Continuous Carbon Fiber Research Articles

Designing Prepregnation and Fused Filament Fabrication Parameters for Recycled PP- and PA-Based Continuous Carbon Fiber Composites.

Continuous carbon fiber (cCF)-based 3D-printed polymer composites are known for their excellent flexural properties; however, the optimization of the overall process is still desired, depending on the material types involved. Here, the improved manufacturing of cCF-based composites is reported, considering virgin polyamide (PA) and postindustrial waste polypropylene (PP), and the parameters affecting the material properties are evaluated. Firstly, the prepregnation technique was optimized to manufacture cCF polymer filaments with various fiber-to-polymer ratios. Secondly, the fused filament fabrication (FFF) technique was optimized. It was observed that the layer height needs to be sufficiently low for proper interlayer adhesion. The influence of the printing temperature is more complicated, with filaments characterized by a lower fiber-to-polymer ratio requiring a higher nozzle diameter and higher temperatures for efficient printing; and for lower diameters, the best flexural properties are observed for parts printed at lower temperatures, maintaining a high interspace distance. Plasma treatment of the cCF was also explored, as was annealing of the produced parts to enhance the flexural properties, the latter being specifically interesting for the PP-based composite due to a lower wetting caused by a higher viscosity, despite supportive interfacial interactions. Eventually, overall guidelines were formulated for the successful production of cCF-based composites.

Effect of slit size on low‐velocity impact behavior of composite laminates with regularly arrayed chopped strands: Experimental and numerical analysis

AbstractShort fiber reinforced polymers (SFRP) based on unidirectionally arrayed chopped strands (UACS) offer exceptional formability and great mechanical properties. To ensure its stability and safety in applications, it is crucial to enhance the impact performance of UACS laminates. This study investigated the low‐velocity impact (LVI) responses and damage evolution of UACS laminates with different slit sizes and continuous carbon fiber reinforced polymer (CFRP) laminates under various impact energies (4, 7, and 11 J). The curves of force and energy were recorded during LVI tests, and the post‐impact damage area was detected by the ultrasonic C‐scan technique. Moreover, a user‐defined subroutine VUMAT, containing a progressive damage model and a Johnson‐Cook constitutive model, was written to mimic the damage evolution. Based on experiments and numerical prediction, it was found that when the size was reduced from 25 to 5 mm, the vertical slits had the effect of suppressing delamination and could restrain the propagation of delamination, which explains the distinct difference in delamination area.Highlights The effect of slit size on the impact behavior of UACS laminate was revealed. The damage mechanism was simulated with a progressive damage model. The novel UACS laminate exhibits excellent energy absorption capacity. The resistance of the slits greatly suppresses the delamination behavior.

Characterization and Simulation of the Interface between a Continuous and Discontinuous Carbon Fiber Reinforced Thermoplastic by Using the Climbing Drum Peel Test Considering Humidity.

The objective of this paper is to investigate the debonding behavior of the interface between continuously and discontinuously fiber reinforced thermoplastics using the climbing drum peel test. The study emphasizes on the importance of considering different climatic boundary conditions on the properties of thermoplastics. Specimens with varying moisture contents, from 0m.% up to above 6m.% are prepared and tested. It is observed that an increase in moisture content from 0m.% to 2m.% results in an increase of the fracture surface energy from 1.07·103J/m2 to 2.40·103J/m2 required to separate the two materials, but a further increase in moisture to 6.35m.% conversely results in a subsequent decrease of the required energy to 1.91·103J/m2. The study presents an explanatory model of increasing plasticization of the polymer due to increased polymer chain mobility, which results in more deformation energy being required to propagate the crack, which is corroborated in SEM investigations of the fracture surface. A further increase in humidity leads to polymer degradation due to hydrolysis, which explains the subsequent reduction of the fracture energy. The experimental set up is modeled numerically for the first time with cohesive surfaces, which could successfully reproduce the effective force-displacement curve in the experiment by varying the interface parameters in the model over an influence length, allowing the conclusion of a process induced variation in the interface properties over a specific consolidation length.

