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1. THE EFFECTS OF DEBULKING ON THE MICROSTRUCTURE OF CARBON FIBER REINFORCED COMPOSITES

   https://doi.org/10.12783/asc36/35951  

   SCHEY, MATHEW; STAPLETON, SCOTT; BEKE, TIBOR (September 2021, Destech Publications, Inc.)

   Carbon fiber reinforced plastics (CFRPs) are widely used due to their high strength to weight ratios. A common process manufacturers use to increase the strength to weight ratio is debulking. Debulking is the process of transversely compacting a dry fibrous reinforcement prior to wet out with the matrix resin, in order to induce fiber nesting, effectively increasing the volume fraction of the sample. While this process is widely understood macroscopically its effects on fibrous microstructures have not yet been well characterized. The aim of this work is to compare the microstructures of three CFRPs, varying only the debulking step in the manufacturing process. The microstructural effects of debulking on three unidirectional CFRPs made from three different levels of debulking were studied. High resolution serial sections of all three samples were taken using the UES ROBO-MET at the NASA Glenn Research Center in Cleveland, Ohio. Using these scans, the fiber positions were measured and connected to make fiber paths. Statistical descriptors such as local fiber and void volume fractions, and void distribution and morphology were then generated for each sample and compared. Using these descriptors, the effects of debulking on the composite microstructure can be measured. 

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2. FIBER AND VOID PROPERTY CORRELATION WITHIN BEAD MICROSTRUCTURE OF LARGE AREA ADDITIVE MANUFACTURING POLYMER COMPOSITES

   Neshat Sayah and Douglas E. Smith (October 2023, Proceedings of ICCM23)

   Short carbon fiber-reinforced polymer composites are widely employed in additive manufacturing (AM) techniques, including Large Area Additive Manufacturing (LAAM) polymer extrusion-deposition, due to their superior mechanical properties compared to neat polymers. The mechanical and thermal properties of these composites are significantly influenced by factors such as fiber volume fraction, orientation, length, and distribution, and void distribution and volume fraction within the microstructure of the printed beads. This paper presents an experimental study that aims to quantitatively assess the relationship between void volume fraction and fiber orientation within the microstructure of both single freely extruded strands and single deposited beads of Short Carbon Fiber reinforced Acrylonitrile Butadiene Styrene (SCF/ABS) manufactured via a LAAM system. The study employs high-resolution 3D microcomputed tomography (μCT) to evaluate the fiber orientation, fiber volume fraction, and void volume fraction within the microstructure of the SCF/ABS composite parts. The findings demonstrate that the print direction 𝐴zz component of the fiber orientation tensor in the regions near the edges of the single freely extruded strand is higher than those near the center, likely due to increased nozzle shear rate near the wall. Furthermore, within a single deposited bead on the print bed, the 𝐴zz component varies throughout the microstructure. Measurements also show that regions with relatively higher void volume fraction have a corresponding lower fiber 𝐴zz and fiber volume fraction for both the single freely extruded strand and the single deposited bead. 

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3. Correlation of Microstructural Features within Short Carbon Fiber/ABS Manufactured via Large-Area Additive- Manufacturing Beads

   https://doi.org/10.3390/jcs8070246  

   Sayah, Neshat; Smith, Douglas E (July 2024, Journal of Composites Science)

   Short carbon fiber-reinforced polymer composites are widely used in polymer extrusion additive manufacturing (AM), including large-area additive manufacturing (LAAM), due to their enhanced mechanical properties as compared to neat polymers. However, the mechanical properties of these composites depend on microstructural characteristics, including fibers and micro-voids, which are determined during processing. In this work, the correlation between fibers and micro-voids within the microstructure of LAAM polymer composites throughout various processing stages of short carbon fiber-reinforced acrylonitrile butadiene styrene (SCF/ABS) is investigated. The processing stages considered here include the incoming pellets, a single freely extruded strand, a single regularly deposited bead, and a single regularly deposited bead pressed by a mechanical roller. A high-resolution X-ray micro-computed tomography (µCT) system is employed to characterize the microstructural features in terms of the fibers (volume fraction, fiber orientation tensor) and micro-voids (volume fraction, sphericity) in the SCF/ABS samples. The results indicate that micro-voids exist within the microstructure of the SCF/ABS composite in all four stages considered here and that the micro-void volume fraction and micro-void sphericity vary among the test samples. Moreover, the results show a considerable variation in fiber orientation and fiber volume fraction within the microstructure throughout all the stages considered; however, all the samples show the highest alignment in the extrusion/print direction. Furthermore, a correlation is identified between the fiber orientation and the micro-void volume fraction within samples from all four stages considered here. This finding suggests that fibers tend to align more in the extrusion/print direction in regions with less micro-void content. 

