Anisotropic Carbon Fiber Research Articles

Detection and Characterization of Defects in Isotropic and Anisotropic Structures Using Lockin Thermography

Lockin thermography is a well-suited method for the characterization of structures made of both metal and fiber reinforced plastic. In most cases, only phase images are analyzed, although the amplitude images might contain useful information as well. Thus, systematic studies of lockin thermography are presented, assessing amplitude and phase images for the detection and quantification of defects in isotropic (steel) and anisotropic (carbon fiber reinforced plastic) materials. Characterized defects are flat bottom holes with different diameters and various remaining wall thicknesses as well as crossed notches at different depths. The excitation frequency was varied while keeping the number of analyzed excitation periods nearly constant for each material. The data analysis was focused on the detectability of the defects both in the amplitude and phase images, including the determination of the signal-to-noise ratio and of the spatial resolution. As a result, the limits of defect detectability and spatial resolution are given for each material.

The effect of pitch-based carbon fiber microstructure and composition on the formation and growth of SiC whiskers via reaction of such fibers with silicon sources

The formation and growth of silicon carbide whiskers (SiCw) have been investigated by reaction of silicon sources at 1400 °C with pitch-based carbon fibers possessing various microstructures. Isotropic and anisotropic pitch-based carbon fibers treated at various temperatures were employed as carbon sources. Silicon sources include silicon powder and a mixture of silicon and silica powder. The reaction of 1000 °C heat-treated carbon fibers containing a certain content of oxygen with silicon powders is also reported. A reasonable yield of monocrystalline and polycrystalline SiC whiskers can be achieved by the reaction of the isotropic and anisotropic pitch-based carbon fibers treated at 1000 and 2000 °C with the mixture of silicon and silica. The critical factors for promoting the formation of SiC whiskers are that the carbon sources should possess turbostratically stacked graphite structure and appropriate grain size. The presence of SiO, which originated from the reaction of silicon with silica or with water vapor derived from pyrolysis of carbon fibers, is another necessary condition for formation of SiCw during these reaction processes.

High Pressure Sensor Based on Fiber Bragg Grating and Carbon Fiber Laminated Composite

A simple anisotropic structure made by carbon fiber laminated composite for fabricating a high pressure sensor is reported. A pressure sensor with good sensitivity over a broad measurement range is fabricated by using fiber Bragg grating and the anisotropic carbon fiber laminated composite structure. The characteristic responses of pressure and temperature of the new pressure sensor are analyzed. Experimental data show that when the pressure changes from 0 to 70 MPa, the wavelength shift of the fiber Bragg grating pressure sensor is up to 7 nm, corresponding to a sensitivity of 10 kPa/pm.

IR thermographic evaluation of thermal diffusivity anisotropy: comparative analysis of some algorithms

Theoretical and experimental results for determining thermal diffusivity components in anisotropic Carbon Fiber Reinforced Plastic laminates up to 5 mm thick are presented. The effectiveness of the theory is evaluated by using a 3D numerical model. IR thermographic implementation of the Parker's technique has been used for determining transverse diffusivity. Both spot- and slit-mask techniques have been applied for analyzing thermal diffusivity components.

Lateral heat flow method for thickness independent determination of thermal diffusivity

A pulsed transient thermography method is described where a high-intensity light pulse is used to heat a long, uniform stripe on the surface of a plate. A high spatial resolution, high frame rate focal plane array infrared camera is used to monitor surface temperature. We explain the theoretical model and data analysis framework used to experimentally determine all three thermal diffusivity components from the temperature measurements. The analysis does not require any fitting to the temperature profile and is based on the creation of thermal time-of-flight (tof) images from the temperature data and the relationship between tof and the distance from the stripe edge. The in-plane components of thermal diffusivity are obtained without the need for thickness information. Experimental validation of this procedure was carried out using anisotropic carbon fiber reinforced polymer composites.

