Plant Fibre Composites Research Articles

Optimising processing conditions of flax fabric reinforced Acrodur biocomposites

Acrodur solution and dispersion have been developed as alternative wood adhesives to phenolic and urea formaldehyde resins. They are non-corrosive and do not emit carcinogenic gases. Contrary to most resins used in natural plant fibre composites, Acrodur has superior tolerance to moisture during composite fabrication and thus drying of the preforms may be minimised or eliminated. The aim of this study was to produce optimised flax fabric reinforced Acrodur biocomposites by varying the ratio between Acrodur solution and dispersion, relative humidity, curing time and temperature. The optimised biocomposites provided a combination of specific tensile strength 57.9 MPa-cm3/g and specific Young’s modulus 5.5 GPa-cm3/g at a low density 0.91 g/cm3. Thermogravimetric analysis and contact angle measurement showed that the biocomposites had higher thermal stability and hydrophobicity than the fabrics. The low loss of tensile properties upon water immersion was explained by tortuous wicking path in the biocomposites.

Developing plant fibre composites for structural applications by optimising composite parameters: a critical review

Plant fibres, perceived as environmentally sustainable substitutes to E-glass, are increasingly being employed as reinforcements in polymer matrix composites. However, despite the promising technical properties of cellulose-based fibres and the historic use of plant fibre reinforced plastics (PFRPs) in load-bearing components, the industrial uptake of PFRPs in structural applications has been limited. Through an up-to-date critical review of the literature, this manuscript presents an overview on key aspects that need consideration when developing PFRPs for structural applications, including the selection of (I) the fibre type, fibre extraction process and fibre surface modification technique, (II) fibre volume fraction, (III) reinforcement geometry and interfacial properties, (IV) reinforcement packing arrangement and orientation and (V) matrix type and composite manufacturing technique. A comprehensive materials selection chart (Ashby plot) is also produced to facilitate the design of a PFRP component, based on the (absolute and specific) tensile properties.

Wood versus Plant Fibers: Similarities and Differences in Composite Applications

The work on cellulose fiber composites is typically strictly divided into two separated research fields depending on the fiber origin, that is, from wood and from annual plants, representing the two different industries of forest and agriculture, respectively. The present paper evaluates in parallel wood fibers and plant fibers to highlight their similarities and differences regarding their use as reinforcement in composites and to enable mutual transfer of knowledge and technology between the two research fields. The paper gives an introduction to the morphology, chemistry, and ultrastructure of the fibers, the modeling of the mechanical properties of the fibers, the fiber preforms available for manufacturing of composites, the typical mechanical properties of the composites, the modeling of the mechanical properties with focus on composites having a random fiber orientation and a non-negligible porosity content, and finally, the moisture sensitivity of the composites. The performance of wood and plant fiber composites is compared to the synthetic glass and carbon fibers conventionally used for composites, and advantages and disadvantages of the different fibers are discussed.

Fatigue life evaluation of aligned plant fibre composites through S–N curves and constant-life diagrams

There is a noticeable lack of fatigue data on plant fibre composites (PFCs) which seriously limits their prospective use in fatigue critical components. The objective of this manuscript is to provide a complete set of fatigue data on vacuum infused aligned PFCs. S–N lifetime diagrams have been constructed to investigate the effect of (i) plant fibre type/quality, (ii) fibre content, (iii) textile architecture, and (iv) stress ratio, on PFC cyclic loading behaviour. At each stage, the fatigue performance of PFCs is compared to that of E-glass/polyester (GFRP) composites. A comprehensive constant-life diagram has been generated to facilitate life prediction of PFC components. While plant fibre type and quality, textile architecture and composite fibre content have a significant impact on the static properties, they have little impact on the material fatigue strength coefficient b (which dictates the slope of the S–N curve). In essence, higher static properties are a sign of superior fatigue loading capacities throughout their lifetime. Increasing stress ratios lead to improved fatigue performance in PFCs. Fracture mechanisms and modes are the same for composites made from three different bast fibres, but depend on fibre content, textile architecture and stress ratios. Although the absolute fatigue performance of GFRPs is far superior to PFCs, it is a revelation to find that fatigue strength degradation rates are lower in PFCs than in GFRP.

