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Effect of hygrothermal ageing on the shear creep behaviour of eco-friendly sandwich cores
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.
In the present study, both the impact and vibration response characteristics of fiber reinforced polymer (FRP) sandwich plates (FRPSPs) with a foam core (FC) reinforced by chopped fiber rods (CFRs) are investigated. Initially, a dynamic model is proposed to predict the vibro-impact characteristics of the FC-CFRs-FRPSP structures. Here, by considering the influence of the dispersion pattern of chopped fiber rods, the equivalent material parameters of this complex core are redefined. Also, by employing the modified mid-plane displacement equation based on the principle of virtual work at the impact point, together with the progressive quasi-static approach and Reddy's high-order shear deformation principle, the key parameters such as impact contact force and displacement related to each failure event are obtained. In addition, with consideration of the impact damage, the natural frequencies and vibration responses are solved by using the energy principle, the proportional damping method, and the Duhamel integral technique. Finally, a number of the FC-FRPSP specimens with and without reinforcement of CFRs are fabricated and tested under different impact energies for verifying the present model. Also, the influences of critical geometric and material parameters on the vibro-impact response of the FC-CFRs-FRPSP structures are discussed. It has been found that the FC-CFRs-FRPSP specimens have a smaller vibro-impact response compared to the specimens without CFRs. For example, when impact energy of 10 J is applied, the impact peak force increases by 19.1%, the impact displacement reduces by 26.6%, and the maximum time-domain vibration response reduces by 5.8%. To reduce the vibro-impact response of such structures, it is recommended to adopt a large thickness ratio of the CFR reinforced foam core to the overall sandwich plate as well as volume fraction of CFRs to the foam core with a uniform dispersion pattern.
The interest in using natural fiber-reinforced composites is now at its highest. Numerous studies have been conducted due to their positive benefits related to environmental issues. Even though they have limitations for some load requirements, this drawback has been countered through fiber treatment and hybridization. Sandwich structure, on the other hand, is a combination of two or more individual components with different properties, which when joined together can result in better performance. Sandwich structures have been used in a wide range of industrial material applications. They are known to be lightweight and good at absorbing energy, providing superior strength and stiffness-to-weight ratios, and offering opportunities, through design integration, to remove some components from the core element. Today, many industries use composite sandwich structures in a range of components. Through good design of the core structure, one can maximize the strength properties, with a low density. However, the application of natural fiber composites in sandwich structures is still minimal. Therefore, this paper reviewed the possibility of using a natural fiber composite in sandwich structure applications. It addressed the mechanical properties and energy-absorbing characteristics of natural fiber-based sandwich structures tested under various compression loads. The results and potential areas of improvement to fit into a wide range of engineering applications were discussed.
This paper investigates the influence of the stress level and hygrothermal conditions on the creep/recovery behaviour of three high-grade composites made of GreenPoxy and flax and hemp fibres. The results show that the levels of instantaneous, time-delayed and residual strains increase with the applied load and the severity of the environment. The time-delayed strains of the materials are higher in the creep phase than in the recovery phase. A stiffening effect is also observed during the recovery phase. The post-creep viscoelastic properties are then identified using an anisotropic viscoelastic law from the recovery function. The relaxation time function is independent of the stress level and the environmental conditions. The viscous parameter varies with the stress level and increases substantially with severe environmental conditions. The dependence of the creep/recovery behaviour on the stress level is due to the dependence of the stiffening phenomenon and irreversible mechanisms rather than to the viscoelastic behaviour.
This work describes the bending and shear mechanical properties of a novel concept of sustainable sandwich panel made from unidirectional prepreg flaxtape skins and bamboo rings as a circular core material. A Design of Experiment (DoE) is used to determine the influence of the bamboo diameter and the type of adhesive bonding between core and skins on the equivalent density, flexural and shear properties of these panels. Numerical analysis are also performed using cohesive surface contacts between skins and bamboo rings to investigate the structural behaviour and failure mechanisms of the sandwich panels. The equivalent density is affected by both factors, with an overall decrease when larger bamboo rings and lower density adhesive are used. Although sandwich panels with the larger bamboo rings as core show superior flexural properties and skin stress, smaller bamboo rings cores shows an increase in the core transverse shear modulus. The physical and mechanical characteristics of the adhesives directly affect the failure mode and the overall structural integrity of the panels.