Isotropic and anisotropic microporosity development upon chemical activation of carbon fibers, revealed by microbeam small-angle X-ray scattering

A sub-micrometer size beam (0.5 μm diameter) in a position-resolved small angle X-ray scattering set-up (μSAXS) has been used for the characterization of chemically activated carbon fibers (ACF). These materials have been prepared from isotropic carbon fibers (pitch carbon fibers) and anisotropic carbon fibers (PAN-based carbon fibers) by chemical activation with KOH and NaOH. The μSAXS experimental set-up made it possible to analyze different regions of a single fiber across its diameter and to distinguish the structural features already existing in the raw fibers or being created during the activation process. The results showed that depending on the precursor, the chemical activation process produces isotropic or anisotropic development of porosity. It was observed that chemically ACF prepared from isotropic carbon fibers present an isotropic development of the porosity and that a high micropore volume is developed not only in the external region of the fiber, but also in the core. On the other hand, in the case of anisotropic PAN-based carbon fibers the existence of two regions with different structure was detected by μSAXS measurements across the fiber diameter: an anisotropic external ring and a more isotropic fiber core. The results showed that these two regions remain after chemical activation and that the activating agents are reaching the fiber core. It seems that the more isotropic fiber core is activated easier by NaOH than KOH.

Structural Changes during Oxidation Process of Anisotopic Mesophase Carbon Fibers(II)-Surface Texture Observation by Scanning Electron Microscopy

Anisotropic mesophase carbon fiber(AMCFs) was exposed to isothermal oxidation in air and atmosphere, and burn-off rates have measured by TGA. The microstructure changes of oxidized carbon fibers, were observed by SEM. It was observed that oxidation rate in the air is over 100 times faster than that in atmosphere. The activation energy obtained in air was about 43.4 Kcal/mole in the temperature range of , and in was about 55.2 Kcal/mole in the temperature range of . Therefore, the oxidation reaction in both atmospheres was under chemical reaction regime in the above temperature ranges. It was shown that the oxidation of the AMCFs is initiated at the end of fibers at high temperature() with developing the large pores, and the small pores are developed on the fiber surface at low temperature(). In conclusion, the oxidation of the AMCFs is progressed through the imperfection.

Unique charge/discharge properties of carbon materials with different structures

The electrochemical properties of isotropic and anisotropic carbon fibers were studied using the two-electrode method in a 1 M LiPF 6 electrolyte solution dissolved in a 1:1 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Isotropic carbon fiber had a larger irreversible capacity than that of the other carbon fibers and might have many sites for the electrolyte decompositon and residual lithium ions. Active carbon fibers prepared from isotropic carbon fibers by steam gasiformation at 850 °C showed large specific surface areas. After steam gasiformation, the active carbon fibers changed into a more crystalline state and had electrochemical characteristics similar to the anisotropic carbon fibers. Impedance profiles of the carbon fibers were fitted by equivalent analogs of a three-resistance—capacitance (RC) circuit in series and the thickness of passivation layers in the carbon fibers was calculated.

A Numerical Study on the Effects of Line Heating in Layered Anisotropic Carbon Fiber Composites

A Numerical Study on the Effects of Line Heating in Layered Anisotropic Carbon Fiber Composites

A Numerical Study on the Effects of Line Heating in Layered Anisotropic Carbon Fiber Composites

Abstract Thermal effects of line heating on layered anisotropic carbon fiber composites were studied numerically. Surface temperature distributions were calculated for delaminations of different sizes placed between different fiber layers using finite difference methods. The anisotropy of thermal conductivity caused detectable effects on the surface temperature distributions that depend on the scanning direction of line heating and the velocity of the scanning. The fiber orientation had a clear effect on the image contrast, whereas the resolution showed only a slight dependency on this. The performance of the imaging method was estimated by comparing the size of the delamination defined in the numerical model to the half-measure size or the 1/e level size obtained from the computed surface temperature distributions. The defect size obtained by using the half-measure values was found to be slightly smaller and the size defined by the 1/e values larger than the actual one. The half measure criterion gave a better estimate compared to the 1/e values. However, in practice the observed size was very close to the actual one when either of the criteria is used. As expected, the lateral resolution deteriorated when the depth of the delamination was increased.

