Discontinuous Carbon Research Articles

Effect of aluminum carbide interphase on the thermomechanical behavior of carbon nanotube/aluminum nanocomposites

The objective of this work is to investigate the coefficient of thermal expansion of carbon nanotube reinforced aluminum matrix nanocomposites in which aluminum carbide (Al4C3) interphase formed due to chemical interaction between the carbon nanotube and aluminum matrix is included. To this end, the micromechanical finite element method along with a representative volume element, which incorporates, carbon nanotube, interphase, and aluminum matrix is employed. The emphasis is mainly placed on the effect of Al4C3 interphase on the coefficient of thermal expansion of aluminum nanocomposites with random microstructures. The effects of interphase thickness, carbon nanotube diameter, and volume fraction on the thermomechanical response of aluminum nanocomposite are discussed. The results reveal that the effect of Al4C3 interphase on the coefficient of thermal expansion of the aluminum nanocomposites becomes more significant with (i) increasing the coefficient of thermal expansion volume fraction, (ii) decreasing the coefficient of thermal expansion diameter, and (iii) increasing the interphase thickness. It is clearly observed that the coefficient of thermal expansion varies nonlinearly with the carbon nanotube diameter; however, it decreases linearly as the carbon nanotube volume fraction increases. Furthermore, the axial and transverse coefficient of thermal expansions of aligned continuous and discontinuous carbon nanotube-reinforced aluminum nanocomposites with Al4C3 interphase are predicted. Also, the presented finite element method results are compared with the available experiment in the literature, rule of mixture, and concentric cylinder model results.

Evaluation and Investigation of Strain Rate and Temperature Dependence Using 3-Point Bending Impact Test for Randomly-Oriented Discontinuous Carbon Fiber Reinforced Thermoplastic Composites

Evaluation and Investigation of Strain Rate and Temperature Dependence Using 3-Point Bending Impact Test for Randomly-Oriented Discontinuous Carbon Fiber Reinforced Thermoplastic Composites

A new numerical method for the mechanical analysis of chopped carbon fiber tape-reinforced thermoplastics

Discontinuous carbon fiber-reinforced thermoplastics (DCFRTP) are promising materials for the fabrication of complex parts. In predicting the mechanical properties of DCFRTP with randomly oriented reinforcement, the application of numerical methods, which employ homogeneous models, is limited because the mesoscopic structural parameters influencing mechanical properties are not adequately represented in the macroscopic numerical model. On the basis of the peridynamic (PD) theory, a new numerical method, which employs a heterogeneous particle model, is proposed in this investigation for the mechanical analysis of DCFRTP. The tensile properties of ultra-thin chopped carbon fiber tape-reinforced thermoplastics (UT-CTT) are investigated using this numerical method. The relationship between PD horizon size and characteristic size of UT-CTT is discussed. The effect of model size on the scattering of the tensile properties is analyzed and the influence of interface properties on crack propagation is investigated. A comparison between PD simulations and experimental results indicated a good agreement.

A general method to improve 3D-printability and inter-layer adhesion in lignin-based composites

We report the utilization of a melt-stable lignin waste-stream from biorefineries as a renewable feedstock, with acrylonitrile-butadiene rubber and acrylonitrile-butadiene-styrene (ABS) polymer to synthesize a renewable matrix having excellent 3D-printability. While the initial low melt viscosity of the dispersed lignin phase induces local thermo-rheological relaxation facilitating the composite's melt flow, thermal crosslinking in both lignin and rubber phases as well as at the lignin-rubber interface decreases the molecular mobility. Consequently, interfacial diffusion and the resulting adhesion between deposited layers is decreased. However, addition of 10wt.% of discontinuous carbon fibers (CFs) within the green composites not only significantly enhances the material performance but also lowers the degree of chemical crosslinking formed in the matrix during melt-phase synthesis. Furthermore, abundant functional groups including hydroxyl (from lignin) and nitrile (from rubber and ABS) allow combinations of hydrogen bonded structures where CFs play a critical bridging role between the deposited layers. As a result, a highly interfused printed structure with 100% improved inter-layer adhesion strength was obtained. This research offers a route toward utilizing lignin for replacement of petroleum-based thermoplastics used in additive manufacturing and methods to enhance printability of the materials with exceptional mechanical performance.

