Elementary Fibres Research Articles

Single-fibre tensile testing of plant fibres: Set-up compliance as a key parameter for reliable assessment of their mechanical behaviour

The use of plant fibres as reinforcements in engineering applications requires reliable determination of their mechanical properties. This work focuses on single-fibre longitudinal tensile testing and the significance of the tensile system compliance for obtaining reliable results. Four types of fibre are tested, namely basalt, aramid, elementary flax fibres and flax fibre bundles. An automated laser scanning method is used to determine their cross-sectional area (CSA) based on circular and elliptical morphological models. The compliance of the tensile system is evaluated by standard and photomechanical analyses, and its influence on the assessment of mechanical properties of the fibres is discussed. Our results show that the evaluation of the compliance is highly dependent on fibre type, but also on the choice of CSA morphological model. The resulting correction on maximum tensile moduli ranges from 6.5 % to 37.6 % for small-diameter fibres, and can reach 55.1 % for large-diameter fibres as flax fibre bundles. Moreover, we evidence that the tensile system compliance is not necessarily constant and evolves with the force range covered during the tensile test. This can have a significant impact on the shape of the stress-strain curves, and hence on the analysis of the overall mechanical behaviour of the fibres.

Mechanical Characterization of Flax and Hemp Fibers Cultivated in Romania.

This study examines the mechanical properties, specifically strength and stiffness, of technical hemp and flax fibers grown in Romania. Tensile testing was employed to determine stress-strain curves and the Young's modulus and to assess the failure strength of both fiber types. Although samples of various lengths were tested, no significant length-dependent variations were observed. However, a strong dependence on fiber diameter was noted, with the smallest diameters approaching the documented strength of elementary fibers. Due to the considerable variability in the experimental results pertaining to the characteristics of the reinforced fibers, a statistical analysis using a two-parameter Weibull distribution was employed. The analysis revealed three distinct stress-strain curve profiles, i.e., linear, bi-linear, and tri-linear patterns, with the average ultimate stress ranging from 412 to 566 MPa for hemp and 502 to 598 MPa for flax.

Exploring eco-friendly pseudo stem fiber from the agricultural byproducts of the Alpinia purpurata plant

This study explored the potential of Alpinia purpurata pseudostems, an agricultural waste product, as a sustainable and eco-friendly source of natural fibers for the textile industry. The aim was to characterize the properties of natural cellulose fibers from the pseudostems of the Alpinia purpurata plant. The research employed water retting to extract the fibers, and the intrinsic fiber properties obtained were evaluated for their suitability in textile applications. The average length of an Alpinia purpurata fiber was 67.80 cm with a fineness value of 7.61 Tex. A bundle of Alpinia purpurata fibers was comprised of many elementary fibers. Alpinia purpurata had an irregular cross-section, with the lumen having a varied oval shape. The fibers exhibited tenacity and elongation values of 2.49 g/denier and 5.72 %, respectively, and a coefficient of friction value of 0.49. Additionally, Alpinia purpurata fibers were hygroscopic, with a moisture regain value of 11.97 %. The findings revealed that the fibers possessed notable tensile strength, moderate elongation, and high moisture regain, making them suitable for non-apparel textile products requiring durability and moisture management. While not the strongest among natural fibers, their sufficient strength and other advantageous properties demonstrated their viability for various applications. This work addressed a significant gap in the literature on the utilization of agricultural by-products, proposing Alpinia purpurata pseudostems as a sustainable alternative that aligned with environmentally responsible manufacturing practices and supported the circular economy. Future research was recommended to optimize processing techniques and explore broader applications.

Experimental and numerical approach to understand the role of defects in damage mechanisms of flax fibers at bundle scale

Flax fibers represent a valuable reinforcement in composite materials. However, the presence of defects known as kink-bands in flax fiber has the capacity to influence their mechanical properties. The primary objective of this research is to gain a comprehensive understanding of the damage mechanism in flax fiber bundles presenting a novel aspect due to the intricate structure of bundle fibers, which is more complex compared to elementary fibers. The study specifically explores flax fiber bundles under tensile loading and investigates the impact of kink-bands on their failure through a combined approach, utilizing both experimental and numerical methods. In the experimental part, X-ray microtomography in situ tensile testing is carried out, which reveals complex failure mechanisms. Furthermore, a 3D-Finite Elements (FE) model is implemented utilizing 3D reconstructed fiber and used for numerical tensile analysis. Our findings highlight that the kink-band region acts as prominent failure site in fiber bundles with significant stress concentration at the kink-band region, mainly induced by local porosity.

