Continuous Fiber Composites Research Articles

Path Planning Method For 3D Printer Heads using Continuous Fiber Composites

Path Planning Method For 3D Printer Heads using Continuous Fiber Composites

3D-printed continuous fiber reinforced polymers considering material and physical variability towards robust design

The non-planar 3D printing of continuous fiber composites contains various uncertainties. To optimize the geometry of 3D printed structures, numerical optimization is increasingly used. However, if uncertainties are not taken into account, optimization can lead to results with low robustness. In this work, we present a novel approach to consider uncertainties in numerical modeling by introducing variability of material and physical parameters. We use the computationally inexpensive Certain Generalized Stresses Method as a stochastic method and we evaluate the robustness of different designs with respect to material and physical variabilities. We demonstrate the benefit of this novel uncoupled approach using the example of 2D-cantilever trusses.

4D printing for continuous fibers reinforced composites

4D printing structure can be applied to the truss or the solar sail of spacecraft. At present, most 4D printing structures often have poor structural stiffness and deformation accuracy. In this study, a 4D printing process based on the composites with embedded continuous fibers is proposed. Due to the different thermal expansion coefficient and elastic modulus of fiber and resin, the structure will produce bending deformation when the temperature changes. The influence of fiber trajectories on deformation curvature is calculated and simulated, and then, some complex structures have been prepared by 4D printing process. The 4D printing structure of continuous fiber composite has better mechanical properties and higher deformation accuracy.

Vector field-based 3D print path generation for continuous fiber composites

Vector field-based 3D print path generation for continuous fiber composites

Compressive strength degradation of the curved sections of 3D-printed continuous carbon fiber composite

In this study, we proposed a test method to evaluate the strength of a curved section of 3D printed continuous carbon fiber reinforced thermoplastics. Curved beam tests were conducted using L-shape specimens to evaluate the strength of the curved section. To test thin L-shape specimens, test jigs were designed. In this process, finite element analyses were conducted to testify that the designed jigs were appropriate for evaluating the strength of the curved section. Several L-shape specimens with different geometries, respectively, were tested to evaluate the effect of radius on the circumferential strength. Additionally, the failure mechanism was investigated by observing the inner structure via X-ray CT. These tests revealed that compressive failure first occurred and that the compressive stress decreased as the radius became smaller.

Combining digital image correlation with X-ray computed tomography for characterization of fiber orientation in unidirectional composites

We apply Digital Image Correlation (DIC) to X-ray Computed Tomography (CT) slices for characterization of individual fibers misalignment in continuous fiber composites. This is an atypical application of DIC since the input is slices of a CT tomogram acquired at one time step, whereas the input of the typical deformation analysis application is 2D images captured over consecutive loading steps. The methodology is demonstrated and validated on synchrotron CT images of a glass/epoxy laminate, where individual fibers can be clearly identified. It is found that the average/standard deviation of the DIC displacement field in every CT slice is a good estimation of the average/standard deviation of the actual displacement of all fibers’ cross-sections in that slice. This provides a shortcut to the measurement of the moments of fiber orientation distribution, compared to the algorithms that reconstruct the fiber center lines. Using this shortcut, the fiber misalignment was also characterized in synchrotron CT images of a carbon/epoxy laminate, where individual fibers were not clearly observed and where, fiber segmentation tools failed to characterize the fibers. Furthermore, being applied to synchrotron images acquired during in-situ tension, the methodology is employed to quantify the evolution of misalignment during tension. The investigated 3D volumes are provided in [1] and described in [2].