Modeling of 3D-printed composite filaments using X-ray computer tomography images segmented by neural networks

This study introduces a novel semi-analytical two-scale approach to model 3D-printed composite filament where both fibers and voids are present. The proposed approach utilizes micro computer tomography (micro-CT) images to visualize fibers and voids. Then, a novel segmentation process based on neural network algorithms (specifically the YOLO v7 algorithm) is employed to segment the distinct fiber and void phases. The volume fractions and the geometries of the phases are then used in the first scale (the micro-scale) of the approach and the effective elastic properties of internal regions of the filament are obtained using the Mori–Tanaka effective field method. These regions are then joined together to form the second scale (the macro-scale), for which the elastic properties are evaluated using the finite element method. The two-scale approach is applied for 3D-printed nylon reinforced with continuous carbon fibers and the predictions of the elastic properties are compared to tensile tests conducted on the single filament. A great agreement is observed for Young’s modulus along the fiber direction of the single printed filament.

Highly porous carbon-SiC composites with continuous carbon fibers for the production of radioisotopes in ISOL facilities

New C/SiC composites with continuous fibers were produced through impregnation of carbon fiber preforms and consolidation by hot pressing. The resulting sintered material was investigated as a potential target for nuclear applications, such as radioisotope production. Desired properties include high porosity and high thermal conductivity to allow isotope release from the targets while working at high temperature. To this end, porosimetry and thermal and mechanical characterizations were conducted up to 1250°C. It was shown that the addition of fibers resulted in high porosity, about 40%, while maintaining a very high thermal conductivity, 40 W/m∙K at 1250°C, a value particularly attractive for the intended application. Emissivity at 1250°C was 0.76, compressive strength at room temperature was 134 MPa, and flexural strength passed from about 90 MPa before thermal conductivity measurements to 50 MPa after the tests.

Enhancement of the bearing strength of pinned-joint composite materials

In this study, a different approach is used to enhance the bearing strength in continuous carbon fiber epoxy composite materials. In order to enhance the bearing strength, metal washers are inserted between laminae of composite material in the region of the pin. Two types of washer, one is stainless steel and the other is aluminum, are used in the preparation of specimens to carry out the bearing strength tests. In addition, three metal washer inserting configurations, as embedding one, two, or three washers in between carbon fiber layers, are tested in each group of specimens. The results show that the first bearing strength values obtained from the specimens with stainless steel washers increase considerably with respect to specimens with aluminum washers and the specimens without washers. The maximum strength of the specimens reinforced with one, two, and three stainless steel washers is approximately 185%, 405%, and 390% higher than the reference specimen, respectively. It is also observed that the maximum strength of the specimens reinforced with one, two, and three aluminum washers is 265%, 285%, and 320% higher than the reference specimen, respectively.

Influence of structural geometry on tensile properties and fracture toughness in 3D printed novel structures

The study employed chopped fiber-reinforced nylon composite, referred to as “Onyx,” along with continuous carbon fiber materials. Using the material extrusion manufacturing process, three sandwich structures were created: a conventional, a concave hexagonal with negative Poisson's ratio, and a feather-inspired structure. Tensile testing and finite element simulations were conducted to analyze their tensile properties and crack propagation paths under tension. Fractal dimensions were calculated for each structure: the standard structure had a dimension of 1.4626, the bionic feather-inspired was 1.4810, and the negative Poisson's ratio concave hexagonal structure measured 1.4973.

A Review of Print Heads for Fused Filament Fabrication of Continuous Carbon Fiber-Reinforced Composites.

The print head is one of the most critical components in an additive manufacturing (AM) system. It can significantly affect the quality of printed parts. Recently, because continuous carbon fiber-reinforced composites can have excellent mechanical properties, a relevant AM technique, fused filament fabrication (FFF), has been attracting increasing attention. This has extended the requirements demanded of print heads. To this end, different FFF extrusion methods have been rapidly developed based on various methods of impregnating fibers into the matrix for the corresponding print heads. Generally, these extrusion methods are of three types: single extrusion, in situ extrusion, and dual extrusion. All these methods face substantial challenges, such as the nozzle clogging and damage to the continuous carbon fibers during extrusion. These common issues still need to be fully addressed. This study's aim is to summarize and discuss the different extrusion methods and their FFF specific components in terms of their advantages and disadvantages for continuous carbon fiber-reinforced composites.

