Carbon Fiber-reinforced Polyamide Research Articles

Experimental validation of numerical model for thermomechanical performance of material extrusion additive manufacturing process: Effect of infill design & density

The optimum selection of process parameters, materials, and product design is essential to achieve the desired response of 3D-printed structures, especially in functional components. The current practices of the experimental optimization process require significant resources, which can be limited through numerical modeling and simulation techniques. In this study, a thermomechanical numerical model is used to predict the performance of the additive manufacturing (AM) process, i.e., fused filament fabrication (FFF). 3D printing (3DP) process simulations were performed for tensile testing coupons using carbon fiber-reinforced polyamide-6 (PA6-CF) material. The numerical model predicted the effect of infill patterns and densities on the deflections and distortions during the FFF process. The numerical model predictions were validated via experiments performed under similar conditions. The results conclude that the numerical model can adequately predict the process-induced deflections and distortions during the FFF process. Generally, higher dimensional control was observed for rectangular infill patterns and increased infill density. However, the numerical model overestimates the shrinkage as the stress-relaxation effect is not considered in the numerical model and underestimates the warpages as perfect build plate adhesion is assumed.

Experimental and numerical analysis of the consolidation process for additive manufactured continuous carbon fiber-reinforced polyamide 12 composites

Substitution of conventional metal structures with fiber-reinforced polymers is possible because of their sustainable performance. One of the primary disadvantages of these composite materials is their high cost due to labor-intensive manufacturing. Because the fiber path can be steered directly along the load path, structures can be manufactured in a near-net shape, and a high degree of reproducibility with low scrap rates can be achieved. Additive manufacturing of these composite structures could enable cost efficiency with a high degree of complexity. However, the high degree of porosity and high void content between the printed fiber filaments results in unacceptable structural performance. Following the printing process, a post-consolidation process (additive fusion) can be performed to improve the mechanical performance of the part and use fiber-reinforced polymers for load-bearing applications. Numerical simulation of the consolidation process enables the production of these complex parts without expensive prototyping iterations. Because of the rapid and local changes in material stiffness, the simulation of the consolidation process is combined with a set of numerical model convergence problems. An advanced finite-element numerical model for simulating the consolidation process of additive manufactured continuous fiber composite parts is presented in this paper. The additive fusion step simulation allows for the evaluation of process-induced deformations, final engineering constants, and porosity. The simulation workflow is demonstrated and validated using experimental data from the manufacturing process of a typical aerospace part, specifically a helicopter hinge element.

Estimation of fiber orientation distribution in carbon-fiber-reinforced polyamide 6 using X-ray diffraction images

The carbon fiber orientation distribution (FOD) of carbon fiber-reinforced polyamide 6 was estimated using wide-angle X-ray diffraction (WAXRD) images of the composites. In the diffraction image of the aligned carbon fibers, the straight line connecting the pair of diffraction spots caused by the graphite crystal is orthogonal to the fiber axis. This property was employed to investigate the FOD. In this study, we attempt to express WAXRD images, that is, the intensity distribution () by the linear combination of the radial basis functions arranged in the diffraction angle () and azimuth angle (). Consequently, FOD corresponds to the weight vector derived through serial waveform separation. Based on this concept, we attempted to identify the laminated composition of the composites, such as unidirectional, cross-ply, and quasi-isotropic composites. The analysis results were in good agreement with the ideal laminated composition.

Stress-adapted fiber orientation along the principal stress directions for continuous fiber-reinforced material extrusion

A proven method to enhance the mechanical properties of additively manufactured plastic parts is the embedding of continuous fibers. Due to its great flexibility, continuous fiber-reinforced material extrusion allows fiber strands to be deposited along optimized paths. Nevertheless, the fibers have so far been embedded in the parts contour-based or on the basis of regular patterns. The outstanding strength and stiffness properties of the fibers in the longitudinal direction cannot be optimally utilized. Therefore, a method is proposed which allows to embed fibers along the principal stresses into the parts in a load-oriented manner. A G-code is generated from the calculated principal stress trajectories and the part geometry, which also takes into account the specific restrictions of the manufacturing technology used. A distinction is made between fiber paths and the matrix so that the average fiber volume content can be set in a defined way. To determine the mechanical properties, tensile and flexural tests are carried out on specimens consisting of carbon fiber-reinforced polyamide. In order to increase the influence of the principal stress-based fiber orientation, open-hole plates are used for the tensile tests, as this leads to variable stresses across the cross section. In addition, a digital image correlation system is used to determine the deformations during the mechanical tests. It was found that the peak load of the optimized open-hole plates was greater by a factor of 3 and the optimized flexural specimens by a factor of 1.9 than the comparison specimens with unidirectional fiber alignment.

