strong hydrophilic behaviour of such materials can lead to their structural modification. Then, understanding moisture sorption mechanisms in these materials is an important issue for their efficient use. In this work, the water sorption on three natural fibres (flax, hemp and sisal) was studied using Fourier transformed infrared spectroscopy. The spectral information allowed both qualitative and quantitative analyses of the moisture absorption mechanisms. The main chemical functions involved in the water sorption phenomenon were identified. The absolute water content of the fibres was also determined by using a partial least square regression (PLS-R) approach. Moreover, typical sorption isotherm curves described by Park model were fitted as well as water diffusion kinetics. These last applications confirmed the validity of the FTIR spectra based predictive models.

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.

Evolution of chemical and thermal curvatures in thermoset-laminated composite plates during the fabrication process

Residual deformations and stresses formation in the thermoset-laminated composite is a frequently studied subject in the recent years. During fabrication, the laminated composites undergo chemical deformation during cross-linking and thermal deformation while cooling. In thin laminates, due to large displacements and complex evolution of shape, these deformations can only be explained by using nonlinear strain–displacement relationship. In the present article, we calculated together for the first time, the thermal and chemical deformations occurring in carbon/epoxy laminates by considering a nonlinear geometrical approach to understand the evolution of shape and hence residual stresses induced during fabrication process. The effect of fibre fraction on the chemical and thermal deformations is studied as well.

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.

Modelling of the moisture concentration field due to cyclical hygrothermal conditions in thick laminated pipes

The paper presents a new method to calculate the moisture concentration field induced by cyclical environmental conditions in thick laminated pipes. The solution which is obtained is composed of a transient solution over the interior of the pipe wall and a fluctuating solution within two thin regions, close to the inner and outer lateral surfaces of the pipe wall. The thickness of these two regions is depending on both materials and frequency conditions. The transient solution is determined by using an analytical method based on the solving in average of the field equation. The fluctuating solution is derived from a finite difference scheme. It is shown that after some period of time the transient solution tends towards a permanent time independent solution. In that case, the fluctuating solution becomes a periodic solution which is conditioned by the cyclical boundary conditions. Finally, the effect of particular cyclical conditions on the moisture concentration in thick wall pipes will be tackled.

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.