Demonstration of computer vision for void characterisation of 3D-printed continuous carbon fibre composites

Microstructural void formation during the processing of continuous fibre-reinforced polymer composites are a significant limitation of processes such as 3D printing, as voids inhibit resultant mechanical properties. Traditional optical microscopy approaches to void characterisation are tedious and are prone to errors due to non-ideal but realistic image conditions, including low contrast typical of microscopic images. This paper proposes a novel application of automated computer vision image processing techniques (i.e. a contrast-limited adaptive histogram equalisation (CLAHE) algorithm and Otsu's method) for detailed void characterisation of continuous fibre-reinforced composites. Microstructural void fraction was determined for both unidirectional and bi-axial laminates of a variety of thicknesses. The void characterisation by computer vision was shown to produce comparable results with manual contrast/brightness control, but with significantly less standard deviation. The computer vision software used in void characterisation is made freely available online at https://github.com/Xiao-ying/VoidDetector.

Influence of Interfacial Interaction and Composition on Fracture Toughness and Impact Properties of Carbon Fiber-Reinforced Polyethersulfone.

In this study, the interlaminar fracture toughness and impact strength of polyethersulfone reinforced with continuous carbon fibers were studied. Interlaminar fracture toughness tests were performed using the double cantilever beam method. It was shown that surface modification using the thermal oxidation method of the carbon fibers can strongly increase the interlaminar fracture toughness of the obtained composites. Thus, the maximum value reached 1.72 kJ/m2, which was 40% higher than the fracture toughness of the composites reinforced with initial carbon fibers. Moreover, fractographic analysis using a scanning electron microscope allowed us to highlight the main reasons for the dependence of fracture toughness on fiber content and surface modification conditions of the carbon fibers. It was shown that the main factor that allowed for an increase in fracture toughness was the enhanced interfacial interaction between the fibers and polymer matrix. Additionally, it was found that expectedly, there was a good correlation between interlaminar fracture toughness and interlaminar shear strength results. However, a negative influence of surface modification on the impact properties of composites was found. Such behavior occurred because of higher structural stability and lower exposure to delamination in multiple layers of the composites reinforced with the modified carbon fibers. It was found that impact energy reached ~150 kJ/m2 for the polyethersulfone-based composites reinforced with initial fibers, while the composites reinforced with modified carbon fibers showed impact energy values of only ~80 kJ/m2. Nevertheless, surface modification of carbon fibers using the thermal oxidation method can be an effective method for improving the performance properties of polyethersulfone-based composite materials.

Tensile fatigue behavior of short and continuous carbon fiber reinforced additively manufactured thermoplastic multiscale composite

Fiber reinforced multiscale composites represent a cutting-edge class of advanced materials that have garnered significant attention in the realm of materials science and engineering. These advanced materials intertwine various fiber reinforcements, meticulously tailored at micro, to macro levels, imparting them with remarkable mechanical, thermal, and functional properties. In this research, an experimental investigation was performed to observe the tensile fatigue behavior of short and continuous carbon fiber reinforced additively manufactured multiscale thermoplastic composites. The multiscale composite was fabricated by combining layers of continuous woven carbon fiber (CCF) with short carbon fiber (SCF) reinforced acrylonitrile butadiene styrene (ABS) laminates which were additively manufactured using the fused filament fabrication technique. It was observed that the multiscale composite exhibits a maximum 127% higher strength and 154% higher Young’s modulus than neat ABS polymer. Based on non-normalized stress-life curves, multiscale composites with four layers of CCF have a higher fatigue strength than multiscale composites with one layer of CCF. However, when the applied stress is normalized by the quasi-static strength, the multiscale composite with one layer of CCF had a longer fatigue life than the four layers of CCF reinforced multiscale composites at high normalized cyclic stress, while the multiscale composite with four layers of CCF exhibited a longer fatigue life at low normalized cyclic stress. The four CCF layers reinforced multiscale composites experienced a different extent of fiber pull-out, matrix cracking, and delamination at different stress levels. Conversely, fatigue failure of the one layer of CCF reinforced multiscale composites primarily occurred due to localized fiber breakage with minimal matrix-related damage.

The role of printing parameters on the short beam strength of 3D-printed continuous carbon fibre reinforced epoxy-PETG composites

It is well known that printing parameters strongly affect the mechanical performance of 3D printed parts. This work explores the role of i) extruder temperature, ii) print speed, and iii) layer height on the interlaminar strength of 3D-printed continuous fibre-reinforced composites. The carbon fibre-reinforced thermoset filament is printed concomitantly with polyethylene terephthalate glycol (PETG) thermoplastic filament in a single nozzle, characterising a continuous fibre co-extrusion (CFC) process. There is a significant variation in the short beam strength for composites printed with different parameters. The load–displacement curves have a similar pattern, with clear load peaks followed by a plastic zone. Optical micrographs and computed tomography (CT) scans reveal that the microstructure is dependent on the printing parameters. Image analysis elucidates the various mechanisms of void formation. Following the application of a three-way ANOVA and statistical tests to quantify the effects and interactions among variables, the analysis concludes that the extruder temperature has the highest influence, followed by print speed and layer height. When considering all possible interactions between the factors, the interaction between print speed and layer height is the most impactful.

