Unidirectional Flax Fiber Research Articles

Damage indicators in unidirectional natural fibre composites under fatigue loading

This paper investigates the occurrence of failure under fatigue loading of unidirectional flax fibre composites with different matrices. Various parameters, such as dissipated energy, residual strain, and stiffness, were measured to evaluate fatigue damage and the fatigue life of composites. The results indicate that absolute dissipated energy may not be the most suitable indicator for assessing damage in natural fibre composites when they are subjected to fatigue before reaching failure. The research proposes to rather consider the “loss factor”, defined as the ratio of dissipated energy to total energy stored in each cycle. This indicator appears to provide a more effective means of evaluating fatigue life of the studied composites, especially when compared to the dissipated energy observed during fatigue tests at low stress levels. Fatigue and acoustic emission results indicate that biocomposites with better fibre–matrix adhesion show a delayed damage initiation and propagation, as well as longer fatigue life.

Glass fibre hybridization to improve the durability of circular flax fibre reinforced composites with off-the-shelf recyclable polymer matrix systems for large scale structural applications

Natural fibre composites (NFCs) are not durable in the long run because of the susceptibility of natural fibres to environmental conditions and specifically moisture. Hybridizing NFC laminates externally, with synthetic fibre reinforcements, may improve durability, due to their inherent environmental resistance. This work aims to investigate the effects of glass hybridization, on flax fibre composites, studied via accelerated ageing. In specific, the durability of hybrid flax/glass fibre reinforced polymer composites, with two recyclable polymer matrices was investigated. Unidirectional (UD) flax and UD glass fibre reinforcements were employed to fabricate laminates, with two fully-recyclable off-the-shelf resin systems, as matrix: (i) a bio-based epoxy resin and (ii) an acrylic liquid thermoplastic (Elium®). In addition, a standard petroleum-based epoxy polymer matrix was for reference purposes. Weathering and hygrothermal ageing were used to test the durability of coupons, exposed to UV radiation/condensation/water spray environment (weathering ageing), and full-immersion in distilled water at 23, 40, and 60°C (hygrothermal ageing). In all cases, ageing was performed for a total duration of 56 days. The performance of the unaged and aged composite coupons was assessed and compared in terms of flexural and viscoelastic performance as well as SEM (Scanning Electron Microscopy) analysis. It was revealed that the addition of glass fibres with flax fibres in the hybrid composites improves the performance and better resistance against ageing environments than their neat flax fibre composites.

Damage evolution in flax fibre composite under creep load

This work aims to reveal the damage mechanisms and evolution in unidirectional flax fibre biocomposites when subjected to creep load. X-ray micro computed tomography and acoustic emission (AE) was used to monitor the failure progress during flexural creep tests. A correlation between the event clusters and directly observed damage modes was established based on coupons with expected failure mechanisms and then validated by computed tomography observations. The damage initiated from matrix cracking in the primary creep stage, and then fibre–matrix interface debonding combined with fibre pull-out occurred sequentially during the steady creep stage. In the tertiary creep stage, the explosive fibre fracture emerged and eventually triggered catastrophic failure. Considerably more AE events were detected during creep tests compared to those in quasi-static tests, which indicates that intensive damage is generated under creep load, and therefore causes the strength degradation. A good agreement was observed between the cumulative number of AE events and the increasing damage volume fraction over time determined by X-ray micro computed tomography during multi-step creep tests.