This paper presents a review of the issues concerning sandwich structures for aeronautical applications. The main questions raised by designers are first recalled and the complexity of sandwich structure design for aeronautics is highlighted. Then a review of applications is presented, starting with early examples from the 1930s and the Second World War. The growth in the use of sandwich materials in civil and military applications is then developed. Recent research and innovations conclude the paper.
The tensile, compressive and shear behavior of standardized specimens cut from a veneered balsa wood used as structural sandwich core was investigated at ambient temperatures of up to 250 °C. The average moisture content was 9.1%. The specimens’ responses were strongly affected by the configuration of the veneer layers. Specimens with a higher number of 0° veneer layers in the loading direction exhibited higher strength and stiffness at each temperature. Independent of the loading type, the specimens gradually lost strength and stiffness up to the wood-burning temperature of 250 °C due to the softening of the hemicellulose and lignin. Smaller cross sections with more surfaces cut perpendicular to the grain dried faster and thus delayed the degradation of properties. The dominant failure modes did not change with increasing temperature if the behavior remained either fiber- or matrix-dominated. The failure mode changed if the behavior shifted from fiber-dominated at lower temperatures to matrix-dominated at higher temperatures. The degradation of the properties of the adhesive between the veneer layers affected specimen behavior only at the highest temperature.
Considering the major role played by sandwich structures in many fields where high stiffness-to-weight ratio is required, the selection of a suitable core material is of paramount importance. In order to face the environmental problems related to waste disposal, the selection of an eco-friendly core material is now included in the design criteria of sandwich structures. Agglomerated cork is recognized as a good solution that combines satisfactory mechanical performances and eco-sustainability. Many research studies individually addressed cork’s morphological, thermal, and mechanical features without providing a comprehensive overview of the relationships that exist between them. In this work, the investigation of the peculiar cork morphology allowed learning more about its good insulation capacity and its impressive recovery capability. The use of dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) clarified the influence of temperature on both flexural and compressive performances. The effect of testing parameters such as temperature and speed on agglomerated cork properties was validated through statistical analysis. Moreover, to highlight agglomerated cork advantages and drawbacks, the work provides also a comparison with more traditional polyvinylchloride (PVC) foams commonly used in industrial applications.
Paper is a fascinating material that we encounter every day in different variants:
tissues, paper towels, packaging material, wall paper or even fillers of doors. Despite
radical changes in production technology, the material, which has been known to
mankind for almost two thousand years, still has a natural composition, being made
up of fibres of plant origin (particularly wood fibres). Thanks to its unique properties,
relatively high compression strength and bending stiffness, low production costs and
ease of recycling, paper is becoming more and more popular in many types of industry.
Mass-produced paper products such as special paper, paperboard, corrugated
cardboard, honeycomb panels, tubes and L- and U-shapes are suitable for use as a
building material in the broad sense of these words – i.e., in design and architecture.
Objects for everyday use, furniture, interior design elements and partitions are just a
few examples of things in which paper can be employed. Temporary events such as
festivals, exhibitions or sporting events like the Olympics require structures that only
need to last for a limited period of time. When they are demolished after a few days or
months, their leftovers can have a significant impact on the local environment.
In the context of growing awareness of environmental threats and the efforts
undertaken by local and international organisations and governments to counter
these threats, the use of natural materials that can be recycled after their lifespan is
becoming increasingly widespread.
Paper and its derivatives fascinate designers and architects, who are always looking
for new challenges and trying to meet the market’s demands for innovative and proecological
solutions.
Being a low-cost and readily available material, paper is suited to the production of
emergency shelters for victims of natural and man-made disasters, as well as homeless
persons.
