The coupling between tensile stress and water diffusion in composites represents a major issue in many marine applications. Even if these two behaviours are well documented as separate subjects, ...there are still very few data on fully coupled mechanisms. The aim of this study is to understand what governs the coupled behaviour, by investigating the water diffusion in carbon/epoxy. The coupling is first evaluated on neat resin samples to characterise the matrix behaviour. Then the study focuses on composites with two types of woven fibre orientation: one at ±45°, in order to understand the coupling effects on materials loaded away from the reinforcement direction and another quasi-isotropic to obtain properties along the fibres. For each material the same approach was applied: first, the tensile behaviour, damage development and water diffusion were studied uncoupled. Then, semi-coupled effects were investigated. The final part of the paper combines both behaviours as fully coupled phenomena.
•Effect of moisture sorption on plant fibres was investigated by FTIR spectroscopy.•Hydroxyl and carboxyl moieties are specific sites for water sorption.•Partial least squares regression quantified ...the water content of natural fibres.•Models fitted the sorption isotherms and diffusion kinetics.
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.
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.
In order to elucidate the hygroscopic effects on impact-resistance of carbon fiber/epoxy quasi-isotropic composite plates, low-velocity impact tests are conducted on dry and hygroscopically ...conditioned plates, respectively, under identical configurations. For the impact tests, plates were immersed in the hot water at 80 °C to absorb a different amount of moisture content (MC). Experimental results reveal that the presence of the MC plays a pivotal role by improving the impact-resistance of composite plates. Plates with higher percentage of MC could behave elastically to a larger strain, yielding larger deflection under impact loading. From SEM fractographies, it is observed that small disbanding grows at the interface of epoxy and carbon fiber due to absorbed MC. After absorbing MC, most of impact energy is dissipated in hygroscopic conditioned composite plates through elastic deformation and overall less damage is induced in wet composite plates compare to the dry plate. We can postulate that the presence of MC increases the elastic limit as well as ductility of the epoxy by promoting chain segmental mobility of the polymer molecules, which eventually leads to the enhancement of the impact-resistance of wet quasi-isotropic composite plates in comparison with the dry plate.
Water diffusion under hydrostatic pressure is critical for many underwater applications. Nevertheless it has rarely been studied, and published data are contradictory. The aim of this study is to ...understand what governs pressure effects by studying different materials (unreinforced resin, and three glass-fibre reinforced epoxy composites). First, kinetics of water diffusion, for unreinforced resin and composite materials, are identified at different pressure levels (1, 50 and 500bar). For the neat epoxy resin the water uptake remained unchanged when pressure was raised. The glass fibre reinforced epoxy composites produced by hand lay-up have a saturation level that increases significantly with increasing pressure, while the diffusion coefficient is unaffected. The infused composites show only a small effect of pressure slowing initial diffusion rate, while the prepreg composite show no effect. In a second part, the present study focuses on the identification of the diffusion law using a numerical method. In the final section X-ray micro-tomography is used and reveals a high level of porosity in the hand lay-up composite. Moreover, as glass fibres are hydrophobic and resin water uptake does not depend on hydrostatic pressure it is concluded that additional water diffuses into voids under pressure.
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•Theories predict lower moisture content in polymers under hydrostatic pressure; the study highlights the opposite phenomenon.•Based on experimental results we propose a theory explaining this behaviour, using voids ratio characterization•This work highlights that water content increases in presence of voids in composites under pressure.•The water content in composites under pressure depends on voids volume ratio and geometry.
Thermogravimetric analyses under air atmosphere were performed up to 973 K on balsa wood to investigate the thermal degradation process of two types of samples aged differently: a dry balsa specimen ...and a hygroscopically aged material. The Kissinger method was used to determinate the kinetic parameters as the activation energy E sub(a), the pre-exponential factor k sub(0) and the order of reaction n. The three constituents of balsa (i.e. hemicellulose, cellulose and lignin) were well identified and the decomposition temperature region well characterized. The presence of a large quantity of water affects the thermal decomposition behavior of wood inducing notably a maximal degradation temperature higher by 314 K in relation to the dry material. The determined experimental kinetic parameters E sub(a), k sub(0) and n are in good agreement with those find in the literature for woods. The role of diffused water in the different thermal behavior between dry and aged balsa is explained from physical and chemical points of view.
In this paper, a multiscale approach is proposed to study the moisture diffusion behavior of polymer/clay nanocomposites. For this purpose, a new method to determine the representative elementary ...volume is proposed for possible nanocomposites structures. For this study, two structures at nanoscale are considered based on micrographs of the actual microstructures of these materials at this scale. The results, obtained for the in-depth profiles of the moisture content as well as the overall moisture diffusion kinetics curves revealed a significant effect of the clay volume fraction on the moisture diffusion in nanocomposites. Furthermore, the predicted results show, that the moisture diffusion behavior at microscale strongly depends on the structure at nanoscale. From these results, it seems that the exfoliated structure is the most critical for the moisture diffusion in polymer/clay nanocomposites.
The hygro-mechanical behaviour of a bio-sourced composite material (MAPP/flax) is experimentally investigated through the characterization of its moisture diffusion and elastic properties, as well as ...the in-plane hygroscopic swelling of a unidirectional ply sequence and curvature in the case of an asymmetric lay-up sequence. From these considerations, we propose a hygro-mechanical model for the material behaviour, based on a Fickian diffusion model solved in 1D with a finite difference method, and coupled to a modified mechanical model based on laminate theory. The proposed model takes into account the evolution of the mechanical properties as well as hygroscopic swelling during moisture uptake to predict the stress state during water sorption of a biocomposite. Results show that sorption kinetics is dependent on the lay-up sequence of the biocomposite structure. The stress state determined from the thickness of the asymmetric lay-up shows that most of the plies (approx. 75% of the whole laminate) are subjected to a compressive stress along their in-plane direction transverse to the fibres. This stress distribution may lead to a decrease in the free-volume of the material, thus modifying the hygroscopic properties by reducing its maximum moisture content compared to the unidirectional laminate (the stress distribution being equal to zero for this latter laminate when saturation is reached).
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