Determining the minimum, critical and maximum fibre content for twisted yarn reinforced plant fibre composites

Abstract The effect of fibre volume fraction on the physical and tensile properties of aligned plant fibre composites (PFCs) produced via vacuum infusion has been investigated. There is no clear correlation between fibre volume fraction and porosity. However, low fibre content PFCs are prone to intra-yarn voids, while high fibre content PFCs are prone to inter-yarn voids. This is due to changing resin flow dynamics with increasing fibre content. The tensile behaviour of PFCs with increasing fibre content is similar to that of conventional fibre reinforced plastics (FRPs). Interestingly, the non-linear stress–strain response of plant fibres has been transferred to the composites. Fibre content and tensile properties are found to be linearly related, as per the rule of mixtures. A void content of up to 4% is found to have minimal effect on the tensile properties of PFCs. The minimum and critical ( v f , crit ) fibre volume fractions for aligned flax and jute–polyester composites are found to be substantially higher than conventional aligned FRPs; v f , crit for jute–polyester and carbon–polyester is 8.1% and 2.4%, respectively. A simple model has also been developed to approximate the theoretical maximum obtainable fibre volume fraction of PFCs reinforced with staple fibre yarns. The absolute theoretical maximum fibre content is found to be 58.9%, which agrees with experimental values in literature. A high v f , crit (∼10%) and low v f , max (∼45%) implies that the range of useful fibre volume fractions for PFCs containing staple fibre twisted yarns is only 35%.

The tensile behavior of off‐axis loaded plant fiber composites: An insight on the nonlinear stress–strain response

AbstractComposites in load‐bearing applications are often exposed to off‐axis loads. For plant fiber composites (PFCs) to be seriously and readily considered in structural applications, knowledge and reliable prediction of their response to off‐axis loads is critical. This article (i) characterizes the stress–strain response, (ii) investigates the tensile properties, and (iii) analyses the fracture modes, of unidirectional flax‐polyester composites subjected to off‐axis tensile loading. A key finding of this study is that due to the nonlinear stress–strain response of PFCs, the apparent stiffness of the composite reduces by ∼30% in the strain range of 0.05 to 0.25%. In addition, through cyclic tests on the composites, the elastic strain limit is found to be only ∼0.15%. This has major implications on the strain range to be used for the determination of the composite elastic Young's modulus. Consequently, it is proposed that the tensile modulus for PFCs should be measured in the strain range of 0.025 to 0.100%. Through comparison with experimental data, conventional composite micromechanical models are found to be adequate in quantitatively describing the tensile behavior of off‐axis loaded PFCs. The application of such models has also enabled the determination of, otherwise difficult to measure, material properties, such as fiber shear and transverse modulus. Off‐axis loaded PFCs fail by three distinct fracture modes in three different off‐axis ranges; each fracture mode produces a unique fracture surface. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers

Modelling the effect of yarn twist on the tensile strength of unidirectional plant fibre yarn composites

The structural potential of plant fibres as reinforcing agents can only be realized when the highest reinforcement efficiency is employed. Hence, aligned plant fibre composites are of interest. However, due to the short length of technical plant fibres, the reinforcement needs to be in the form of staple fibre yarns, which have a twisted structure. Although twist facilitates yarn processability, it has several detrimental effects on the composites produced from such twisted yarn reinforcements; one of which is fibre obliquity and misalignment. This results in a drastic drop in composite mechanical properties. No analytical model currently exists to accurately predict the effect of yarn twist on aligned plant fibre composite tensile strength. In this paper, a novel mathematical model is developed. The model is based on (i) a modified rule of mixtures for plant fibre composites, (ii) well-defined structure-property relationships in an idealised twisted staple fibre yarn and (iii) the Krenchel orientation efficiency factor. The developed model includes a corrected orientation efficiency factor of cos2(2 α), where α is the yarn surface twist angle. The model is validated with extensive experimental data from Goutianos and Peijs (Goutianos S, Peijs T (2003) Adv Compos Lett 12(6):237) and is found to be a near-perfect fit ( R2 = 0.960). Experimental data from other studies are also used for further verification.