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4. Effect of the Print Bed Temperature on Void Distribution within the Microstructure of Short Carbon Fiber Reinforced/ABS Manufactured via Large Area Additive

   Neshat Sayah and Douglas E. Smith (October 2023, Proceedings 2023 International Solid Freeform Fabrication Symposium)

   Short carbon fiber-reinforced polymer composite structures produced using Large Area Additive Manufacturing (LAAM) have garnered significant attention due to the design flexibility, energy savings, and materials selection associated with this process. However, the physical and mechanical properties of the additively manufactured composite parts often fall below expectations due to void formation between printed beads and within the microstructure of individual beads. This study aims to investigate the effect of bed temperature on the microstructure within the beads of two-layer Short Carbon Fiber reinforced Acrylonitrile Butadiene Styrene (SCF/ABS) beads manufactured via the LAAM system. This study employs high-resolution 3D micro-computed tomography (μCT) to evaluate the void shape and distribution within the microstructure of composite parts printed at various bed temperatures. The results of this study demonstrate substantial variation in the void volume fraction among four bead sets deposited at different print bed temperatures. Moreover, within each part, a noticeable discrepancy in void volume fraction is observed between the top and bottom bead of the two-bead test samples. Preliminary results indicate that increasing the bed temperature from 25°C to 75°C reduces void volume fraction within the microstructure of the composite parts. However, an opposite trend emerges when the bed temperature is further increased to 100°C, increasing void volume fraction, which needs further investigation to understand. This study also evaluated the void shapes through the calculation of their sphericity. The preliminary results reveal that as the bed temperature increases from 25°C to 75°C, the voids exhibit higher sphericity within the printed parts as the interconnected voids decrease. 

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5. Enhancing the Interlaminar Shear Strength and Void Control of 3D-Printed Continuous Carbon-Fiber-Reinforced Polymer Composites Using a Robotic Magnetic Compaction Force-Assisted Additive Manufacturing (MCFA-AM) Process and Carbon-Nanofiber Z-Threads

   https://doi.org/10.3390/app13105914  

   Islam, Mohammad Rakibul; Taylor, Wyatt; Warren, Ryan; Hsiao, Kuang-Ting (May 2023, Applied Sciences)

   Manoj Gupta (Ed.)

   Three-dimensional (3D) printing with continuous carbon-fiber-reinforced polymer (C-CFRP) composites is under increasing development, as it offers more versatility than traditional molding processes, such as the out-of-autoclave-vacuum bag only (OOA-VBO) process. However, due to the layer-by-layer deposition of materials, voids can form between the layers and weaken some of the parts’ properties, such as the interlaminar shear strength (ILSS). In this paper, a novel mold-less magnetic compaction force-assisted additive manufacturing (MCFA-AM) method was used to print carbon nanofiber (CNF) z-threaded CFRP (ZT-CFRP) laminates with significantly improved ILSS and reduced void content compared to traditional C-CFRP laminates, which are printed using a no-pressure 3D-printing process (similar to the fused-deposition-modeling process). The radial flow alignment (RFA) and resin-blending techniques were utilized to manufacture a printing-compatible fast-curing ZT-CFRP prepreg tape to act as the feedstock for a MCFA-AM printhead, which was mounted on a robotic arm. In terms of the ILSS, the MCFA-AM method coupled with ZT-CFRP nanomaterial technology significantly outperformed the C-CFRP made with both the traditional no-pressure 3D-printing process and the OOA-VBO molding process. Furthermore, the mold-less MCFA-AM process more than doubled the production speed of the OOA-VBO molding process. This demonstrates that through the integration of new nanomaterials and 3D-printing techniques, a paradigm shift in C-CFRP manufacturing with significantly better performance, versatility, agility, efficiency, and lower cost is achievable. 

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