Development of a closed-loop recycling process for discontinuous carbon fibre polypropylene composites

Abstract In this study the effects of a closed-loop recycling methodology are evaluated for degradation using a discontinuous carbon fibre polypropylene (CFPP) composite material. The process comprises two fundamental steps, reclamation and remanufacture. The material properties are analysed over two recycling loops. For neat polypropylene, the molecular weight analysis indicates evidence of minimal matrix degradation that does not affect the material behaviour, as demonstrated by the shear tests. CFPP specimens show no decrease in mechanical properties over repeated loops, the final specimens show an increase of 26% and 43% in ultimate tensile strength and ultimate strain, respectively. These are attributed to cumulative matrix residue on the fibre surface after reclamation and subsequently increased fibre-matrix adhesion. The improvement of CFPP properties and insignificant variability in the tensile properties and molecular weight distribution of neat polypropylene validate the potential of this proof-of-concept, closed-loop recyclable material. Future studies will investigate alternative, higher performance matrices.

Billet flow formation of discontinuous carbon fiber ribbed square panels from continuous carbon fibers

This paper presents a newly-developed billet flow formation method, which produces discontinuous carbon fiber thermoplastic ribbed panels by using the die cushion motion of a press. Billets of discontinuous fiber reinforced thermoplastic were extruded into rib cavities by die cushion motion while press-forming a panel of continuous fiber reinforced thermoplastic. Using billet flow formation, 200 mm long, 46 mm high, 1.4 mm thick ribs comprised of 30 mm long discontinuous fibers were formed on 400 mm wide, 300 mm depth, 50 mm high panels. The rigidity of a ribbed panel was five times greater than a panel without ribs. The tensile strength of the formed ribs was approximately 250 MPa, the same strength of the material before forming. Material behavior such as fiber flow and billet deformation during formation is discussed. This study has shown that the billet flow formation can create ribs with high mechanical strength using only a press with a die cushion.

Topology Optimization of Lightweight Lattice Structural Composites Inspired by Cuttlefish Bone

Lattice structural composites are of great interest to various industries where lightweight multifunctionality is important, especially aerospace. However, strong coupling among the composition, microstructure, porous topology, and fabrication of such materials impedes conventional trial-and-error experimental development. In this work, a discontinuous carbon fiber reinforced polymer matrix composite was adopted for structural design. A reliable and robust design approach for developing lightweight multifunctional lattice structural composites was proposed, inspired by biomimetics and based on topology optimization. Three-dimensional periodic lattice blocks were initially designed, inspired by the cuttlefish bone microstructure. The topologies of the three-dimensional periodic blocks were further optimized by computer modeling, and the mechanical properties of the topology optimized lightweight lattice structures were characterized by computer modeling. The lattice structures with optimal performance were identified.

Evaluation of measurement method for carbon fiber length using an optical image scanner

Fiber length and its distribution are important factors to determine the mechanical properties of discontinuous carbon fiber-reinforced composites. For the efficient and easy measurement of fiber length, an optical image scanner was introduced into the measurement procedure. The scan image allowed to measure approximately 25,000 carbon fibers. The measured values () and mean value of these (<>) were evaluated by comparison with reference values () obtained using an optical microscope. were dependent on the angle between fiber and sub-scanning direction. Furthermore, the relative errors between <> and were less than 2.5% in case that were 0.226 mm or more. The sample standard deviations of were approximately constant and their values were ca. 0.010 mm. Therefore, it was found that the relative error of measurement value derived from the scan image was dependent on the measured fiber length.

Simulating the effects of carbon nanotube continuity and interfacial bonding on composite strength and stiffness

Molecular dynamics simulations of carbon nanotube (CNT) composites, in which the CNTs are continuous across the periodic boundary, overestimate the experimentally measured mechanical properties of CNT composites along the fiber direction. Since the CNTs in these composites are much shorter than the composite dimensions, load must be transferred either directly between CNTs or through the matrix, a mechanism that is absent in simulations of effectively continuous CNTs. In this study, the elastic and fracture properties of high volume fraction discontinuous carbon nanotube/amorphous carbon composite systems were compared to those of otherwise equivalent continuous CNT composites using ReaxFF reactive molecular dynamics simulations. The simulation results quantify the dependence of composite mechanical properties on the number of nanotube-matrix interfacial covalent bonds. Furthermore, the mechanical impact of interfacial bonding was decomposed to reveal its effect on the properties of the CNTs, the interfacial layer of matrix, and the bulk matrix. For the composites with continuous reinforcement, it was found that any degree of interfacial bonding has a negative impact on axial tensile strength and stiffness. This is due to disruption of the structure of the CNTs and interfacial matrix layer by the interfacial bonds. For the discontinuous composites, the modulus was maximized between 4% and 7% interfacial bonding and the strength continued to increase up to the highest levels of interfacial bonding studied. Areas of low stress and voids were observed in the simulated discontinuous composites at the ends of the tubes, from which fracture was observed to initiate. Experimental carbon nanotube yarn composites were fabricated and tested. The experimental results illustrate the knockdown factors that reduce composite mechanical properties relative to those of the tubes themselves.

