Hygroelastic Behaviour Research Articles

A 3D multi-scale hygro-mechanical model of oak wood

A multi-scale framework is proposed for the prediction of the macroscopic hygro-elastic properties of oak wood. The distinctive features of the current multi-scale approach are that: (i) Four different scales of observation are considered, which enables the inclusion of heterogeneous effects from the nano-, micro-, and meso-scales in the effective constitutive behavior of oak at the macro-scale, (ii) the model relies on three-dimensional material descriptions at each considered length scale, and (iii) a moisture-dependent constitutive assumption is adopted at the nano-scale, which allows for recovering the moisture dependency of the material response at higher scales of observation. In the modeling approach, oak wood is assumed as homogeneous at the macro-scale. The meso-scale description considers the cellular structure of individual growth rings with three different densities. At the micro-scale, the heterogeneous nature of cell walls is described by the characteristics of the primary and secondary cell wall layers. Finally, the nano-scale response is determined by cellulose micro-fibrils embedded in a matrix of hemicellulose and lignin. The oak properties at the four length scales are connected via a three-level homogenization procedure, for which, depending on the geometry of the fine-scale configuration, an asymptotic homogenization procedure or Voigt averaging procedure is applied at each level to determine the effective hygro-elastic properties at the corresponding coarse scale. In addition, the moisture adsorption isotherms at each scale are constructed from a volume-weighted averaging of the moisture adsorption characteristics at the scale below. The computational results demonstrate that the macro-scale moisture-dependent, hygro-elastic behavior of oak wood is predicted realistically, thereby revealing the influence of the material density, the micro-fibril orientation, and the hygro-elastic properties from the underlying scales. The computed macro-scale properties of oak are in good agreement with experimental data reported in the literature.

Estimation of the hygro-elastic behavior of polymeric composites during moisture aging considering hygromechanical coupling

This study aims to combine, using a numerical tool, different phenomena of hygromechanical coupling in a unidirectional carbon/epoxy composite and to compare the results obtained with the cases where these couplings are considered separately. The first case of coupling considers the effect produced by the mechanical state on the diffusive behavior of the material. This was accomplished using the free volume theory, which links the trace of the matrix strain tensor to the moisture absorption capacity and the diffusion coefficient. The second case takes into account plasticization, that is, the effect of humidity on the mechanical properties of composites, without modifying the diffusion parameters. Finally, these two mechanisms are combined in a third configuration, which represents the original contribution of this present work. The results include the water content profiles, the diffusion coefficient, the transverse Young's modulus, the transverse hygroscopic expansion coefficient, as well as the induced local mechanical states. The interest of the work carried out is to pave the way for the extension to more complex materials, such as plant fiber biocomposites, in which the water absorption by the hydrophilic fibers should not be neglected anymore.

Influence of the cross-linking of the cellulosic fibrillar network on the effective hygro-mechanical behavior of the wood cell wall

This study aims to analyze the influence of the fibrils oscillations and connections on the effective hygro-elastic behavior of the wood cell wall. For that, two different models of microstructure describing the cell wall are introduced: the model 1S, for which the fibrils are allowed to oscillate in one direction like a sine curve, and the model 2S, for which the fibrils oscillate in two directions as a helix. The effective hygro-elastic behavior of the cell wall associated with each of the two models is computed by numerical periodic homogenization. Then, we analyze the effects of the fibers oscillations on the effective behavior by making varying some parameters such as the shape ratio of the unit cell, the fibrils volume fraction, or the elastic contrast between the matrix and the fibrils. It is shown that the fibrils oscillations have a significant effect on some effective moduli of the cell wall, such as the effective shear moduli \({\widetilde{G}}_{12}\) (for both model 1S and 2S) and \({\widetilde{G}}_{13}\), \({\widetilde{G}}_{23}\) (for model 2S only).