Prosthetic limb sockets from plant-based composite materials

There is a considerable demand for lower limb prostheses globally due to vascular disease, war, conflict, land mines and natural disasters. Conventional composite materials used for prosthetic limb sockets include acrylic resins, glass and carbon fibres, which produce harmful gasses and dust in their manufacture. To investigate the feasibility of using a renewable plant oil-based polycarbonate-polyurethane copolymer resin and plant fibre composite, instead of conventional materials, to improve safety and accessibility of prosthetic limb manufacture. Experimental, bench research. Test pieces of the resin with a range of plant fibres (10.0% by volume) were prepared and tensile strengths were tested. Test sockets of both conventional composite materials and plant resin with plant fibres were constructed and tested to destruction. Combinations of plant resin and either banana or ramie fibres gave high tensile strengths. The conventional composite material socket and plant resin with ramie composite socket failed at a similar loading, exceeding the ISO 10328 standard. Both wall thickness and fibre-matrix adhesion played a significant role in socket strength. From this limited study we conclude that the plant resin and ramie fibre composite socket has the potential to replace the standard layup. Further mechanical and biocompatibility testing as well as a full economic analysis is required. Using readily sourced and renewable natural fibres and a low-volatile bio-resin has potential to reduce harm to those involved in the manufacture of artificial limb sockets, without compromising socket strength and benefitting clinicians working in poorer countries where safety equipment is scarce. Such composite materials will reduce environmental impact.

Post-impact static and cyclic flexural characterisation of hemp fibre reinforced laminates

The suitability for use of plant fibre composites after an impact event is still an open issue in literature. In this work, hemp fibre reinforced laminates have been subjected to cyclic flexural tests following falling weight impact at 12, 16 and 20J. At these energies, still quite far from penetration, which was at about 40J, damage starts to be increasingly apparent on both laminate surfaces. Post-impact flexural tests have been monitored using acoustic emission. Laminates with a sufficiently strong fibre–matrix interface have been obtained, as revealed from the impact hysteresis cycles and electron microscopy damage characterisation. However, a quite significant decrease of flexural properties and an increase of unrecoverable deformation after cyclical loading were also revealed with growing impact energies, in particular passing from 12 to 16J.

Wear Behavior of Plant Fiber (Pine-Bark) and Cement Kiln Dust-Reinforced Polyester Composites Using Taguchi Experimental Model

Parametric appraisal of a dry sliding wear process is presented for a set of new composites consisting of polyester as the matrix, flakes of pine-bark as the fibrous reinforcing component, and the kiln-dust of a cement plant as the filler. The filler content in the composites is fixed at 50 wt%, while the weight fraction of fiber reinforcement is varied (0—12 wt%) so as to obtain composite samples of three different compositions. Wear tests are carried out with the help of a pin-on-disc test rig employing the design of experiments approach based on Taguchi’s orthogonal arrays. The findings of the experiments indicate that the rate of wear is greatly influenced by various control factors. An optimal parameter combination is determined, which leads to minimization of wear rate. Analysis of variance is performed on the measured data and signal-to-noise (S/N) ratios. A mathematical correlation, consistent with the experimental observations is proposed as a predictive equation for estimation of sliding wear rate of these composites. Finally, optimal factor settings for minimum wear are determined using genetic algorithm.

Plant fibre composites – porosity and stiffness

Plant fibre composites contain typically a relatively large amount of porosity which influences their performance. A model, based on a modified rule of mixtures, is presented to include the influence of porosity on the composite stiffness. The model integrates the volumetric composition of the composites with their mechanical properties. The fibre weight fraction is used as an independent parameter to calculate the complete volumetric composition. A maximum obtainable stiffness of the composites is calculated at a certain transition fibre weight fraction, which is characterised by a best possible combination of high fibre volume fraction and low porosity. The model is validated with experimental data from the literature on several types of composites. A stiffness diagram is presented to demonstrate that the calculations can be used for tailoring and design of composites with a given profile of properties.