Winter flax: Stem structure, fibre properties and reinforcement potential for composite materials

Climate change is characterized by rising temperatures and an increased risk of spring water deficit, both of which threaten the cultivation of textile flax. With the multiplication and intensification of these phenomena, fibre plant yields have become increasingly uncertain in recent years, leading to shorter stems as well as reduced yields. In this context, winter flax cultivation emerges as a solution to sustain the exploitation of this plant. Sown before winter, it can resist periods of spring water stress, owing to the early development of it root system, and is also protected from the summer heat. In this study, we investigate the stem architecture and fibres properties of two winter flax varieties, in comparison to spring flax. Cross-sections reveal that both winter and spring flax have a similar plant structure. The part located at mid-height of the winter stem contains the most fibres, accounting for around 23% of the biomass. At the middle of the stem, the elementary fibres of the Cirrus and Olga varieties have an average length of 20.9 ± 7.7 mm and 30.0 ± 10.8 mm, respectively. At this same plant height, winter flax fibres of the Cirrus and Olga varieties have an average Young’s modulus of 63.6 ± 18.8 GPa and 57.6 ± 20.8 GPa, and an average strength at break of 1248 ± 554 MPa and 1187 ± 627 MPa, respectively. As a conclusion, winter flax has a similar length and mechanical properties to spring flax and therefore offers the same potential for textile applications. In addition, as demonstrated in this study, winter flax can be similarly useful as a reinforcement in composite materials.

Synchrotron X-ray microtomography and finite element modelling to uncover flax fibre defect’s role in tensile performances

Flax fibres offer performance capabilities comparable to glass fibres, thereby enhancing their potential in the biobased composites industry. However, these fibres have morphological defects affecting their mechanical features. In the present work, flax elementary fibres geometries with defects assessed by synchrotron X-ray microtomography were meshed to simulate a tensile test using finite element analysis. For the first time, the distribution of stresses in the vicinity of defects is revealed. The geometrical irregularities at the surface of the fibre and the delamination of cellulose layers within fibre cell wall turned out to concentrate stress up to 7.5 times compared to defect-free regions. These results demonstrate why flax fibres cannot reach their full potential in comparison to what could be expected from a structure mainly constituted from crystalline cellulose microfibrils, and why fracture in a composite is likely to initiate in those defect zones.

Insights into the assembly process and properties of regenerated cellulose beads prepared in alkali/urea aqueous solutions

Taking the perspective of cellulose molecular chain assembly via the “bottom-top” route, we delve into the influence of both the cellulose solution and the coagulation bath on the assembly process and structure of regenerated cellulose beads (RCBs). The results show that cellulose molecular weight, mass fraction, and the presence of surfactant have an impact on RCBs. Contrary to traditional views where the structures of material are determined by solvent-nonsolvent exchange rate, ion-cellulose binding capacity also affects RCBs. Overall, the influence of ions follows the Hofmeister sequence. Kosmotropes promote the assembly of cellulose chains and elementary fibers, leading to “salting out” effects, reduced pore size of RCBs, increased crystallinity, and enhanced mechanical properties. In contrast, chaotropes induce “salting in” effects, resulting in opposite outcomes. The average pore size of RCBs coagulated in NaSCN solution was approximately 15-folds larger than those prepared in sodium citrate solution. Anions have a greater impact than cations, and both “salting out” and “salting in” effects strengthen with concentration. Temperature variations primarily affect solvent and nonsolvent exchange speed during cellulose regeneration. These findings provide new insights into regulating RCBs, enabling tailored performance for different applications.