In order to gain a better understanding of paper’s potential in terms of architecture, its
material properties were researched on a micro, meso and macro level. This research of
the possible applications of paper in architecture was informed by two main research
questions:
- What is paper and to what extent can it be used in architecture?
- What is the most suitable way to use paper in emergency architecture?
To answer the first research question, fundamental and material research on paper
and paper products had to be conducted. The composition of the material, production
methods and properties of paper were researched. Then paper products with the
potential to be used in architecture were examined. The history of the development of
paper and its influence on civilisation helped the author gain a better understanding
of the nature of this material, which we encounter in our lives every day. Research
on objects for everyday use, furniture, pavilions and architecture realised in the last
150 years allowed the author to distinguish various types of paper design and paper
architecture. Analysis of realised buildings in which paper products were used as
structural elements and parts of the building envelope resulted in a wide array of
possible solutions. Structural systems, types of connections between the various
elements, impregnation methods and the functionalities and lifespan of different
types of buildings were systematised. The knowledge thus collected allowed the author
to conduct a further exploration of paper architecture in the form of designs and
prototypes.
To answer the second research question, the analysed case studies were translated into
designs and prototypes of emergency shelters.
During the research-by-design, engineering and prototyping phases, more than a
dozen prototypes were built. The prototypes differed in terms of structural systems,
used materials, connections between structural elements, impregnation methods,
functionality and types of building. The three versions of the Transportable Emergency
Cardboard House project presented in the final chapter form the author’s final answer
to the second research question.
Paper will never replace traditional building materials such as timber, concrete, steel,
glass or plastic. It can, however, complement them to a significant degree.
This paper investigates the environmental impact of cardboard, investigating its integration into buildings. It is part of a broader research, which aims to select a set of criteria in order to analyse the environmental sustainability of cardboard as a building material, assessing its production, its characteristics and its integration into buildings. As factors such as the embodied energy and greenhouse gases emissions, should not be considered in isolation, but rather in the context of the total energy needs of a building, the focus of this paper is the energy efficiency of cardboard in buildings. As part of the investigation, buildings that use cardboard as their primary building material are analysed in order to identify the main environmental issues associated with its use in buildings. The analysis consisted in literature review, a field study survey and an analysis of the information gained. The results made it possible to compare and analyse different approaches to using cardboard in building construction and during occupational phases. During the occupational phase, the energy in use for thermal comfort (heating and cooling) was studied using simulations software in order to compare cardboard with typical construction materials in each of the case studies countries. Moreover, the energy in use is compared with the embodied energy of cardboard in order to understand potential energy savings during the life cycle of the material. In some cases, improvements were suggested in order to inform and improve the environmental sustainability of cardboard. The results of this paper show that cardboard buildings, compared to traditional building materials, can result in more energy efficient buildings.
In restoration plantings in degraded pastures, initial soil nutrient status may lead to differential growth of tropical tree species with diverse life history attributes and capacity for N 2 fixation. In 2006, we planted 1,440 seedlings of 15 native tree species in 16 fenced plots (30 × 30 m) in a 60-year-old pasture in Los Tuxtlas, Veracruz, Mexico, in two planting combinations. In the first year, we evaluated bulk density, pH, the concentration of organic carbon (C), total nitrogen (N), ammonia (NO − 3), nitrate (NH + 4), and total phosphorus (P) in the upper soil profile (0–20 cm in depth) of all plots. The first two axes of two principal component analyses explained more than 60% of the variation in soil variables: The axes were related to increasing bulk density, NO − 3 , NH + 4 , total N concentration, and pH. Average relative growth rates in diameter at the stem base of the juvenile trees after 6 years were higher for pioneer (45.7%) and N 2-fixing species (47.6%) than for nonpioneer (34.7%) and nonfixing species (36.2%). Most N 2-fixing species and those with the slowest growth rates did not respond to soil attributes. Tree species benefited from higher pH levels and existing litter biomass. The pioneers Ficus yoponensis, Cecropia obtusifolia, and Heliocarpus appendiculatus, and the N 2-fixing nonpioneers Cojoba arborea, Inga sinacae, and Platymiscium dimorphandrum were promising for forest restoration on our site, given their high growth rates.
