On the effect of stacked fabric layers on the stiffness of a woven composite

Most micromechanical models for stiffness prediction of woven composites assume independence of the Q-matrix on the number of fabric layers in the composite. For example, the moduli of single and 10 layer composites are assumed to be equal in the case when all layers have the same in-plane orientation. Although this statement is likely to be true for isotropic materials or even for unidirectional laminated composites, it may not be valid in some cases of woven composites.

This paper contains experimental and theoretical investigations of plain weave carbon fiber/polyester composites. Specimens with one single and eight layers of fabrics are tested and observable differences of mechanical properties are obtained.

The theoretical part of this article consists of derivation and application of several micromechanical models on these particular composites. The use of those simplified models finally allows us to find the main mechanisms which cause the observed effects.     

Computational up-scaling of anisotropic swelling and mechanical behavior of hierarchical cellular materials

The hygro-mechanical behavior of a hierarchical cellular material, i.e. growth rings of softwood is investigated using a two-scale micro-mechanics model based on a computational homogenization technique. The lower scale considers the individual wood cells of varying geometry and dimensions. Honeycomb unit cells with periodic boundary conditions are utilized to calculate the mechanical properties and swelling coefficients of wood cells. Using the cellular scale results, the anisotropy in mechanical and swelling behavior of a growth ring in transverse directions is investigated. Predicted results are found to be comparable to experimental data. It is found that the orthotropic swelling properties of the cell wall in thin-walled earlywood cells produce anisotropic swelling behavior while, in thick latewood cells, this anisotropy vanishes. The proposed approach provides the ability to consider the complex microstructure when predicting the effective mechanical and swelling properties of softwood.     

Towards a better understanding of wood cell wall characterisation with contact resonance atomic force microscopy

Nowadays, the multi-scale modelling of wood has a great need for measurements of structural, chemical and mechanical properties at the lowest level. In this paper, the viscoelastic properties in the layers of a wood cell wall are investigated using the contact resonance mode of an atomic force microscope (CR-AFM). A detailed experimental protocol suitable for obtaining reproducible and quantifiable data is proposed. It is based on three main steps: sample preparation to obtain a good surface state, calibration of the contact modulus using reference samples, and image processing to produce the viscoelastic images. This protocol is applied on chestnut tension wood. The obtained topography and semi-quantitative viscoelastic maps are discussed with respect to the cell wall structure, sample preparation effects, and AFM measurement specificity compared with nanoindentation.     

Effects of fiber orientation and anisotropy on tensile strength and elastic modulus of short fiber reinforced polymer composites

An experimental study was conducted to investigate anisotropy effects on tensile properties of two short glass fiber reinforced thermoplastics. Tensile tests were performed in various mold flow directions and with two thicknesses. A shell–core morphology resulting from orientation distribution of fibers influenced the degree of anisotropy. Tensile strength and elastic modulus nonlinearly decreased with specimen angle and Tsai–Hill criterion was found to correlate variation of these properties with the fiber orientation. Variation of tensile toughness with fiber orientation and strain rate was evaluated and mechanisms of failure were identified based on fracture surface microscopic analysis and crack propagation paths. Fiber length, diameter, and orientation distribution mathematical models were also used along with analytical approaches to predict tensile strength and elastic modulus form tensile properties of constituent materials. Laminate analogy and modified Tsai–Hill criteria provided satisfactory predictions of elastic modulus and tensile strength, respectively.     

Bounds on the Elastic Brittle solution in bodies reinforced with FRP/FRCM composite provisions

In the paper the behavior of composite-reinforced masonry structures is discussed. One focuses on the problem of the reinforced structure under different modeling of the basic material keeping into account possible strength in tension of the masonry material. Actually the presence of the reinforcement requires the development of a specialized treatment, that is presented in the paper, with the purpose of exploring the dependence of the solution on more or less refined hypotheses about the masonry material in the presence of reinforcement. The original set up leads to the formulation of new bounding theorems for masonry structures reinforced by composites, with the masonry possibly modeled by an elastic brittle assumption.     