Mechanical behaviors of composites made of natural fibers through environmentally friendly treatment

Natural fiber reinforced composites have garnered significant interests as potential substitutes for conventional materials because of their eco-friendly attribute and favorable physical and mechanical properties. Typically the natural fiber undergoes chemical treatment before processing with the matrix to produce composites, however, the chemical treatment can have a negative impact on the environment. This research work presents an environmentally friendly treatment method for hemp fibers by using boiling water and shear force for specific time periods. The purpose of the treatment is to break down the technical fiber bundles into elementary fibers, which creates a fourfold increase in bonding surface area between the fibers and matrix. The change in fiber length and size before and after the debundling treatment were analyzed using optical microscope, confocal microscope, and scanning electron microscopy. The treated fibers were then made into mats through a wet-laid process and compression molded with low density polyethylene via film stacking. The effects of different fiber treatment variables, including debundling time, on mechanical properties were compared with composites reinforced with conventional alkali treated fibers. The results presented show that the composites reinforced by hemp fiber using the new treatment method have equivalent or improved tensile, flexural and impact properties than the composite reinforced with alkali treated fibers.

Effect of extension speed on the mechanical performance of sisal and coir fiber bundles

Quasi-static tensile tests are commonly used to determine the mechanical properties of fiber bundles. These tests are generally carried out at a low standard speed of 1 mm.min−1, which is recommended for elementary fibers, although other moderate and high speeds are rarely used. In this study, tensile tests were carried out on coir and sisal fiber bundles from the coastal region of Cameroon using four different speeds: 1, 2, 5, and 10 mm.min−1 to assess mechanical properties. Samples were prepared by attaching the two ends of each randomly selected fiber bundle to a paper frame to obtain a length of 30 mm. The results showed a significant effect of extension speed, indicating a decrease in tensile strength and Young’s modulus, and an increase in strain at break with increasing speed. These trends were correlated with the power functions. A strong dependence between fiber bundle diameter and tensile strength was also observed. Furthermore, statistical analysis based on the Weibull distribution highlighted the dispersion of mechanical properties, demonstrating a reduction in dispersion with increasing extension speed for coir fibers. These data can be used to develop specific standards for these natural fibers and contribute to the advancement of composites based on plant fibers.

The impact of sodium periodate oxidation and alginate coating on the capillarity of jute fabric

This study explores the impact of sodium periodate oxidation time (30 min, 60 min, or 120 min) and alginate coating on the capillarity of jute fabrics. Raw jute fabric exhibits a rapid increase in capillary rise height ( h) over time, reaching 72 mm at 1200 s. Fabrics oxidized for 30, 60, and 120 min show significantly lower h values of 19, 18, and 28 mm at 1200 s, respectively, which can be ascribed to the oxidation process itself. Precisely, oxidation-induced conversion of hydroxyl groups of cellulose (the main constituent of jute fiber) to aldehyde groups contributed to the fabric reduced affinity for water, hence negatively affecting its capillarity. Approximately 47% and 55% lower h values of fabrics oxidized for 30 and 60 min compared to that oxidized for 120 min can be explained by their lower weight loss after oxidation as well as less pronounced liberation of elementary fibers. Oxidized fabrics characterized by low capillarity can be used for the manufacturing of packaging materials, outdoor gear, and protective covers for crops. To expand jute fabric applications in biocarpet engineering, that is, to obtain polysaccharide-based supports, raw and oxidized jute fabrics were treated with a 1% sodium alginate solution. At 1200 s, oxidized fabrics coated with alginate exhibited 4.3–5.8 times higher h values compared to solely oxidized counterparts. The diffusion coefficients of oxidized fabrics coated with alginate increase up to 35% with prolonged oxidation, emphasizing the oxidation impact on alginate binding.

Use of a commingling process for innovative flax fibre reinforced unidirectional composites

Optimisation of textile preforms play a crucial role in the development of high-performance biobased composites materials. In this context, the main ambition of this work is to quantify and assess the mechanical properties and behaviour of biocomposite materials made from unidirectional commingled preforms based on flax and poly-(propylene) fibres. To the best of our knowledge, there is no literature examining the effect of the commingling process on the ultrastructure and the mechanical properties of flax fibres. At the scale of the elementary fibre, fibre mechanical properties are observed to be stable after commingling. However, repeated drawing in the commingling process leads to increased cellulose crystallinity and a larger fraction of elementary fibres exhibiting quasi-linear tensile behaviour. The composite materials produced with these commingled flax/poly-(propylene) preforms contain few cortical residues and show a remarkable degree of fibre individualisation. Moreover, they exhibit high Young's modulus and a stress at break of 24 GPa and 194 MPa, respectively, for a fibre volume fraction of 36 %. A substantial drop in properties is however noted at high fibre fractions due increased heterogeneity of the materials. Remarkably, the biocomposites achieved unprecedented transverse modulus and stress at break of 2.3 GPa and 16.5 MPa, respectively. Our results validate the potential and interest in commingling processes for designing a new family of plant fibre composite materials.