Toward the development of a new smart composite structure based on piezoelectric polymer and flax fiber materials: Manufacturing and experimental characterization

Although technologies and processes for creating smart composite structures have been established, significant limitations still necessitate additional research to overcome. Indeed, numerous studies have highlighted the challenges of embedding active elements, often leading to compromises in the structural integrity of smart composite materials. In contrast, this study introduces a novel perspective, offering innovative insights and valuable contributions to the existing knowledge. The primary contribution of this study lies in addressing the critical need for the seamless integration of active materials within composite structures. To demonstrate this, our study considers the potential integration of Polyvinylidene Fluoride (PVDF), a polymer-based piezoelectric material, with pre-impregnated unidirectional flax fibers to produce a smart composite structure. The consolidation molding process was adopted to manufacture the smart composite samples for investigations. Experimental analysis were then considered to evaluate the influence of the embedded PVDF film on the mechanical performance of the structure and to assess the resulting functional properties. X-ray micro-computed tomography and an additional strength-of-material test approach, Interlaminar Shear Strength (ILSS), were performed to explore the impact of the insertion on the integrity of the laminate structure. On the other hand, we conducted vibration tests to assess the electromechanical properties. The tomography results reveal void elimination in the edges of the active material insert, which could be attributed to the choice of thin film materials. Moreover, the ILSS results highlight the minimal impact of PVDF material on mechanical performance, showcasing a mere 0.69% degradation of the material strength. These results are comparable to the recently reported works but with an appreciable margin of improvement. The vibration test response demonstrates the ability of the embedded piezoelectric material to generate voltage, which validates our integration procedure. The smart composite material’s sensitivity to ambient vibrations and its power generation potential show promise for sensing and energy harvesting.

Effect of accelerated weathering on the performance of natural fibre reinforced recyclable polymer composites and comparison with conventional composites

This work investigated the long-term performance of unidirectional (UD) flax fibre reinforced recyclable polymer composites (FFRPs), after accelerated weathering for a total of 56 days, in laboratory environment. The employed recyclable polymer matrices were a bio-based epoxy and a liquid thermoplastic acrylic resin. The performance of the developed composites, were compared with traditional UD glass fibre composites (GFRPs), employing a standard petro-based epoxy resin, in terms of flexural and viscoelastic properties, while visual inspection and Scanning Electron Microscopy (SEM) were employed to investigate further the effects of ageing after exposure. It was revealed that the reinforcing fibres dominated the performance of the composites after ageing, whilst the choice of polymer matrix exhibited less influence.

Exploring the flexural and impact properties of pure flax/epoxy and Kevlar/flax/epoxy composites through experimental and numerical analysis

Hybrid composites offer a viable alternative to conventional composites in terms of reduced cost and environmental impact. This study investigated the effect of hybridizing flax with Kevlar on the flexural and impact properties of Kevlar/flax/epoxy composites using three-point bending, drop-weight, and Charpy impact tests. The tested specimens were made of unidirectional flax fibers (F) and woven Kevlar fibers (K) in four configurations: unidirectional flax/epoxy [016F] (UFE), angle-ply flax/epoxy [±454F]S (AFE), woven Kevlar/unidirectional flax/epoxy [0–902K/06F]S (UKFE), and woven Kevlar/angle-ply flax/epoxy [0–902K/±453F]S (AKFE). The three-point bending test results showed that the ultimate strength and the flexural modulus of the UKFE increased by 15% compared to pure UFE and by more than threefold for AKFE compared to AFE laminates. Additionally, the results of the drop-weight test revealed a significant increase in the impact force by 30% for both unidirectional and angle-ply laminate configurations. Similarly, Charpy test results indicated a 60% improvement in the impact energies of the flax laminates. The predictions of the proposed finite element model agreed very well with the experimental results. These findings demonstrate the effectiveness of hybridizing two layers of Kevlar onto flax/epoxy composites in enhancing the composite's impact and flexural properties.

