Continuous Fibre-reinforced Thermoplastics Research Articles

Mechanical performances of continuous carbon fiber reinforced PEEK (polyether ether ketone) composites printed in a vacuum environment

Continuous fiber-reinforced thermoplastic (CFRTP) composites have attracted significant attention in various industries due to their exceptional mechanical properties and lightweight nature. Various methods have been applied to improve the mechanical properties of parts produced by fused deposition modeling (FDM), such as printing in a vacuum environment. Vacuum is utilized during printing to minimize heat transfer by convection, improve interfacial bonding, and reduce void formation. This study investigates the effects of vacuum and process parameters on the mechanical and microstructural properties of continuous fiber-reinforced thermoplastic composites. The goal was to investigate the effects of the printing parameters in a vacuum environment and therefore, enhance the mechanical properties. CFRTP test specimens were produced under various vacuum levels and processed at nozzle temperatures, heated bed temperatures, and printing speeds. Three-point bending tests were conducted to evaluate the flexural properties, and microstructural analysis was performed using optical microscopy. The results demonstrate the positive influence of vacuum on flexural performance and highlight the importance of controlling parameters, such as nozzle temperature and heated bed temperature for achieving enhanced mechanical properties. This study provides valuable insights into the production of higher mechanical properties of CFRTP composites and contributes to advancements in their manufacturing.

Numerical Modelling of the Thermoforming Behaviour of Thermoplastic Honeycomb Composite Sandwich Laminates.

Lightweight component design is effectively achievable through sandwich structures; many past research studies in the aerospace and racing sectors (since the 1920s) have proven it. To extend their application into the automotive and other transport industries, manufacturing cycle times must be reduced. This can be achieved by sandwich materials made of continuous fibre-reinforced thermoplastic (CFRTP) cover layers and thermoplastic honeycomb cores. To widen the application of flat thermoplastic-based sandwich panels into complex parts, a novel forming technology was developed by the Fraunhofer Institute of Microstructure of Materials and Systems (IMWS). Manufacturing defects like wrinkling and surface waviness should be minimised to achieve high reproducibility of the sandwich components. Studying different manufacturing parameters and their influence on the final part is complex and challenging to analyse through experiments, as it is time-consuming. Therefore, a finite element (FE) modelling approach is implemented to reduce such efforts. Initially, a thermoforming model is developed and validated with experimental results to check its reliability. Further, different simulations are performed to optimise the novel sandwich-forming process. In this study, a thermoplastic sandwich made of polypropylene (PP) honeycomb core and polypropylene glass fibre (PP/GF) cross-ply as cover layers was used, and its numerical model was executed in LS-DYNA software release R11.2.1.

Influence of the Textile Reinforcement on the Joint Formation of Pin-Joined Composite/Metal Parts

AbstractHybrid components consisting of continuous fiber reinforced thermoplastic (CFRT) and steel components exhibit promising potential in advanced lightweight construction. However, the joining operation presents a significant challenge due to the materials’ distinct physical and chemical properties. This paper studies a joining method in which dual pin arrays protruding from the surface of the metal component are inserted into the locally heated CFRT component to create a form-fitting joint. The primary objective is to scrutinize the influence of various CFRT materials on joint formation and quantify the resulting properties. The fiber type (glass and carbon) and fiber architecture (unidirectional and bidirectional reinforcement) are varied. All materials could successfully be joined via the direct pin pressing process, while depending on the CFRT material, distinct characteristic fiber morphologies could be identified. Bidirectionally reinforced carbon fiber reinforced samples showed the highest overall strength, while unidirectionally glass fiber reinforced samples showed the highest energy absorption and second highest ultimate strength.

