Joris Baets

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.

Characterization of flax/epoxy prepregs before and after cure

This paper aims to better understand the nature of flax/epoxy hot melt prepreg systems as well as the mechanical properties of their cured composites. Two uncured systems were subjected to thermal gravimetric analysis and compaction experiments. Composites were then produced in an autoclave at pressures of 1, 3 and 5 bars. Void analysis was carried out on the cured laminates by optical microscopy and void contents ranging from below 1% to over 20% were obtained by image analysis. The primary source of voids was shown to be resin starvation based on the results of the compaction tests. Tensile properties were shown to be strongly degraded by fabric crimp. Short beam tests revealed a 16% decrease in interlaminar shear strength with a 3.5% increase in void content. Similarly, water absorption tests showed a significant increase in rate of absorption likely due to the presence of voids. The results suggest that in order for flax/epoxy prepregs to reach their full potential, the level of crimp and the evacuation of moisture during cure should be of primary concern.

Impregnated fibre bundle test for natural fibres used in composites

In this study, the impregnated fibre bundle test, a common method used by carbon and glass fibre manufacturers to determine the properties of fibres used in composites, was adapted for natural fibres and validated by a round robin test on one type of natural fibres, namely flax fibres. Five European laboratories have carried out in parallel the impregnated fibre bundle test, on the same batch of hackled flax (long fibres), to check the applicability and reliability of this modified method on natural fibres. The results were compared to the more traditional single fibre test on elementary fibres. The back-calculated fibre stiffness shows a very low scatter between the five laboratories of less than ±5% (59.8 ± 2.4 GPa, as measured between 0 and 0.1% strain). The fibre ultimate tensile strength of 527 ± 138 MPa has a higher scatter, compared to stiffness values, as this property is highly sensitive to imperfections and flaws.

Tension-tension fatigue behaviour of woven flax/epoxy composites

Understanding the fatigue performance of biocomposites is critical in order to increase their acceptance, but current literature in this area is mostly limited to nonwoven reinforcements. This paper considers the tension–tension fatigue of three different woven flax/epoxy composites, for which two of them are prepreg-based and the other is manufactured using the Vacuum Assisted Resin Transfer Moulding (VARTM) process. Good fatigue performance of flax fibres comparable to those exhibited by glass fibres has shown the potential of this material to be implemented in load-bearing applications. The results suggest that minimizing the crimp in the yarns is a major concern to increase the resistance to fatigue damage in this class of materials. In addition to the three mentioned composites, two hybrids of flax/glass/epoxy were manufactured using the same VARTM process to check if the fatigue stability of flax fibre is extendable to its hybrids. The results show that an increase in the strength is possible, while maintaining similar fatigue behaviour as the plain flax/epoxy composites.