A numerical study on the effects of line heating in layered anisotropic carbon fiber composites

Thermal effects of line heating on layered anisotropic carbon fiber composites were studied numerically. Surface temperature distributions were calculated for delaminations of different sizes placed between different fiber layers using finite difference methods. The anisotropy of thermal conductivity caused detectable effects on the surface temperature distributions that depend on the scanning direction of line heating and the velocity of the scanning. The fiber orientation had a clear effect on the image contrast, whereas the resolution showed only a slight dependency on this. The performance of the imaging method was estimated by comparing the size of the delamination defined in the numerical model to the half-measure size or the 1/e level size obtained from the computed surface temperature distributions. The defect size obtained by using the half-measure values was found to be slightly smaller and the size defined by the 1/e values larger than the actual one. The half measure criterion gave a better estimate compared to the 1/e values. However, in practice the observed size was very close to the actual one when either of the criteria is used. As expected, the lateral resolution deteriorated when the depth of the delamination was increased.

The elastic stress field arising in the single-fiber pull-out test

The stress field arising in the single-fiber pull-out test is analyzed within the framework of the linear theory of elasticity. Two fiber types with different elastic properties, an isotropic glass and an anisotropic carbon fiber, are studied. Both fibers are embedded in a polycarbonate matrix with a length to diameter ratio of 15. The solution of the boundary value problem is performed by means of the finite element method. It is shown that the stress distribution in the vicinity of the fiber is strongly inhomogeneous and exhibits severe concentrations of shear and radia stresses at the fiber entry and at the fiber end. The level of the stress concentrations is highly influenced by the axial stiffness ratio of fiber and matrix. The potential failure processes resulting from these stress concentrations are discussed. The von Mises yield stress and the principal stresses are also calculated. It is shown that the commonly calculated interface strength, utilizing experimental data only, results in an underestimate of the actual interface strength in the case of linear elastic material behavior.

Mechanism of formation of the texture and microtexture in as-spun then oxidized anisotropic pitch-based carbon fibers, part I: Radial with wedge type i1

An anisotropic pitch made of a mixture of isotropic phase 1 (containing BT mesophase spheres) and a two-phase-disperse system 2.3 (edge-to-edge type of gel) was used for spinning fibers (radial with wedge type I1). The as-spun, then oxidized fibers were studied by optical microscopy (OM) and transmission electron microscopy (TEM). The sample preparation technique was thin sectioning (cross, oblique and longitudinal) for TEM and thin-sectioning residues for OM. The microtexture of a fiber is that of an edge-to-edge type of gel elongated uniaxially by spinning. The structure finally obtained is highly ordered. It is a combination of two orthogonal orientations of the aromatic layers. A dominant radial texture 2 (containing radial aromatic layers) is constantly prolonged parallel to the fiber axis. It is associated to sinuous channels 3, running radially in the fiber cross-sections. Inside them, the aromatic layer orientation is concentric. The sinuous channels themselves are constantly prolonged along the fiber axis as dotted slits, with well-aligned dots. The isotropic phase of the initial pitch is elongated as fibrils 1. A central pole is occupied by another fibril 1. The other fibrils are increasingly deformed when approaching the periphery of the fiber.