Investigating the mechanical properties of 3D fused deposition modeling composites reinforced with continuous fibers: Effects of fiber number and negative Poisson structure

The use of 3D printers is expanding across various industries. The incorporation of reinforcing fibers and structures, such as negative Poisson, has the potential to enhance the mechanical properties of three-dimensional products. In this study, a specialized nozzle is developed for 3D Fused Deposition Modeling (FDM) printers dedicated to the production of continuous fiber-reinforced thermoplastic (CFRT) composites. The primary objective is to enhance the tensile strength and impact resistance of 3D-printed samples. It is achieved through the investigation of strategies such as fiber reinforcement, variation in the number of fibers within the reinforcing yarn, and the creation of samples featuring an internal structure with a negative Poisson’s ratio (NPRS). In this regard, four types of 3D samples are prepared including polymer only, polymer with a negative Poisson’s ratio structure, polymer reinforced with continuous fibers (glass and carbon fibers), and polymer reinforced with continuous fibers featuring a negative Poisson’s ratio structure. The mechanical properties of these samples, including tensile strength and impact resistance, are also compared. The results indicate that incorporating fibers as reinforcement can enhance the mechanical properties of 3D-printed products. Moreover, continuous fibers with more fibers within the same yarn count have an increased strengthening effect. The use of negative Poisson structures significantly improves impact resistance but adversely affects tensile strength.

A new method of preparing lattice structures of continuous carbon fiber-reinforced thermoplastics

The three-dimensional lattice sandwich structure of composite has attracted extensive attention in the engineering field due to its superior mechanical property and multifunctional designability. Developing technology that can guarantee both production cycle and cost remains challenging. In this work, an innovative forming technology based on 3D printing and hot folding process is presented to fabricate lattice sandwich structures of continuous fiber-reinforced thermoplastics. The prepared lattice sandwich structures show obvious competitiveness and superior material efficiency, which is reflected in high compressive strength (23.55 MPa/(g/cm3)) and specific compressive modulus (457.82 MPa/(g/cm3)). The simplified theoretical model and 3D progressive failure model are carried out to predict the compressive behaviors of the lattice sandwich structures, and the results are consistent with the experimental results. Additionally, the flexibility of the proposed method is demonstrated by fabricating various 3D structures.

Additive manufacturing of high‐strength polyamide 6 composites reinforced with continuous carbon fiber prepreg

AbstractThe additive manufacturing of continuous fiber reinforced thermoplastics (CFRTPs) paves way for the high‐strength, light‐weight components for variety of load‐bearing applications. In this work, the continuous carbon fiber reinforced PA6 (CCF‐PA6) composites was successfully printed from the prepreg filament. The prepreg filament was prepared in‐house by impregnating the heat‐and‐acid treated 1 K carbon fiber bundle with the molten PA6. The tensile strength of the prepreg filament, which contained with 40 vol% CF, reached 984 MPa. The unidirectional CCF‐PA6 specimens were subsequently 3D‐printed with the prepreg filament, and the mechanical strength of those 3D‐printed specimens were tunable by adjusting a set of printing parameters, such as layer thickness, hatch spacing and printing temperatures. The highest tensile strength of the specimen reached 555 MPa. Those specimens also exhibited outstanding mechanical strength at elevated temperatures, still reaching 184 MPa at 150°C. The mechanical strength of those specimens was dependent on the content of the fiber. This study can hopefully provide new insights for feedstock design and spur novel ideas in tailoring the mechanical properties of the 3D‐printed CFRTPs.

Investigation of the Fiber Length and the Mechanical Properties of Waste Recycled from Continuous Glass Fiber-Reinforced Polypropylene

This paper explores the mechanical recycling of continuous fiber-reinforced thermoplastics (CFRTPs) waste into injection molded products, focusing on the influence of recycling parameters on fiber length and mechanical properties. CFRTPs are gaining attention for their promising attributes, including weight-specific mechanical properties, short cycle times, storability, and recyclability, making them suitable for diverse applications. However, as CFRTP production rates rise, recycling strategies become crucial for sustainability. This study investigates the processability of CFRTP waste, defines size reduction conditions, and evaluates the impact of various compounding parameters such as temperature, screw speed, and fiber volume content during extrusion. The research findings indicate that higher screw speeds lead to fiber length reduction, whereas elevated temperatures result in longer fibers. Increased fiber volume intensifies interactions, resulting in shorter lengths. Additionally, the study examines the influence of injection molding parameters such as back pressure, screw speed, and initial fiber length on the resulting fiber length and mechanical properties of injection molded specimens, emphasizing the need for precise parameter control to optimize performance in recycled CFRTPs. Key findings are that increasing the initial fiber length from 260 μm to 455 μm results in an average fiber length after injection molding of 225 μm and 341 μm, respectively. This implies that longer initial fibers are more prone to breakage. Regarding the mechanical properties, increasing back pressure from 20 bar to 60 bar results in a reduction in Young’s modulus of approximately 40 MPa. Higher screw speed also reduces modulus by approximately 70 MPa due to intensified fiber–screw interactions. However, back pressure and screw speed have neutral effects on the tensile strength and the elongation at break.