Evaluation of elastic-plastic response of discontinuous carbon fiber-reinforced thermoplastics: Experiments and considerations based on load-transfer-based micromechanical simulation

The present study investigated nonlinear elastic-plastic stress-strain relationships of discontinuous carbon fiber-reinforced thermoplastics (CFRTPs) using experiments and numerical simulations. In the experiments, we conducted uniaxial tensile tests and three-point bending tests for two types of CF/PA6 (carbon fiber/polyamide 6) specimen: injection-molded specimens with short fiber length and aligned fiber orientation, and compression-molded specimens with long fiber length and random fiber orientation. Comparison of the experiment results indicated that the injection-molded specimens exhibited a nonlinear stress-strain response, while the compression-molded specimens exhibited an almost linear response. These results implied that long discontinuous fibers effectively increased the yielding point of composites, even if the composites had random fiber orientation, which eliminated orientation dependence on mechanical properties of the composites. Furthermore, we attempted to simulate elastic-plastic stress-strain relationships of discontinuous CFRTPs in an effort to understand the effect of the microstructure, including fiber length. For this purpose, we employed fiber-based simulations to deal with the microstructure of fibers and matrix and the constitutive law of the matrix. The simulated results indicated that fiber length influences the nonlinearity of the stress-strain relationships of discontinuous CFRTP composites.

Pearl-Chain Formation of Discontinuous Carbon Fiber under an Electrical Field

The purpose of this paper is to develop a theoretical derivation on aligning discontinuous carbon fiber with an applied electric field, and prove the theory with experiment. A principle with regard to the occurrence of carbon fiber alignment is presented after an introduction of the electromechanical quantities of dielectrics. Based on this principle, an estimation of the polarizability tensor is employed to calculate the required electric field to achieve fiber alignment in liquid solution (e.g., water, resin, etc.). Individual carbon fiber is modeled as a polarizable dielectric cylinder in liquid resin and its motion under direct current (DC) electrical field is decomposed into a polarization effect and rotation effect. A value of 20.12 V/mm is required to align short carbon fibers (0.15 mm) long in liquid resin and is experimentally validated. Finally, an expression to include weight percentage as a means of controlling pearl-chain formation is derived to change the composite’s electrical conductivity.

Thermal conductivity and mechanical properties of pitch-based carbon fibre reinforced aluminium composites fabricated by squeeze casting

Pure aluminium and high-silicon aluminium alloy were reinforced with the discontinuous pitch-based carbon fibres by squeeze casting, then the thermal conductivity and the mechanical properties of the composites were investigated. Optical microscopy revealed that the fibres were in a random planar arrangement, and the transmission electron microscopy revealed that there is no interfacial reaction between the matrices and the fibres. The random planar arrangement of the fibres leads to the anisotropy of the composite. The fibre-reinforcement increased the thermal conductivity in the parallel direction for both pure aluminium and its alloy matrices, while the thermal conductivity decreased in the vertical direction. The increase in the elastic modulus by the reinforcement was not observed for both matrices. The proof stress of the pure aluminium increased by the reinforcement especially in the parallel direction, while that of the high-silicon alloy decreased by the reinforcement.

FRP for seismic strengthening of shear controlled RC columns: Experience from earthquakes and experimental analysis

Abstract The high vulnerability of existing Reinforced Concrete (RC) structures, even to moderate seismic events, has been confirmed from recent post-earthquake surveys. Short and wall-like RC columns are particularly prone to brittle failures, governed by concrete crushing. To reduce the vulnerability of existing RC columns, the use of externally bonded Fibre Reinforced Polymer (FRP) reinforcement has been recognized as an effective method for preventing the aforementioned brittle failure and, hence, increasing members' lateral capacity and ductility. In the first part of this study, the results of an observational analysis on columns shear failures in RC buildings severely damaged after the L'Aquila earthquake are presented. The second part of the study presents and discusses the results of an experimental program carried out on seven short RC columns governed by shear failure under load reversal and compressive axial load. Both columns in “as built” configuration and strengthened in shear with discontinuous carbon FRP (CFRP) strips have been tested. Two classes of concrete have been used, in order to simulate structures with medium or poor material quality, and different external reinforcement ratios have been investigated. The specimens' responses have been analysed in terms of failure modes, strength/deformation capacity and strain distribution in CFRP strips.

