Flax Fibre-reinforced Composites Research Articles

Low-velocity impact damage characteristics of flax/glass epoxy hybrid laminates on the influence of different temperatures: Experimental and numerical analysis

This study investigated the effects of different temperatures on the low-velocity impact damage behaviour of flax fibre-reinforced epoxy composites and their glass/flax hybrids. Composites reinforced with flax, glass, and hybrid flax/glass onto epoxy matrix Subjected to low-velocity drop weight impact tests at 5 J of incident impact energy at sub-zero temperatures (−10 °C and −20 °C) and at room temperature (RT) are presented. Under the different temperatures, the experimental findings showed a beneficial hybrid effect where the temperature played a significant role. At RT, the Lam-GFGFGFG exhibit improved impact resistance, with enhanced energy absorption capabilities compared to glass-only laminates (Lam-G). Besides, Lam-GFFFFG laminates exhibit a significant difference in the force–displacement curves at − 20 °C, with a maximum load of 801.95 N in contrast to RT and − 10 °C resulting in a gradual increase in force with increasing displacement. This indicates that Lam-GFFFFG laminates can resist the impact and maintain structural integrity at sub-zero temperatures. The alternation of glass and flax layers in the hybrid structure contributes to the synergistic effects, resulting in improved damage resistance and tolerance. Also, the highest impact tolerance in a laminate is achieved through the hybridisation of flax fibre-reinforced composites with glass-reinforced layers on the outer surfaces (Lam-GFFFFG) at − 10 °C. Subsequently, experimental results were compared with finite element analysis (FEA) results, derived from a model built using a VUMAT subroutine integrated with ABAQUS/Explicit for a more accurate representation of the damage characterisation of the composite laminates under low-velocity impact.

On the Response to Hygrothermal Ageing of Fully Recyclable Flax and Glass Fibre Reinforced Polymer Composites.

The present work studies the response to hygrothermal ageing of natural fibre composites (NFCs) against synthetic fibre composites when using three different types of polymers as matrices. For ageing, coupons were fully immersed in distilled water at 23, 40, and 60 °C for a total ageing period of 56 days. Flax fibre-reinforced composites, using two recyclable polymer systems: (i) a bio-based recyclable epoxy and (ii) an acrylic-based liquid thermoplastic resin, were tested against conventional glass fibre-reinforced composites employing a synthetic (petroleum-based) epoxy. Different fibre/polymer matrix material combinations were tested to evaluate the effects of hygrothermal ageing degradation on the reinforcement, matrix, and fibre/matrix interface. The hygrothermal ageing response of unaged and aged composite coupons was assessed in terms of flexural and viscoelastic performance, physicochemical properties, and microscopy (SEM-Scanning Electron Microscopy).

Inorganic Fillers and Their Effects on the Properties of Flax/PLA Composites after UV Degradation.

The present investigation seeks to assess the impact of fillers on the mechanical characteristics of entirely biodegradable composites, introducing an advanced solution to fulfil long-term durability demands within point-of-purchase (POP) industries. The inclusion of calcium carbonate (CaCO3) fillers on the various properties of the flax fibre-reinforced composites, after accelerated irradiation in an ultraviolet (UV) radiation exposure has been investigated in the present study. Different types of flax fibre-reinforced poly lactic acid (PLA) biocomposites (with and without filler) were fabricated. The mechanical (tensile and flexural), and physical properties of the specimens were assessed after 500 h of exposure to accelerated UV irradiation of 0.48 W/m2 at 50 °C and were compared with those of the unexposed specimens. The results indicate that the presence of the inorganic filler significantly improved the performance of the biocomposites compared to the unfilled biocomposites after UV exposure. After adding 20% of fillers, the tensile strength was increased by 2% after UV degradation, whereas the biocomposite without filler lost 18% of its strength after UV exposure. This can be attributed to the change in the photo-degradation of the PLA due to the presence of the CaCO3 filler, which acts as a safeguard against UV light penetration by creating a protective barrier. The scanning electron microscopy (SEM) images of the degraded specimen surface show substantial difference in the surface topography of the composites with and without fillers.

Damping under Varying Frequencies, Mechanical Properties, and Failure Modes of Flax/Polypropylene Composites.