Estimation of modulus of elasticity of plastics and wood plastic composites using a Taber stiffness tester

This study measured the modulus of elasticity (MOE) of various plastics and composite materials with a Taber stiffness tester as an alternative to conventional universal testing machines. The proposed approach presents an expedited means to assess MOE for a wide range of plastics and wood plastic composites (WPCs) with various shapes. The Taber stiffness units and the geometry of the samples acted as the basis for the calculation of the MOE. The results showed a high correlation between the MOE calculated from Taber units and that obtained on a universal testing machine (Instron). Concurrently, Taber units showed the potential to assess stiffness of samples with irregular shapes, such as in the case of extruded rods, which exhibit this characteristic.     

Prediction of transport properties of wood below the fiber saturation point - A multiscale homogenization approach and its experimental validation. Part II: Steady state moisture diffusion coefficient

In this publication a multiscale homogenization model for moisture transport in wood is developed and validated. The model aims at prediction of macroscopic transport properties of clear wood samples from their microstructure and the physical properties of a few microscale constituents. In the first part of this two-part paper, the theoretical background and fundamentals of the model were presented, and its specification for the estimation of macroscopic thermal conductivities was shown. In this second part the model is applied to steady state moisture diffusion below the fiber saturation point. The model starts on a scale of about 50 μm, where the wood cells form a honeycomb-like structure. In a first homogenization step the effective moisture transport behavior of the cell structure is determined from moisture diffusion properties of the cell walls and the (moist) air in lumens, respectively. Further homogenization steps account for the larger vessels that exist in hardwood species, the annual rings which are a succession of layers with different densities, and finally wood rays, that form pathways in the radial direction throughout the stem. The model validation rests on experiments as in the case of heat conduction: The macroscopic diffusion coefficients predicted by the multiscale homogenization model for tissue-specific composition data (input data set II) are compared to corresponding experimentally determined tissue-specific diffusion coefficients under steady state conditions (experimental data set). As for thermal conductivity, the good agreement of model predictions and test data underlines the suitability of the presented multiscale model.     

Prediction of transport properties of wood below the fiber saturation point - A multiscale homogenization approach and its experimental validation: Part I: Thermal conductivity

This two-part paper covers the development and validation of a multiscale homogenization model for macroscopic transport properties of wood. The starting point is the intrinsic structural hierarchy of wood, which is accounted for by several homogenization steps. Starting on a length scale of a few nanometers the model ends up with macroscopic properties by including the morphology of the intermediate hierarchical levels. In this first part this is done for thermal conductivity, based on a six-level homogenization scheme. The used homogenization technique is continuum micromechanics in terms of self-consistent and Mori-Tanaka schemes. Model validation rests on statistically and physically independent experiments: the macroscopic thermal conductivity values predicted by the multiscale homogenization model on the basis of tissue-independent (universal) phase conductivity properties of hemicellulose, cellulose, lignin, and water (input data set I) for tissue-specific data (input data set II) are compared to corresponding experimentally determined tissue-specific conductivity values (experimental data set).     

Effective moduli of multi-scale composites

The effective moduli of a multi-scale composite are evaluated by a bottom-up (hierarchical) modeling approach. We focus on a two-scale structure in which the small scale includes a platelet array inside a matrix, and the large scale contains fibers inside a composite matrix. We demonstrate that the principal moduli of the multi-scale composite can be fine-tuned by the platelet arrangement and orientation. As a case study, we consider the phenomenon of fiber micro-buckling within the multi-scale composite. It is found that the compressive micro-buckling strength can be considerably increased for specific platelet orientations. The multi-scale design approach presented here can be used to generate novel families of composite materials with tunable mechanical properties.     

Effects of anatomical and chemical properties of wood on the quality of particleboard

The objective of this study was to investigate the effects of anatomical and chemical structures of wood on the quality properties of particleboard containing different mixture of wood species. Urea–formaldehyde adhesive was used as a binder for manufacturing of test panels. Anatomical and chemical properties of wood species, and physical and mechanical properties particleboards were evaluated. The anatomical and chemical structures were found to be effective on the all of the properties of particleboards. Panels made from the particles including more amount of pine wood had highest mechanical strength properties and lowest thickness swelling values. Cellulose, hemicellulose and lignin contents, acidity and solubility values (in hot–cold water, dilute alkali and alcohol benzene) of wood significantly affected all of the properties of particleboards. The physical and mechanical properties of particleboards showed statistically differences related to the length, thickness and number of the cells and fibers.     