Finite element (FE) models for the flexural creep of the sandwich panels with various Kraft paper honeycomb cores and wood composite skins were established. The creep constants of these FE models' core and skin were determined by simulating the experimental results of the flexural creep of the corresponding skin layer and sandwich panels individually. The influence of the core orientation, core shape, core and skin thickness, and core cell size was studied using these established FE models. The results indicated that the panel's flexural creep in the primary stage was smaller when the panel was thinner, longer, and wider as well as when the shelling ratio (thickness ratio between panel's core layer and skin layer) was smaller. The panel that had a higher stiffness skin layer, or the core's ribbon direction was parallel to the panel span, or was loaded at a lower level had a smaller flexural creep. However, there was no observed influence of honeycomb core's cell size on the flexural creep behavior of the sandwich panels.
This work investigates the flexural behavior of a composite sandwich made of flax fibers reinforced skin facings and an agglomerated cork core, to be employed as an eco-friendly solution for the making of structural components of small sailing boats. An experimental mechanical characterization of the strength and stiffness flexural behavior of the proposed sandwich is carried out, providing a comparison of performances from three implemented assembling techniques: hand-lay-up, vacuum bagging and resin infusion. Sandwich beams have been tested under three point bending (TPB) at various span lengths. A procedure is also proposed and implemented to consider the potential influence of the local elastic indentation in the experimental evaluation of the flexural stiffness. This procedure is based on the analytical solution of an indented beam resting on a fully backed Winkler foundation.
L’objectif de cette étude est de proposer, pour les composites stratifiés, un indicateur de confiance caractéristique de leur fiabilité.
Pour obtenir un résultat pertinent, il est indispensable de disposer d’un modèle de comportement mécanique le plus complet possible. Dans ce travail, l’approche méso-macro mise en oeuvre permet de prédire, à partir de la modélisation du pli élémentaire, le comportement visco-élastoplastique endommageable de n’importe quel empilement.
Etant donnée sa complexité, ce type de modèle réclame, pour l’identification des paramètres, la réalisation d’essais multiaxiaux et l’utilisation d’un algorithme numérique adapté : la procédure développée, en combinant les avantages des algorithmes génétiques et des méthodes locales, permet d’éviter les optima locaux. L’éprouvette utilisée comme exemple de travail est un stratifié tubulaire verre-époxy d’empilement [±55]12.
L’approche retenue pour définir l’indicateur de confiance repose sur la notion de probabilité de défaillance. La méthode utilisée (FORM : first order reliability method) permet de définir un indice de fiabilité qui lui est directement relié. Malheureusement, compte tenu du manque d’informations expérimentales et de la difficulté d’identification des paramètres, cet indice est incertain. La stratégie utilisée pour évaluer un indicateur fiable consiste à déterminer le cas le plus défavorable dans le domaine de possibilité des
paramètres statistiques. L’indice de fiabilité minimum ainsi calculé est caractéristique de la confiance que l’on peut raisonnablement accorder au stratifié, étant donnés les scénarios de défaillance considérés et les connaissances disponibles.
In order to understand the influence of the thicknesses of Kraft paper honeycomb core and medium density fiberboard skins on the stiffness of the sandwich panel, the corresponding finite element models for the resulting sandwich panels were developed. The material properties for the core and skin components of these finite element models were determined using the published data and specifications. It was found that a decrease in the thickness ratio of the core to skin layer (shelling ratio) resulted in an increase in the modulus of elasticity and shear modulus of the sandwich panels. The increase was significant when the shelling ratio was smaller than six. Cell size only affected the modulus of elasticity of the sandwich panels under the flat-wise compression and panel’s inter-laminar shear modulus. Regression equations relating the stiffness of the sandwich panels to the shelling ratio and core cell size were obtained using the finite element model simulated results and were found to compare well with the existing models for layered wood composites.