Investigation on the Influence of Process Parameters on the Mechanical Properties of Extruded Bio-Based and Biodegradable Continuous Fiber-Reinforced Thermoplastic Sheets.

The use of bio-based and biodegradable matrix materials in fiber-reinforced polymers (FRPs) is an approach to reduce the consumption of fossil resources and the amount of polymer waste. This study aims to assess the influence of the process parameters on the resulting mechanical properties of extruded bio-based and biodegradable continuous fiber-reinforced thermoplastics (CFRTPs) in the form of sheets. Therefore, the impregnation temperature during the production of PLA/flax fiber composites is varied between 220 °C and 280 °C, and the consolidation pressure, between 50 bar and 90 bar. A design of experiments approach is used. Fiber contents of 28.8% to 34.8% and void contents of 6.8% to 15.5% are determined for the composites by optical measurements. To assess the mechanical properties, tensile tests are performed. Using the evaluation software Minitab, a strong negative influence of the consolidation pressure on the tensile modulus and the tensile strength is observed. Increasing the pressure from 50 bar to 90 bar results in a reduction in the tensile modulus of 50.7% and a reduction in the tensile strength of 54.8%, respectively. It is assumed that this is due to fibers being damaged by the external force exerted onto the materials during the consolidation process in the calender. The influence of the impregnation temperature on the mechanical properties cannot be verified.

In-Situ Computed Tomography — Analysis of a Single-Lap Shear Test with Composite-Metal Pin Joints

Lightweight design in the form of intelligent multi-material structures that combine the advantages of high strength steel and continuous fibre reinforced thermoplastics (CFRTs) gain increasing relevance. In this context, the joining operation is a major challenge as it has to be time and cost efficient and the resulting joint has to exhibit a high mechanical durability. One possible approach is the use of cold formed pin structures, which can be inserted into the CFRT to create a form fitting joint under avoidance of fibre damage as it is commonly the case for bolted or riveted joints. The deformation phenomena of pin joints are usually investigated by macrosectioning or (ex-situ) computed tomography. However, due to resetting elastic deformations and cracks that close after unloading an inaccurate state of the inner joint structure is measured. Furthermore, an investigation of different stages with increasing load and progressing failure is very time consuming, because multiple samples have to be tested and investigated. Alternatively, in-situ computed tomography (in-situ CT) can be used to investigate the testing of pin joints. In this paper, a method for in-situ CT analysis of a single-lap shear test with composite-metal pin joints is presented. The pins are plastically extruded to a height of approx. 1.8 mm from the metal sheet (1.5 mm thick) and are pressed into a locally heated glass fibre reinforced thermoplastic (FRT) sheet (approx. 2 mm thick) creating a form fit. Specimens with quasi-unidirectional fibre reinforcement in 0° and 90° direction are tested. With this procedure, the three-dimensional deformation of the joint can be observed and failure phenomena can be identified for each reinforcement direction respectively. Thus, this method can also be used for validating numerical simulations.

Process chain for forming and consolidating fiber-reinforced thermoplastics and metallic sheets in a two-stage isothermal tool

Due to their good mechanical properties and low structural weight, multi-material structures are a promising approach in the automotive industry to lightweight design, body construction and functionalization. Especially metal and plastic are mainly combined to achieve improved properties of the final component compared to mono-material structures. This paper describes the development of a manufacturing cell for the joint forming and heat-assisted press joining of steels and continuous fiber-reinforced thermoplastics in the form of unidirectional carbon-fiber tapes. In order to achieve shorter cycle times and to ensure economical production, a manufacturing cell, supplemented with automated handling by means of two robots and an isothermal, two forming stages tool concept was developed and tested. The composite components produced were tested with regard to their mechanical performance. The feasibility of the production was demonstrated. All composite components had a higher specific load capacity than a pure steel component. Cycle times of well under 60 seconds were achieved. An enormous reduction in process time compared to variothermal tool concepts could be achieved with the new manufacturing cell.