Effects of nonlinear strain components on the buckling response of stiffened shear-deformable composite plates

A detailed investigation of the weight of each non linear term of the Green–Lagrange strain displacement equation is presented, with reference to the buckling of orthotropic, both flat and prismatic, Mindlin plates. Usually in the literature, in buckling analysis only the second order terms related to the out-of-plane displacement are considered. Such heuristic simplification, known as von Kármán hypothesis, starts by the consideration that the buckling mode of a flat plate is described by dominant out-of-plane displacement and disregards the non-linear terms of the Green–Lagrange strain tensor depending on the in plane displacement components, whose role is confined to first order, say pre-critical, deformation. The present paper shows that disregarding the non linear terms related to the in-plane strain–displacement is equivalent to neglect shear induced rotation. In the work, the governing equations are derived using the principle of strain energy minimum and the differential equations solution is gained by using the general Levy-type method. The obtained results show that the von Kármán model overestimates the critical load when, in buckling mode, magnitudes of shear rotation, in-plane and out-of-plane displacements are comparable.     

Flexural stiffness modelling of RC slab strengthened by externally bonded FRP

This paper describes the behaviour of strengthened and unstrengthened reinforced concrete slabs, subjected to a single, specific, local, load at their centres. In the experimental stage, the influence of the reinforcement on the various slabs tested was analysed by studying their behaviour at failure and by making a bending stiffness analysis. In order to predict the mechanical behaviour of the slab, a model was developed to predict the value of the bending stiffness, the mid-span displacements and the strain on each material making up the slabs. The experimental results were compared to those of the model and there was good agreement.     

Design analysis of three-layered structural composites based on post-consumer recycled plastics and wood residues

Three-layered structural composites were produced from municipal plastic wastes and wood flour residues to investigate the effects of design parameters on their flexural and impact performance. The studied parameters include wood content, thickness of individual composite layers, as well as stacking sequence and configuration (symmetric and asymmetric structures). The results indicate that the core layer has a lower influence on the flexural properties of structural beams in comparison with the skins. But depending on beam configuration (stacking sequence), different flexural characteristics can be obtained using the same composite layers. The classical beam theory was used to predict the flexural modulus with high precision. In addition, performance of the beams under impact tests was shown to be independent from their stacking sequences and layer thicknesses for each configuration.     

Effect of TiO2 and nanoclay on the properties of wood polymer nanocomposite

High density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and poly(vinyl chloride) (PVC) with Phragmiteskarka wood flour (WF) and polyethylene-co-glycidyl methacrylate (PE-co-GMA) was used to develop wood polymer composite (WPC) by solution blending method. The effect of addition of nanoclay and TiO₂ on the properties of the composite was examined. The exfoliation of silicate layers and dispersion of TiO₂ nanopowder was studied by X-ray diffractometry and transmission electron microscopy. The improvement in miscibility among polymers due to addition of compatibilizer was studied by scanning electron microscopy (SEM). WPC treated with 3 phr each of clay and TiO₂ showed an improvement in thermal stability. Mechanical, UV resistance and flame retarding properties were also enhanced after the incorporation of clay/TiO₂ nanopowder to the composites. Both water and water vapor absorption were found to decrease due to inclusion of nanoclay and TiO₂ in WPC.     

Crosslinked natural rubber nanocomposites reinforced with cellulose whiskers isolated from bamboo waste: Processing and mechanical/thermal properties

Crosslinked natural rubber (NR) nanocomposites were prepared using cellulose nanowhiskers (CNWs) that were extracted from bamboo pulp residue of newspaper production, as the reinforcing phase. The coagulated NR latex containing bamboo nanowhiskers (master batch) was compounded with solid NR and vulcanizing agents using a two-roll mill and subsequently cured to introduce crosslinks in the NR phase. No evidence of micro-scaled aggregates of cellulose nanowhiskers in NR matrix was observed in Scanning Electron Microscopy (SEM) images. The addition of CNWs had a positive impact on the tensile strength, E-modulus, storage modulus, tan delta peak position and thermal stability of the crosslinked NR. Theoretical modeling of the mechanical properties showed a lower performance than predicated and therefore further process optimization and/or compatibilization are required to reach the maximum potential of these nanocomposites.     