Today mechanical requirements and weight targets demand a lightweight sandwich design in many application areas. The potential of honeycomb sandwich construction in furniture applications is, like in many other application areas mainly determined by the production cost of cores and panels. In the last decade the traditional honeycomb production processes for low cost paper honeycomb cores have been optimised towards concepts with a fully automated continuous in-line sandwich panel production. Sandwich selection charts, a graphical presentation of the effects of sandwich constructions on weight and cost are shown for paper honeycomb sandwich panels. This allows a transparent selection and comparison of sandwich materials for furniture applications. It can be shown that a paper honeycomb sandwich panel can offer cost savings in comparison to uniform chipboard panels.
Measurement of heat, air and moisture (HAM) phenomena in building assemblies under both controlled conditions and field conditions are difficult to achieve with uniform accuracy and reliability. Care is needed in selecting the measurement sensors and instrumentation to achieve an acceptable degree of accuracy. When the experiments are planned to answer specific questions or to confirm expected responses, the degree of accuracy needed is pre-determined. Proper calibration coupled with appropriate selection of materials can improve the reliability of the measurements, enhance the accuracy achieved, and ensure that the installation has the durability to survive for as long as is needed by the experiment.
The field and laboratory experience of the authors in undertaking HAM measurements, particularly those involving transient conditions arising from exposure to real weather, are the basis for the recommendations provided here. The limitations in both undertaking certain measurements and in the interpretation of some data are addressed. The complex of interactions related to the driving forces and changes in material properties prevents experimenters from attributing certain outcomes to particular theoretical assumptions. However, field studies are complementary to carefully executed laboratory studies. As the accuracy of theory increases, there will be an increased need for detailed and accurate field measurements.
This article presents an experimental study into thermal softening and thermal recovery of the compression strength properties
of structural balsa wood (Ochroma pyramidale). Balsa is a core material used in sandwich composite structures for applications where fire is an ever-present risk, such
as ships and buildings. This article investigates the thermal softening response of balsa with increasing temperature, and
the thermal recovery behavior when softened balsa is cooled following heating. Exposure to elevated temperatures was limited
to a short time (15 min), representative of a fire or postfire scenario. The compression strength of balsa decreased progressively
with increasing temperature from 20° to 250°C. The degradation rates in the strength properties over this temperature range
were similar in the axial and radial directions of the balsa grains. Thermogravimetric analysis revealed only small mass losses
(<2%) in this temperature range. Environmental scanning electron microscopy showed minor physical changes to the wood grain
structure from 190° to 250°C, with holes beginning to form in the cell wall at 250°C. The reduction in compression properties
is attributed mostly to thermal viscous softening of the hemicellulose and lignin in the cell walls. Post-heating tests revealed
that thermal softening up to 250°C is fully reversible when balsa is cooled to room temperature. When balsa is heated to 250°C
or higher, the post-heating strength properties are reduced significantly by decomposition processes of all wood constituents,
which irreversibly degrade the wood microstructure. This study revealed that the balsa core in sandwich composite structures
must remain below 200°–250°C when exposed to fire to avoid permanent heat damage.
Key wordsBalsa-Thermal-Mechanical properties-Decomposition
The viscoelastic properties of wood have been investigated with a dynamic mechanical analyser (DMA) specifically conceived for wooden materials, the WAVET device (environmental vibration analyser for wood). Measurements were carried out on four wood species in the temperature range of 0°C to 100°C at frequencies varying between 5 mHz and 10 Hz. Wood samples were tested in water-saturated conditions, in radial and tangential directions. As expected, the radial direction always revealed a higher storage modulus than the tangential direction. Great differences were also observed in the loss factor. The tan peak and the internal friction are higher in tangential direction than in radial direction. This behaviour is attributed to the fact that anatomical elements act depending on the direction. Viscoelastic behaviour of reaction wood differs from that of normal or opposite wood. Compression wood of spruce, which has higher lignin content, is denser and stiffer in transverse directions than normal wood, and has lower softening temperature (Tg). In tension wood, the G-layer is weakly attached to the rest of the wall layers. This may explain why the storage modulus and the softening temperature of tension wood are lower than those for the opposite wood. In this work, we also point out that the time-temperature equivalence fits only around the transition region, i.e. between Tg and Tg + 30°C. Apart from these regions, the wood response combines the effect of all constitutive polymers, so that the equivalence is not valid anymore.