Effect of freeze-thaw cycling on the mechanical properties of continuous carbon fiber-reinforced polyamide 6 composites

This paper deals with the effects of different freeze-thaw cycling environments on the mechanical properties, dimensional stability, and water absorption behaviors of continuous carbon fiber reinforced polyamide 6 composites (CF/PA6). In a nominally dry state, freeze-thaw (FT) exposure can slightly increase the mechanical properties due to the elimination of residual stress. However, FT exposure in a water immersion environment causes a linear decrease in transverse tensile properties and flexural properties and an exponential decrease exposure in water immersion environment. This difference in degradation rate can be attributed to the varying water content, and the relationship between retention rate of mechanical properties and FT cycles was further developed to predict its long-term aging performance. In addition, water absorption behaviors, dimensional measurement, and micro-characterization were also applied to analyze the mechanism of mechanical properties degradation.

Carbon Fiber-Reinforced Polyamide 6 Composites Formed by In Situ Polymerization—Experimental and Numerical Analysis of the Influence of Polymerization Temperature

In this study, carbon fiber-reinforced polyamide 6 composites were prepared by in situ polymerization via vacuum-assisted resin transfer molding (VARTM) in order to evaluate the effect of processing parameters on the mechanical properties and mold filling ratio. At the molding temperature of 140 °C, a high degree of polymerization of the monomer and impregnation of the resin in the preform were empirically observed. The effect of the molding temperature was analyzed by monitoring the mechanical properties (tensile, bending, and impact strength). The mechanical tests showed that the tensile strength, bending strength, bending modulus, and impact strength were highest for the specimens molded at 140 °C compared to those treated at other temperatures. The experimental results were compared with numerical simulations in order to evaluate the relationship between the mold filling ratio and filling time for large-scale automotive parts with various shapes and sizes. The comparison is of great significance for predicting the mold design as well as processing parameters for large-scale automotive parts.

Optimization of additive manufacturing for the production of short carbon fiber-reinforced polyamide/Ti-6Al-4V hybrid parts

The present work is aimed at utilizing an adapted version of the Fused-Filament Fabrication process as a means to produce hybrid joints comprised of sandblasted, rolled Ti-6Al-4V substrates and additively manufactured short carbon fiber-reinforced polyamide (PA-CF). Layer height (h), printing speed (v) and printing bed temperature (Tbed) for the coating layer (i.e. initial layer with unreinforced polyamide) were varied. Using the ultimate single-lap shear strength (ULSS) of the produced joints as a response, linear and polynomial regressions were fit to the experimental dataset using an approach based on Machine Learning. The linear model achieved a better accuracy, with a test R2 of 0.76. It was possible to conclude that the ULSS is strongly dependent on the actual coating layer height (hreal), which in turn depends on h and v. The optimal set of parameters resulted in an ULSS of 23.9 ± 2.0 MPa. Additionally, specimens for a three-point bending test based on the ISO 14679:1997 were produced and tested to further evaluate the influence of coating layer on mechanical behavior of hybrid joints, this time focusing exclusively on their interphase component. For this test, v did not play a statistically significant role, whereas h did.

Infill Density Influence on Mechanical and Thermal Properties of Short Carbon Fiber-Reinforced Polyamide Composites Manufactured by FFF Process

In three-dimensional (3D) printing, one of the main parameters influencing the properties of 3D-printed materials is the infill density (ID). This paper presents the influence of ID on the microstructure, mechanical, and thermal properties of carbon fiber-reinforced composites, commercially available, manufactured by the Fused Filament Fabrication (FFF) process. The samples were manufactured using FFF by varying the infill density (25%, 50%, 75%, and 100%) and were subjected to tensile tests, three-point bending, and thermal analyses by Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). It was shown that the samples with 100% ID had the highest values of both tensile, 90.8 MPa, and flexural strengths, 114 MPa, while those with 25% ID had the lowest values of 56.4 MPa and 62.2 MPa, respectively. For samples with infill densities of 25% and 50%, the differences between the maximum tensile and flexural strengths were small; therefore, if the operating conditions of the components allow, a 25% infill density could be used instead of 50%. After DSC analysis, it was found that the variation in the ID percentage determined the change in the glass transition temperature from 49.6 °C, for the samples with 25% ID, to 32.9 °C, for those with 100% ID. TGA results showed that the samples with IDs of 75% and 100% recorded lower temperatures of onset degradation (approximately 344.75 °C) than those with infill densities of 25% and 50% (348.5 °C, and 349.6 °C, respectively).