Electrothermally controlled origami fabricated by 4D printing of continuous fiber-reinforced composites

Active origami capable of precise deployment control, enabling on-demand modulation of its properties, is highly desirable in multi-scenario and multi-task applications. While 4D printing with shape memory composites holds great promise to realize such active origami, it still faces challenges such as low load-bearing capacity and limited transformable states. Here, we report a fabrication-design-actuation method of precisely controlled electrothermal origami with excellent mechanical performance and spatiotemporal controllability, utilizing 4D printing of continuous fiber-reinforced composites. The incorporation of continuous carbon fibers empowers electrothermal origami with a controllable actuation process via Joule heating, increased actuation force through improved heat conduction, and enhanced mechanical properties as a result of reinforcement. By modeling the multi-physical and highly nonlinear deploying process, we attain precise control over the active origami, allowing it to be reconfigured and locked into any desired configuration by manipulating activation parameters. Furthermore, we showcase the versatility of electrothermal origami by constructing reconfigurable robots, customizable architected materials, and programmable wings, which broadens the practical engineering applications of origami.

Real-time in-situ process monitoring method based on the self-conductivity of carbon fiber prepreg for automated fiber placement (AFP)

Real-time online monitoring is of great significance for the manufacturing of carbon fiber reinforced composites, contributing to improving and ensuring the quality of the final manufactured part. An in-situ real-time process monitoring method based on the conductivity of continuous carbon fiber reinforced thermoplastic matrix prepreg was proposed for automated fiber placement (AFP). The detection capability of several different defects was investigated, including foreign object debris, tape breakage, weak edge bonding, gaps/overlaps, and even debonding. In addition, the influence of placement speed and laser power on the detection capacity was also discussed. The number, position, length, and width of various interlayer defects were detected through the resistance change between the current placement layer and the laid layers. Furthermore, accurate detection of defects in different layers was also achieved, indicating the detection capability of existing defects. This method of timely defect detection during the manufacturing process allows for an early manual intervention to improve the quality of manufactured structures and the intelligence of AFP technology.

Continuous carbon fiber recycling technology using the electrolytic sulfuric acid method

Our objective is to achieve the societal implementation of the Circular Economy Program for Automobile Carbon Fibers. This involves recycling carbon fibers sourced from carbon fiber reinforced plastics/carbon fiber reinforced thermoplastics (CFRP/CFRTP) discarded from automobiles and reintegrating them into the manufacturing processes of automobiles. Although, the existing carbon fiber recycling technology recycles the carbon fiber into a finely chopped state; it is not the original continuous carbon fiber product. A novel recycling technology is elucidated herein, referred to as the electrolytic sulfuric acid method (ESAM). The ESAM recycles carbon fibers by decomposing only the resin component of CFRP/CFRTP into CO2 and water using oxidative active species generated by electrolytic sulfuric acid. This method can (1) be applied to all resins, (2) maintains strength of the recycled carbon fibers, and (3) regenerates continuous carbon fibers. Moreover, it is the only technology applicable to CFRP pressure tanks. The successful recycling of continuous carbon fiber from pressure tanks has been achieved, enabling the production of new tanks and unidirectional CFRTP tape. This technology recycles the original continuous carbon fiber product, thus enabling a “close” resource circulation cycle. We have not yet confirmed the number of times recycling can be performed. However, according to the method described in this paper, the recycling process can yield continuous carbon fibers without a decrease in physical properties. Therefore, theoretically, it becomes possible to recycle indefinitely and revert the fibers to their original state, provided that the strength reduction during product use is not a significant consideration.