Mechanical characterization of hybrid biocomposites reinforced with nonwoven hemp and unidirectional flax fibers

AbstractFiber‐reinforced polymers are widely used in many applications where high specific strength and high specific stiffness are required. Biocomposites have replaced synthetic‐based fiber‐reinforced polymers as a preferred option due to their environmental friendliness, ease of supply, and affordability. Comparable strength synthetic‐fiber‐reinforced polymers and biocomposites both have lower specific weights. This study aims to characterize hybrid biocomposites produced from unidirectional prepregs made of flax/polypropylene fibers and nonwoven mats made of hemp/polypropylene fibers. Research has also been conducted on how the number of layers and the stacking sequence affect the mechanical performance of hybrid biocomposites. Three different designs of biocomposite plates have been produced using compression molding. They were then subjected to tensile, compressive, shear, bending, Charpy impact, and drop‐weight impact tests. According to the test results, it is found that each design has its own characteristics, and the characterized static and dynamic behaviors are very different from each other. Therefore, each biocomposite mentioned here may be a good candidate for engineering design based on the given engineering design criteria.

Comparative study on the low-velocity impact properties of unidirectional flax and carbon fiber reinforced epoxy plates

Flax and carbon fibers reinforced polymer composites have high potentials to be used as sustainable building materials. To understand the impact resistance of these fiber composites, the present article studied the impact resistance of unidirectional flax fiber and carbon fiber reinforced polymer (FFRP and CFRP) composite plates. The impact response, damage resistance, and impact tolerance were investigated using drop hammer impact and compression after impact (CAI) tests. The internal damage of FFRP and CFRP due to impact loads was studied using acoustic emission technology. In addition, the development process of impact damage was analyzed. CFRP plates show a higher specific energy absorption rate than that of FFRP, because of the more serious internal damage and brittle fracture tendencies. Matrix cracking was the primary damage in FFRP and CFRP under impact load. Compared with CFRP, FFRP possesses a higher CAI due to less damage. Further expansion of the matrix crack caused by the original impact occurs in the impacted fiber-reinforced polymer composites (FRP) under a compressive load.

UV accelerated aging of unidirectional flax composites: Comparative study between recycled and virgin polypropylene matrix

Previous studies on the photooxidation of natural fiber composites have focused on using short fibers and virgin matrices. In this work, unidirectional flax fiber composites with virgin and recycled polypropylene matrix were prepared by thermocompression to compare their accelerated aging. These composites were exposed to two accelerated xenon arc UV exposures where only the irradiance was modified. A multi-scale characterization was carried out to evaluate the consequences of the photooxidation on their mechanical and physicochemical properties with time. Results show that the photooxidation was limited to the surface. In addition, the photo-degradation of the surfaces of both composites was identified by the appearance of cracks, the variation of crystallinity, the decrease in weight average molar mass, and the appearance of new chemical products. These physicochemical variations are more evident with increasing irradiance. Despite these variations, the mechanical tensile properties of recycled matrix composites remain relatively indifferent to the two UV exposures compared to those with virgin polypropylene. It can be concluded that recycled polypropylene, if well selected, can replace virgin polypropylene in natural fiber composites for better mechanical resistance to photooxidation.

Modal Analysis of Delaminated Flax Fibre Reinforced Epoxy Composite Plate

This paper describes the influence of fibre orientation, delamination size and location on the natural frequencies of the single mid-plane delaminated unidirectional flax fibre reinforced epoxy (FFRE) composite plates under different boundary conditions numerically by using ANSYS MAPDL. The delaminated composite plate was modelled as two separate volumes divided at the midplane of the plate, with the nodes on the intact surfaces merged. In contrast, the nodes on the delaminated region remained separate. The results show that the CCCC delaminated composite plate has the highest fundamental natural frequency (219.996 Hz). Furthermore, for increasing fibre orientation from 0° to 45°, the fundamental natural frequencies decreased by 10.12% for the CCCC condition and increased by 6.01% for the SSSS condition. The fundamental natural frequency for the cantilever condition decreased 60.91% when fibre orientation increased to 90°. Moreover, the CCCC condition significantly reduces the fundamental natural frequency (up to 38.95%) with increasing delamination size. For CCCC and SSSS conditions, the centre delamination possesses the highest fundamental natural frequency (219.996 Hz and 116.525 Hz, respectively). The highest fundamental natural frequency for cantilever conditions is 33.081 Hz, with delamination located at the middle of the width and near the free end.

