This study presents a new method with improved accuracy for measuring the tensile properties of elementary flax fibres using an automated single-fibre tester with Digital Image Correlation (DIC) for strain tracking, validated with glass fibres of known properties. Modulus values were obtained for glass fibres (83±5.17 GPa, 12mmGL; 79.4±1.33 GPa, MidFibre) and elementary flax fibres (77.0±15.3 GPa, 12mmGL; 74.8±20.2 GPa, MidFibre) using speckle patterns on both end tabs and on optical flags attached to the fibres. This study highlights the advantages of automated single-fibre testing and optical extensometry for reliable and efficient measurement of tensile properties of single fibres.

This study focuses on interlayer hybridisation of flax with silk fibres and the resulting damage mechanisms controlled by the hybrid composite configuration, can lead to an improved balance between stiffness, strength and toughness/ductility. The results demonstrate that a sandwich design configuration with flax layers at the outside of the laminate exhibit the highest increase in (pseudo-)ductility compared to monolithic flax fibre composites. X-ray computed tomography (XCT) revealed that fragmentation and debonding of the flax fibre layers can be achieved by optimising the hybrid laminate configuration and the volume fraction ratio between the two fibres, explaining the increased toughness of the hybrid composites.

This study focuses on interlayer hybridisation of flax with silk fibres and the resulting damage mechanisms controlled by the hybrid composite configuration, can lead to an improved balance between stiffness, strength and toughness/ductility. The results demonstrate that a sandwich design configuration with flax layers at the outside of the laminate exhibit the highest increase in (pseudo-)ductility compared to monolithic flax fibre composites. X-ray computed tomography (XCT) revealed that fragmentation and debonding of the flax fibre layers can be achieved by optimising the hybrid laminate configuration and the volume fraction ratio between the two fibres, explaining the increased toughness of the hybrid composites.

The aim of this study was to determine the effect of moisture cycling (environmental relative humidity cycles) on the durability of flax-epoxy composites and investigate the influence of cycling duration and temperature on the stiffening and strengthening of flax fibres. Four moisture cycling protocols for flax fibres were employed in this research which includes 4D21 (4days per cycle at 21ºC), 4D60 (4days per cycle at 60ºC), 3H27 (3hours per cycle at 27ºC) and 3H60 (3hours per cycle at 60ºC). To measure the impact of high-low humidity cycling at different cycling durations and temperature, tensile testing of impregnated fibre bundle test (IFBT) samples was done. Results of the back-calculated properties revealed that the applied cycling protocols enhanced both the tensile strength and modulus of the fibres. Better improvement of tensile properties was observed in fibres cycled at longer duration. The fibres undergoing 4 days of cycling at 21ºC (4D21 fibres) showed the highest improvements in tensile strength (18%), as well as tensile moduli E1 (19%) and E2 (18%) after 10 cycles. Interestingly, all fibres showed increased stiffness (E1) in the range of 8-20% after 10 cycles and 4-8% after 20 cycles. This fibreimprovement in mechanical strength and stiffness of the fibres can possibly be attributed to a phenomenon similar to a hornification effect in wood or possibly by fibre repair due to pectin migration, which produces the strengthening and stiffening effect.

In this study, the hybridization of flax and silk fibre reinforced composites, at ply level, were studied and compared to their monolithic counterparts. Hybrid FRP laminates were produced via the filmstacking compression moulding method using the highly ductile thermoplastic high-density polyethylene grafted with maleic-anhydride as the matrix. The fibre volume fraction ratio between the two fibres and the lay-up configuration were studied, resulting in hybrid composites with different degrees of distribution of the flax and silk plies within the laminate. The results showed that more balanced properties in terms of stiffness, strength, strain to failure, and impact energy absorption can be achieved by varying those two parameters, thus increasing the designing freedom tailored to specific engineering applications. Additionally, the findings revealed that by optimizing those parameters, multiple fractures of the flax plies or “fragmentation” can be achieved as a toughening mechanism, leading to a pseudo-ductile hybrid composite with a gradual or delayed failure development. The fibre fragmentation mechanism, apart from the increased ductility, can be potentially used as a failure detection criterion for structural health monitoring.