Influence of anisotropic fiber properties on tensile elastic modulus estimation for carbon fiber reinforced thermoplastics

The influence of anisotropic fiber properties on estimation of the elastic modulus of carbon fiber-reinforced polyamide 6 (CFRPA6) was studied using two kinds of micromechanics models; Nairn et al. and Hashin (N–H) models, and Mori–Tanaka (M–T) model. First, these models’ accuracy was verified by comparison with an elastic modulus measured for injection molded dumbbell specimens of CFRPA6. Although there was a slight difference between the elastic moduli estimated from the N–H models and the M–T model, the difference was discussed in terms of model properties. Next, estimation errors caused by assuming carbon fiber (CF) as isotropic were calculated under several conditions. The maximum error occurred in the condition that the tensile direction was perpendicular to the fiber axis. Despite CF had large anisotropy, it was approximately 10% for both models. We infer that the small error was caused by a specific stress field.

Process parameters effect on environmental sustainability of composites FFF technology

ABSTRACT The present study aims at investigating the effect of process parameters on environmental sustainability of 3D printing of short fiber-reinforced polymer composites. A preliminary study allowed defining the printing parameters, in terms of printing speed, extrusion temperature and layer thickness, which guarantee low energy consumption. Then, the environmental behavior of two 3D printable composite materials, glass fiber-reinforced (GlassPA) and carbon fiber-reinforced (CarbonPA) polyamide, was investigated using the Life Cycle Assessment methodology, to provide a comprehensive overview of the considered materials and to support the sustainable development of industrial additive manufacturing processes. The functional unit was chosen taking into account the mechanical properties of the two short fiber-reinforced composites. To this purpose, tensile and flexural tests were performed on specimens produced by Fused Filament Fabrication process to evaluate the mechanical properties of printed materials. Scanning electron microscopy was used to observe the different filament morphology affecting the materials performances. Experimental tests showed that CarbonPA exhibits mechanical performances higher than those of GlassPA. Due to the weight reduction that can be accomplished by means of carbon fibers, CarbonPA results the most environmentally friendly alternative in tensile loads applications. On the contrary, for flexural loads applications, GlassPA exhibits lower environmental footprint.

Experimental Characterization and Analysis of the In-Plane Elastic Properties and Interlaminar Fracture Toughness of a 3D-Printed Continuous Carbon Fiber-Reinforced Composite.

The use of continuous fiber as reinforcement in polymer additive manufacturing technologies enhances the mechanical performance of the manufactured parts. This is the case of the Carbon-Fiber reinforced PolyAmide (CF/PA) used by the MarkForged MarkTwo® 3D printer. However, the information available on the mechanical properties of this material is limited and with large variability. In this work, the in-plane mechanical properties and the interlaminar fracture toughness in modes I and II of Markforged’s CF/PA are experimentally investigated. Two different standard specimens and end-tabs are considered for the in-plane properties. Monolithic CF/PA specimens without any additional reinforcement are used for the interlaminar fracture toughness characterization. Two different mode I specimen configurations are compared, and two different test types are considered for mode II. The results show that prismatic specimens with paper end-tabs are more appropriate for the characterization of the in-plane material properties. The use of thick specimens for mode I fracture toughness tests complicates the characterization and can lead to erroneous results. Contrary to what has been reported in the literature for the same material, fracture toughness in mode I is lower than for mode II, which agrees with the normal tendency of traditional composite materials.

Interlaminar fracture toughness of 3D printed continuous carbon fibre- reinforced polyamide

In this study, the interlaminar fracture toughness of 3D printed continuous carbon fibre-reinforced polyamide was characterized under Mode I and II loadings. 3D printing of fracture test samples presents difficulties, mainly shrinkage and excessive warping. To minimize these effects, two solutions were developed. The results show that the Mode I interlaminar fracture behaviour of the printed samples is similar to that obtained by hot compression moulding. In mode II, however, propagation is not effective due to the type of microstructure of the 3D printed material. The hot pressing of the printed material reduces the fracture toughness under Mode I, but significantly improves the fracture toughness under Mode II. The causes of these behaviours are discussed in the paper.