Analysis of the hygroscopic and hygroelastic behaviours of water aged flax-epoxy composite

In this paper, an analysis of the hygroscopic and hygroelastic behaviours of unidirectional flax fibre-reinforced epoxy composite is presented. Samples of this composite material were elaborated by the vacuum infusion process, then cut and prepared to be aged in tap water until saturation. To describe their hygroscopic behaviour, Fick’s model is considered and an optimisation procedure is developed to identify their macroscopic water diffusion parameters. After that, a plane finite element modelling of the hygroscopic behaviour of the aged flax-epoxy samples is conducted to identify the flax fibre radial and longitudinal water diffusion parameters using an inverse approach. Finally, a plane finite element analysis of their hygroelastic behaviour is proposed allowing the estimation of the hygroscopic internal stress in flax fibres and epoxy matrix. In particular, the hygroscopic analysis reveals that water diffusion is more pronounced in the direction of flax fibres compared to the transverse and thickness directions. In addition, the hygroelastic finite element modelling highlights the high compressive and tensile stress subjected to flax fibres and epoxy matrix, respectively, due to water absorption.

Hygroelasticity analysis of an elastically restrained functionally graded porous metamaterial circular plate resting on an auxetic material circular plate

The main objective of this research is to investigate the hygroelastic behavior of a non-homogeneous circular plate made up of porous metamaterial resting on an auxetic material plate. The mechanical properties of the main plate, as well as moisture concentration, vary as an exponential function in the transverse direction. Poisson’s ratio is constant. The elastic supporting medium is developed by considering the structure-structure coupling. Based on the linear hygroelasticity theory, the governing state equations in terms of displacements and moisture concentration are acquired. At first, the Fickian equation is solved to compute the nonlinear distribution of moisture through the plate thickness, and then the state equations are semi-analytically solved using the state-space (SS) method and the differential quadrature (DQ) rule to predict the elastic field quantities. A comprehensive parametric analysis is accomplished to elucidate the effects of key parameters on the steady-state response of the plate under the mechanical and hygral loads.

Molecular dynamics approach on the hygroelastic behavior of epoxy/graphene nanocomposites

Hygroscopic aging causes swelling, degradation of mechanical and interfacial properties in thermosetting epoxy polymers, which eventually lead to a fast fracture at the interface in nanocomposite. Since the epoxy is a widely used high performance thermoset polymer for aerospace structures, durability analysis is necessary for the sake of maintenance and stability of the structures. In this study, the hygroelastic behavior of pure epoxy and epoxy/graphene nanocomposite is investigated through atomistic molecular dynamics (MD) simulation. To determine the hygroelastic behavior of epoxy materials, amorphous molecular unit cells of pure epoxy and epoxy/graphene nanocomposites with its highest possible cross-linking ratio and moisture contents of 0 and 2 wt% are considered. Through the classical ensemble simulations, diffusion coefficient of water in matrix, coefficient of moisture expansion (CME) and elastic modulus of pure epoxy and epoxy/graphene nanocomposites are determined.

Molecular dynamics and micromechanics study of hygroelastic behavior in graphene oxide-epoxy nanocomposites

The hygroelasticity of oxygen-functionalized graphene (GO)-epoxy nanocomposites is studied. Two different transversely isotropic nanocomposite molecular models are constructed: with uniformly distributed and interface-concentrated water molecules, respectively. The stress-strain curves and coefficients of moisture expansion (CMEs) are determined according to moisture content. The degradation of the GO/epoxy interface due to the infiltrated moisture is characterized by interfacial decohesion tests. The micromechanics model is used to derive new closed-form solutions for the effective elastic stiffness and CME of multi-phase composites with interfacial imperfections. Regardless of the moisture distribution, the overall hygroelastic behavior of nanocomposites clearly degrades upon moisture absorption.

3D printing of wood fibre biocomposites: From mechanical to actuation functionality

Natural fibres are increasingly used as reinforcements for thermoplastic composites. Additive manufacturing, also known as 3D printing, is a common material extrusion process using (bio)polymers reinforced with natural fibres. However, there is a lack of understanding of the effect of printing parameters on the mechanical properties involved in this new process, and more particularly in the case of Fused Deposition Modeling (FDM). Hygromorphic biocomposites represent a novel use of natural fibres for the production of original self-bending devices that actuate in a moisture gradient. By mimicking natural actuators and their bilayer microstructure adapted for seed dispersal, hygromorphic biocomposites take advantage of the hygro-elastic behaviour of natural fibres.The FDM of wood fibre reinforced biocomposites leads to mechanical properties that are strongly dependent on printing orientation (0 or 90°) due to fibre anisotropy. Mechanical properties depend also on printing width (overlapping of filaments), with a lower Young's modulus than in the compressed samples. Indeed, printed biocomposites have a microstructure with relatively high porosity (around 20%) that conjointly leads to damage mechanisms but also water absorption and swelling.The FDM of hygromorphic biocomposites enables a shift towards 4D printing since the material is able to evolve over time in response to an external stimulus. Typical microstructures achieved by printing could be used advantageously to produce biocomposites with a faster moisture-induced bending response compared to compressed samples.