This work investigates the effects of fibre content, fibre orientation, and frequency on the dynamic behaviour of flax fibre-reinforced polypropylene composites (FFPCs) to improve understanding of the parameters affecting vibration damping in FFPCs. The effects of fibre content and fibre orientation on the mechanical performances of FFPCs, along with fracture characteristics, are also investigated in this study. Laminates of various fibre contents and orientations were manufactured by a vacuum bagging process, and their dynamic and static properties were then obtained using dynamic (dynamic mechanical analysis (DMA) to frequencies of 100 Hz) and various mechanical (tensile and flexural) analyses, respectively. The findings suggest that of all the parameters, fibre orientation has the most significant impact on the damping, and the maximum loss factor (i.e., 4.3-5.5%) is obtained for 45° and 60° fibre orientations. However, there is no significant difference in loss factors among the composites with different fibre contents. The loss factors lie mainly in the range of 4-5.5%, irrespective of the fibre volume fraction, fibre orientation, and frequency. A significant improvement (281 to 953%) in damping is feasible in flax fibre/polypropylene composites relative to more widespread glass/epoxy composites. The mechanical properties of composites are also strongly affected by fibre orientation with respect to the loading direction; for example, the tensile modulus decreases from 20 GPa to 3.45 GPa at an off-axis angle of 30° for a fibre volume fraction of 0.40. The largest mechanical properties (tensile and flexural) are found in the case of 0° fibre orientation. For composites with fibre volume fractions in the range 0.31-0.50, tensile moduli are in the range 16-21 GPa, and tensile strengths are in the range 125-173 MPa, while flexural moduli and strengths are in the ranges 12-15 GPa and 96-121 MPa, respectively, making them suitable for structural applications. The obtained results also suggest that flax fibre composites are comparable to glass fibre composites, especially in terms of specific stiffness. The ESEM analysis confirms the tensile failures of specimens due to fibre debonding, fibre pull-out and breakage, matrix cracking, and inadequate fibre/matrix adhesion. The outcomes from this study indicate that flax fibre-reinforced composite could be a commercially viable material for applications in which noise and vibration are significant issues and where a significant amount of damping is required with a combination of high stiffness and low weight.

Low-Velocity Impact and Compression-After-Impact Behaviour of Flax Fibre-Reinforced Composites

This study details the low-velocity impact and compression-after-impact (CAI) behaviour of flax fibre-reinforced polymer (FFRP) composites. The impact resistance, energy absorption efficiency and residual compressive strength as well as the damage pattern of the FFRP composites are compared with the corresponding features of glass fibre-reinforced polymer (GFRP) composites, and the effect of the stacking sequence of FFRP composites is also investigated. The results show that the cross-ply FFRP composites have the highest impact resistance, whereas the multi-directional ply composites have the lowest impact resistance but the highest energy absorption efficiency. The energy absorption efficiency of the FFRP composites is greater than that of the GFRP composites, but the penetration resistance and residual compressive strength of the FFRP composites are lower than those of the GFRP composites with the same stacking sequence, mainly due to the lower tensile strength and elongation at fracture of the FFRP composites. It is also reported that the damage pattern of the FFRP composites is localised cracking and delamination, unlike the overall delamination failure exhibited in the GFRP composites after CAI testing. Finally, the failure mechanisms of the FFRP and GFRP composites are detailed.

Fibre architecture modification to improve the tensile properties of flax-reinforced composites

As far as the tensile properties of natural fibres as reinforcements for composites are concerned, flax fibres will stay at the top-end. However, an efficient conversion of fibre properties into their corresponding composite properties has been a challenge, due to the fibre damages through the conventional textile methods utilised to process flax. These techniques impart disadvantageous features onto fibres at both micro- and meso-scale level, which in turn degrade the mechanical performances of flax fibre-reinforced composites (FFRC). Undulation of fibre is one of those detrimental features, which occurs during traditional fibre extraction from plant and fabric manufacturing routes. The undulation or waviness causes micro-compressive defects or ‘kink-bands’ in elementary flax fibres, which significantly undermines the performances of FFRC. Manufacturing flax fabric with minimal undulation could diminish the micro-compressive defects up to a substantial extent. In this research, nonwoven flax tapes of highly aligned flax fibres, blended with a small proportion of polylactic acid have been manufactured deploying a novel technique. Composites reinforced from those nonwoven tapes have been compared with composites reinforced with woven Hopsack fabrics and warp knitted unidirectional fabrics from flax, comprising undulating fibres. The composites reinforced with the highly aligned tapes have shown 33% higher fibre-bundle strength, and 57% higher fibre-bundle stiffness in comparison with the composites reinforced with Hopsack fabric. The results have been discussed in the light of fibre undulation, elementary fibre individualisation, homogeneity of fibre distribution, extent of resin rich areas and impregnation of the fibre lumens.