A new concept of shock mitigation by impedance-graded materials

Over the last several decades, homogenous single-layer armor has been replaced by multi-layer integral armor to improve ballistic penetration resistance. This has led to better attenuation of shock wave energy by multiple interface reflections and transmissions. Efforts have been reported to improve the penetration resistance by providing higher energy dissipation at higher levels of impedance mismatch. However, high stress concentrations and stress reversals have made these interfaces the primary sources of failure. This paper discusses a concept for a new class of blast and penetration resistant (BPRM) materials which are layer-less but designed to have a continuous gradient of impedance that can dissipate the shock energy without material failure. In a simplistic approach by applying the classical theory of uniaxial stress propagation, it has been shown that attenuation of the stress wave energy would be possible by controlling the impedance distribution within the body of such a material. The development of such material to resist blast or impact will overcome the current common difficulty of interfacial delamination failure in any protective barrier system or armors.     

The effect of reprocessing on the mechanical properties of polypropylene reinforced with wood pulp,flax or glass fibre

Composites of polypropylene, substitutable for a given application and reinforced with: Medium Density Fibreboard fibre (MDF) (40 wt%); flax (30 wt%); and glass fibre (20 wt%), were evaluated after 6 injection moulding and extrusion reprocessing cycles. Of the range of tensile, flexural and impact properties examined, MDF composites showed the best mean property retention after reprocessing (87%) compared to flax (72%) and glass (59%). After 1 reprocessing cycle the glass composite had higher tensile strength (56.2 MPa) compared to the MDF composite (44.4) but after 6 cycles the MDF was stronger (35.0 compared to 29.6 MPa for the glass composite). Property reductions were attributed to reduced fibre length. MDF fibres showed the lowest reduction in fibre length between 1 and 6 cycles (39%), compared to glass (51%) and flax (62%). Flax fibres showed greater increases in damage (cell wall dislocations) with reprocessing than was shown by MDF fibres.     

Die drawn wood polymer composites. I. Mechanical properties

Oriented wood polymer composites (WPC) have been prepared by the Leeds die drawing process. Softwood and hardwood powder were used at 40% weight concentration (32% volume concentration) and in both cases materials with significantly increased stiffness (from 1.9 to 8.2 GPa) and strength (from 13 to 127 MPa) were obtained. Although the moduli of the drawn filled composites were lower than the equivalent unfilled polypropylene, the specific moduli, which take into account the lower density of the die drawn materials due to void formation were very similar. The type of wood particles and the use of polypropylene grafted with maleic anhydride had only a marginal influence on the mechanical properties of the die drawn composites. The morphology of the wood composites was studied by electron microscopy.     

Transverse shear stiffness of thickness gradient honeycombs

This paper describes the transverse shear stiffness of a novel topology of gradient honeycomb structures. Opposite to classical honeycomb configurations, gradient honeycombs feature elements of their unit cells with a regular geometry variation across the whole honeycomb panel. The tessellation of the cells is not periodic, but is dictated by geometric constraints between adjacent units. Gradient honeycombs with wall thickness linearly increasing along the panel are described using experimental data and Finite Element models. The gradient behaviour of the cellular structure provides additional complexity, and the possibility of tailoring design properties, such as the stiffness per unit of weight. We observe a good agreement between the Finite Element and the experimental results, with maximum percentage errors <7% for the shear moduli of the honeycombs.     

Relationship between wood elastic strain under bending and cellulose crystal strain

Wood is a natural composite material with a complex multi-scale structure. Its stiffness is mainly due to crystalline cellulose fibrils reinforcing the cell walls. In order to quantify the contribution of cellulose to wood elastic properties in both tension and compression, the change in cellulose (0 0 4) lattice spacing (cellulose crystal strain) was measured by X-ray diffraction during a bending test on poplar specimens. A detailed methodology is presented to accurately quantify this cellulose crystal strain. Results show that during elastic loading, cellulose crystal strain is roughly proportional to wood strain. The strain ratio (cellulose crystal strain/wood strain) was close to 0.75, and did not differ significantly in tension and compression. Interpretation of the strain ratio with respect to cellulose orientation shows that part of the wood strain occurs without inducing cellulose crystal strain. This contribution amounts to 10–15% of wood strain, and its possible origin at different levels of wood ultra-structure is discussed.