Mechanical and falling weight impact properties of unidirectional phormium fibre/epoxy laminates

The falling weight impact properties of composites obtained by introducing 16 wt% of phormium tenax fibres extracted with two different methods i.e., either manually or by paddocking and scutching and in both cases chemically treated with a 1% solution of sodium hydroxide, have been investigated. The effect of the two extraction methods on fibre characteristics is compared by the tensile properties of the fibres and the flexural properties, fracture modes and hysteresis cycles parameters (impact energy partition, linear stiffness and normalised penetration energy) of final laminates. Laminates obtained using paddocked and scutched fibres are clearly superior to those manufactured using the manually extracted fibres: this is due to the more effective fibre impregnation in the former case than in the latter, which results in an improved dissipation of energy during the damping phase of the impact event. It is noteworthy, however, that the low vol- ume of fibres introduced in the laminate with the manufacturing method adopted does not allow obtaining properties com- parable with other semi-structural plant fibre composites, such as e.g., hemp fibre reinforced laminates.

Plant fibre composites – porosity and volumetric interaction

Plant fibre composites contain typically a relative large amount of porosity, which considerably influences properties and performance of the composites. The large porosity must be integrated in the conversion of weight fractions into volume fractions of the fibre and matrix parts. A model is presented to predict the porosity as a function of the fibre weight fractions, and to calculate the related fibre and matrix volume fractions, as well as the density of the composite. The model predicts two cases of composite volumetric interaction separated by a transition fibre weight fraction, at which the combination of a high fibre volume fraction, a low porosity and a high composite density is optimal. Experimental data from the literature on volumetric composition and density of four types of plant fibre composites are used to validate the model. It is demonstrated that the model provides a concept to be used in fabrication, process optimisation and design of plant fibre composites.

Post-impact flexural tests on jute/polyester laminates monitored by acoustic emission

A number of industrial sectors, including automotive, building and leisure, have recently shown interest on natural fibers, as reinforcement for low-weight, environmental-friendly materials. The introduction of natural fiber composites in components for large volume applications requires studies aimed at predicting their in-service behavior, in particular fatigue and impact testing. Some recent investigations have raised concerns on the performance of these materials, especially when subjected to a high number of cycles, as an effect of a long-service life, in harsh environmental conditions [1‐3]. The use of nondestructive testing techniques, such as acoustic emission, as monitoring methods during mechanical tests, proved useful in previous studies on mechanical characterization of natural fiber composites [4, 5]. Two approaches are used to predict impact damage on laminated composites reinforced with artificial fibers [6]. The first one is based on estimating the overall size of impact-damaged area and considering stress distribution in the region surrounding the impact point, whilst the second one is aimed at detecting the appearance of the first matrix crack, then the study of the initiation and propagation of delamination. When dealing with plant-fiber composites, both approaches are viable, at least in principle: however, a number of difficulties can be perceived. First, the measurement of impact-damaged area has been revealed to be particularly difficult, as an effect of the fibers becoming loose and suffering early debonding around the impact point, even at low stress [7]. Second, the study of impact damage initiation is based on the assumption that the laminate shows limited presence of defects prior to impact and that the direction of impact, whether mono- or bi-dimensional, determines the damage propagation mode. However, in biological materials, such as plant fibers, the combined presence of stronger and weaker parts is a natural layout, aimed at obtaining the maximum possible impact resistance. In other words, natural materials work effectively through the limited and controlled occurrence of defects [8]. In plant fiber reinforced composites, because of the larger dimensional variability of fibers and fiber bundles, defects can more easily lead to disruptions in the laminate geometry than in glass fiber reinforced composites [9]. In addition, the capability of detecting defects and especially measuring their level of criticality is crucial to arrive to the possible production of components with plant fibers reinforcement. In this context, the acoustic emission (AE) technique can present some interest, for its capability of providing data from real-time monitoring during post-impact testing [10]. In the present work, plain woven jute fabric/polyester plates (100 mm×100 mm), manufactured using a resin transfer moulding (RTM) process with a 60% wt. reinforcement content, have been impacted and then subjected to cyclic post-impact three-point bending tests. The impact energy was changed, by varying the mass of the hemispherical drop-weight steel impactor, whilst maintaining the impact velocity constant at 2 m/s (±5%). Impact tests were conducted on a CEAST Fractovis impact tower fitted with an anti-rebound device. The plates were divided into eight categories with five specimens in each category. One group of specimen was not impacted, while the remainder were impacted at energies of 5, 7.5, 10, 12.5, 15 and 20 J, and the last one was impacted to penetration according to ASTM D3763, always using a hemispherical impactor tip diameter 12.7 mm. Depending on the impact energy, the total mass of the impactor varied: 2.53 kg (5 J), 3.89 kg (7.5 J), 4.99 kg (10 J), 6.26 kg (12.5 J), 7.54 kg (15 J), and 9.97 kg (20 J). The penetration energy measured on these laminates was equal to 25 (±2.2) J. On the impacted specimens, a 25.4 mm diameter indentor was used to undertake cyclic three-point bend tests, using an Instron 9400 universal testing machine, and following the scheme in Fig. 1. A loading rate of 4 mm/min was used. This condition was the