En génie civil, les matériaux polymères utilisés pour la fabrication des géotextiles doivent présenter de bonnes propriétés mécaniques et être stables chimiquement, afin de pouvoir résister durablement à des contraintes mécaniques et environnementales potentiellement importantes. Le choix des géotextiles utilisés pour des applications telles que le renforcement des sols s'est porté entre autres sur des polyesters (les polyéthylènes téréphtalates) car ces matériaux présentent des propriétés mécaniques initiales appropriées à ce type d'applications. Cependant, dans certains cas, des risques importants de défaillances peuvent apparaître à long terme, en particulier en milieu fortement alcalin, phénomène pouvant être accentué à des pH moins agressifs par l'action de contraintes couplées (température, niveau de contraintes mécaniques). Cette étude bibliographique a pour objectif d'effectuer un état de l'art sur la durabilité des polyéthylènes téréphtalates et de déterminer les valeurs critiques relatives à chaque paramètre, afin d'en informer les maîtres d'ouvrage.
The last 80 years have seen significant changes in industrial technologies, with the development of composite materials being particularly striking. These were hardly present before the Second World War but were made possible by the availability of new polymers, fibre reinforcements and innovative manufacturing techniques. In parallel, the traditional flax farming practices for the textile industry have benefited from major improvements in seed selection, retting and fibre extraction methods. This aims of this paper are twofold; first to describe the early work to develop flax fibre reinforced composites and compare their mechanical properties with those of contemporary flax fibres. And second, to understand the impact of significant efforts made recently to improve flax fibre production. The simple substitution of synthetic fibres is no longer sufficient to justify their use; their environmental benefits must be demonstrated, through life cycle analyses to support sustainable societal choices, and these are also discussed.
Balsa wood is an appropriate core material for structural sandwich applications due to its high strength- and stiffness-to-weight ratios. However, the mechanical properties vary considerably owing to the inherent scattering of natural wood materials. One approach to reduce this scatter and tailor the mechanical properties according to specific application needs is to recompose the natural material into a veneered material consisting of veneer layers of different grain orientations, which are adhesively bonded together. The mechanical properties of such a veneered balsa wood, composed of alternating 0°/90° grain orientations, were investigated at ambient temperature according to corresponding standards. The properties were significantly influenced by the orthotropy on the material scale within one veneer layer and on the system scale within the assembled veneer layers. Standardized experimental set-ups and specimen geometries may produce artifacts such as buckling or strain hardening which deviate from the material behavior in real structures. The thin adhesive between the veneer layers did not negatively affect the mechanical behavior since failure occurred within the veneer layers and not in the interfaces.
A sandwich panel based on upcycled bottle caps core and sustainable components is investigated to contribute to advances in lightweight and environmentally friendly structural solutions. Ecological alternatives to the panel skin and adhesive, such as a recycled PET-bottle foil and a castor oil bio-polyurethane, respectively, are tested and compared to commercial components (aluminium skin and epoxy polymer). Bottle caps are characterised using a small punch test specially developed to obtain the properties of the bottle caps. Additionally, low-cost reinforcement (Portland cement) is added to adhesives to enhance the mechanical behaviour of the panel. The sustainable panels achieve enhanced efficiency compared to aluminium-based panels for core shear strength and stiffness, besides having similar specific flexural properties compared to those of epoxy-based PET panels. Despite their higher strength and stiffness, epoxy polymer-based panels show visible adhesive peeling off to bottle caps core and aluminium skin. In contrast, the biopolymer exhibits larger deformation and debonding of both substrates, indicating a progressive and ductile failure. The satisfactory efficiency of sustainable panels confirms the promising reuse of recycled bottle caps in structural applications.