In this study, the tensile and bending properties of silk fibre composites using three (0, 90) woven fabrics with different architectures are investigated. The tensile results show that the silk composites can achieve high strain to failure (more than 20%) and toughness (up to 13 MJ/m3), which can be further manipulated based on the architecture of the fabrics, thus providing more tailored properties and design freedom in applications. XCT and SEM characterization are used to investigate and explain the outstanding toughness of these composites. In tension, a high density of microcracking was observed away from the failed location, which could explain the intrinsic high ductility and energy absorption of silk fibre composites by means of damage spreading throughout its volume in contrast to inherently brittle materials. In bending, significantly lower properties were observed with the more striking being the strain at failure reaching only 30% of the tensile value, thus limiting the potential of silk fibres in bending-dominated loading configurations. XCT revealed that the lower performance is due to failure on the compressive side of the composites, where a clear characteristic kink-band was observed in all composites subjected to bending, while there was no visible damage on the side under tension. This behaviour is also linked to the soft HDPE polymer matrix used, which provides little resistance to fibre buckling.

Natural fibre-based materials offer various advantages compared to synthetic fibres, however their applications are limited mainly due to their hygroscopic properties, which are affected by their chemical composition, microstructure and the porosity of the plant cells of which the fibre is composed. Therefore, this work investigates the hygroscopic behavior of natural fibres to obtain a better understanding of the relation of the chemical composition of the fibres, their crystallinity, and their equilibrium moisture content. The crystallinity index was determined to include amorphous cellulose into the developed models. Nine biomass samples were selected (flax, hemp, jute, spruce, bamboo, corn stalks, palm leaves, rice husk and wheat straw) to construct models via linear regression to predict the moisture sorption behavior of natural fibres. Thorough statistical (ANOVA, RMSE) analysis showed that the developed models are relevant and descriptive. From all major plant cell wall constituents (lignin, crystalline cellulose, amorphous cellulose and hemicellulose), it is hemicellulose's hygroscopicity that is largely responsible for the moisture uptake of the fibres, with (amorphous) cellulose and lignin playing a (much) smaller role. This study has improved the understanding of the hygroscopic behavior of natural fibres, and is important for optimal application of these fibres in composite materials.

Rotational acceleration of the head is known to be the cause of traumatic brain injuries. It was hypothesized that by introducing anisotropy in a foam liner in head protection applications, for example, protective helmets, rotational acceleration transmitted to the head can be further mitigated. Therefore, composite foam with a cylinder/matrix configuration with anisotropy at “macro level” is proposed as a smart structural solution to replace single layer foam headliners of the same weight and thickness. In this paper, a parametric study on the cylinder/matrix configuration is performed and the results are compared with these of single layer expanded polystyrene foam. The structure is subsequently optimized for the best performance in mitigation of rotational acceleration and velocity. Oblique impact results show that the parameters such as the number of cylinders in a given structure, and the compliance of the matrix foam significantly affect the extent of rotational acceleration and velocity mitigation. Optimized composite foam configurations are subsequently proposed and they demonstrate a mitigation of rotational acceleration and velocity up to 44 and 19%, respectively. Moreover, relevant global head injury criteria such as HIC (Head Injury Criterion), RIC (Rotational Injury Criterion), HIPmax (Head Impact Power), GAMBIT (Generalised Acceleration Model for Brain Injury Threshold), and BrIC (Brain Injury Criterion) demonstrated reduction up to 27, 67, 31, 26, and 19%, respectively.

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.

This study investigates the impact-energy absorbing potential of steel fibre reinforced composites with different matrices. The composites were subjected to low-velocity impact using the drop-weight impact test. The interfacial adhesion strength between matrices and steel fiber was characterized by a transverse three point bending test. It was found that factors such as fibre architecture, matrix strain to failure, and the level of adhesion can play a crucial role in the absorbed energy for penetration. A cross-ply non-crimp fabric showed higher energy absorption than a plain woven fabric. Absorbed energy increased with polymer matrix strain to failure and lower fibre-matrix adhesion.

(1) (PDF) On the potential of fine steel fibres to create stiff but tough polymer composites. Available from: https://www.researchgate.net/publication/293222932_On_the_potential_of_fine_steel_fibres_to_create_stiff_but_tough_polymer_composites [accessed Apr 28 2020].