Modelling of the hygroelastic behaviour of normal and compression wood tracheids

Compression wood conifer tracheids show different swelling and stiffness properties than those of usual normal wood, which has a practical function in the living plant: when a conifer shoot is moved from its vertical position, compression wood is formed in the under part of the shoot. The growth rate of the compression wood is faster than in the upper part resulting in a renewed horizontal growth. The actuating and load-carrying function of the compression wood is addressed, on the basis of its special ultrastructure and shape of the tracheids. As a first step, a quantitative model is developed to predict the difference of moisture-induced expansion and axial stiffness between normal wood and compression wood. The model is based on a state space approach using concentric cylinders with anisotropic helical structure for each cell-wall layer, whose hygroelastic properties are in turn determined by a self-consistent concentric cylinder assemblage of the constituent wood polymers. The predicted properties compare well with experimental results found in the literature. Significant differences in both stiffness and hygroexpansion are found for normal and compression wood, primarily due to the large difference in microfibril angle and lignin content. On the basis of these numerical results, some functional arguments for the reason of high microfibril angle, high lignin content and cylindrical structure of compression wood tracheids are supported.

Micromechanics investigation of hygro-elastic behavior of cellular materials with multi-layered cell walls

In this paper, the hygro-elastic behavior of two-dimensional periodic honeycombs composed of multi-layered cell walls is investigated using a computational micromechanics approach. Detailed numerical results for the effective hygro-expansion coefficients and the elastic moduli of honeycombs are obtained. The influence of the arrangement of the cell wall layers and the geometrical parameters of the honeycombs on the effective hygro-elastic properties is examined. Limiting cases are considered, and the validity of the model is established by comparison with the analytical solutions available in the existing literature. The obtained results suggest that the layered architecture of the cell wall enhances the anisotropy in swelling behavior of honeycombs with irregular configuration which is reflected in their transverse hygro-expansion coefficients while regular honeycombs show isotropic behavior. The proposed model explains the complex thermo-hygro-mechanical behavior of natural cellular materials and provides a predictive tool for bio-mimetic material design.

Hierarchical modelling of microstructural effects on mechanical properties of wood. A review COST Action E35 2004–2008: Wood machining – micromechanics and fracture

Abstract Wood exhibits a hierarchical architecture. Its macroscopic properties are determined by microstructural features at different scales of observation. Recent developments of experimental micro-characterisation techniques have delivered further insight into the appearance and the behaviour of wood at smaller length scales. The improved knowledge and the availability of increasingly powerful micromechanical modelling techniques and homogenisation methods have stimulated rather comprehensive research on multiscale modelling of wood. Linking microstructural properties to macroscopic characteristics is expected to improve the knowledge of the mechanical behaviour of wood and to serve as the basis for the development of innovative wood-based products and for biomimetic material design. Moreover, understanding fundamental aspects of wood machining requires multiscale approaches which can take into account the heterogeneity, anisotropy and hierarchies of wood and wood composites. In this review, recent developments in the field of hierarchical modelling of the hygroelastic behaviour of wood are discussed, and a short outline of the theoretical background is given. Much focus is placed on composite micromechanical models for the wood cell wall and on multiscale models for wood resting upon hierarchical finite element models and on the application of continuum micromechanics, respectively. These models generally lead to the specification of equivalent homogeneous continua with effective material properties. Finally, current deficiencies and limitations of hierarchical models are sketched and possible future research directions are specified.

