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Abstract

The development of sustainable sandwich materials is needed in the transportation sector to address environmental concerns related to the production and operation of vehicles. In addition to biobased composite skins, alternatives to classic synthetic core materials must be found to reduce the ecological footprint of whole sandwich-structured composites. This study focused on three eco-friendly lightweight core materials: balsa wood, paper honeycomb, and recycled PET foam. The effect of the hygrothermal ageing on their shear creep/recovery behaviour has been here investigated. Two different environmental conditions were tested: 23 °C-50% RH and 70 °C-65% RH. The results indicate that the maximum shear strain, the time-delayed strain and the residual strain increase for the three core materials with the severity of the hygrothermal conditions. This was attributed to the softening of the constitutive polymeric materials of the cell walls at temperatures close to 70 °C. The balsa wood exhibits the best creep resistance under the two environmental conditions. The identification of the viscoelastic properties highlights that the release times and the shear viscous parameters of the balsa wood and the PET foam depend on the stress level and the hygrothermal conditions.

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Composite materials have grown rapidly both in their applications and their economic importance, and they will no doubt continue to do so. With this growth has come increased attention in engineering curricula, but most coursework tends to focus on laminate theory and the analysis of composites, not on the practical design aspects most important to engineers. Composite Materials: Design and Applications fills that gap. Updated and translated from the successful French text Materiaux Composites, it offers comprehensive coverage of composites and their use in a broad range of applications. Part I provides a detailed introduction to composite materials, including fabrication processes, properties, design concepts, assembly, and applications. This section could also be used by itself in a course on advanced materials. Part II discusses elastic anisotopic properties, the directional dependence of different properties, and the mechanical properties of thin laminates. Alone, this section is suitable for a course on the mechanics of composite materials. Part III addresses the orthotropic coefficients needed for design activities, the Hill-Tsai failure criterion, the bending and torsion of composite beams, and the bending of thick composite plates. While somewhat more theoretical than the preceding chapters, it helps students better understand the behavior of composite parts. Part IV contains 41 detailed, numerical examples illustrating the design and use of composites. These are presented on three levels and cover the mechanical properties of composite structures in different forms, thermoelastic properties and failure analysis and the bonding of cylinders, sandwich beam buckling and flexure shear, and vibrations in composite plates. Clearly written and filled with more than 500 illustrations, Composite Materials: Design and Applications forms an outstanding textbook for senior undergraduate and beginning graduate-level course work-one that can make a significant contribution to the training of future engineers.
Chapter
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Thesis
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This paper presents an experimental and analytical study about the effect of temperature on the shear creep response of a rigid polyurethane (PUR) foam within the scope of sandwich panel application in building floors. Shear creep tests were carried out on a foam (87.4 kg/m3) subjected to shear stress levels of 11%, 22% and 44% of its shear strength at temperatures of 20 °C, 24 °C and 28 °C – a range likely to be found in the envisaged application – for more than 1300 h. The results obtained show that the foam’s creep response increases with both stress level and temperature. Findley’s power law, extended to include Arrhenius equations describing the temperature dependency of its viscoelastic parameters, was fitted to the experimental creep curves, thus allowing to model the time-temperature-stress dependent shear creep behaviour of the PUR foam. The proposed model provided a good fit to the experimental creep curves within the linear viscoelastic range. Practical design equations were also derived for the time-temperature dependent (i) shear modulus, (ii) creep coefficient, and (iii) shear modulus reduction factor. Finally, the time-temperature-stress superposition principle (TTSSP) and the time-stress superposition principle (TSSP) were used to yield “master curves” that compared well with the proposed model’s predictions.
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Moisture in wood acting as a plasticizer will strongly affect the wood's viscoelastic properties. However, achieving the desired moisture content (MC) at elevated temperatures during creep tests is difficult. The aim of this study is to accurately and systematically investigate the creep behavior of birch wood at high temperatures. Experiments were conducted using a dynamic mechanical analyzer with a relative humidity accessory coupled with polyvinylidene chloride (PVDC) film for wrapping samples. Creep behavior was examined at six MCs (0%, 6%, 12%, 18%, 24%, >30%) and 11 temperatures (5 to 105 ° C). The MC of wood was strictly and accurately controlled during creep tests. Instantaneous compliance (IC) and creep compliance (CC) increased with the increase of both temperature and MC, with significant changes at higher temperatures and MCs. The effects on IC and CC were more pronounced when the subject was influenced by MC, with readings approximately three times and one time greater than those influenced by temperature, respectively. Dramatic increases in CC were found at certain temperatures and MC values. There was a complex interaction between temperature and MC on IC and CC.
