Intermediate Modulus Carbon Fibers Research Articles

Dip-Coating of Carbon Fibers for the Development of Lithium Iron Phosphate Electrodes for Structural Lithium-Ion Batteries

This study describes a dip-coating method for applying an active material to commercially available intermediate modulus carbon fibers (CFs). A suite of tools were developed to assist with the handling and coating of CF tows to create disc electrodes. CF electrodes were fitted into 2025-type coin cells, for electrochemical analysis, first to determine their performance as anodes. Specimens of CFs were dip-coated with a slurry consisting of lithium iron phosphate, carbon black, and polyvinylidene fluoride, then dried, and fitted into half-cells to analyze the cathode performance. A cyclic voltammetry sweep was performed on each half-cell to determine suitable cycling potential limits, followed by galvanostatic cycling for a minimum of 30 cycles. Measured capacities for anode and cathode half-cells yielded 92 and 52.3 A h kg–1, respectively. A full-cell with CFs in both electrodes was assembled and tested, revealing a capacity of 24.7 A h kg–1 after 30 cycles. Finally, both the anode and cathode were examined with a scanning electron microscope to establish a benchmark (anode) to compare surface topologies and to analyze the quality of the active material applied via dip-coating (cathode).

Damping analysis of stiffened laminated composite plates in thermal environment

High-strength and lightweight composites used in high-speed vehicles and aircraft experience increased temperature owing to aerodynamic friction. Because of the rise in temperature, the elastic moduli of the composites are degraded and inherent viscous damping is increased to the glass transition temperature. In this work, the damping performance of unstiffened and stiffened polyetheretherketone (PEEK) based intermediate modulus (IM7) carbon fiber laminated composite plates is evaluated at various temperatures using displacement- and energy-based approaches. In the displacement-based approach, the logarithmic decrement of the damped vibration is determined to investigate the effect of lamina sequences, stiffener orientation, and stiffener depth at different temperatures. A first-order shear deformation theory considering the viscoelastic property of the composite lamina is implemented to simulate the damped dynamic response of the laminated composite plates in a thermal environment. The best suitable damping model, among Rayleigh damping and modal expansion damping, has been identified to evaluate the damping matrix. Furthermore, the energy-based damping analysis appeared to be a robust approach to evaluate the damping performance as compared to the displacement-based analysis. It is concluded that stiffened plates effectively suppress the dynamic response at elevated temperature, whereas the unstiffened plates show relatively better damping performance at lower temperature.

Dynamic response of stiffened laminated composite plate in thermal environment

Composite materials are increasingly used in aerospace and automobile sectors. These materials are characterized by high stiffness and light-weight in comparison to conventional structural materials. During high-speed maneuvering of these vehicles, associated composite structures are subjected to dynamic loadings and are often exposed to varying thermal environments. To ensure high load carrying capacity and to avoid thermal buckling, stiffened laminated composite plates are preferred in some important structural components, e.g., aircraft’s fuselages, wings etc., than unstiffened laminated composite plates. For safe designing of these structures, it is thus necessary to investigate the dynamic behavior of stiffened laminated composite plates in thermal environment. In this paper, the dynamic response of stiffened laminated composite plates made of polyetheretherketone (PEEK) with intermediate modulus (IM7) carbon fibers in thermal environment has been investigated. Furthermore, development of a generalized finite element (FE) formulation using an 8-node isoparametric plate element employing first-order shear deformation theory (FSDT) has also been presented to analyze unstiffened and stiffened laminated composite plates in thermal environment. Detailed parametric study reveals that the dynamic deflection and the corresponding normal stress decrease with adding stiffener, and decreases further with increasing depth of the stiffener in thermal environment.

The Effect of Carbon Fiber Types on Tool Wear During Edge Trimming of 0 deg, 45 deg, 90 deg, and 135 deg Carbon Fiber-Reinforced Plastic Laminates

Abstract Carbon fiber-reinforced plastics (CFRP) composites are difficult-to-machine materials due to extensive tool wear. This article investigates the impact of carbon fiber types on tool wear in edge-trimming CFRPs, each with the ply angle of 0 deg, 45 deg, 90 deg, or 135 deg, using uncoated tungsten carbide endmills at the high spindle speed of 6000 rpm and the constant feed of 0.0508 mm/rev. Three distinct types of carbon fiber tows, including T300 as standard modulus (SM), IM-7 as intermediate modulus (IM), and K13312 as high modulus (HM), have been vacuum infused into CFRP laminates and edge trimmed to investigate wear characteristics. Three wear criteria measured are flank wear, edge rounding radii, and worn area. The results show that tool wear is influenced by carbon fiber properties, such as fiber tensile strength, tensile modulus, and fiber microstructure. Overall, IM carbon fibers with the highest tensile strength produced the most extensive tool wear due to the abrasive carbon fibers. Of four fiber directions, the largest tool wear was obtained from the 45 deg angle, while the lowest tool wear occurred in the 0 deg angle.