On the use of the geometric mean approximation in estimating the effective hygro-elastic behavior of fiber-reinforced composites

A geometric averaging is employed in a scale transition model dedicated to determining the effective hygro-elastic macroscopic properties of a fiber-reinforced composites from the properties of its constituents (i.e., the fiber and the matrix), for the first time. Calculations of macroscopic and local stresses due to hygroscopic loads are also carried out. Numerical computations performed according to the geometric mean are compared to the corresponding predictions given by the arithmetic average, classically used in the field of micro-mechanical modeling.

Modelling of the hygroelastic behaviour of a wood-based composite for construction

OSB is a construction panel made with layers of precision manufactured wood strands that are aligned, formed into panels and pressed with an exterior grade adhesive resin. The aim of the paper is to provide a theoretical analysis of the hygromechanical behaviour of this material by using a two-step method. First, the mechanical and morphological parameters characterizing the wood particles, as well as the information concerning their orientations, are used in a multi-scale approach for the prediction of the behaviour of a single layer. Then, the classical theory for the analysis of laminates is used to obtain the stiffness constants of the panel. The flexural Young moduli of the panel as well as its in-plane hygroelastic constants are studied in relation with the orientation distribution followed by the particles. The study permits to establish that the orientation distribution function is a key factor in obtaining high structural properties for OSB. It is shown that an improved strand alignment may help to gain significant performance with respect to stiffness and behaviour under moisture conditions.

Ultrastructural features affecting mechanical properties of wood fibres

The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure–property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure–property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.

The effect of the morphology on the hygroelastic behaviour of polyester and epoxy resins

The effect of the morphology on the hygroelastic behaviour of polyester and epoxy resins is studied. The morphology is expressed by two factors, namely, the free volume of the polymer and a two-phase formation. The first factor is varied in styrene-crosslinked polyester using different styrene/alkyd proportions, and by using different immersion temperatures. The second is investigated in polyesters by replacing styrene with bromostyrene inducing a two-phase structure, and in epoxies by changing the hardener/epoxy ratio. It is shown that the hygroelastic response is affected markedly by the morphology. In general, both the moisture content and its rate of absorption are higher when the polymer network is more open and when more free volume is available. In some cases, however, the hygroelastic parameters are dominated by a two-phase structure where present.

Anisotropic hygroelastic behavior of poly(methyl methacrylate) under case II diffusion

AbstractThe hygroelastic behavior of polymers, in which the diffusion mechanism is characterized by a sharp advancing boundary between the swollen shell and the core, was studied. A highly anisotropic swelling response was observed. The an‐isotropy of the swelling strains was caused by the mechanical constraints exerted by the stiff core on the penetrated shell. The swollen systems were viewed as analogous to a composite material comprising a stiff fiber embedded hi a soft matrix. The size and relative dimensions of the core were found to be the crucial factor determining the antsotropic response en‐countered. The experimental system employed was of poly(methyl methacrylate).

Angular dependence of hygroelasticity in unidirectional glass-epoxy composites

Swelling and hygroelasticity of unidirectional glass fibre-reinforced epoxy composites in boiling water is studied with respect to the angle θ between the direction of orientation and the direction of measured dimensional change. The results are expressed in terms of the dependence of θ of a hygroelasticity coefficient. A very good agreement is observed between the experimental results and the theoretical equation, already verified in the case of the angular dependence of the coefficient of thermal expansion. This is taken as additional support for the analogy between thermoelastic and hygroelastic behaviour in composites. The experimental longitudinal and transverse hygroelasticity coefficients are found to be different from predictions based on Schapery's equation. However, it is shown that these equations can produce possible valid estimates of μL and μT provided the mechanical properties of the swollen constituent materials are used.

Anisotropic hygroelastic behaviour of oriented poly(methyl methacrylate)

A series of samples of oriented poly(methyl methacrylate) was prepared by drawing at a range of temperatures above T g to a constant extension ratio. The birefringence was used as a measure of orientation. The specimens were immersed in water at 21°C and the coefficients of hygroelasticity were calculated from the dimensional change and weight gain data. Two coefficients, namely the longitudinal and the transverse, were measured for each sample. The longitudinal coefficients of hygroelasticity exhibit a linear decrease and the transverse exhibit an increase as the birefringence increases from zero. The results are analogous to those obtained for thermoelasticity (Wilson, J. D. and Treloar, L. R. G. J. Phys. (D) 1972, 5, 1614).