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In this study, the glass transition behavior and mechanical properties of poly(ethylene terephthalate) (PET), PET/silica nanocomposite and PET/hydroxylated silica nanocomposite were studied through molecular dynamics (MD) simulations. It was found that the density and the specific volume of PET, PET/silica nanocomposite and PET/hydroxylated silica nanocomposite regularly changed along with the changes of temperature and the transition occurs at the Tg point, the simulation results show the addition of nanosilica decreased Tg of PET in PET/silica nanocomposite, but the addition of hydroxylated silica increased Tg of PET in PET/hydroxylated silica nanocomposite,and the underlying mechanism resulting in the change of the glass transition temperature of nanocomposite was discussed. Moreover, the thermal and mechanical properties of the systems were characterized, The MD simulations showed that the addition of silica particle increases Young's modulus, bulk modulus, Poisson's ratio, lame constants and compressibility of PET/silica nanocomposite in comparison with those in the pure PET system.
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In the paper at hand, a modelling approach for the simulation of the anisotropic long-term behaviour of wooden structures subjected to mechanical loading at constant moisture content for the use within the framework of the Finite Element Method is presented. Using a standard-solid body-model in serial combination with a Bingham-element allows for the differentiation between linear viscoelastic and non-linear viscoelastic–viscoplastic behaviour with respect to the applied stress level. Creep failure is considered by means of the concept of strain-energy density. Due to the cylindrical anisotropy of wood, the four material properties required for the description of the creep behaviour are determined depending on material direction and loading type (tension, compression, shear) by means of the re-calculation of experimental results. Finally the approach is applied to the re-calculation of two test-series in bending and to the analysis of a face staggered joint. The paper at hand is the first part of a more complex model and deals with the mechanical long-term behaviour of wood. In the second part, the hygro-mechanical coupling is covered with respect to the consideration of the influence of moisture content on mechanically induced creep and mechano-sorptive effects caused by simultaneous mechanical loading and moisture changes.
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Mechanical properties and global stability of foam core sandwich structures are highly controlled by the shear response of the core material. In this work, we have studied the shear deformations of three common structural core materials with the aid of full-field optical analysis. The chosen core materials are namely extruded PET foam (ρ=105kg/m 3 , G xz =21MPa,) and cross-linked PVC foam (ρ=60kg/m 3 , G xz =22MPa) which have comparable shear properties, as well as Balsa wood with the lowest density commercially available (ρ=94kg/m 3 , G xz =106MPa) as a reference core material. Both global and local shear strains in the core materials are calculated and graphically visualized. In the elastic region, foam cores showed more uniform deformations than Balsa. Yielding and shear failure of the two foam core materials were quite different. The PVC foam experienced a high local deformation under the load introduction bars, from which sub-interface shear failure initiated. The PET foam, in contrast, showed no sign of stress concentrations, resulting in a homogenous evolution of shear deformations in the mid-core regions. A comparison between the direct foam shear test and sandwich specimen bending suggested that the former method might not be capable of capturing a full picture of the in-service core shear response.
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Balsa, with its low density and relatively high mechanical properties, is frequently used as the core in structural sandwich panels, in applications ranging from wind turbine blades to racing yachts. Here, both the cellular and cell wall structure of balsa are described, to enable multi-scale modeling and an improved understanding of its mechanical properties. The cellular structure consists of fibers (66–76 %), rays (20–25 %) and vessels (3–9 %). The density of balsa ranges from roughly 60 to 380 kg/m3; the large density variation derives largely from the fibers, which decrease in edge length and increase in wall thickness as the density increases. The increase in cell wall thickness is predominantly due to a thicker secondary S2 layer. Cellulose microfibrils in the S2 layer are highly aligned with the fiber axis, with a mean microfibril angle (MFA) of about 1.4°. The cellulose crystallites are about 3 nm in width and 20–30 nm in length. The degree of cellulose crystallinity appears to be between 80 and 90 %, considerably higher than previously reported for other woods. The outstanding mechanical properties of balsa wood in relation to its weight are likely explained by the low MFA and the high cellulose crystallinity.
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Balsa wood is one of the preferred core materials in structural sandwich panels, in applications ranging from wind turbine blades to boats and aircraft. Here, we investigate the mechanical behavior of balsa as a function of density, which varies from roughly 60 to 380 kg/m3. In axial compression, bending, and torsion, the elastic modulus and strength increase linearly with density while in radial compression, the modulus and strength vary nonlinearly. Models relating the mechanical properties to the cellular structure and to the density, based on deformation and failure mechanisms, are described. Finally, wood cell-wall properties are determined by extrapolating the mechanical data for balsa, and are compared with the reduced modulus and hardness of the cell wall measured by nanoindentation.
