Feasibility and Characterization of Large-Scale Additive Manufacturing With Long Fiber Reinforced Composites

A new additive manufacturing (AM) approach to fabricate long fiber reinforced composites (LFRC) was proposed in this study. A high deposition rate was achieved by the implementation of a single-screw extruder, which directly used thermoplastic pellets and continuous fiber tows as feedstock materials. Thus, the proposed method was also used as a large-scale additive manufacturing (LSAM) method for printing large-volume components. Using polylactic acid (PLA) pellets and continuous carbon fiber tows, the feasibility of the proposed AM method was investigated through printing LFRC samples and further demonstrated by fabricating large-volume components with complex geometries. The printed LFRC samples were compared with pure thermoplastic and continuous fiber reinforced composite (CFRC) counterparts via mechanical tests and microstructural analyses. With comparable flexural modulus, the flexural strength of the LFRC samples was slightly lower than that of the CFRC samples. An average improvement of 28% in flexural strength and 50% in flexural modulus were achieved compared to those of pure PLA parts, respectively. Discontinuous long carbon fibers, with an average fiber length of 20.1 mm, were successfully incorporated into the printed LFRC samples. The carbon fiber orientation, distribution of carbon fiber length, and dispersion of carbon fiber as well as porosity were further studied. The carbon fibers were highly oriented along the printing direction with a relatively uniformly distributed fiber reinforcement across the LFRC cross section. With high deposition rate (up to 0.8 kg/hr) and low material costs (< $10/kg), this study demonstrated the potentials of the proposed printing method in LSAM of high strength polymer composites reinforced with long carbon fibers.

Theoretical Description of Carbon Felt Electrical Properties Affected by Compression

Electro-conductive carbon felt (CF) material is composed by bonding together different lengths of carbon filaments resulting in a porous structure with a significant internal surface that facilitates enhanced electrochemical reactions. Owing to its excellent electrical properties, CF is found in numerous electrochemical applications, such as electrodes in redox flow batteries, fuel cells, and electrochemical desalination apparatus. CF electro-conductivity mostly arises from the close contact between the surface of two electrodes and the long carbon fibers located between them. Electrical conductivity can be improved by a moderate pressing of the CF between conducting electrodes. There exist large amounts of experimental data regarding CF electro-conductivity. However, there is a lack of analytical theoretical models explaining the CF electrical characteristics and the effects of compression. Moreover, CF electrodes in electrochemical cells are immersed in different electrolytes that affect the interconnections of fibers and their contacts with electrodes, which in turn influence conductivity. In this paper, we investigated both the role of CF compression, as well as the impact of electrolyte characteristics on electro-conductivity. The article presents results of measurements, mathematical analysis of CF electrical properties, and a theoretical analytical explanation of the CF electrical conductivity which was done by a stochastic description of carbon filaments disposition inside a CF frame.

Mechanical structural design based on additive manufacturing and internal reinforcement

The design of modern mechanical components often requires the use of low-density and high-strength parts. Additive manufacturing presents competence in obtaining format complexity internally (voids, ducts, channels) and externally (shape, holes). However, parts obtained by material extrusion additive manufacturing are highly anisotropic and relatively weak. This paper aims to present a new mechanical design technique that combines the high geometry flexibility of additive manufacturing with internal structuring reinforcement by high-strength materials, which enables optimized parts with reinforcement in the most mechanical stressed areas during service, through adopting structured internal geometry filled with reinforcement material. Dense test specimens and test specimens with internal structural canals filled with reinforcement material (epoxy resin and carbon fibers) were designed, fabricated and tested physically and virtually. The obtained results provide property values for 3D-printed acrylonitrile butadiene styrene (typical material of additive manufacturing) and for this polymer reinforced with various reinforcement material configurations (useful for mechanical design). The reinforcement decreased anisotropy and improved mechanical properties. Optimized parts filled with resin and long carbon fibers had maximum flexural resistance of 112 MPa, with a specific weight of 1.1 g/cm3. This reinforcement provided parts with specific flexural strength similar to structural aluminum alloys, preserving the geometry and external dimension of the printed parts. The technique presented here shows the possibility of new conceptions in mechanical components design and strength optimization by internal reinforcement canals in parts. The technique is useful for mechanical design activity and allows for new product conceptions based on additive manufacturing.

Lightweight Cellulose/Carbon Fiber Composite Foam for Electromagnetic Interference (EMI) Shielding

Lightweight electromagnetic interference shielding cellulose foam/carbon fiber composites were prepared by blending cellulose foam solution with carbon fibers and then freeze drying. Two kinds of carbon fiber (diameter of 7 μm) with different lengths were used, short carbon fibers (SCF, L/D = 100) and long carbon fibers (LCF, L/D = 300). It was observed that SCFs and LCFs built efficient network structures during the foaming process. Furthermore, the foaming process significantly increased the specific electromagnetic interference shielding effectiveness from 10 to 60 dB. In addition, cellulose/carbon fiber composite foams possessed good mechanical properties and low thermal conductivity of 0.021–0.046 W/(m·K).

Theoretical and experimental analysis of multifunctional high performance cement mortar matrices reinforced with varying lengths of carbon fibers

An effective scheme to formulate high performance and multifunctional cement based mortar composites reinforced with varying lengths of carbon fibers has been devised. The detailed investigations pertaining to the fracture response of composites in cracks initiation and progression phases, their conducting mechanism and volumetric stability were performed with varying loads of 6mm and 12mm long carbon fibers at two different w/c ratios i.e. 0.45 and 0.50. The experiments concluded that an optimum addition of carbon fibers results in substantial improvement of fracture properties alongside significant reduction in electrical resistivity and total plastic shrinkage. The field emission scanning electron microscopy of the cryofractured specimen revealed crack arresting actions of uniformly distributed carbon fibers through successful crack bridging and branching phenomenon.