Sandwich composite material structures arevery important factors because the advantages of light weight and high strength and stiffness of the sandwich core composite structure. Honeycomb sandwich composite materials and corrugated cores are mostly used in the aerospace, marine structure, automobile, structural and construction etc. To withstand high temperature and low temperature applications are also on important factor to manufacture a new material sandwich structure. To find a new method and to adopt a new technology for a new material sandwich structures and skin materials are the main objective of this paper. The prepared sandwich core to work in high temperatures and low temperatures applications and withstand heavy loads is important factor. This review paper aims to find a new method and materials for sandwich core materials for all suitable applications of composite structures with cost effectiveness suitable for the next advancement.
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.
Exterior walls play a significant role in buildings thermal behaviour, and utilization of proper insulation materials with high thermal performance and low adverse environmental impacts is of great importance. The main aim of this study is to assess thermal and environmental benefits and drawbacks of honeycomb cardboard application in external wall configuration of prefabricated buildings. Its thermal conductivity was measured by guarded hot plate method, and its steady-state as well as periodic thermal transmittances were obtained. Furthermore, main construction junctions were simulated, and their linear thermal transmittance was studied. Secondly, with a life cycle assessment (LCA) approach, the adverse impacts of cardboard on environment in production phase and within a limited impact category set were studied. Additionally, the same procedures of thermal and environmental assessments were performed on a number of functionally equivalent insulation materials to be compared with cardboard. The results demonstrate an overall image of positive and negative consequences of cardboard application as insulation for wall envelopes.
Les éco-composites s'imposent progressivement comme une alternative à certains matériaux classiques. L'utilisation de fibres végétales en guise de renfort permet en effet d'améliorer les performances environnementales de ces matériaux ainsi que leurs propriétés spécifiques élevées. Dans ce contexte, cette étude propose d'élaborer un éco-composite sandwich dont les peaux sont constituées d'une résine thermoplastique innovante associée à des fibres de lin et à une âme en balsa. Tout d'abord, les comportements statiques de la résine et de l'âme sont analysés. Par la suite, le composite renforcé de fibres de lin constituant les peaux du sandwich est étudié. Les caractéristiques élastiques principales du pli UD en contraintes planes sont déterminées. De plus, une analyse des mécanismes d'endommagement est effectuée au moyen de la technique d'émission acoustique. Le comportement des poutres sandwiches sollicitées en flexion est ensuite étudié. Une attention particulière est portée à la compréhension des modes de rupture et à l'influence des variations locales des propriétés mécaniques de l'âme. Par ailleurs, certaines propriétés dynamiques de cette structure sont explorées, notamment son comportement en fatigue et sa réponse à l'impact afin de discuter de sa durabilité. Enfin, une étude expérimentale du comportement vibratoire des poutres composites et sandwiches est réalisée. Le rôle des différents constituants dans l'amortissement global des vibrations est discuté au moyen d'une modélisation par élément finis. L'ensemble des propriétés déterminées sont comparées à celles des matériaux non bio-sourcés, afin de situer ses performances.
Fiber reinforced polymer (FRP) sandwich systems as primary load-bearing elements are relatively new concepts in lightweight civil infrastructure. These systems offer a combination of light weight, high strength, thermal insulation for some types, and service-life benefits. Recent developments and applications have demonstrated that these composite systems have emerged as a cost-effective alternative, especially when each material component is appropriately designed. Still, some issues and challenges need to be addressed if FRP systems are to gain widespread use in civil infrastructure. This paper provides an overview of the state-of-the-art research, development, and applications of FRP sandwich systems. It also identifies the challenges and future opportunities for the broad use of these advanced systems in civil engineering and construction.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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)
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.
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.