Physical and mechanical properties of unidirectional plant fibre composites—an evaluation of the influence of porosity

Unidirectional composites were made from filament wound non-treated flax yarns and polypropylene foils. With increasing composite fibre weight fractions from 0.56 to 0.72, porosity fractions increased from 0.04 to 0.08; a theoretical model was fitted to the data in order to describe the composite volumetric interaction between contents of fibre, matrix and porosity. In the model two porosity components were proposed, a process governed component and a structurally governed component. The composite axial stiffness and strength were in the range 27–29 GPa and 251–321 MPa, respectively. A modified version of the “rule-of-mixtures”, supplemented with parameters of composite porosity content and anisotropy of fibre properties, were developed to improve the prediction of composite tensile properties.

Self-reinforced melt processable composites of sisal

Through slight benzylation treatment, skin layers of sisal fibers were converted into thermoplastic material while the core of the fiber cells remained unchanged. On the basis of these modified sisal fibers, self-reinforced composites were prepared using hot pressing, in which the plasticized parts of sisal serve as matrix and the unplasticized cores of the fibers as reinforcement. The paper discussed the influence of various benzylation conditions on the structure, thermal flowability and mechanical properties of modified sisal and the composites. It was found that a balance of melt processability and reinforcing effect of the benzylated sisal fibers should be considered. Unlike the conventional plant fiber composites using petro-polymers as matrices, the current self-reinforced composites based on sisal are characterized by inherent interfacial compatibility and full biodegradability.

Reinforced Biocomposites from Flax and Hemp

Abstract Defibrated flax and hemp fibres form new compatible composites substituting cabinets, car-inner panels etc. The aim is to produce composites from Danish flax and hemp that are competitive to composites reinforced with fibres such as polypropylene, glass fibre, and metals. The plant fibre composites can be used in many applications with different purposes. The composites are based on a new process, where flax or hemp are defibrated into shortened fibres and subsequently formed into mats through a unique air-forming technique mixing the plant fibre and polymers in a strong web. The mats are then moulded in a hot-press for products in all kinds of shapes. The process is based on shortened fibres from flax and hemp. The stiffness of the composites and tensile strength properties are equal in all directions, and their values are higher than those of pure plastic composites. In the new Danish composites the tensile strength seems lower than in typical composites from flax and hemp, where mats are deriv...

The penetration of epoxy resin into plant fibre cell walls increases the stiffness of plant fibre composites

A treatment is described that enables epoxy resin to penetrate into the cell walls of plant fibres. This ensures a very good interface in the cured composites, mimicking the way that interfaces are formed in animal and plant materials. The treatment involves swelling the plant cell walls with urea solution, washing out the excess urea and then replacing the water with alcohol in a graded series. Epoxy resin can then penetrate into the swollen cell wall and mix with the alcohol. The alcohol is probably driven off during curing of the resin. This treatment increased the stiffness of composites made from flax fibres and epoxy resin.