Anisotropic behaviors of moisture absorption and hygroscopic swelling of unidirectional flax fiber reinforced composites

This work aims to study the anisotropic moisture absorption characteristics of unidirectional flax fiber reinforced composites (FFRCs) and evaluate the variations of the internal stress caused by the hygroscopic deformation, which might lead to the deterioration of the mechanical properties of the composites. FFRCs were prepared using vacuum-assisted resin infusion molding (VARIM) technology. Moisture absorption tests were designed by sealing the designated surfaces with a polyurethane waterproof coating to keep the moisture absorption of FFRCs only in the desired directions. The deformation caused by moisture absorption in the width direction was also monitored. It confirms that the moisture absorption of FFRCs conformed to the 3D Fick's law, and the influence of moisture absorption in the width direction cannot be ignored, especially for the unidirectional transverse tensile test specimen (UTFRC). The hygroscopic deformation of the unidirectional FFRCs showed that the material expands perpendicular to the fiber direction but shrinks along the fiber direction. The modified 3D Fick’s law based on heat conduction equation was performed to reveal the mechanism of the above moisture absorption and hygroscopic deformation. According to the finite element inversion, it was found that the axial diffusion coefficient along flax fiber bundles was much larger than the transverse diffusion coefficient perpendicular to the fiber bundle direction due to the existence of the unique lumen structure inside flax fibers. In addition, the high internal stress caused by the mismatched swelling properties between the fiber and the matrix was the main reason for the damage of the FFRCs, especially at the interface, which may seriously affect the service life of the composites. The outcome of this study can be used to accurately analyze the hydro-elastic behaviors of plant fiber reinforced composites, which is a prerequisite for predicting the mechanical properties affected by moisture absorption.

Effect of machining processes on the damage response and surface quality of open hole hybrid carbon/flax composites: An experimental study

The influence of conventional drilling (CD) and abrasive water jet (AWJ) machining on the mechanical behavior of hybrid carbon/flax composites was investigated. The specimens were manufactured using woven carbon fiber (C) and unidirectional flax fiber (F) in three configurations: unidirectional [0-90C2/0F6]S, cross-ply [0-90C2/(0/90)F6]S, and angle-ply [0-90C2/(±45)F6]S laminates. The examination of the hole surface revealed severe damage in the form of peel-up, push-out, and secondary delamination of AWJ machined holes, and only push-out delamination at the exit side of the CD drilled hole. A delamination factor was developed to quantify the delamination at the hole surface. The specimens were subjected to quasi-static load-unload tensile tests to assess their damage response and surface fracture. Unidirectional and angle-ply specimens drilled with AWJ accumulated a higher damage compared to CD specimens. This correlates with the delamination factor, indicating that the delamination at the entry and exit of the hole and secondary delamination greatly affect the damage evolution of the laminates.

Durable mechanical properties of unidirectional flax fiber/phenolic composites under hydrothermal aging

Natural fibers show great potential in polymer composites reinforcement, it remains challenging to tackle the mechanical stability of the composites under moist/water condition for long-term outdoor applications. Herein, unidirectional flax fiber reinforced phenolic (FFRP) composites were developed to systematically study the effects of water absorption on their mechanical properties. Water absorption tests were conducted by immersing composite specimens into distilled water at three different temperatures (23, 37.8, and 60 °C) for a period up to 3 years. Water absorption curves indicate FFRP achieved adsorption equilibrium within 64 days, exhibiting diffusion coefficients (D) of 7.3 mm2/s and maximum moisture contents (Mm) of 18.3%, which reveal the water absorption by FFRP composites in the initial stages followed Fickian behavior. After aging for 3 years, the mean values for tensile strength and Young's modulus were 57 MPa and 15 GPa, respectively, 77.7 and 47% lower, respectively, and the shear strength decreased by about 71.7% compared to the unaged specimens. The effects of hydrothermal aging on the tensile and interlaminar shear characteristics of the composites Long-term hydrothermal aging was found to weaken interlaminar shear characteristics of the composites, causing degradation/deformation of the flax fibers and degradation of the phenolic matrix. A hydrothermal aging mechanism defined as “swelling-debonding-matrix degradation-fiber degradation” at molecular scale is proposed for FFRP composites, promising to steer the future design and exploitation of natural fiber reinforced polymer composites to accommodate the prolonged adaptability at water conditions.