Analysis of the hydro-mechanical behaviour of flax fibre-reinforced composites: Assessment of hygroscopic expansion and its impact on internal stress

The present work aims at modelling the hydro-elastic behaviour of twill flax fabric-reinforced epoxy composites. These latter were manufactured using the vacuum infusion technique and aged into tap water until saturation. Their heterogeneity is taken into consideration by modelling the twill weave fabrics with two geometric paths: [0/90] and elliptical undulation. Moreover, the water diffusion coefficient and the hygroscopic expansion parameter of the flax fibre are estimated by an inverse approach exploiting the experimental results. In particular, the finite element simulations reveal high mechanical stress concentrations especially at the fibre-matrix interface caused by the differential swelling between the flax fabrics and the epoxy resin. This water absorption-induced internal stress is the main cause of damage initiation in the flax-epoxy composites, which leads to high variations of their mechanical properties and reduces their long-term sustainability.

Vibration damping of flax fibre-reinforced polypropylene composites

This work investigates the effects of fibre content and fibre orientation on the damping of flax fibre-reinforced polypropylene composites. Laminates of various fibre contents were manufactured by a vacuum bagging process; their dynamic behaviour were then found from the vibration measurements of beam test specimens using an impulse hammer technique to frequencies of 1 kHz. The frequency response of a sample was measured and the response at resonance was used to estimate the natural frequency and loss factor. The single-degree-of-freedom circle-fit method and the Newton’s divided differences formula were used to estimate the natural frequencies as well as the loss factors. The damping estimates were also investigated using a “carpet” plot. Experiments were subsequently conducted on a range of samples with different fibre volume fractions and orientations. The results show significant variations in natural frequencies and loss factors according to the variations in fibre orientation. Composites containing 45°, 60° and 90° fibre orientation exhibit approximately the same natural frequencies. Composites with differing fibre orientations exhibit different loss factors for the various modes of vibration, and the maximum loss factor is obtained for the case of 45° fibre orientation, with the loss factor generally lying in the range of 2-7 %. It was found that the loss factor increases with increasing frequency and decreases slightly with increasing fibre content. These outcomes indicate that flax fibre-reinforced composite could be a commercially viable material for applications in which noise and vibration are significant issues and where a significant amount of damping is required.

Effect of fibre volume fraction and fibre direction on crack paths in flax fibre-reinforced composites

Mechanical and fracture mechanical behaviour of composites reinforced with unidirectionally aligned flax fibres is investigated experimentally and numerically. Thereby, the main points are the determination of crack paths in compact tension specimens with three different fibre directions under static as well as fatigue loading and their fatigue crack growth rate curves. Due to the pronounced orthotropic behaviour of those materials the crack path is not only governed by the stress state, but affected by the fibre direction and fibre volume fraction as well. Therefore, the well-known stress intensity factor solutions for the standard specimens are not applicable. Consequently, extensive numerical simulations have been carried out to evaluate the stress intensity factor evolution along the growing crack in order to be able to determine fatigue crack growth rate curves. An approach for calculating stress intensity factors analytically with geometry correction factor functions is conducted exemplified for one fibre direction.

Investigation of crack paths in natural fibre-reinforced composites

Nowadays, fibre-reinforced composite materials are widely used in many fields, e.g. automotive and aerospace. Natural fibres such as flax and hemp provide good density specific mechanical properties. Additionally, the embodied production energy in natural fibres is much smaller than in synthetic ones. Within this paper the fracture mechanical behaviour of flax fibre-reinforced composites is discussed. Especially, this paper focuses on the determination and investigation of crack paths in compact tension specimens with three different fibre directions under a static as well as fatigue load. Differences and similarities in the obtained crack paths under different loading conditions are presented. Due to the pronounced orthotropic behaviour of those materials the crack path is not only governed by the stress state, but practically determined by the fibre direction and fibre volume fraction. Therefore, the well-known stress intensity factor solutions for the standard specimens are not applicable. It is necessary to carry out extensive numerical simulations to evaluate the stress intensity factor evolution along the growing crack in order to be able to determine fatigue crack growth rate curves. Those numerical crack growth simulations are performed with the three-dimensional crack simulation program ADAPCRACK3D to gain energy release rates and in addition stress intensity factors.