Incremental design process for vehicle structural components using thermoplastic-based composites: A simulation-guided approach for door anti-intrusion beams

The design of safety-related structural components in automobiles is a critical phase. Most car manufacturers are aiming at reducing vehicle weight considering continuous fiber-reinforced thermoplastics as candidate to replace metal-based structural components. However, this replacement cannot compromise the safety level already achieved by metal-based designs. A good example is the side door anti-intrusion beam, which is a passive safety component to protect the passengers under a lateral impact. This paper presents an incremental design process to achieve a functional side door anti-intrusion beam fully made of glass fiber reinforced thermoplastic conceived to replace metal-based ones. The reader is guided along the design challenges, where the mechanic-based reasons to adapt the part geometry and composite layout are explained in detail. This incremental design process uses a fully FEM-based approach allowing to reach a standard-defined absorbed energy of 1000 J in a lateral collision. From the simulation analysis, it was found that by implementing essential modifications in the beam’s cross-section corrugation, the damage progression can be slowed down leading to an increase of absorbed energy. Additionally, the enhancement of one extra ply in the proper direction solves severe fracture in the clamping area. The final composite design not only performs similarly to its metal-based counterpart, but also considers aspects of production feasibility. This trip along the design process concludes with the production of the simulate-designed component and its effectiveness under lateral impact is demonstrated. To meet the safety regulations, the model is thoroughly validated against the real impact test by analyzing the response in force, intrusion, energy and damage.

Experimental characterization and phenomenological modeling of nonlinear viscoelasticity, plasticity and damage of continuous carbon fiber-reinforced thermoplastics

The presented work addresses the experimental characterization and phenomenological modeling of the complex rate dependent mechanical response of continuous fiber-reinforced thermoplastics (cFRTP). The theoretical basis is given by the concept of an equilibrium curve of the material which provides a comprehensive framework for separating the mechanical response into a rate dependent overstress and a rate independent equilibrium stress. In long-term creep and relaxation tests, it is shown that relaxation tests provide several advantages over creep tests when determining the equilibrium response. A stepwise loading–unloading relaxation (SR) test is proposed which makes it possible to extract nonlinear viscoelasticity, damage and plasticity present in the material from a single test. The application to cFRTP is exemplified by using the proposed method for the experimental characterization of a continuous carbon fiber-reinforced polycarbonate (CF-PC). Based on the obtained experimental data, a phenomenological material model is developed which describes the mechanical response of a unidirectional ply. The model consists of an overstress branch addressing the nonlinear viscoelastic material response and an equilibrium branch that utilizes a coupled damage-plasticity approach. After calibrating the model using the experimental data, it is shown that the proposed model is able to accurately reproduce the mechanical response of cFRTP.

The Relationship Between Fiber Bundle Size and Mechanical Performance of Additively Manufactured Continuous Carbon Fiber Reinforced Thermoplastic Composites.

Additively manufactured continuous fiber-reinforced thermoplastic (CFRTP) composites are still in the early stages of reaching competitive mechanical properties compared with conventional composites. The main reason for this is that their mechanical properties are limited due to weak interlayer strength, porosity, and low fiber fraction. Therefore, the effects of many parameters, such as layer thickness, temperature, printing speed, and fiber fraction, have been extensively studied to improve mechanical properties. With a different perspective on these parameters, this study aimed to investigate the effect of fiber bundle size on the mechanical properties of CFRTP composites. For this purpose, 3K and 6K fiber bundle filaments with the same fiber volume fractions (∼41%) were produced utilizing a polymer impregnation setup. CFRTP samples were printed using fused deposition modeling with polylactic acid as the matrix. The mechanical properties were investigated via three-point bending, interlaminar shear strength (ILSS), and tensile tests. The results showed that fiber bundle size does not particularly influence tensile strength but dominates flexural and ILSS performance. Although increased flexural strength and modulus were observed, the bundle size effect was much more dominant in ILSS tests, and 6K bundle size samples with the same fiber fraction showed much higher strength.