Effects of hygrothermal aging on the thermomechanical properties of a carbon fiber reinforced epoxy sheet molding compound: An experimental research

AbstractCarbon fiber sheet molding compounds (C‐SMCs) are discontinuous fiber reinforced composite materials. Among them, epoxy‐based C‐SMCs are becoming relevant materials due to their high thermomechanical performance and better formability than continuous fiber reinforced composites. The thermomechanical performance of epoxy resins and epoxy based continuous carbon fiber composites have shown to be influenced by hygrothermal aging. In this work, this influence is studied for an epoxy‐based C‐SMC. Epoxy‐based C‐SMC samples were hygrothermally aged by means of accelerated conditioning, exposing them to 65% relative humidity, and 80°C in a climatic chamber. The equilibrium moisture content, as well as the moisture diffusion coefficient has been determined. The thermomechanical properties of epoxy C‐SMC have been analyzed by dynamic mechanical analysis, tensile, 3‐point bending, and short beam tests in dry and aged samples. The results showed that epoxy C‐SMC is affected by hygrothermal aging in the cases of moisture intake and its effects on Tg value, but interestingly, the hygrothermal aging did not generate any degradation effects in the mechanical response of epoxy C‐SMC.

Calculating Filament Feed in the Fused Deposition Modeling Process to Correctly Print Continuous Fiber Composites in Curved Paths.

Fused deposition modeling (FDM) is a popular additive manufacturing (AM) method that has attracted the attention of various industries due to its simplicity, cheapness, ability to produce complex geometric shapes, and high production speed. One of the effective parameters in this process is the filament feed presented in the production G-code. The filament feed is calculated according to the layer height, the extrusion width, and the length of the printing path. All required motion paths and filling patterns created by commercial software are a set of straight lines or circular arcs placed next to each other at a fixed distance. In special curved paths, the distance of adjacent paths is not equal at different points, and due to the weakness of common commercial software, it is not possible to create curved paths for proper printing. The creation of a special computer code that can be used to make various functions of curved paths was investigated in this study. The filament feed parameter was also studied in detail. Next, by introducing a correction technique, the filament feed was changed on the curved path to uniformly distribute the polymer material. Variable-stiffness composite samples consisting of curved fibers can be produced with the proposed method. The high quality of the printed samples confirms the suggested code and technique.

Evaluating the effect of variable fiber content on mechanical properties of additively manufactured continuous carbon fiber composites

Fiber volume fraction is a driving factor in mechanical properties of composites. Micromechanical models are typically used to predict the effective properties of composites with different fiber volume fractions. Since the microstructure of 3D-printed composites is intrinsically different than conventional composites, such predictions need to be evaluated for 3D-printed composites. This investigation evaluates the ability of the Voigt, Reuss, and Halpin–Tsai models to capture the dependence of modulus and strength of 3D-printed composites on varying fiber volume fraction. Tensile coupons were printed with continuous carbon fiber-reinforced Onyx matrix using a Markforged Mark Two printer. Specimens were printed at five different volume fractions with unidirectional fibers oriented at either [Formula: see text] to obtain longitudinal, shear, and transverse properties, respectively. It is shown that the Voigt model provides an excellent fit for the longitudinal tensile strength and a reasonable fit for the longitudinal modulus with varied fiber content. For the transverse direction, while the Reuss model fails to capture the transverse modulus trend, the Halpin–Tsai model provides a reasonable fit as it incorporates more experimental parameters. Like conventional composites, addition of fibers degrades the transverse strength, and the transverse strength decreases with increasing fiber volume fraction. The shear modulus variation with fiber content could not be fitted reasonably with either Halpin–Tsai model or Reuss model.

3D-Printed Carbon Fiber Reinforced Polymer Composites: A Systematic Review

Fiber reinforced composites offer exceptional directional mechanical properties, and combining their advantages with the capability of 3D printing has resulted in many innovative research fronts. This review aims to summarize the methods and findings of research conducted on 3D-printed carbon fiber reinforced composites. The review is focused on commercially available printers and filaments, as their results are reproducible and the findings can be applied to functional parts. As the process parameters can be readily changed in preparation of a 3D-printed part, it has been the focus of many studies. In addition to typical composite driving factors such as fiber orientation, fiber volume fraction and stacking sequence, printing parameters such as infill density, infill pattern, nozzle speed, layer thickness, built orientation, nozzle and bed temperatures have shown to influence mechanical properties. Due to the unique advantages of 3D printing, in addition to conventional unidirectional fiber orientation, concentric fiber rings have been used to optimize the mechanical performance of a part. This review surveys the literature in 3D printing of chopped and continuous carbon fiber composites to provide a reference for the state-of-the-art efforts, existing limitations and new research frontiers.