Biomimetic approaches towards lightweight composite structures for car interior parts

Due to good density specific mechanical properties and low carbon footprint, natural fibre-reinforced plastic composites made from fleeces and felts are frequently used as automotive interior parts. The industry targets further mass reduction. Following a biomimetic technology-pull approach, natural fibre-reinforced polypropylene plates with improved lightweight potential were produced using compression moulding. Best results were obtained for a sandwich structure with thin, unidirectional flax fibre layers surrounding a needle felt based core. It was inspired by the carapace structure of the red-eared slider Trachemys scripta elegans. The lay-up performed well, not only with a high density specific flexural stiffness of 2.33GPa1/3 cm3/g, strength of 9.46 MPa1/2 cm3/g, and energy dissipation but also through less abrupt failure due to an asymmetric orientation of unidirectional fibres in the two face sheets. Combined with a mass-reduced needle felt as a core, this concept displays a promising and unconventional approach for further mass reduced, sustainable interior panels used as non-visible and visible design elements in the vehicle.

Influence of CNT fillers in the vibration characteristics of natural fiber reinforced composite plates

Flax epoxy composites can be massively adopted in internal applications of automobiles and aircraft. This research work mainly deals with flax fiber composite in which unidirectional flax fibers are reinforced in epoxy matrix and are used in combination with CNT fillers. Such composite plate structures are manufactured and are tested using a smart actuation system. This experimental research work aims to find out the variations of the natural frequencies of composite plates made of flax fibers and CNT fillers. The consequence of various parameters like size, number of layers, etc. on CNT reinforced flax epoxy composite is discussed. The percentage of CNT fillers is varied and results for several parametric studies are presented.

Characterizing flax fiber reinforced bio-composites under monotonic and cyclic tensile loading

Traditional composites such as glass and carbon fiber reinforced polymers (G- and CFRP) can be effectively approximated to linear-elastic materials with brittle failure in most of the cases. On the contrary, natural fiber-reinforced composites (NFRCs) are characterized by non-linear elasticity, viscous effects, and plastic strains far before failure. The present study presents the results of an extensive experimental campaign aimed to the mechanical characterization of unidirectional flax fiber composites (FFRCs) produced through Light Resin Transfer Molding. The nonlinear phenomena characterizing the stiffness evolution and its relations with damage propagation and accumulated inelastic strain is investigated and analyzed through pure monotonic loading tension, cyclic tension, and repeated progressive loading tests. The analysis of the experimental results concentrates on the evaluation of the evolution of the mechanic response as the inelastic strain is accumulated in the sample. Results highlight the need for a complex material model in order to predict the mechanical behavior of flax-based composites.

Mechanical Characterization and Damage Analysis of Carbon/Flax Hybrid Laminates with Holes Machined by Abrasive Water Jet