Tensile Response of Fibre-Reinforced Plastics Produced by Additive Manufacturing for Marine Applications

The present study makes a consistent attempt to evaluate promising additive manufacturing (AM) processes and materials for marine structural applications, paving the way for the development of additively manufactured light-weight composites. The main objective is to analyse the structural performances of fibre-reinforced plastics (FRP) produced by AM for marine applications. In particular, the tensile response of chopped and continuous carbon-fibre-reinforced thermoplastics have been investigated through destructive and non-destructive testing, considering the influence of AM process settings and thermal post-manufacturing treatments. The results demonstrate that continuous fibre-reinforced thermoplastics produced by AM are potentially suited to marine structural applications, since their tensile capacity is superior to the minimum imposed by the Classification Society Rules. However, the mechanical properties of additively manufactured FRP are currently lower than conventional composites. The continuous carbon fibre reinforcement is far more effective than the chopped one, and the additive manufacturing deposition pattern significantly influences the structural capacity. The annealing post-manufacturing treatment enhances the mechanical properties by approximately 10%, decreasing material ductility and manufacturing defects.

Investigation of the influence of multiple thermoforming processes on the properties of continuous fiber-reinforced thermoplastics to enable structural reuse

Fiber-reinforced plastics (FRP) are finding more and more applications in many industrial sectors. The growth in material use simultaneously leads to larger volumes of end-of-life materials and FRP are generally difficult to recycle. This also applies to FRP with a thermoplastic matrix. One possibility is the approach of structural reuse by reshaping continuous fiber-reinforced thermoplastics (organosheets). In this process, end-of-life (EOL) components are remelted and formed back into semi-finished products. However, the multiple thermal processes can damage the matrix. In addition, the frequent forming processes lead to fiber breaks and displacements of the reinforcement architecture. In order to assess the level and impact of matrix and fiber damage, various forming and reforming processes were performed on selected materials, which were then investigated using attenuated total reflectance (ATR) spectroscopy and tensile tests. The results of the ATR spectroscopy show no evidence of damage to the matrix. In contrast, slight to moderate decreases in the characteristic values as a result of multiple deformations can be observed in the results of the tensile tests. However, the experiments and material tests show the potential that this approach can produce high quality secondary materials. A comparison is made here with mechanically recycled and compounded injection molding materials.

On correlation of stamping process with fiber angle variation and structural performance of thermoplastic composites

This study aims to experimentally explore the influences of stamping conditions on the variations in fiber yarn angle and material properties of continuous fiber reinforced thermoplastics (CoFRTP). First, the upright-orthogonal (O-), biased-orthogonal (B-) and non-orthogonal (N-) specimens were manufactured with thermo-stamping process. Second, the X-ray computed tomography (CT) was employed to characterize fiber yarn angle variations within each layer of thin-walled CoFRTP structures nondestructively. The results indicate that fiber yarn angle varied in different layers at the same position and also changed at different positions in the same layer for the N-specimens. In addition, the 6-ply, 8-ply and 10-ply N-specimens exhibited a similar average variation in fiber yarn angle. Finally, the tensile and three-point bending (TPB) tests were conducted to investigate the effects of stacking sequence and fiber yarn angle variations on material properties and structural performances of these consolidated CoFRTP specimens. In the off-axis tensile tests, the modulus, yield strength and failure strength of laminates decreased but the failure strain increased with increasing fiber yarn angle. In the TPB tests, the O-specimen showed the smallest failure displacement, the B-specimen presented the highest peak force, and the N-specimen exhibited the largest failure displacement. This study is anticipated to gain insights into the relationship between stamping process and forming characteristics as well as structural performances of CoFRTP structures.

Injection over-molding warpage prediction of continuous fiber-reinforced thermoplastic composites considering yarn reorientation

The injection over-molding process combines thermo-stamping of continuous fiber reinforced thermoplastic (CFRTP) laminates and injection of plastic onto the surface of the CFRTP insert. A challenge for this process is to predict the warpage due to the influence of CFRTP insert. This work proposed a simulation method for predicting the warpage considering yarn reorientation induced in the thermo-stamping. In detail, the geometry and yarns reorientation of the deformed CFRTP inserts in thermo-stamping were calculated by a hybrid lamination model. Then the local thermo-mechanical properties were calculated according to the directions of reorientated yarns. Finally, the geometric and material information of CFRTP insert were imported into injection analysis, especially warpage prediction. The simulation method was validated by experiments on centrally injected bilayer plate and stiffened square box. This work highlights the importance of anisotropy induced from thermo-stamping on the final warpage of injection over-molded composite parts.