Evolution of stiffness in flax yarn within flax fiber reinforced composites during moisture absorption

This study aims to investigate the evolution of stiffness in flax yarn within flax fiber reinforced composites (FFRCs) during moisture absorption, focusing on the influence of moisture content on the microstructure of flax fibers. To characterize the hygroscopic mechanical behaviors of flax yarns in FFRCs, the hygroscopicity and tensile properties of dried and impregnated flax yarns were tested at various humidity levels. A multi-scale modeling approach was utilized to simulate the stiffness in flax yarn within FFRCs, encompassing the modeling of cell wall layers of the flax elementary fiber, flax elementary fiber and twisted flax yarn. Specifically, the variation trends of the microfiber angle (MFA) in the S2 layer and the stiffness degradation of the combined amorphous matrix (CAM) with respect to relative moisture content (RMC) were proposed and determined through simulation and inversion calculation. The study reveals that the MFA in the S2-layer is the most crucial parameter affecting the longitudinal elastic properties of flax yarn in FFRCs, while the stiffness degradation of the CAM significantly influences the transverse elastic properties. Finally, this study establishes a relationship between the overall stiffness of flax yarn in FFRCs and the RMC. This relationship provides a parameter foundation for accurately predicting the mechanical properties of FFRCs during moisture absorption.

Elucidating links between the mechanical performance of flax fibres and their structural defects

Single flax fibres develop morphological deformations, called kink-bands, which are considered to be areas of weakness. The correlation between the occurrence of kink-bands, fibre extraction method and fibre mechanical properties still needs to be clarified. To this end, an extensive multi-scale statistical study was conducted on 96 elementary fibres extracted from four distinct batches processed with different scutching and combing conditions. First, kink-bands were observed and quantified through polarized light microscopy (PLM), scanning electron microscopy (SEM) and synchrotron X-ray microtomography. PLM quantification demonstrated that uncombed fibres have more cortical residues and larger surface kink-bands with many small internal porosities. Then, the mechanical characteristics of each PLM-quantified single fibre were measured by tensile tests. A larger number of kink-bands was observed for the most intensively combed batch, 18.2 kink-bands/mm, as well as a surprisingly low average kink-band area, of the order of 158 µm² per kink-band. In addition, the same batch had a higher tensile strength and modulus than the other three batches with a negative Stearman correlation of − 0.64 and − 0.78 with the proportion of defects, respectively. Finally, 74% of the fibres in this intensively combed batch, had a ‘Type I’ linear elastic behaviour, highlighting a significant change of behaviour in comparison to the uncombed batch, attributed to local reorientation and pre-stretching. The results suggest that changes in kink-bands structure are principally responsible for these evolutions; due to both new distribution of pores in kink-bands regions and reduction in kink-band size possibly induced by local cellulose realignment.

ANN-based structure peciliaties evaluation of polymer composite reinforced with unidirectional carbon fiber

At the moment, there is a growing interest in composite materials with matrices based on thermoplastic polymers; these materials are superior to traditional carbon fiber plastics due to higher fracture toughness and impact strength, low smoke generation during combustion, and high thermal and chemical resistance. The deformation behavior of such composite materials due to the high plastic deformation of the matrix differs from the behavior of traditional composites, which must be considered when performing calculations. In our study, as a model object, the features of the microstructure of single carbon thread impregnated with polysulfone were studied in order to evaluate the distribution of misorientation angles of elementary fibers, the degree of their damage, the thickness of the polymer matrix interlayers between elementary fibers in threads impregnated with more viscous thermoplastic binders. Statistical analysis of a large array of micrographs was carried out using specially developed algorithms for computer processing of electron microscopic images of a composite material. Algorithms for processing arrays of images using machine vision algorithms are proposed. Based on the analysis of the data array, the distributions of carbon fibers impregnated with polysulfone were plotted over the misorientation angle of the filaments and over the interfilament distance in the longitudinal and cross sections of the fiber. Using the machine learning method, an algorithm for detecting violations of the filament structure was implemented. The data obtained can be used to refine the calculations of the strength and deformation characteristics of composite materials with thermoplastic matrices.