A strength optimization approach for manufacturable 3D printed variable stiffness laminates with a strain-based approximation

The 3D printed continuous carbon fiber reinforced polymers (CFRP) are promising lightweight structures with outstanding mechanical performance. In this work, we propose a structure-manufacture integrated optimization approach for strength maximization of variable stiffness 3D printed CFRP composites by tailoring the steered printing paths. The rule of mixture is modified to evaluate material properties and strength of the printed laminates considering the effect of fiber volume fraction. To make this approach applicable to CFRP with asymmetric layups, a strain-based two-level convex approximation for the failure index based on the Tsai–Wu failure criterion is proposed, which can take the tension-bending coupling effect into consideration. The local failure indices are aggregated using the p-norm formula to reduce the size of the strength optimization model. In order to guarantee the manufacturability of the optimal design, the curvature of the steered printing fiber paths is constrained using a hybrid control method within the approach. The numerical cases show that the strength enhancement of the 3D printed variable stiffness laminates depends on the load, where an improvement between 35% and 46% can be achieved under the tension. Moreover, the proposed method can not only converge stably, but also be capable of optimizing structures with complex geometry.

High thermal conductivity continuous pitch carbon fiber 3D printed using a 6-axis robot arm

High thermal conductivity continuous pitch carbon fiber 3D printed using a 6-axis robot arm

Design and Optimization of 3D-Printed Variable Cross-Section I-Beams Reinforced with Continuous and Short Fibers.

By integrating fiber-reinforced composites (FRCs) with Three-dimensional (3D) printing, the flexibility of lightweight structures was promoted while eliminating the mold's limitations. The design of the I-beam configuration was performed according to the equal-strength philosophy. Then, a multi-objective optimization analysis was conducted based on the NSGA-II algorithm. 3D printing was utilized to fabricate I-beams in three kinds of configurations and seven distinct materials. The flexural properties of the primitive (P-type), the designed (D-type), and the optimized (O-type) configurations were verified via three-point bending testing at a speed of 2 mm/min. Further, by combining different reinforcements, including continuous carbon fibers (CCFs), short carbon fibers (SCFs), and short glass fibers (SGFs) and distinct matrices, including polyamides (PAs), and polylactides (PLAs), the 3D-printed I-beams were studied experimentally. The results indicate that designed and optimized I-beams exhibit a 14.46% and 30.05% increase in the stiffness-to-mass ratio and a 7.83% and 40.59% increment in the load-to-mass ratio, respectively. The CCFs and SCFs result in an outstanding accretion in the flexural properties of 3D-printed I-beams, while the accretion is 2926% and 1070% in the stiffness-to-mass ratio and 656.7% and 344.4% in the load-to-mass ratio, respectively. For the matrix, PAs are a superior choice compared to PLAs for enhancing the positive impact of reinforcements.

Continuous carbon fiber 3D printing partitioning method based on stress distribution

3D printing of continuous carbon fiber composites presents a need for the directivity of fiber distribution. For parts with actual working conditions, the stress distribution of the same part under the same stress condition is different, but the existing partitioning algorithm mainly fills the path of the subregion according to the shape of the part, and does not combine the working condition of the part. To solve this problem, a partitioning algorithm based on stress contour density is proposed. Based on the results of finite element analysis, the stress contour of the parts is obtained, and the stress concentration area is partitioned by calculating the stress contour density. The stress concentration area is filled with a more suitable path to improve the performance of the parts.

Integrating continuous mineral-impregnated carbon fibers into digital fabrication with concrete

Digital concrete technologies, such as 3D concrete printing, have the potential to be game changers in the construction industry by reducing material consumption and offering a high level of automation. This not only provides a solution to the shortage of qualified workers but also makes construction sites more attractive workplaces. However, a significant challenge lies in integrating reinforcement, as traditional methods quickly reach their limits, particularly in geometrically complex, material-minimized structures. A potential solution is the use of mineral-impregnated carbon fibers (MCF) reinforcement, which offers high flexibility for 3D printing and better bonding to concrete compared to polymer coating, among other advantages. This article presents the integration of both freshly impregnated and cured MCF into two different concretes in the context of digital fabrication: 3D-printed concrete (3DPC) and self-compacting concrete (SCC). A particular focus was on the mechanical performance of MCF and MCF-reinforced concrete, as well as on the bond behavior between MCF and concrete. The second focus was on the production method. Therefore, samples were conventionally cast with SCC, while layered pouring or printing were used for producing specimens with 3DPC. It was found that the cured MCF exhibited a notably better bond, enhanced by 50%, to both 3DPC and SCC compared to freshly impregnated yarns. Subsequently, when subjected to uniaxial tensile loading, a significantly higher crack density and more uniform crack distribution could be observed in composites with cured MCF using digital image correlation (DIC). This effect was particularly pronounced in printed specimens and confirmed with computed tomography (CT) and environmental scanning electron microscope (ESEM) images.