Long discontinuous carbon fibre/polypropylene composites for high volume structural applications

A processing route is presented to manufacture discontinuous carbon fibre-reinforced polypropylene composites, using much longer fibre lengths (25 mm) and higher volume fractions (up to 45%) than previously reported in the literature. Carbon fibre tows are coated with different ratios of polypropylene, blended with a maleic anhydride coupling agent, to investigate the influence of the interfacial shear strength at the microscale on the macroscale composite properties. Improvements in the tensile performance at the macroscale (70% increase) are not as high as those reported for the interfacial shear strength at the microscale (300%), following the addition of the coupling agent. Consequently, the tensile strength of the carbon fibre-reinforced polypropylene material is only 45% of values reported for carbon fibre/epoxy systems, however, the tensile stiffness is comparable. This demonstrates the potential for using carbon fibre-reinforced polypropylene for structural applications, following further process optimisation to overcome the current high levels of porosity (3.3% at 0.45 Vf) to improve the tensile strength.

Tensile Strength Prediction of Discontinuous Carbon Fiber Reinforced Thermoplastics with an Open Circular Hole

Tensile Strength Prediction of Discontinuous Carbon Fiber Reinforced Thermoplastics with an Open Circular Hole

Aligned discontinuous intermingled reclaimed/virgin carbon fibre composites for high performance and pseudo-ductile behaviour in interlaminated carbon-glass hybrids

Highly aligned intermingled fibre composites are produced from reclaimed and virgin carbon fibres using the High Performance Discontinuous Fibre (HiPerDiF) method. The stiffness and strength characteristics of these materials are studied as a function of the reclaimed to virgin fibres ratio. Interlaminated hybrid composites with discontinuous carbon fibre preforms sandwiched between continuous glass fibres are designed to demonstrate pseudo-ductility and allow investigation of the effect of the mixing ratio of reclaimed and virgin carbon fibres on the nonlinear stress-strain curve shape. The pseudo-ductile behaviour is explained by adapting the Damage Mode Map to describe the failure process of interlaminated hybrid specimens with different low elongation material strength. It is concluded that the HiPerDiF method is a valuable platform to remanufacture reclaimed carbon fibres into a high performance and potentially economical value recycled composite material. The Damage Mode Maps can be used to optimise the pseudo-ductile response of the interlaminated hybrid material.

Effect of thin-ply on damage behaviour of continuous and discontinuous carbon fibre reinforced thermoplastics subjected to simulated lightning strike

This study experimentally investigates the damage behaviour of a quasi-isotropic (QI) laminate of carbon fibre reinforced thermoplastics and chopped carbon fibre tape reinforced thermoplastics (CTT) under lightning strikes. Simulated lightning strike tests were performed and the damage was evaluated using a high-speed camera, ultrasonic testing, and optical microscopy of cross-sectioned samples. The shape of the vaporized resin and projected damage area resulting from the joule heating was dependent on the electrical anisotropy of composite surfaces; the shape of the vaporized resin was circular for QI laminate with thinner prepreg and CTT materials. In addition, the damage could be categorized into three modes: fibre sublimation, resin vaporization, and delamination. Finally, the application of thin-ply prepreg suppressed the projected damage area and the removal of the composite surface due to fibre sublimation and resin vaporization in both samples. Our findings clearly demonstrate the reduction of lightning damage when using thin-ply prepreg.

Analytical Evaluation on Mechanical Characteristics of Discontinuous Carbon Fiber Reinforced Thermoplastic Composites

Analytical Evaluation on Mechanical Characteristics of Discontinuous Carbon Fiber Reinforced Thermoplastic Composites

Optimal Structural Design of Discontinuous Carbon Fiber Reinforced Thermoplastic Plates Subjected to Bending Loads

Optimal Structural Design of Discontinuous Carbon Fiber Reinforced Thermoplastic Plates Subjected to Bending Loads

Recovery and reuse of discontinuous carbon fibres by solvolysis: Realignment and properties of remanufactured materials

Discontinuous carbon fibre tows were recovered after solvolysis of an aeronautic type composite made with RTM6 epoxy resin. A Sohxlet extraction method was used to quantify the organic residue on the fibre tows and showed that less than 3 wt% was remaining on the surface. The recovered tows were therefore reused directly to manufacture a plate with randomly distributed carbon fibres and then three plates with realigned carbon fibres. The latter were then characterised and tested and the results obtained were compared to the material manufactured using the same type of virgin fibres by the same method. The materials made from recycled carbon fibres showed very good properties in comparison to the virgin fibre material, despite the presence of flaws such as quality of the fibre surface after solvolysis, alignment and voids). This is the first time in the open literature that carbon fibres recovered from solvolysis were reused in this way together with characterisation of the resulting materials.