Identification of new potential molecular actors related to fiber quality in flax through Omics.

One of the biggest challenges for a more widespread utilization of plant fibers is to better understand the different molecular factors underlying the variability in fineness and mechanical properties of both elementary and scutched fibers. Accordingly, we analyzed genome-wide transcription profiling from bast fiber bearing tissues of seven different flax varieties (4 spring, 2 winter fiber varieties and 1 winter linseed) and identified 1041 differentially expressed genes between varieties, of which 97 were related to cell wall metabolism. KEGG analysis highlighted a number of different enriched pathways. Subsequent statistical analysis using Partial Least-Squares Discriminant Analysis showed that 73% of the total variance was explained by the first 3 X-variates corresponding to 56 differentially expressed genes. Calculation of Pearson correlations identified 5 genes showing a strong correlation between expression and morphometric data. Two-dimensional gel proteomic analysis on the two varieties showing the most discriminant and significant differences in morphometrics revealed 1490 protein spots of which 108 showed significant differential abundance. Mass spectrometry analysis successfully identified 46 proteins representing 32 non-redundant proteins. Statistical clusterization based on the expression level of genes corresponding to the 32 proteins showed clear discrimination into three separate clusters, reflecting the variety type (spring-/winter-fiber/oil). Four of the 32 proteins were also highly correlated with morphometric features. Examination of predicted functions for the 9 (5+4) identified genes highlighted lipid metabolism and senescence process. Calculation of Pearson correlation coefficients between expression data and retted fiber mechanical measurements (strength and maximum force) identified 3 significantly correlated genes. The genes were predicted to be connected to cell wall dynamics, either directly (Expansin-like protein), or indirectly (NAD(P)-binding Rossmann-fold superfamily protein). Taken together, our results have allowed the identification of molecular actors potentially associated with the determination of both in-planta fiber morphometrics, as well as ex-planta fiber mechanical properties, both of which are key parameters for elementary fiber and scutched fiber quality in flax.

Radiation induced graft polymerization of fluorinated monomers onto flax fabrics for the control of hydrophobic and oleophobic properties

Functional textiles have been one of the most studied topics in recent decades. Due to toxicity concerns, the treatment of fabrics for non-wettable properties with fluorinated molecules is nowadays limited to short fluoroalkyl chains. In this work, several fluorinated (meth)acrylic monomers such as 1,1,1,3,3,3-hexafluoroisopropyl methacrylate, 2-(perfluorobutyl)ethyl methacrylate, 1H,1H,2H,2H-perfluorooctyl methacrylate, 1H,1H,2H,2H-perfluorooctyl acrylate and 1H,1H,2H,2H-perfluorodecyl methacrylate (denoted as M2, M4, M6, AC6 and M8, respectively) were grafted onto flax fabrics to improve their hydrophobic and oleophobic properties by radiografting using a pre-irradiation procedure. The effects of the length of the fluoroalkyl group of the monomer, the irradiation dose and the reaction time on the grafting rate were studied. The functionalization of flax fabrics by these five monomers was confirmed by FTIR spectroscopy analysis. The morphology of the fibers before and after treatment was studied by scanning electron microscopy (SEM). The location of fluorine element in the flax elementary fibers was assessed by SEM-EDX. The hydro- and oleophobic properties were determined by contact angle measurement using water and diiodomethane as solvents, respectively. The sliding angle was also measured to assess the water repellency of hydrophobic fabrics. Significant fluorine contents were obtained for flax fibers irradiated and treated with 10 wt% solutions of monomer. Hydro- and oleophobic fabrics were obtained for flax irradiated even at a low dose of 5 kGy and treated with M4, with water and diiodomethane contact angles (CAs) of about 139° and 120°, respectively. Superhydrophobic fabrics (CA = 150°) were also achieved for samples irradiated at 20, 50 and 100 kGy and treated with 10 wt% of M8. For this monomer, SEM analysis showed the formation of spherical microparticles of fluoropolymer covering the surface of elementary fibers. The formation of these particles is assumed to result from a dispersion polymerization mechanism. This is due to the fewer affinity of M8 with methanol and to the incompatibility of the formed polymer chains with this solvent used for the grafting reaction.