3D printed continuous CF/PA6 composites: Effect of microscopic voids on mechanical performance

The production of continuous carbon fibre composites using a fused deposition modelling (FDM) method has addressed the problem of low mechanical performance of raw- or short-fibre reinforced polymer parts fabricated by the same process, due to the excellent specific strength and stiffness of continuous fibres. However, one key issue of 3D printed polymers or fibre-reinforced polymers is the formation of microscopic voids between individual filaments and within the filaments during the FDM process. This study aims to quantify the adverse effects of voids on 3D printed continuous fibre-reinforced polymer composites. Optical microscopy and micro-CT are used to quantify the void content in continuous CF/PA6 composites fabricated on a 3D printing platform. As a benchmark, 3D printed CF/PA6 composites with the same printing configurations were further processed by compression moulding (CM) with thickness controlled to achieve the minimum void content. Apart from tensile and three-point bending tests in the longitudinal and transverse directions, the study also evaluated the Mode I interlaminar fracture toughness of CF/PA6 composites. By revealing the substantial adverse effects of the microscopic voids in 3D printed composites, this study articulates the critical importance of developing in-process techniques during 3D printing to decrease the void content within the continuous fibre reinforced composites, for the sake of expanding practical applications of 3D printed continuous fibre composites.

Additive manufacturing of short fiber reinforced thermoset composites with unprecedented mechanical performance

Additively manufactured, short fiber reinforced polymer composites have advantages over traditional continuous fiber composites, which include low cost and design flexibility. However, these composites suffer from low strength and stiffness as compared to their continuous fiber counterparts due to the limitation of low fiber volume. In this study, we overcame this limitation by developing a vibration integrated auger extrusion system. This direct write additive manufacturing technique allowed us to fabricate short fiber reinforced thermoset composites in intricate geometries, with unprecedented high compression strength (673 MPa), flexural strength (401 MPa), flexural stiffness (53 GPa), and fiber volume ratio (46 %). Milled carbon fibers were used as the reinforcing fibers, which were considered too short to have the ability to enhance the mechanical strength of composites. However, in this study we show for the first time that milled carbon fibers have the ability to significantly reinforce the thermoset matrix and the composites reinforced with these fibers achieve mechanical performances similar to those of composites reinforced with longer fibers. We believe that a transformation takes place at high fiber volumes on the load transport mechanism within the composites, leading to higher levels of strength and a stiffness enhancement. This pseudo transformation can give rise to short fibers that act as if they are longer, which aids in the effective transfer of tensile loads from the matrix phase to the fibers. This study also showed that the mechanical properties of the additively fabricated thermoset composites match those of ubiquitous, denser structural metals, and these properties show nearly isotropic behavior. Therefore, these systems have great potential to find immediate applications where weight reduction and component complexity are both desired.

A Dimensionless Characteristic Number for Process Selection and Mold Design in Composites Manufacturing: Part I—Theory

The present article introduces a dimensionless number devised to assist composite engineers in the fabrication of continuous fiber composites by Liquid Composite Molding (LCM), i.e., by injecting a liquid polymer resin through a fibrous reinforcement contained in a closed mold. This dimensionless number is calculated by integrating the ratio of the injection pressure to the liquid viscosity over the cavity filling time. It is hereby called the “injectability number” and provides an evaluation of the difficulty to inject a liquid into a porous material for a given part geometry, permeability distribution, and position of the inlet gate. The theoretical aspects behind this new concept are analyzed in Part I of the article, which demonstrates the invariance of the injectability number with respect to process parameters like constant and varying injection pressure or flow rate. Part I also details how process engineers can use the injectability number to address challenges in composite fabrication, such as process selection, mold design, and parameter optimization. Thanks to the injectability number, the optimal position of the inlet gate can be assessed and injection parameters scaled to speed up mold design. Part II of the article completes the demonstration of the novel concept by applying it to a series of LCM process examples of increasing complexity.

