Jan Ivens

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.

INTERLAMINAR FRACTURE TOUGHNESS OF CFRP INFLUENCED BY FIBRE SURFACE TREATMENT: PART 1. EXPERIMENTAL RESULTS

The bond between a fibre and the matrix is very important for the mechanical behaviour of CFRP. The quality of the fibre/matrix interface will influence the initiation and development of damage. This is certainly true for interlaminar fracture toughness. The present study is focused on the influence of the interface properties on the fracture toughness of CFRP. Carbon fibres with four different wet oxidative surface treatment levels have been investigated. The fibres were embedded in a thermoset resin and in a thermoplastic system. Mode I, mode II and mixed-mode tests were performed to determine critical strain-energy release rates. Results show an important increase in the fracture toughness initiation value with increasing fibre surface treatment level. For tests dominated by mode I loading, however, the propagation value can reach a maximum at intermediate treatment levels as a consequence of fibre bridging effects.

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.

The fatigue behaviour and damage development of 3D woven sandwich composites

In this paper, the bending fatigue behaviour and damage development during fatigue of polyurethane (PUR)-epoxy 3D woven sandwich composites is investigated. 3D sandwich fabrics are produced by a velvet weaving technique. It allows relatively easy production of delamination-resistant sandwich panels, compared to more traditional sandwich structures with honeycomb or foam cores. In this paper, the 3D sandwich fabrics made of glass fibre are impregnated with epoxy resin while the empty core can be foamed up with PUR to improve shear resistance of the panels. Core properties and static bending strength of the panels were evaluated. Three-point bending (3PB) fatigue tests were performed on four different materials (two unfoamed epoxy panels and two PUR foamed epoxy panels) with varying static properties. After fatigue testing, the Wohler curves for each panel were determined. The relation between the observed damage development, the property degradation during bending fatigue and the static properties of the panels was investigated.

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.

Determination of the optimal flax fibre preparation for use in unidirectional flax–epoxy composites

The specific stiffness of flax fibres is comparable to that of glass fibres. However, the application of flax fibres in composites still poses two important problems. The orientation of the fibres in the composite is not impeccable and the fibre–matrix adhesion is too weak. This research focuses on the former. To produce a continuous yarn for the textile industry, technical flax fibres are spun. The twisting of the fibres increases the yarn strength but the orientation of the fibres with regard to the fibre axis decreases. Experiments show that the slightly twisted yarns and the spun yarns possess the best processability but have lower mechanical properties than untwisted ribbons. The values fit very well to certain models which predict stiffness with regard to the twist angle.

INTERLAMINAR FRACTURE TOUGHNESS OF CFRP INFLUENCED BY FIBRE SURFACE TREATMENT: PART 2. MODELLING OF THE INTERFACE EFFECT

The bond between a fibre and the matrix is very important for the mechanical behaviour of CFRP. The quality of the fibre/matrix interface will influence the initiation and development of damage. This is certainly true for interlaminar fracture toughness. The present study is focused on the modelling of the influence of the interface properties on the fracture toughness of CFRP. Carbon fibres with four different wet oxidative treatment levels were investigated. The fibres were embedded in an epoxy resin and in a thermoplastic system. Mode I, mode II and mixed-mode tests were performed to determine critical strain-energy release rates. A micromechanical model was developed to explain how fracture toughness of the interface influences the initiation fracture toughness of the composite. A second model explains the R curve behaviour for mode I tests by analysing the strain in a single fibre bridging a crack.

Influence of carbon fibre surface treatment on composite UD strength

Abstract Carbon fibre surface treatments can affect the composite properties in the fibre direction. Two effects play an important role in the longitudinal tensile strength. First, the carbon fibre surface treatment has some influence on the individual fibre strength. Second, an improved interface strength leads to better load transfer in the composite. Using an elastic-slip model, it is shown that the ineffective length increases dramatically with decreasing degree of surface treatment. It is also shown that, as the debonded length of a broken fibre decreases, the strain in the matrix becomes very high, leading to matrix cracking and additional stress concentrations on the neighbouring fibres. This leads to a maximum strength at intermediate treatment levels. This conclusion is supported by UD tensile tests on [0]9 carbon/epoxy laminates.

Low velocity impact characteristics of 3D integrated core sandwich composites

In this paper, the low velocity impact characteristics and impact damage of sandwich composites, produced at four different core thicknesses from 3-dimensional (3D) integrated sandwich fabrics, with and without foam filling, have been examined. The 3D sandwich fabrics have been produced using the same yarn and weaving densities. Thus, the impact characteristics are only affected by the core thickness and whether foam filling is used or not. Low velocity impact tests have been conducted at 32 and 48 J energy levels. The impact behavior has been determined as a function of the peak load, the energy to peak load, the time to peak load and the absorbed energy. The impact damage and the change in the compressive strength after impact have been analyzed. The findings obtained indicate that core-skin delamination on 3D sandwich composites has been fully prevented. Impact tests carried out on integrated 3D sandwich structures have shown that impact damage is limited to the vicinity of the point of impact and does not affect the integrity of the structure. This indicates that such damage can be easily repaired and the service life of the product can be sustained.

Quasi-static behavior of three-dimensional integrated core sandwich composites under compression loading:

In the current study, the effect of the thickness and the foam density in three-dimensional integrated woven sandwich composites on quasi-static properties was investigated. For this purpose, produced samples were subjected to uniaxial flatwise compression tests and their compression strength and moduli were determined. Obtained results were optimized by taking core thickness, foam density and panel weights into consideration. Damages that occurred on the tested samples were reported. When compared to conventional foam core sandwich composites, it was found that three-dimensional integrated sandwich composites have better compression properties and due to the fact that the pile yarns in the core and the foam support each other.