Elementary Liber Fibres Characterisation: Bias from the Noncylindricity and Morphological Evolution along the Fibre

In this work, we investigate the influence of noncircularity along with cross-sectional area evolution on the measurement of the mechanical properties of elementary fibres. First, we focus on the cross-sectional area measurement and compare the circular assumption with the elliptical one using an ombroscopic device that allows the measurement of the projected diameters along the fibre as the fibre rotates around its axis, the fibre dimensional analysis system (FDAS). The results highlight important approximations to the cross-sectional area evaluation for fibres with noncircular cross sections, leading to reduced elastic modulus and stress at failure evaluated by the standard method. Additionally, results from the FDAS are used to evaluate the twist inside an individual fibre when the cross sections are sufficiently elliptical. A numerical model based on the real measured dimensions of the fibres is developed to illustrate and visualize this nonuniformity and to more accurately identify the elastic modulus. The results obtained lead us to an analytical approach that takes into account the evolution of the cross-sectional area along the fibre for a better identification of the stiffness and modulus of elasticity, which maximizes the identified mechanical properties on average by 12% for the modulus and 200% for the stress at failure. Finally, recommendations are formulated to better account for the variability along a fibre in order to evaluate the cross-sectional area.

Properties and structural features of epoxycomposites reinforced with modified PAN- flagellum

The performance characteristics of composite materials reinforced with polyacrylonitrile technical flagellum (PAN-TF), which has the reactivity of functional groups, can be improved by modifying it. The aim of the work was to study the effect of polyacrylonitrile fibrous materials, applied with AGM-9, A-187, A-174 and Duron OS 3151 modifiers, on the kinetics of the ED-20 epoxy resin curing process, structural features of the developed composites and their operational properties. The methods of kinetic research, differential scanning calorimetry (DSC), scanning electron microscopy and mechanical testing of composite materials were used in the work. The effect of modified PAN-TF on the change in the kinetics of the curing process of the epoxy binder is shown, characterized by an increase in the curing time, a decrease in the maximum curing temperature and a decrease in the activation energy of the curing ED-20. The results of kinetic studies are confirmed by the data of DSC epoxy compositions based on modified PAN-TF, which show a decrease in the maximum temperature and an increase in the thermal effects of the ED-20 curing process in the presence of the studied reinforcing systems compared with epoxy composite reinforced with unmodified flagellum. The structure formation of epoxy compositions under milder conditions ensures the formation of a contact zone between the elementary fibers and the binder, which contributes to an increase in the solidity of plastics compared to a composite based on the original PAN-TF. A comparative analysis of the strength properties of the studied composites showed that when applied to ED-20, there is an improvement in strength indicators. The assessment of kinetic parameters, structural features and strength properties of the developed composites reinforced with modified PAN-TF indicates an increase in the surface activity of fibrous materials as a result of their modification.

Experimental determination of the adhesive characteristics of the “elementary fiber-epoxy matrix” system

The paper considers the method of testing the elementary fiber-polymer matrix system. Results of determining the following adhesive characteristics are provided: apparent and local adhesive strength, start and end in the adhesive destruction of the fiber-matrix joint under study. Basalt fiber and epoxy matrix based on the epoxy resin and amine hardener are considered as the study objects. Influence of the samples geometric characteristics on their adhesive characteristics is established. The paper shows that at the depth of the elementary fiber emersion in the polymer matrix equal to 200 µm, the scatter of all determined adhesive characteristics is significantly reduced.

A Method for Assessing the Adhesion Strength of an Elementary Fiber–Epoxy Matrix System

A Method for Assessing the Adhesion Strength of an Elementary Fiber–Epoxy Matrix System