Analyzing carbon fiber structures observed by helium ion microscopy and their mechanical properties

In this work we present images of carbon fiber fractured surfaces observed for the first time by Helium Ion Microscopy (HIM) at high magnifications, aiming to capture defects ranging in size between 10 nm and 100 nm. Nominal critical defect sizes of various commercial carbon fibers were calculated from the Griffith Equation and their critical defect size distributions were calculated from their Weibull strength distributions. HIM images of the carbon fibers with the standard modulus (~230 GPa), intermediate modulus (~280 GPa), high modulus (>350 GPa) and ultra-high strength (>6.8 GPa) are shown to have contrasting microstructures. Some of the three dimensional (3D) structural features, like crystalline sheets, flakes, and sets of amorphous material, have not been previously captured by Scanning Electron Microscopy (SEM). Furthermore, a representative intermediate modulus carbon fiber was mechanically tested by the single filament test method and its strength characteristics were corelated to structural features observed by HIM. HIM imaging offers a new, reproducible, and feasible technique to analyze the original 3D nanostructures of carbon fibers.

Gas permeability of three-dimensional stitched carbon/epoxy composites for cryogenic applications

3D through-the-thickness stitching can be used to improve interlaminar properties of polymer matrix composites (PMCs) and produce lightweight, unitized structures for cryogenic storage tanks. To fully utilize stitched composite structures for these applications, their inherent gas permeability challenges must be understood. Therefore, the effects of cryogenic cycling on the gas permeation of three-dimensional stitched PMCs are investigated. Stitched and unstitched carbon/epoxy composites of standard modulus (237 GPa) and intermediate modulus (290 GPa) carbon fibers were thermally cycled and the helium gas permeability was measured at selected cryogenic cycles. Results show that stitching increases gas permeability in PMCs, with the standard modulus stitched composites having approximately twice the permeability of the intermediate modulus stitched specimens. Optical microscopy images show that cracks develop in the resin rich areas around the stitching seams. These internal cracks produce the increase in the midspan microcrack density that leads to the increase in permeability of stitched composites.

Understanding compressive strength improvement of high modulus carbon-fiber reinforced polymeric composites through fiber-matrix interface characterization

Low fiber-direction compressive strength of high-modulus (HM) carbon fiber-reinforced polymers (CFRPs) has been their major weakness prohibiting implementation of such materials in aircraft primary structures despite improving mechanical stiffness at a lower weight. A new HM CFRP achieving fiber-direction compressive strength of intermediate-modulus (IM) CFRPs but with more than 30% higher axial modulus has been recently developed. This work looks into fiber-matrix interface shear strength as a potential mechanism driving the compression strength improvement of the new material system. In-situ scanning electron microscopy (SEM) based single fiber push-out experiments addressing standing challenges associated with manufacturing high-quality samples as well as distinguishing same-diameter HM and IM carbon fibers in the hybrid composite system are used to measure fiber-matrix interface shear strength. The experiments show a 29% lower average value of the fiber-matrix interface shear strength for the HM carbon fibers compared to the IM carbon fibers in the new material system. Such a significant reduction corresponds to a 22% lower fiber-direction compressive strength of the HM CFRP without the integrated IM fibers. The results support the idea of integrating off-the-shelf IM carbon fibers with a stronger fiber-matrix interface and a higher shear modulus into HM CFRPs to improve their compressive strength.