A Dimensionless Characteristic Number for Process Selection and Mold Design in Composites Manufacturing: Part II—Applications

The dimensionless “injectability number” was devised to assist composite engineers in the fabrication of continuous fiber composites by Liquid Composite Molding (LCM), i.e., by injecting a liquid polymer resin through a fibrous reinforcement contained in a mold cavity. Part I of this article introduced the injectability number as the integral of the ratio of the injection pressure to the resin viscosity over the cavity filling time and analyzed the theoretical aspects behind this new concept. For a given mold configuration and reinforcement material characteristics, the invariance of the injectability number with regard to process parameters was demonstrated, and an initial verification in unidirectional injection cases was conducted. Part II completes the analysis by evaluating the injectability number in more complex application cases, confirming its invariance properties. The investigation, which was carried out using numerical simulations of different LCM processes and injection strategies, examined the fabrication of various composite parts: a rectangular laminate, a hood for automotive applications, a reservoir box and a fuselage section for the aerospace industry. The results indicate that more efficient injection strategies lead to lower values of the injectability number, thus enabling the use of this dimensionless number as a tool to assess the difficulty to manufacture a given part by LCM as well as to guide process selection and compare different mold configurations.

Mechanical properties of continuous fiber composites of cubic silicon carbide (3C-SiC) / different types of carbon nanotubes (SWCNTs, RSWCNTs, and MWCNTs): A molecular dynamics simulation

The composites made from cubic silicon carbide (3C-SiC), as a matrix, and carbon nanotubes (CNTs), as reinforcements, have a wide range of applications in the industry because of their unique mechanical properties. However, to make effective and efficient composites of 3C-SiC/CNTs, the manner composition of 3C-SiC and CNTs and their volume fraction are among the biggest challenges. Therefore, in this study, to clarify the effects of different volume fractions of various CNT structures on 3C-SiC, continuous fiber composites of 3C-SiC/CNTs are investigated via the molecular dynamics (MD) method. The utilized reinforcements include Single Wall Carbon Nanotubes (SWCNT), Ropes Single Wall Carbon Nanotubes (RSWCNTs), and Multi-Walled Carbon Nanotubes (MWCNTs). The results reveal that the CNTs in the 3C-SiC/CNTs under axial stress are able to bear the most stress and yield before the 3C-SiC, the values of the Young's modulus of the 3C-SiC/SWCNTs, 3C-SiC/RSWCNTs, and 3C-SiC/MWCNTs boost as the chiral indices, the number of ropes, and the number of layers increase, respectively, and the 3C-SiC/CNTs, as continuous fiber composites, have the reinforced Young's modulus and the weakened fracture stress. In addition, among this type of composites, the 3C-SiC/MWCNTs with almost the same weight have the greatest value of the Young's Modulus; thus, they are the most appropriate composites for the industrial applications with high mechanical stiffness.

Forging C/Thermoplastic Printed Composite, Shaping Parameters Impact

The innovative EPITHER process makes it possible to produce massive C/Thermoplastic composite parts with oriented continuous fibres in order to meet specific mechanical loading constraints for structural parts. This fibre orientation allows, as in the case of forging fibre pull-outs, to increase the mechanical strength of the composite product. Composite structural parts must also contribute to the reduction of CO2 emissions by reducing the weight of parts in mobile systems, particularly those in motion for power transmission. This process consists of two or three steps, the preforming necessarily for the placement and orientation of the continuous carbon fibre composites required to increase their characteristics, pre-consolidation and shaping. In this study, the first two steps are performed simultaneously by 3D printing. In order to obtain finished products with good dimensional and material characteristics. This study provides elements for optimizing certain process parameters and the associated results, particularly at different steps in the forming process. In addition, this paper focuses on the movement of the material of the parts obtained by the shaping process. It links proceeding conditions and health of the material: for example, by determining the void ratio, fibre ratio.