ABSTRACT This paper experimentally examines the influence of abrasive water jet (AWJ) machined holes on the mechanical performance and damage evolution of flax/epoxy and carbon/flax/epoxy hybrid composites. The specimens were made of unidirectional flax fiber (F) and woven carbon fiber (C) in four configurations: unidirectional flax [0 F]16 (UD-F), unidirectional hybrid [0–90C2/0 F6]S (UD-CF), cross-ply [0–90C2/(0/90)F6]S (CF [0/90]), and angle-ply [0–90C2/(45)F6]S (CF [±45]). The specimens were subjected to static and quasi-static load-unload tensile tests until failure and the damage evolution and plastic strain were monitored. Results revealed severe delamination induced by AWJ at the entry and the exit of the hole in UD-CF and CF [±45] samples, and no delamination was evident in the flax/epoxy plies. The specimens with holes showed pronounced stiffness degradation compared to hole-free specimens. A similar trend was observed in their permanent plastic strain. The cross-ply hybrids showed the lowest increase in damage. While all hybrids exhibited failure by carbon fiber breakage and delamination at the carbon–flax interface, only the laminates with unidirectional flax plies failed by fiber splitting around the hole.

Tensile fatigue response of a novel carbon/flax/epoxy hybrid composite under strain-controlled and stress-controlled amplitude

This is the first study on the fatigue tensile performance of a novel hybrid composite made as Type A (i.e. woven carbon fibers plus unidirectional flax fibers [WC2/0F12/WC2]) or Type B (i.e. woven carbon fibers plus oblique flax fibers [WC2/(±45)F6S/WC2]) in epoxy resin. Composite plates were examined under constant strain amplitude using conventional fatigue tests and constant stress amplitude using thermographic stress analysis at 5 Hz cycling with a constant amplitude ratio of 0.1. For conventional fatigue, the max strain εmax versus cycles to failure Nf curve for Type A was log (Nf) = 9.7–588.2εmax, while for Type B it was log (Nf) = 14.9–1000εmax; neither material had a true endurance limit since curves never became horizontal. For thermographic stress analysis, high-cycle fatigue strength for Type A was 231.7 MPa (i.e. 56% of ultimate tensile stress σut), while for Type B it was 203.2 MPa (i.e. 60% of σut).

Visco-elastoplastic Characterization of a Flax-fiber Reinforced Biocomposite

In the presented study, the load induced long-term behavior of a biocomposite material is analyzed. The studied material is a unidirectional flax fiber reinforced epoxy resin, material, whose quasi-static mechanical properties can compare with those of glass fiber composites. Samples with a fiber direction of 0� were subjected to two types of multi-level creep-recovery tests, one with a varying creep duration, and the other with a varying creep stress, with the purpose of discriminating the viscoplastic and viscoelastic behavior of the composite. Results show a significant viscous response in time, dependent on both creep duration and creep stress, up to 20% of the elastic one. Sample damage is absent, leading to the conclusion that the viscoplastic response is caused by the permanent reorganization of the fiber�s internal structure.

Wear under brittle removal regime of an under-expanded cryogenic nitrogen jet machining of bio-composites

Machining of biocomposites using traditional techniques has shown some limitations due to the multiscale complex cellulosic structure of natural fibrous reinforcement. This paper aims to demonstrate the feasibility of the cryogenic nitrogen jet, which is a novel cutting process that combines sustainable resources and cryogenic temperatures from −175 °C to 150 °C, as a machining process for biocomposites made of unidirectional flax fibers and polylactic-acid polymer (PLA). A high-velocity stream of liquid nitrogen with and without abrasives (400–700 m/s) is hence directed onto the workpiece surface. For the abrasive jet, both conventional garnet abrasives and bio-based abrasives made from walnut shells were used for the nitrogen jet process. The kerf depth, which is representative of the erosion wear mechanisms, was calculated to assess the erosion rate of the biocomposite structure after the nitrogen jet cutting operation. Then, scanning electron microscope observations are performed to characterize the induced damages on the machined biocomposite surfaces. Results show that the pressure and the traverse speed of the nitrogen jet are the relevant process parameters that control the mechanical and thermal stresses viewed by the biocomposite during the machining operation. Each impinging abrasive particle removes a small amount of biocomposite material during the jet cutting process, which depends on the brittleness of the anisotropic work structure and the abrasiveness of the incorporated particles into the jet stream.