Graphitic microstructure and performance of carbon fibre Li-ion structural battery electrodes

Carbon fibres (CFs), originally made for use in structural composites, have also been demonstrated as high capacity Li-ion battery negative electrodes. Consequently, CFs can be used as structural electrodes; simultaneously carrying mechanical load and storing electrical energy in multifunctional structural batteries. To date, all CF microstructural designs have been generated to realise a targeted mechanical property, e.g. high strength or stiffness, based on a profound understanding of the relationship between the graphitic microstructure and the mechanical performance. Here we further advance this understanding by linking CF microstructure to the lithium insertion mechanism and the resulting electrochemical capacity. Different PAN-based CFs ranging from intermediate- to high-modulus types with distinct differences in microstructure are characterised in detail by SEM and HR-TEM and electrochemical methods. Furthermore, the mechanism of Li-ion intercalation during charge/discharge is studied by in situ confocal Raman spectroscopy on individual CFs. Raman G band analysis reveals a Li-ion intercalation mechanism in the high-modulus fibre reminiscent of that in crystalline graphite. Also, the combination of a relatively low capacity of the high-modulus CFs (ca. 150 mAh g−1) is shown to be due to that the formation of a staged structure is frustrated by an obstructive turbostratic disorder. In contrast, intermediate-modulus CFs, which have significantly higher capacities (ca. 300 mAh g−1), have Raman spectra indicating a Li-ion insertion mechanism closer to that of partly disordered carbons. Based on these findings, CFs with improved multifunctional performance can be realised by tailoring the graphitic order and crystallite sizes.

An investigation into the surface heterogeneity of nitric acid oxidized carbon fiber

The carbon fiber surface plays a critical role in the performance of carbon fiber composite materials and, thus it is important to have a thorough understanding of the fiber surface. A series of nitric acid treated intermediate modulus carbon fibers with increasing treatment level was prepared and characterized using a range of surface sensitive techniques including Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. The results, which were found to be consistent with increasing treatment levels, were compared to the literature. Raman spectral mapping has been used to investigate the heterogeneity of the carbon fiber surface after nitric acid oxidation. The mapping enabled the effects of surface treatment on carbon fiber to be investigated at a spatial resolution unattainable by XPS and provided chemical structure information not provided by SEM or AFM. The highest level of treatment resulted in the most heterogeneous surface. Raman mapping, while time consuming, can provide valuable information which can lead to an enhanced understanding of the heterogeneity of the carbon fiber surface.

Failure simulations of open-hole IM7/977-3 coupons subjected to fatigue loading using Autodesk Helius PFA

Finite element simulations of three laminates in open-hole configuration subjected to constant amplitude tension–tension fatigue loading are investigated as part of the Damage Tolerant Design Principles program organized by the Air Force Research Laboratory. All coupons were made from unidirectional IM7/977-3 plies, which are composed of intermediate modulus carbon fibers and a toughened epoxy matrix. Government furnished experimental data from an assortment of fatigue loaded unnotched coupons were used to characterize the behavior of the composite material in the simulations. The commercial software Autodesk Helius PFA was used to model the non-linear response of the material. Blind simulations of coupon stiffness and damage at several cycle numbers and residual coupon tensile and compressive strengths are benchmarked against experimental measurements and X-rays. Upon review of the experimental results, a second round of simulations was performed where the modeling strategy was updated to improve correlation to experiment.

Failure simulations of open-hole and unnotched IM7/977-3 coupons subjected to quasi-static loading using Autodesk Helius PFA

Finite element simulations of three laminates in open-hole and unnotched configurations subjected to tension and compression quasi-static loading are investigated as part of the Damage Tolerant Design Principles program organized by the Air Force Research Laboratory. The coupons are made from unidirectional IM7/977-3 plies, which are a composite material composed of intermediate modulus carbon fibers and a toughened epoxy matrix. Blind simulations of coupon stiffness, nominal coupon stress at failure and damage evolution are benchmarked against experimental measurements and X-rays. The blind simulations are followed by a second round of simulations where the modeling strategy is modified to improve agreement between the simulations and experiments. In the present article, the commercial software Autodesk Helius PFA is used to model the non-linear response of the composite material. Within Helius PFA, failure is evaluated at the constituent level by extracting the fiber and matrix volume average stress state from the homogenized composite stress state. The relationships between the composite and constituents are developed using multicontinuum theory and a high-fidelity micromechanics model.

Sublimation and oxidation zone ablation behavior of carbon/carbon composites

Three-dimensional carbon/carbon (C/C) composites comprising four reinforcement directions (4D) were fabricated using intermediate modulus carbon fibers and densified using a hybrid process. This consists of a pre-densification step using a thermal-gradient chemical vapor infiltration process followed by a high-pressure pitch impregnation and carbonization process. The specimens machined along Z-axis of the preform architecture were tested in an arc plasma heater for studying its ablation behavior at different temperatures. Regimes from ultra-high temperature (4750K) sublimation to high-temperature (2467K) oxidation zones were created by varying the mass flow rate of secondary air in the heater. The ablation rate showed a progressive increase as the environment changed from oxygen-lean sublimation to oxygen-rich oxidation conditions while the back-face temperature showed a similar temperature profile during the plasma exposure period. The thermal diffusivity value decreased with the rise in temperature till 1173K and later on became fairly flat till 1523K and onwards. In the compression test, 4750K exposed specimen showed toughening in the plasma-affected zone and crushed with shear mode from the opposite face while the 2467K exposed specimen showed end brushing in the plasma heat-affected face with a lower residual strength and Young’s modulus.