An enhanced elastoplastic damage coupled model for compression characteristics analysis of continuous fiber-reinforced thermoplastic composite stiffened panel

In this work, the elastoplastic behaviors, damage characteristics, and debonding forms of continuous fiber-reinforced thermoplastic (CFRTP) composite stiffened panel under uniaxial compression load have been studied. The enhanced elastoplastic damage coupled model (EPDM) was established to model the mechanical properties of CFRTP composites which were based on generalized Hill yield criterion, the associated flow rules, the power hardening law, Chang-Chang failure criterion, and continuous damage mechanics (CDM). The forward Euler algorithm was implemented in Abaqus via user-defined subroutine VUMAT. A series of experiments including tensile, compression, and interlaminar shear tests of laminates were performed to validate the material model. Subsequently, the developed EPDM combined with cohesive zone model (CZM) was applied to simulate the nonlinear mechanical responses of CFRTP composite stiffened panel. Moreover, an axial compression test of CFRTP composite stiffened panel was also carried out, which confirmed that the simulation results were in agreement with the corresponding experimental results. Furthermore, based on the established finite element model, the buckling knockdown factor (KDF) and plastic region development of the CFRTP composite stiffened panel were also analyzed. Overall, the obtained results demonstrate the efficiency of the proposed EPDM for the compression characteristics analysis of the CFRTP composite structures.

The Effect of Heat Treatments Applied to Continuous Fiber Reinforced Thermoplastic Composites on Mechanical Properties

This study investigated the effects of different heat treatments on continuous fiber-reinforced thermoplastic (CFRTP) 's. CFRTP composite is produced using fused deposition modeling (FDM), which is one of the additive manufacturing methods. Polylactic acid (PLA) was used as a matrix, and carbon fibers (3K) were utilized as reinforcement material. First, CFRTP filament was produced on a specially designed melt impregnation line. Afterward, test samples were manufactured via a conventional 3D printer. Then, heat treatments (re-melting in salt, microwave oven, oven) were applied to the produced samples, and the effects of these processes on mechanical properties were investigated. Three-point bending tests were used to investigate the mechanical properties of the test samples. As a result of the heat treatments applied to the CFRTP specimens, flexural stresses between 200 and 220 MPa was achieved. The highest bending stress was obtained by re-melting in salt. As a result of the heat treatments, the stress values are similar, but the re-melting in salt application exhibited a more rigid behavior.

Geometrical optimization of thermoforming continuous fibers reinforced thermoplastics with Finite Element Models: A case study

The manufacturing of thermoformed components of continuous fiber-reinforced thermoplastics (CFRTPs) relies on an optimization process owing to a demand for quality and the mechanical requirements. Geometrical optimization is typically based on trial-and-error processes, and delays due to the manufacturing of different prototypes increases the development cost of the final product. In this study, the mechanical behavior of a CFRTPs top mount damping component made of polyamide 6 (PA6) reinforced with long glass fibers (47% in volume) and installed in an automobile differential system was evaluated. This was achieved by developing ad hoc tooling for axial testing. The experimental results were used to validate those obtained using a finite element (FE) model based on the fabricated geometry and mechanical properties of the CFRTPs laminate. Furthermore, a complete mathematical procedure was implemented to address the lack of information on the mechanical moduli and Poisson's ratio in the material datasheet for the FE simulation. This method was based on a Halpin-Tsai model and classical lamination theory with a 3D expansion for out-of-plane mechanical property calculation, wherein Tsai's modulus was obtained for checking purposes. The obtained results were fitted with the test results to validate the developed FE model. In addition, this model was used to further optimize the component geometry. The implementation of in silico models based on FE techniques can be useful for accelerating the development of recyclable CFRTPs for large-scale production, allowing significant weight reduction and lower greenhouse emission without diminishing their reliability and safety throughout their useful life.