Influence of Fiber Orientation on Deformation of Additive Manufactured Composites

Anisotropy caused by flow-induced fiber orientation of discontinuous fibers within the extrudate in the Extrusion Deposition Additive Manufacturing (EDAM) process gives rise to anisotropic shrinkage in printed parts and thereby, final part deformations. Three fiber orientation states, described by the second-order orientation tensor, were investigated to determine their influence upon final geometry. Two material systems were investigated including a short glass fiber-reinforced polyamide and a short carbon fiber-reinforced polyamide. A curvilinear geometry was modeled and printed virtually in the thermo-mechanical simulation, ADDITIVE3D©. The scale of the printed geometry was in the range of 300–400 mm and contained three semicircular geometry regions connected to linear regions, thereby inducing significant magnification of the spring-in deformation. The predicted deformation of the printed geometry for the three fiber orientation states and two material systems were compared. The deformation predicted for glass fiber-filled polyamide and carbon fiber-filled polyamide of the same fiber orientation states showed comparable deformations. The largest residual deformation was shown to correspond to the orientation tensor with the greatest degree of anisotropy.

Additive manufacturing of short carbon fiber-reinforced polyamide composites by fused filament fabrication: Formulation, manufacturing and characterization

Short carbon fiber-reinforced Polyamide 12 composite materials were prepared and used as filaments for additive manufacturing (AM) of structures using the Fused Filament Fabrication (FFF) method. The effect of carbon fibers concentration and type, infill pattern and environmental temperatures on mechanical properties of the printed test samples were investigated. The measured tensile modulus of the printed composite ranged from 1.4 to 8.8 GPa, an increase of up to 6.3 times the value of the neat printed polymer. The tensile strength ranged from 40 to 90 MPa, for an increase of up to 2.15 times. Optimization of the environmental temperature for improved coalescence of filaments led to a fair increase in values of the tensile modulus and strength, with an improvement up to 1.5 and 2 times, respectively, for the printed samples with pattern orthogonal to the loading directions. Microstructure characterizations were performed for mechanical results interpretations, with the help of a specialized homogenization model. The combination of FFF and carbon fiber-reinforced composite shows high promises for applications in the transportation industry.

Tensile behavior of additively manufactured carbon fiber reinforced polyamide-6 composites

ABSTRACT Carbon fiber-reinforced polymer composites have been mainly used for several applications owing to their thermo-mechanical properties. Fused deposition modeling is a rapidly growing additive manufacturing technology owing to its ability to create a functional part with complex geometries. The tensile properties of polyamide-6 and carbon fiber-reinforced polyamide (PA6/CF) composite parts were compared. The influence of printing parameters as feed rate, build orientations, and layer thickness on the tensile characteristics of the printed parts were studied. The tensile properties of the fabricated materials were investigated at the piece scale considered as macroscopic scale and at the filament scale considered as mesoscopic scale. Scanning electron micrography of the fracture surfaces of the specimens revealed the origin of rupture at the microscopic scale. The density of the printed specimens was also compared to correlate the process parameters to the microstructure and therefore mechanical properties of the final part.

Carbon fiber-reinforced polyamide composites with efficient stress transfer via plasma-assisted mechanochemistry

Carbon fiber-reinforced polymers (CFRPs) show excellent mechanical strength and lightweight. However, conventional CFRPs require high loading of carbon fibers (CFs) to improve their mechanical properties due to poor polymer–CF interfacial adhesion. We propose a dry and ecofriendly method to fabricate high-performance CFRPs by improving interfacial adhesion. The plasma-assisted mechanochemistry (PMC) process can create physical and chemical linkages between polymers and carbon fillers via application of a mechanical force and plasma treatment. We use carbon nanotubes as bridge materials to increase the effective interfacial area and promote mechanical interlocking. The PMC-treated CFRPs exhibit reduced C-factor and increased degree of entanglement, indicating efficient stress transfer the polymer to the filler. Additionally, the PMC process can provide the significantly improved mechanical properties of CFRPs even at low CF loading. Notably, the PMC-treated CFRPs showed continuously increasing tensile strength, Young's modulus, and elongation at break with increasing CF content up to 30 wt%, which has not been observed previously for CFRPs. As a result, the tensile strength, Young's modulus, and elongation at break of the PMC-treated composites with a CF content of 30 wt% increased by 47.1%, 43.0%, and 91.7%, respectively, compared to those of the conventional composites. We believe that this simple approach opens a new route to the fabrication of high-performance CFRPs exhibiting extraordinary mechanical properties.

