Aart Willem Van Vuure

Damage in flax/epoxy quasi-unidirectional woven laminates under quasi-static tension

The damage initiation and development in flax/epoxy laminates under quasi-static tension is studied. The laminates are made of quasi-unidirectional woven prepregs in different configurations [0°]8, [0°, 90°]2S, [−45°, 45°]2S and [0°, 90°, +45°, −45°]S, and processed using an autoclave. The damage was monitored during the tensile test using acoustic emission and observed by post-mortem microscopy of the samples. The stress–strain curves illustrate the ductile behaviour of the [+45°, −45°]2S composite, whereas in the other composites a more brittle behaviour was observed. Non-linearity of the stress–strain curves is explained by the intrinsic non-linearity of flax fibres in tension. The combination of the stress–strain data and the registered acoustic emission data is used to identify the damage initiation and propagation thresholds. The damage thresholds are the lowest in the [0°]8 laminate and the highest in the [+45°, −45°]2S laminate. The observed fracture zones and damage mode are cracks inside and on the boundary of technical fibres, cracks on the boundary of tows, matrix cracking, fibre pull-out and fibre breakage. A notable feature of the damage behaviour is almost full absence of transverse matrix cracks inside tows in 90° plies, which are the major damage modes in glass- and carbon-reinforced plastics. This is attributed to the low stress concentrations in transverse direction due to the low transverse modulus of flax fibres.

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.

Wettability of a Single Carbon Fiber

Wettability as determined from contact angle measurements is a suitable parameter for characterizing the physical bonding of a polymer matrix and reinforcing fibers, but it is very challenging to measure the capillary force exerted by a probe liquid on a fiber accurately for very fine fibers such as single carbon fibers. Herein, we propose an innovative method for measuring dynamic contact angles with a tensiometer, considering both the intrinsic variability of the carbon fiber diameter and the extremely small amplitude of the capillary forces, allowing the measurement of reliable dynamic contact angles over a large range of contact line velocities. The analysis of the contact angle dynamics by the molecular-kinetic theory permits us to check the relevancy of the measured contact angles and to obtain the static contact angle value, improving the prospect of employing tensiometry to better understand the wetting behavior of carbon fibers.

Penetration impact resistance of novel tough steel fibre-reinforced polymer composites

Conventional composites are susceptible to impact loading due to their low toughness. Novel annealed steel fibres possess a unique combination of high stiffness and high strain to failure. A range of composite laminates incorporating various steel fibre architectures with thermoplastic and thermoset matrices were produced and the low-velocity penetration impact resistance was characterized by falling weight impact tests, showing the high potential toughness of steel fibre composites. The effect of different parameters was determined. High matrix strain to failure allows a better exploitation of the steel fibre ductility. The higher absorbed energy up to penetration, in case of laminates with lower levels of fibre/matrix adhesion is attributed to fibre debonding, potential for more plastic deformation, and pull-out mechanisms. Thinner fibres are more sensitive to premature failure due to inclusions in the steel fibres. Cross-ply laminates show higher energy absorption, probably due to increased delamination, whereas woven fibre structures lead to more localized damage. A preliminary study shows that layer by layer (‘macro’) hybridization of carbon and steel fabrics leads to penetration impact resistance close to the pure steel fibre composite, in case the steel plies are placed on the outside surfaces, showing a significant positive effect of hybridization.

Spreading Dynamics of Molten Polymer Drops on Glass Substrates

Wetting dynamics drive numerous processes involving liquids in contact with solid substrates with a wide range of geometries. The spreading dynamics of organic liquids and liquid metals at, respectively, room temperature and >1000 °C have been studied extensively, both experimentally and numerically; however, almost no attention has been paid to the wetting behavior of molten drops of thermoplastic polymers, despite its importance, for example, in the processing of fiber-reinforced polymer composites. Indeed, the ability of classical theories of dynamic wetting, that is, the hydrodynamic and the molecular-kinetic theories, to model these complex liquids is unknown. We have therefore investigated the spreading dynamics on glass, over temperatures between 200 and 260 °C, of two thermoplastics: polypropylene (PP) and poly(vinylidene fluoride) (PVDF). PP and PVDF showed, respectively, the highest and lowest slip lengths due to their different interactions with the glass substrate. The jump lengths of PP and PVDF are comparable to their Kuhn segment lengths, suggesting that the wetting process of these polymers is mediated by segmental displacements. The present work not only provides evidence of the suitability of the classical models to model dynamic wetting of molten polymers but also advances our understanding of the wetting dynamics of molten thermoplastics at the liquid/solid interface.

Evaluation of the Extraction Efficiency of Enzymatically Treated Flax Fibers

This study evaluates the efficiency of the extraction of enzymatically treated flax fibers for application in composite materials. Three extraction parameters were introduced. Fiber Efficiency (FE) represents the yield of long fibers extracted after enzymatic treatment. Time Efficiency (TE) is introduced in order to evaluate the ease of extraction. Taking into account both Fiber Efficiency and Time Efficiency results in the introduction of the overall Extraction Efficiency (EE). The Extraction Efficiency is determined for flax fibers treated with pectate lyase, pectin lyase, polygalacturonase, endoxylanase and Viscozyme L as reference while comparing with green and dew retted fibers. Moreover, fiber fineness and transverse properties of the resultant composite materials were investigated. Polygalacturonase treatment appears to be the most promising. Results demonstrate the potential of enzymatic extraction of flax fibers with an increased transverse bending strength of the composite reinforced with polygalacturonase treated fibers (26.4 ± 3.8 MPa) compared to green flax fiber composite (12.1 ± 1.4 MPa) and dew retted fiber composite (16.9 ± 2.2 MPa).

Fracture toughness of unidirectional flax fiber composites with rigid gliadin matrix

The aim of this study is to improve the interlaminar fracture toughness of flax/gliadin composites. Some key parameters are used such as processing conditions, matrix plasticization, and fiber treatment. These parameters affect the crack-growth resistance since they are, respectively, related to the cross-linking density by varying the cooling conditions, to the reduction of the brittleness by adding a low amount of plasticizer, and to the quality of the fiber–matrix interface. Results show that interface quality is more dominant than the effect of a low amount of plasticizer for the initiation fracture toughness value. While crack initiation is closely related to weak links such as the fiber–matrix interface, crack propagation appears to be mainly in the matrix.

Deformability of a flax reinforcement for composite materials

In this work, the deformability of a flax fabric adopted as composite reinforcement is experimentally investigated. The fabric (commercialized as FLAXPLY UD 180 by LINEO) is a quasi-unidirectional woven fabric with thin weft yarns connecting thick slightly twisted warp using satin weave interlacing pattern. The study is dedicated to the understanding and measurement of the main deformation mechanisms occurring during forming processes. The deformation during extension is investigated under uniaxial and biaxial loading in the in-plane tow directions (i.e. warp and weft). Particular attention is dedicated to the behavior during shear loading because this is considered the primary deformation mechanism in the reinforcement forming. Uniaxial bias extension and picture frame tests are adopted to measure the shear deformation. The tests are assisted by digital image correlation (DIC) technique to have a continuous measurement of the local deformation in the fabric plane during loading.