Large Strain Four-Point Bending of Thin Unidirectional Composites

A large deformation four-point bending fixture was used to test thin unidirectional composite laminas primarily used in strain energy deployable space structures. The tests allowed investigation of the large strain elastic constitutive behavior of two aerospace-grade intermediate modulus carbon fibers, a high modulus carbon fiber, and a structural glass fiber. Thermoset plastic coupons reinforced with these fibers and ranging in thickness from 0.1 to 0.5 mm were tested. A nonlinear empirical constitutive model was used to represent fiber axial tensile and compressive behavior and through a structural model, predict coupon flexural response and estimate constitutive model parameters. Intermediate modulus carbon fibers were found to be significantly nonlinear with modulus linearly increasing with strain. At failure, the fiber tangent modulus in tension was 2 to 3.1 times higher than in compression. The modulus of glass fibers was essentially constant. High modulus carbon fibers exhibited a more complex response with flexural stiffness initially slightly increasing followed by a moderate reduction in stiffness and finally, a sharp reduction in stiffness and failure.

The effect of gamma ray irradiation on PAN-based intermediate modulus carbon fibers

Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were conducted on PAN-based intermediate modulus carbon fibers to investigate the structure and surface hydrophilicity of the carbon fibers before and after gamma irradiation. Two methods were used to determine Young’s modulus of the carbon fibers. The results show that gamma ray irradiation improved the degree of graphitization and introduced compressive stress into carbon fiber surface. Gamma ray also improved the carbon fiber surface hydrophilicity through increasing the value of O/C and enhancing the quantity of oxygen functional groups on carbon fibers. No distinct morphology change was observed after gamma ray irradiation. The Young’s modulus of the fibers increased with increasing irradiation dose.

Effect of fiber surface on flexural strength in carbon fabric reinforced epoxy composites

The effect of carbon fiber surface characteristics on flexural properties of structural composites is studied in this work. Two types of intermediate modulus carbon fibers were used: T800HB and IM7. Results revealed that higher mechanical properties are linked with higher interfacial adhesion. Morphologies and chemical compositions of commercial carbon fibers (CF) were characterized by Fourier Transformed Infra Red (FTIR) and Scanning Electronic Microscopy (SEM). Comparing the results, the T800HB apparently has more roughness, since the IM7 seems to be recovered for a polymeric film. On other hand, the IM7 one shows higher interactivity with epoxy resin system Cycom 890 RTM. Composites produced with Resin Transfer Molding (RTM) were tested on a flexural trial. Interfacial adhesion difference was showed with SEM and Dynamic Mechanical Analyses (DMA), justifying the higher flexural behavior of composites made with IM7 fibers.

Degradation behavior of 4D carbon/carbon composites under supersonic oxidative air plasma

Abstract Four-directional (4D) carbon/carbon (C/C) composites were prepared using an intermediate modulus carbon fiber as the reinforcement and two kinds of coal tar pitches as the source for carbon matrix. After repeated high-pressure impregnation, carbonization and graphitization at 2250 °C, a high density of 1.86 g/cm3 was achieved. A plasma arc jet was applied to the composite for studying its degradation behavior. The oxidative supersonic jet was generated by an air plasma torch. For studying various stages of degradation during a long exposure time of 90-s, the intermittent ablation and erosion rates were measured for the C/C composite at time intervals of 70, 50 and 30 s. The mass ablation rate was increased rapidly with the time, while the erosion showed a bilinear response with a decreasing trend. The results were discussed based on the machined and eroded surfaces of the test samples. For studying the residual ablative performance, the samples of 30–70 s were then re-ablated and re-eroded in steps of 20-s so that all the samples are equal in total exposure time. The samples showed decreasing trends in the re-ablation and re-erosion rates and ultimately became zero for the last 20-s exposure time. There was a progressive increase in the char layer on the samples with the re-ablation. The porous char layer protected the samples against chemical and mechanical actions of the plasma stream. Compressive tests were carried out, and the decrease in the modulus was due to the presence of microcracks resulting from the mismatch in thermal expansion of the fiber and the matrix. The SEM micrographs reveal that inter- and intra-bundle cracks tend to grow through the weaker matrix pockets.

