A.W. Van Vuure

Morphological aspects and mechanical properties of single bamboo fibers and flexural characterization of bamboo/ epoxy composites:

A novel mechanical extraction process was developed to obtain long bamboo fibers to be used as reinforcement in structural composites. A single-fiber tensile test at four different span lengths for fibers of the bamboo species Guadua angustifolia was performed. Strength values of 800 MPa and Young’s modulus of 43 GPa were obtained. Unidirectional bamboo fiber/epoxy composites (BFC) were produced with untreated and alkali-treated fibers to evaluate the effectiveness of the new reinforcing material. Flexural tests were performed with two fiber orientations (longitudinal and transverse). The longitudinal flexural strength is higher when untreated fibers are used while the treatment benefits the longitudinal flexural stiffness of the composite. Transverse strength increases at lower alkali concentrations, but the transverse three-point bending strength of untreated bamboo in epoxy is already quite high at around 33 MPa. The results illustrate that these bamboo fibers present a natural and renewable option to reinforce composites in several applications where glass fiber and traditional natural fibers are used nowadays.

Mechanical properties of composite panels based on woven sandwich- fabric preforms

An overview of the mechanical properties of the core of woven sandwich-fabric panels is given. These materials provide a new type of sandwich structure with a high skin‐core debonding resistance and the potential for cost-effective sandwich construction. The sandwichfabric preforms are produced by a large-scale textile weaving process (velvet weaving). The basic mechanical properties of the sandwich core (compression and shear) were evaluated and compared with those of other core materials. There is a large variety in possible core layouts and thus mechanical performance for sandwich-fabric panels. Acceptable mechanical properties for cores of higher thickness (higher than 10 mm) can be obtained by weaving part of the pile fibres in the core under angles of^458, by creating networks of piles at lower degrees of stretching and sufficient pile density, or by filling the core with foam. There appears to be a strong synergistic effect between pile and foam properties in the sandwich core. q 2000 Elsevier Science Ltd. All rights reserved.

Sandwich-fabric panels as spacers in a constrained layer structural damping application

Sandwich-fabric panels can provide for an alternative spacer material in a constrained layer damping configuration. Constraining layer configurations with sandwich-fabric spacers can be a weight efficient replacement for full composite constraining layers, if the shear stiffness of the rubber used is not too high. It seems that in any case the use of sandwich-fabric spacers can lead to a more cost-effective damping treatment. To predict the damping of multilayer materials a strain energy method was used. The damping of multilayer beams could be accurately modelled by calculating the distribution of strain energies in the structure with the help of finite elements and knowledge of the loss factors of the individual layers (as a function of frequency).

Modelling the core properties of composite panels based on woven sandwich-fabric preforms

A finite-element preprocessing program was developed to predict the mechanical performance of the cores of woven sandwich-fabric panels. These materials, which are produced from velvet-weave sandwich-fabric preforms, provide a new type of sandwich structure with a high skin-core debonding resistance and the potential for cost-effective sandwich construction. There is a large variety in possible core layouts and thus mechanical performance. To model the core behaviour a detailed geometrical modelling of the core lay-out of unfoamed panels was carried out. A unit-cell of the sandwich-fabric panel under consideration is determined and the shape of the piles and pillars and the resin distribution inside the unit cell are modelled. As inputs, only weaving data and some panel production parameters are required. The predicted microstructure for the cases which were modelled had a good resemblance to reality. For the FE analysis a homogenization principle has been used. Linear static analyses for the basic core properties (flexural shear and flat compression) have been performed. The results for the shear modulus as a function of the pile shape were very good. Model predictions at higher resin contents and for the compression modulus were less accurate.

Interlaminar fracture toughness of flax-epoxy composites

The purpose of this study was to determine the influence of fibre architectures on the interlaminar fracture toughness and tensile toughness of flax fibre epoxy composites. The fracture toughness was investigated for both Mode I (GIC) and Mode II (GIIC) for seven flax-epoxy architectures: one plain weave, two twill 2 × 2 weaves, a quasi-unidirectional and a unidirectional architecture, the UD’s being tested in both [0,90] and [90,0] composite lay-ups. The results of the Mode I and Mode II showed promising results of the flax-epoxy composite performance. The addition of flax fibre increases the GIC and GIIC of the composites over that of the unreinforced brittle polymer by at least two to three times. Further improvements are made with the use of woven textiles. The tensile toughness was found to be a good indicator of the capacity of a material to sustain perforation or non-perforation impact.

In-depth study of the microstructure of bamboo fibres and their relation to the mechanical properties

The mechanical properties of bamboo technical fibre, from the species Guadua angustifolia, have been studied showing values of strength up to 800 MPa and E-modulus up to 43 GPa, proving their adequate tensile properties that make this natural fibre suitable as reinforcement in composite materials. To fully explore the good mechanical properties and to make an adequate use of this new reinforcement, it is indispensable to comprehensively understand the fibre behaviour as a function of the microstructure. Microscopic observations have provided us with an extensive knowledge of the complex microstructure of this natural fibre from the macroscale down to the microscale level where different features like the distribution of the elementary fibres within the fibre bundle, dimensions and layering pattern of the elementary fibres and the main microfibrillar angles could be measured. The Young’s modulus of the elementary fibre is analysed based on the micromechanics of composite materials, commonly used for unidirectional short fibre composites, and the fibre microstructure. The predicted results are in reasonable agreement with experimental data, showing the appropriateness of the model for describing the elementary fibre stiffness. Also, the failure modes of single fibres after tensile testing are analysed by microscopic observations, to have an indication of the stress development in the elementary fibres and the different failure mechanisms.

Microstructural analysis using X-ray computed tomography (CT) in flax/epoxy composites

Among natural fibres which have recently become attractive to researchers, flax is probably the most commonly used bast-type fibre today. Due to its properties and availability, flax fibre has potential to substitute glass in polymer composites. A flax fibre has a complex structure; it can be classified into elementary fibres, which are grouped into so-called technical fibres. These technical fibres themselves are actually composite structures. Several works [1, 2, 3] were focussed on the study of damage behaviour in unidirectional flax fibres reinforced composites, where materials were subjected to tensile loading. At the microscopic level and at low stress, microcracks arise within the material and by growing they may lead to other forms of damage such as delamination, fibre breakage, interfacial debonding...etc. In order to better understand the damage phenomena and to better control the parameters which lead to the failure, several methods and techniques have been developed on natural fibre reinforced composites [2, 3]. In the present work, X-ray computed tomography (CT) technique has been used to observe damage in flax/epoxy quasi-unidirectional woven laminates, loaded in uniaxial tension. The tensile tests show that these composites offer good mechanical properties. X-ray computed tomography technique allowed us, on the one hand to determine the microstructure parameters of the studied composites and to observe the damage occurring during loading, on the other. The inspection of the several tomography images showed cracks on interface of the yarns and technical fibres.

Modelling of the Core Properties of Sandwich-Fabric Panels with the Help of Finite Elements

Sandwich-fabrics are produced with a velvet weaving technique. Due to the integrally woven nature of the fabric, panels and structures produced from the fabrics have a very high skin-core debonding resistance. Due to the one-step production of a sandwich preform the cost of sandwich panels and structures based on the fabric can stay limited.