Stiffness-based optimization framework for the topology and fiber paths of continuous fiber composites

Recent advancements of additive manufacturing technology have enabled the fabrication of complex geometries and fiber placement along customized paths. The proposed approach introduces a novel framework for the optimized topology and the fiber paths in order to create variable stiffness designs. The optimized distribution of the material is achieved by two different methods: a density-based method and a level-set method for orthotropic materials. The fiber orientation of each element or the fiber path is considered as the design variable. The optimized orientation is obtained using an energy-based method and a level-set based method. In addition to a fiber angle filtering scheme, three new approaches for the design of fiber infill pattern are introduced to enforce the fiber path continuity across the domain, ensure manufacturability, and achieve reduced compliance. The effects of fiber infill pattern techniques on the compliance is demonstrated in three benchmark case studies.

Electrical and thermal conductivities of the Cu–CF composite

The paper presents a new approach in the field of metal–matrix composite characterisation where an electrical conductivity measurement was used to calculate the electron part of composite thermal conductivity by using the Wiedemann–Franz law. The electrical and thermal conductivities of the composite were characterised and their relationship was analysed. Results showed that in comparison with simple analytical models, this method can also be used for predicting the thermal conductivity of the copper matrix–continuous carbon fibre composite in a transverse direction. The unidirectional composite was produced by diffusion bonding and contained 40–60 vol.% of unidirectional fibres. Experiments were performed in directions parallel and normal to the fibre orientation and showed that with an increasing ratio of fibre volume, both thermal and electrical conductivities decreased from 221.6 W/m·K to 38.7 W/m·K and from 35.8 MS/m to 5.3 MS/m, respectively.

Tensile property evaluations of 3D printed continuous carbon fiber reinforced thermoplastic composites

Continuous carbon fiber composites can be printed using 3D printers. Many studies detailing the mechanical properties of such 3D printed composites have been published, all of which employed a conventional tensile specimen configuration with surface resin layers. In the present study, 0°, 90°, ±45°, and lay-up direction type specimens have been designed for 3D printed composites without surface layers. Both conventional and newly designed specimens with serpentine folded fiber bundles were fabricated and investigated experimentally using a 3D printer. The lay-up direction specimen was fabricated using 800 layers. The specimens without the serpentine folded fiber bundles were experimentally shown to be adequate for tensile tests. The lay-up direction specimen had the lowest strength and stiffness, which appears to be related to its surface roughness.

Thermomechanical characterization of a low viscosity PA66 thermoplastic matrix and associated continuous glass fibre composite under processing conditions

Characterizing the thermomechanical properties of thermoplastic resins and associated composites is of outmost importance to understand the development of process-induced stresses. To that extent, the characterization of a low viscosity PA66 matrix is proposed thanks to a homemade volumetric dilatometer named PvT-XT and a dynamic mechanical analyser on a wide temperature range. The PvT-XT results, reported for the first time, permit to identify the evolution of the coefficients of thermal expansion, of crystallization shrinkage and of the bulk modulus with temperature. Dynamic mechanical analyser experiments lead to the estimation of the Young’s modulus. The shear modulus as well as the Poisson’s ratio are then estimated thanks to analytical relations for isotropic and homogeneous materials. These properties are used to feed a new analytical model estimating the coefficients of thermal expansion of quasi-unidirectional composites. All the results are compared with the values from the literature or from complementary experiments, showing a good agreement which permits to validate the developed methodology.