Rheological investigation of nylon‐carbon fiber composites fabricated using material extrusion‐based additive manufacturing

AbstractFused filament fabrication (FFF) has seen broad industrial adoption as it is capable of manufaturing large complex parts from robust thermoplastics in a cost‐effective manner. However, the mechanical performance of the printed parts is limited due to poor interlayer bonding and the presence of voids. In order to overcome these drawbacks, the addition of short or continuous fibers into the polymer matrix has been investigated, as the fibers can act as a mechanical reinforcement while also mitigating residual stress resulting from the material's rapid solidification following extrusion. Therefore, understanding the implications of process parameters and fiber reinforcements on printed part properties through detailed crystallization analysis and rheological characterizations is of paramount importance. The goal of this study is to understand the process–structure–property relationships of short carbon fiber‐reinforced polyamide 6 (CF‐PA6) by comparing the melt rheology and crystallinity of CF‐PA6 versus a neat PA6 polymer. Differences in the melting and crystallization behavior resulting from the reinforcing fibers revealed an increased time window for crystallization in the fiber‐reinforced matrix. Rheological characterizations at the recommended printing temperatures demonstrate the shear‐thinning behavior of the samples at shear rates relevant to FFF. From a statistical design of experiments analysis, the layer thickness was found to be the most significant parameter affecting the tensile properties of a printed part at a constant printing temperature and printing speed. The tensile fracture surfaces of the printed specimens using scanning electron microscopy were analyzed to provide insights into the failure mechanisms as a function of AM processing variables.

Simulation on the mechanical performance and fracture behavior of unidirectional carbon fiber-reinforced composites

Unidirectional carbon fiber reinforced polyamide 6 (CF/PA6) laminates after tensile tests exhibited two different types of fracture modes, namely, splitting and step-like. In this study, the mechanical properties and fracture behavior of the unidirectional CF/PA6 laminates were simulated and analyzed by using finite element simulation of a tensile test. The simulations considered the actual deformation and damage mechanism (matrix plastic deformation, stochastic fiber fracture, and interfacial debonding between the fiber and the matrix) experimentally observed in previous research. Results of the numerical simulations were in good with the experimental stress–strain curves. Moreover, the above-mentioned two types of fracture mode were well simulated by changing the interfacial properties. The simulated results showed that the unidirectional laminates with weak interface properties exhibit the splitting fracture mode. Meanwhile, the unidirectional laminates with strong interface properties show the step-like fracture behavior.

Modified Halpin–Tsai equation for predicting interfacial effect in water diffusion process

Abstract Interfacial degradation is the main reason for deterioration of mechanical properties of composites in hydrothermal environments. In this study, the effect of the interphase on water diffusion in two types of unidirectional continuous carbon fiber-reinforced polyamide 6 (CF/PA6) composites is investigated through experimental measurements, theoretical analysis, and numerical simulation. The moisture diffusion coefficient of composite at different environmental temperatures is characterized by water immersion tests for analyzing the barrier and accelerating effects of the interphase layer. Based on the experimental results, the three-phase Halpin–Tsai model is derived and validated, and then the critical diffusivity is obtained to quantify the interfacial effect during the diffusion process. To further validate the present three-phase Halpin–Tsai model, the stable and transient finite element models of moisture diffusion are developed. It is found that the critical diffusivity coefficient of the interphase for the CF/PA6 composite system is 7.31 times higher than that of the matrix.

Effect of Geometric Parameters and Moisture Content on the Mechanical Performances of 3D-Printed Isogrid Structures in Short Carbon Fiber-Reinforced Polyamide

The present paper aims at studying the effect of geometric parameters and moisture content on the mechanical performances of 3D-printed isogrid structures in short carbon fiber-reinforced polyamide (namely Carbon PA). Four different geometric isogrid configurations were manufactured, both in the undried and dried condition. The dried isogrid structures were obtained by removing the moisture from the samples through a heating at 120 °C for 4 h. To measure the quantity of removed moisture, samples were weighted before and after the drying process. Tensile tests on standard specimens and buckling tests on isogrid panels were performed. Undried samples were tested immediately after 3D printing. It was observed that the dried samples are characterized by both Young modulus and ultimate tensile strength values higher than those provided by the undried samples. Similar results were obtained by the compression tests since, for a given geometric isogrid configuration, an increase in the maximum load of the dried structure was detected as compared to the undried one. Such discrepancy tends to increase as the structure with the lowest thickness value investigated is considered. Finally, scanning electron microscopy was carried out in order to analyze the fractured samples and to obtain high magnification three-dimensional topography of fractured surfaces after testing.