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An experimental study was performed using Iosipescu specimens to evaluate the effects of parameters such as shear plane, density and adhesive joints on the shear stiffness and strength of balsa wood panels as well as the variation of ductility with respect to the shear planes. The shear planes exerted a significant effect on shear stiffness and strength. Highest values were obtained for the shear plane parallel to the end grain, intermediate values for the plane parallel to the flat grain and lowest values for the plane transverse to the flat grain. Shear stiffness and strength increased with increasing density of the balsa. The thin adhesive joints in the balsa panels between the lumber blocks increased the shear stiffness and strength with one exception. The strength of specimens with the shear plane transverse to the flat grain was reduced because of a change in the failure mode. Due to plastic deformations in the tracheids, specimens with shear planes parallel to the end grain and transverse to the flat grain exhibited significant ductility. The ductility of specimens with the shear plane parallel to the flat grain was less pronounced as it was affected by the relatively brittle lignin material of the middle lamella.
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In this chapter, some recent studies on moisture transport processes in paper materials are reviewed. The equilibrium aspect of moisture interaction with paper shows significant hysteresis which can be estimated by an application of Everett’s theory of independent domain complexions. Thus, when a paper sheet is subjected to arbitrary cycles of humidity all the while allowed to reach equilibrium at each state, the sheet’s moisture content evolution may be predicted by an analysis of the sorption isotherms and the interior of the sorption hysteresis loop. It is shown that the theoretical predictions of equilibrium moisture content are in good agreement with experimentally determined values.
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Commercial poly(ethylene terephthalate) (PET) was treated at R. H.>80% and room temperature for a set time. The glass transition temperature (Tg) decreases with the time of exposure to high humidity. The decrease in Tg is a result of plasticization. Our data indicate that the Tg of dry PET of 76-78°C may decrease to as low a temperature as 65-67°C when it is wet. Induced crystallization of PET in the presence of water reduces the cold crystallization temperature (Tc). The structure of water-induced crystals is imperfect and can be improved in perfection by annealing.
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Balsa wood is a natural cellular material with excellent stiffness-to-weight and strength-to-weight ratios as well as superior energy absorption characteristics. These properties are derived from the microstructure, which consists of long slender cells (tracheids) with approximately hexagonal crosssections that are arranged axially. Parenchyma are a second type of cells that are radially arranged in groups that periodically penetrate the tracheids (rays). Under compression in the axial direction the material exhibits a linearly elastic regime that terminates by the initiation of failure in the form of localized kinking. Subsequently, under displacement-controlled compression, a stress plateau is traced associated with the gradual spreading of crushing of the cells through the material. The material is less stiff and weaker in the tangential and radial directions. Compression in these directions crushes the tracheids laterally but results in a monotonically increasing response typical of lateral crushing of elastic honeycombs. The elastic and inelastic properties in the three directions have been established experimentally as a function of the wood density. The microstructure and its deformation modes under compression have been characterized using scanning electron microscopy. In the axial direction it was observed that in the majority of the tests, failure initiated by kinking in the axial–tangential plane. The local misalignment of tracheids in zones penetrated by rays ranged from 4° to 10° and axial compression results in shear in these zones. Measurement of the shear response and the shear strength in the planes of interest enabled estimation of the kinking stress using the Argon–Budiansky kinking model. The material strength predicted in this manner has been found to provide a bounding estimate of the axial strength for a broad range of wood densities. The energy absorption characteristics of the wood have also been measured and the specific energy absorption was found to be comparable to that of metallic honeycombs of the same relative density.
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Binary and ternary copolymers of poly(ethylene terephthalate) (PET), poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) and poly(p-hydroxybenzoic acid) (PHB) were synthesized and dynamic mechanical measurements at temperatures ranging from −140°C up to the melting point were performed. A linear dependence of the glass transition temperature of the isotropic materials on composition was obtained. Copolymers containing more than about 30 mol% PHB are partly liquid-crystalline, and those containing more than 50 mol% PHB are completely liquid-crystalline. The PET/PEN/PHB ternary copolyester containing equal parts of each of the three components can be obtained at room temperature as a liquid-crystalline glass and as an isotropic glass, depending on thermal history. The glass transition temperature Tg of the material in the liquid-crystalline state was found to be about 30°C lower than that of samples in the isotropic state. By extrapolation to 100 mol% PHB, a glass transition temperature of about 120°C was estimated for the PHB homopolymer. Samples containing more than 70 mol% PHB show an additional peak in tan σ, which is attributed to a relaxation process in a disturbed hexagonal crystalline phase. At temperatures below Tg in all copolymers investigated several β relaxation maxima were observed. The temperature positions of these maxima do not depend on the composition.
