Ignaas Verpoest

Natural fibres: can they replace glass in fibre reinforced plastics?

In this work, natural fibres (sisal, kenaf, hemp, jute and coir) reinforced polypropylene composites were processed by compression moulding using a film stacking method. The mechanical properties of the different natural fibre composites were tested and compared. A further comparison was made with the corresponding properties of glass mat reinforced polypropylene composites from the open literature. Kenaf, hemp and sisal composites showed comparable tensile strength and modulus results but in impact properties hemp appears to out-perform kenaf. The tensile modulus, impact strength and the ultimate tensile stress of kenaf reinforced polypropylene composites were found to increase with increasing fibre weight fraction. Coir fibre composites displayed the lowest mechanical properties, but their impact strength was higher than that of jute and kenaf composites. In most cases the specific properties of the natural fibre composites were found to compare favourably with those of glass.

Influence of processing and chemical treatment of flax fibres on their composites

Abstract During the consecutive decortication stages of flax fibres (retting, scutching, hackling, …), the fibre properties change tremendously due to mechanical and chemical modifications. This study points out the influence of the flax processing parameters, and consequently of the fibre properties, on the mechanical properties of flax fibre reinforced epoxy composites. The better the retting degree of the fibres, the better the composite properties are. The use of long flax slivers does not necessarily result in better composite properties. The influence of certain chemical treatments, performed on the flax fibres, on the composite properties has been studied as well. Treatments with alkali, dilute epoxy, acetone and silane were carried out. A treatment consisting of a combination of alkali and dilute epoxy gives the highest improvement of the flexural properties. The longitudinal properties of the UD composites (both strength and modulus) enhanced with 40% or more, transverse strength increased with 250%, transverse modulus could improve up to 500%.

Textile composites: modelling strategies

Textile materials are characterised by the distinct hierarchy of structure, which should be represented by a model of textile geometry and mechanical behaviour. In spite of a profound investigation of textile materials and a number of theoretical models existing in the textile literature for different structures, a model covering all structures typical for composite reinforcements is not available. Hence the challenge addressed in the present work is to take full advantage of the hierarchical principle of textile modelling, creating a truly integrated modelling and design tool for textile composites. It allows handling of complex textile structure computations in computer time counted by minutes instead of hours of the same non-linear, non-conservative behaviour of yarns in compression and bending. The architecture of the code implementing the model corresponds to the hierarchical structure of textile materials. The model of the textile geometry serves as a base for meso-mechanical and permeability models for composites, which provide therefore simulation tools for analysis of composite processing and properties.

A round-robin programme on interfacial test methods

Abstract A round-robin programme has been undertaken to assess the compatibility in the micromechanical techniques used to evaluate the interfacial shear strength of the fibre/matrix bond in composite materials. The tests selected for evaluation were the single-fibre pull-out test, the microdebond test, the fragmentation test and the micro-indentation test. Twelve laboratories were invited to participate in this programme. Each laboratory was supplied with Caurtaulds XA fibre in the untreated condition and with a standard surface treatment, and a quantity of epoxy resin, hardener and catalyst, all from the same batch. Some laboratories were supplied with composite bars made with the same materials. A common cure cycle was chosen for sample preparation. Each laboratory conducted the tests to its own procedures. The results showed that the scatter within each laboratory was acceptable but the scatter between laboratories for a particular test was high. The results are discussed and possible explanations are presented for these observations. The indications are that the fundamental procedures used in each laboratory are sound. The results also suggest that there is great potential for achieving standard procedures and reducing the inter-laboratory scatter. A further round-robin programme is proposed to generate test protocols.

New set-up for measurement of permeability properties of fibrous reinforcements for RTM

Resin transfer molding (RTM) is becoming increasingly important as a production technique for FRPs. However, the design of molds that assures the production of parts with acceptable quality remains a difficult task. Simulation codes developed for design need reliable input data such as reinforcement permeability. The measurement of this material parameter is still not standardized and thus many different set-ups have been proposed. In this paper a new method will be presented that is much faster and much less operator-dependant than other set-ups. It is a 2D central injection set-up, based on the automatic detection of the flow front position as a function of time. For this purpose, simple electrical sensors were designed and an automated calculation procedure was programmed on a PC-based system. As will be shown, the elements of the permeability tensor are not single valued at a particular fiber volume fraction. Rather, each element displays a broad distribution. More importantly, the variations in the principal permeability values are not correlated since their ratio, the anisotropy, also displays a broad distribution.

Modelling the elastic properties of knitted-fabric-reinforced composites

In this paper the elastic properties of knitted-fabric-reinforced composites are investigated. Tensile and shear tests were performed to determine the in-plane Youngs modulus, Poissons ratio and shear modulus of different types of glass/epoxy warp-knitted fabric composites. Some models are discussed for the prediction of the elastic constants, on the basis of the fibre orientation distribution, the constituent material properties and the fibre content. Apart from the Krenchel approach, two basic variants of simple three-dimensional models are evaluated. Depending on the averaging procedure used, upper and lower bounds for the elastic moduli are found with the Voigt and Reuss method, respectively. These bounds are shown to agree well with the experimental results for the Youngs moduli and the shear moduli. Closer bounds are obtained by assuming that the matrix is uniformly distributed through the composite. These closer bounds are shown to be valid for the Youngs modulus but not for the shear modulus. The in-plane Poissons ratio could not be predicted successfully.

Permeability measurement of textile reinforcements with several test fluids

In liquid composite moulding (LCM) techniques, the liquid resin has to flow a long distance to impregnate the dry fibres. The measure for the resistance of the fibre preform to the resin flow is the permeability of the fibre preform. Because of the dual-scale porous structure of the textile preforms, test fluid can influence the unsaturated permeability values through the interaction of the fluid and fibres. In this study, the influence of test fluid on the permeability measurement of several types of textile reinforcements is investigated. First the contact angle of various fluids and fibres was measured. Then the permeability measurement of the textile reinforcements was carried out. The results showed that the influence of test fluid is small under the test conditions.

Designing new materials from wheat protein

We recently discovered that wheat gluten could be formed into a tough, plasticlike substance when thiol-terminated, star-branched molecules are incorporated directly into the protein structure. This discovery offers the exciting possibility of developing biodegradable high-performance engineering plastics and composites from renewable resources that are competitive with their synthetic counterparts. Wheat gluten powder is available at a cost of less than dollars 0.5/lb, so if processing costs can be controlled, an inexpensive alternative to synthetic polymers may be possible. In the present work, we demonstrate the ability to toughen an otherwise brittle protein-based material by increasing the yield stress and strain-to-failure, without compromising stiffness. Water absorption results suggest that the cross-link density of the polymer is increased by the presence of the thiol-terminated, star-branched additive in the protein. Size-exclusion high performance liquid chromatography data of molded tri-thiol-modified gluten are consistent with that of a polymer that has been further cross-linked when compared directly with unmodified gluten, handled under identical conditions. Remarkably, the mechanical properties of our gluten formulations stored in ambient conditions were found to improve with time.

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.

Round-robin interlaminar fracture testing of carbon-fibre-reinforced epoxy and PEEK composites

Abstract This paper summarizes results from a series of tests performed in eighteen laboratories with three specific aims: first, to establish the reproducibility of values from mode I and mode II tests carried out on two materials in different laboratories; secondly to investigate the differences between two data analyses; and finally to examine the influence of specimen thickness on mode I and mode II values. The materials tested, both unidirectional, were a relatively brittle carbon-fibre/epoxy laminate and a thermoplastic composite, carbon fibre/PEEK.