•Continuous flax fibers increase notched and plain specimens' tensile and fatigue strength.•Fatigue behavior at low and high cycles changes with notch type and fiber content.•The hysteresis of ...3D-printed flax fiber composites differs in tension and compression.
Nowadays, additive manufacturing is increasingly used for prototypes and low-volume products. Firstly, it offers great design freedom; however, the components must inevitably have certain design artifacts that cause stress concentrations. To this end, composites of biodegradable PLA (polylactic acid) matrix and continuous flax fibers were fabricated in this study using material extrusion 3D printing. Static characterization and mechanical fatigue tests between 103 and 2·106 load cycles were performed for plain specimens, three types of notches for the matrix, and two composite configurations. The notches only slightly affected the static performance of the tested materials. Accordingly, compared to the unnotched versions, the PLA matrix and composites exhibited a reduction in tensile strength of up to 8% and 19%, respectively. It was found that compared to the PLA matrix, the fatigue strength at high cycles increased by 1.8 and 2.3 times for the plain composite with a fiber volume fraction of 7.8% and 14% fibers, respectively. In the fatigue of notched composites, the notch effect is reduced by fibers in the tip. The fatigue strength is up to 1.9 and 2.9 times higher compared to the notched matrix for the composite with a smaller and larger volume fraction, respectively.
Fabricating additively manufactured thermoplastic parts with increased load-bearing capacity using material extrusion, namely fused deposition modeling, requires the incorporation of continuous ...reinforcing fibers. In this study, composites of PLA matrix and continuous flax fibers were additively fabricated using a specially designed 3D printer with multiple deposition toolheads. Unidirectional tensile and flexural specimens with a component-like shape and a fiber volume fraction of up to 18% were fabricated. The mechanical properties were determined for four different composite and matrix layers arrangements. The tensile modulus of elasticity and strength values increased by 3.6 and 2.9 times, respectively, compared to the specimens without the reinforcing fibers. In contrast, the flexural modulus of elasticity improved by 2.4 times, while the flexural strength increased by 1.4 times. It was found that it is possible to influence the mechanical properties and the type of damage by changing the volume fraction of the fibers and the arrangement of the composite layers.
The mechanical properties and accuracy of 3D-printed composites can be significantly affected by printing parameters. This work discovered a noteworthy recovery phenomenon during 3D printing of ...continuous flax fiber-reinforced composites (CFFRCs). The yarn count and printing speed are the critical factors that influence the recovery behavior, which, in turn, affects the thickness, the fiber volume fraction, and the void content of CFFRCs, ultimately impacting the mechanical properties. Smaller yarn counts have a greater effect on the recovery behavior of CFFRCs. Increasing the printing speed from 5 mm/s to 15 mm/s leads to the loss of tensile properties and flexural modulus in CFFRCs up to 30 % and 58 %, respectively. The recovery mechanism of the CFFRCs is further elucidated with the observation of temperature distribution during the printing process. This indicates that recovery is more likely to occur in the compressed yarn with a twisted structure when the deposited thermoplastic matrix exceeds the glass transition temperature. The results and findings have the potential to enhance the process optimization of 3D-printed continuous plant fiber composite parts, ensuring high mechanical properties and dimensional precision.
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•A noteworthy recovery phenomenon is discovered during 3D printing of continuous flax fiber reinforced composites (CFFRCs).•The yarn count and printing speed are the critical factors that influencing the recovery behavior.•The recovery is occurred in the compressed yarn with a twisted structure when the deposited resin matrix exceeds its Tg.
To address the issue of high void contents in 3D printed continuous flax fibre reinforced composites (CFFRCs), an efficient fibre pre-treatment method was proposed to improve the compatibility ...between plant fibre and resin matrix so that the high-quality manufacturing and excellent mechanical performance were achieved. The surface of flax yarns was treated by silane coupling agents to improve the wettability and interfacial performance. The CFFRCs were then prepared by the customised 3D printing technology, and the mechanical tests were conducted to measure their mechanical properties. It was found that the mechanical properties of the CFFRCs were greatly improved after the treatments which induced very low void contents of less than 1.1%, due to the improved wettability and the interfacial bonding between the flax yarns and resin matrix. The mechanical properties of the CFFRCs prepared by 3D printing in the present work were comparable to those manufactured by compression moulding in literature.
Compared with compression moulding and autoclave, 3D printing has lower pressure and shorter time during moulding. So, prior to the preparation of 3D-printed continuous plant fibre reinforced composites, it is necessary to modify the surface of the plant fibres to improve the wettability and the interfacial compatibility, while ensuring the continuity of the yarn. The silane coupling agent can eliminate the hydroxyl group through chemical reaction with the fibre surface, and establish a chemical bond with the matrix. To improve the wettability of the fibre and the interface performance of the 3D printed composites, thereby improving the mechanical properties of continuous flax fibre reinforced composites (CFFRCs) manufactured by 3D printing Display omitted
•An efficient pre-treatment method for 3D printing is proposed to improve mechanical properties of CFFRCs.•Improved fibre wettability helps to reduce voids in CFFRCs arisen from low printing pressure and twisted nature of yarns.•Mechanical properties of 3D printed CFFRCs are comparable to those by compression moulding in literature.
The mechanical properties of 3D-printed continuous plant fibre-reinforced poly(lactic) acid (PLA) composites are insufficient due to the low fibre volume fraction (∼30 %). Further increase of fibre ...volume fraction of composites by reducing the line width may result in a composite with low fibre impregnation degree or the clogging of printer nozzle because of the twisted nature and uneven diameter of plant yarns, which also degrades the mechanical properties. This study proposed a method to obtain 3D printed continuous flax fibre reinforced composites (CFFRCs) with a high fibre volume fraction and excellent mechanical performance by statistical analysis of the dimensions of flax fibre yarns as well as surface modification. The CFFRCs were printed by a fibre prepreg-based 3D printing method, and an in-situ impregnation method was also used to prepare CFFRCs for comparison. The results showed that the CFFRCs by the fibre prepreg method had a fibre volume fraction of 44.1 % and a void content of ∼1.9 % due to the high impregnation degree. Correspondingly, the tensile modulus and strength of the CFFRCs were the highest among the existing reported values of the composites made by 3D printing and even greater than those of the CFFRCs with a fibre volume fraction of ∼51.4 % prepared by the in-situ impregnation method.
The mechanical properties and printing efficiency (i.e. the printing time) are often contradictory goals in 3D printing of continuous fibre composites. In this work, we presented a multi-objective ...optimization method for improving both the mechanical properties and printing efficiency of 3D-printed continuous flax-fibre-reinforced composites (CFFRCs). It was found that the printing efficiency of the CFFRCs with the optimal processing parameters was remarkably improved by 40% without sacrificing the tensile strength of the composites. This study provides an effective method for the improvement of the production efficiency of high-quality 3D-printed continuous fibre reinforced composites.
•A multi-objective optimization is proposed to obtain optimal parameters for 3D printing of continuous flax fibre composites.•Fast printing speed results in short impregnation time and thus an increase in void contents inside the composites.•Printing efficiency is improved by 40% without sacrificing tensile strength of composites after optimization.