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Poly(ethylene terephthalate) (PET) is known to be a hygroscopic thermoplastic, which absorbs moisture from its environment at a rapid rate. The water absorption characteristics of PET as a function of relative humidity, exposure time, temperature, thickness, and molecular weight are reported here. Results indicate that absorbed moisture has significant influences on the physical properties of PET, leading to large decreases in the glass transition temperature, crystallization temperature, and degree of molecular orientation.
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Dilation of polysulfone (PSUL) and crystalline poly(ethylene terephthalate) (PET) films accompanying sorption of carbon dioxide is measured by a cathetometer under high pressure up to 50 atm over the temperature range of 35–65°C. Sorptive dilation isotherms of PSUL are concave and convex to the pressure and concentration axes, respectively, and both isotherms exhibit hysteresis. Each dilation isotherm plotted versus pressure and concentration for the CO2-PET system shows an inflection point, i.e., a glass transition point, at which the isotherm changes from a nonlinear curve to a straight line. Dilation isotherms of PET below the glass transition point are similar to those of the CO2-PSUL system, whereas the isotherms above the glass transition point are linear and exhibit no hysteresis. Partial molar volumes of CO2 in these polymers are determined from data of sorptive dilation. On the basis of the extended dual-mode sorption model and the current data, primitive equations for gas-sorptive dilation of glassy polymers are proposed.
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Although not the lightest known wood, balsa is the lightest commercial timber, and as such has found utilization in the manufacture of airplanes, life preservers, insulating equipment and packing crates. During recent war years Ecuador supplied 95 percent of world production but must now meet competition from wild and cultivated sources in Central America and Ceylon.
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In the present paper the environmental impact of biocomposites and bio-sandwich materials production are evaluated, using simplified Life Cycle Analysis (LCA) following the procedure recommended in the ISO 14044 standard. The materials are dimensioned and evaluated by comparing with reference materials, glass mat reinforced unsatured polyester and glass mat/unsatured polyester/balsa sandwich. The results indicate that bio-sandwich materials are very attractive in terms environmental impact. However further improvements in biocomposite and bio-sandwich mechanical strength are necessary if they are to be used in transport application compared to glass/polyester and glass/polyester/balsa sandwich. KeywordSandwich–Natural fibre–Biopolymer–Balsa–Simplified Life Cycle Analysis (SLCA)
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In this work several numerical techniques for modelling the transverse crush behaviour of honeycomb core materials were developed and compared with test data on aluminium and Nomex™ honeycomb. The methods included a detailed honeycomb micromechanics model, a homogenised material model suitable for use in FE code solid elements, and a homogenised discrete/finite element model used in a semi-adaptive numerical coupling (SAC) technique. The micromechanics model is shown to be suitable for honeycomb design, since it may be used to compute crush energy absorption for different honeycomb cell sizes, cell wall thicknesses and cell materials. However, the very fine meshes required make it unsuitable for analysis of large sandwich structures. The homogenised FE model may be used for such structures, but gives poor agreement when failure is due to core crushing. The SAC model is shown to be most appropriate for use in structural simulations with extensive compression core crushing failures, since the discrete particles are able to model the material compaction during local crushing.
Article
A meso–macro modelling is proposed for laminates made of unidirectional layers of a polymer matrix reinforced with long fibres. The time-independent behaviour introduced in the first part of this article is improved herein to account for viscous phenomena through viscoelasticity and viscoplasticity. A spectrum-type viscoelastic model is considered, which is based on the definition of elementary viscoelastic mechanisms. Its mathematical formulation is simplified by using a relaxation times triangular layout. A generalised Norton-type model integrating the elastic domain concept is used to report the plastic strains delay. A zero-valued “dynamic” yield function is incorporated into the traditional viscoplastic format, allowing a same treatment of plastic and viscoplastic problems. The integration of the layer behaviour through the thickness is obtained within a Kirchhoff shell finite element. The constitutive equations are integrated using two families of algorithms that generalise the well-known trapezoidal and mid-point rules, for which accuracy and non-linear stability analysis are carried out. Significant robustness of the local iterative solution is provided by complementing the basic Newton’s scheme with a local line-search strategy. In the case of a fully coupled plastic–viscoplastic behaviour, the local Newton’s iterative scheme is associated with a grid-search method in order to define available initial solutions. A perturbation technique is suggested to evaluate an algorithmic tangent operator since the viscoelasticity renders non-trivial an explicit determination of a consistent tangent operator. The proposed formulation has been implemented in the finite element code CASTEM2000® in order to test its validity. The obtained results are compared with semi-analytical ones in the case of progressive repeated loading tests by applying pure traction and pure pressure. Creep tests are also considered.
Effect of Absorption of Liquids on the Mechanical Properties of Balsa Wood
  • J A Stahlhut
Bhangu SS. Effect of Moisture Absorption on the Mechanical Properties of Balsa Wood. ADFA J Undergrad Engeenering Res 2012;5:11.