Carbon fiber non-crimp multi-axial reinforcement and epoxy mono-component system composite: Fatigue behavior

Fiber reinforced polymer composites have been widely applied in the aeronautical field. However, composite processing, which uses unlocked molds, should be avoided in view of the tight requirements and also due to possible environmental contamination. To produce high performance structural frames meeting aeronautical reproducibility and low cost criteria, the Brazilian industry has shown interest to investigate the resin transfer molding process (RTM) considering being a closed-mold pressure injection system which allows faster gel and cure times. Due to the fibrous composite anisotropic and non homogeneity characteristics, the fatigue behavior is a complex phenomenon quite different from to metals materials crucial to be investigated considering the aeronautical application. Fatigue sub-scale specimens of intermediate modulus carbon fiber non-crimp multi-axial reinforcement and epoxy mono-component system composite were produced according to the ASTM 3039 D. Axial fatigue tests were carried out according to ASTM D 3479. A sinusoidal load of 10 Hz frequency and load ratio R=0.1. It was observed a high fatigue interval obtained for NCF/RTM6 composites. Weibull statistical analysis was applied to describe the failure probability of materials under cyclic loads and fractures pattern was observed by scanning electron microscopy.

Fractography analysis and fatigue strength of carbon fiber/RTM6 laminates

The Brazilian industry has shown interest in resin transfer molding process (RTM) to produce aeronautical structural frames with low cost. Literature shows such manufacturing process as adequate to produce simple geometry polymer composite airframes, fairly thickness, requiring high quality of finish without manufacturing scale limitations. An experimental methodology regarding a carbon epoxy composites landing gear sub-component has been developed and a 7050 T7451 stalk was selected to be produced by RTM process. Fatigue sub-scale specimens of intermediate modulus carbon fiber non-crimp multi-axial reinforcement and epoxy mono-component system composite were produced according to ASTM D3039. Axial fatigue tests were carried out according to ASTM D3479 with a sinusoidal load of 10 Hz and load ratio R = 0.1. Delamination process occurred independently on the applied load. In order to confirm the replacement advantages, composite properties were compared to those of anodized 7050 T7451 aluminum alloy.

Carbon fibers and composites with epoxy resins: Topography, fractography and interphases

Interphases exist in hybrid materials and significantly influence their mechanical performance. To find a bridge between the microscopic and macroscopic mechanical properties, this work investigates the nanoscopic nature of surface/interphases in terms of topography, fractography, adhesion and stiffness. Here, we show that variations in both adhesive and attractive forces on oxidized high modulus (HM) and intermediate modulus (IM) carbon fiber surfaces appear to result from the coating layer. The coating layer is critical for adhesive interaction with two different epoxy resins. The HM fiber has the apparently higher roughness but lower surface area than the IM fiber on the scanning scale of 200 nm. The surface roughness on a few tens of nanometer scale has no significant contribution to interphase adhesion from ‘mechanical interlocking’. In contrast, the true contact area on the nanometer scale plays a dominant role in interfacial adhesion. Using force volume nanoindentation, the stiffness of the resin region near the finished fiber surface was found to not depend on the distance from the fiber surface. Our observations suggest an energy-geometry link between critical interphase energy release rate by micromechanical testing and detailed fracture surface features.

Experimental and numerical investigation into fatigue damage mechanisms in multifibre microcomposites

Polarised light microscopy has been used to investigate the influence of stress level, interfibre spacing, and fibre–matrix adhesion on the fatigue micromechanisms in carbon–epoxy model composites consisting of a planar array of five intermediate modulus carbon fibres. It was found that an increase in fatigue stress results in an increase in the number of fibre breaks, a more coordinated fibre fracture pattern as a result of fibre–fibre interaction, and extensive interfacial damage. In addition, it was shown that a smaller interfibre spacing results in a higher level of fibre–fibre interaction. Finally, in the case of surface treated carbon fibres (good fibre–matrix adhesion), a more coordinated fibre failure pattern was observed owing to stronger fibre–fibre interaction, whereas in the case of untreated carbon fibres (poor fibre–matrix adhesion), extensive debonding was observed which resulted in a more random fibre failure pattern. Finally, the experimental results were validated by means of a three-dimensional finite element analysis.