Anisotropic pitch-based carbon fibers—carbonization and graphitization (radial-with-wedge i1 and radial i2 types)

Anisotropie pitch-based fibers (radial-with-wedge I1 and radial types I2) were carbonized and graphitized. The initial pitch B was a mixture of a gel 2.3 (edge-to-edge type) and of an isotropic phase-1 rich in Brooks and Taylor mesophase spheres. The continuous part 2 of the gel is a network of edge-toedge associated carbon BSU (micelles) including microdroplets 3 of the finely divided part. The latter has an orthogonal orientation of its carbon layers, relative to the former. During heat treatment, the nanotexture changes follow those of other carbons, whereas the texture undergoes minor changes. The fibrils 1 yield a microporous non-graphitizing carbon. The gel 2.3 yields zig-zag continuous layers. The fibers of the radial-with-wedge and the radial types have the same ability to graphitize.

Mechanism of formation of the texture and microtexture in as-spun then oxidized anisotropic pitch-based carbon fibers, part II: Radial type I2

The microtexture of a radial type of fiber 12 is that of an edge-to-edge type of gel elongated uniaxially by spinning. The structure finally obtained follows a model derived from that of the radial-withwedge type of fiber I1, inside which an increasing disorder is introduced. It is still a combination of two orthogonal orientations of the aromatic layers in the core of the fiber, disrupted near the surface. In the core, a dominant radial texture 2 (containing radial aromatic layers) turns to spiral at the periphery; 2 is constantly prolonged parallel to the fiber axis. The maximum disordering is introduced in the channels 3. They still have a concentric orientation of their aromatic layers and are still dotted along the fiber axis. However, they are strongly distorted in either cross- or longitudinal sections and the dots are not well aligned along the fiber axis. The isotropic phase of the initial pitch is elongated as fibrils. There is no central isotropic pole.

Rheology of Pitches

Rheological behavior of optically isotropic pitches derived from petroleum crude oils and coal tars were reviewed in relation to their dependence on molecular weight and temperature. Studied were the hydrodynamic characteristics (intrinsic viscosity, [η], and diffusion coefficient) of dilute pitch solutions, glass transition behavior, viscoelastic properties, and dielectric dispersion-electric conductivity characteristics of fractionated pitches with different molecular weights, which were prepared using the difference in solubility of the pitches in n-hexane and benzene. The hydrodynamic studies yielded an important information on the shape and dimension of constituent pitch molecules, i. e., oblate ellipsoid with the axial ratio of 1-3, and 10-20Å in equatorial diameter. The glass transition temperature, Tg, which decreased linearly with the inverse of number averaged molecular weight, was found to play an important role on the molecular weight dependence of steady-state shear viscosity, η, and steady-state shear compliance, Je0. The dependence of Je0 on temperature suggests that the elastic process in pitches is enthalpic, not entropic as observed in usual organic polymers. The dielectric behavior of pitches were described well by so-called Cole-Cole relation except prominent energy dissipation due to electrically conductive carriers in the region of low electrical frequency at high temperature. The importance of the rheological studies of mesophase pitches was emphasized in relation to the technical development of anisotropic carbon fibers with high elastic modulus and high strength

Finite element analysis of a barrel‐stave underwater sound projector

The barrel‐stave projector comprises several concave metal staves joining two cylindrical head pieces which are driven axially by a stack of piezoelectric ceramic. This radiating system has been analyzed using an axially symmetric finite element model. The metal staves are modelled as an anisotropic material having normal axial and radial stiffness but low hoop stiffness. The performance of an experimental unit, 12 cm long and 7.6 cm in diameter, which resonates near 1500 Hz, is compared with predictions of the finite element model. Additional computer modelling has been carried out with an anisotropic carbon fiber composite replacing the metal staves.

Irradiation-induced dimensional changes of poorly crystalline carbons

Data are presented on irradiation-induced dimensional changes of poorly crystalline carbons at high temperatures (>900°C). The materials surveyed include: 1. (1) carbon fibers, 2. (2) glassy carbons, 3. (3) carbonaceous matrix materials for HTGR fuel rods and 4. (4) pyrocarbons. The materials above are listed in order of increasing stability, with maximum strains ranging from more than 50% for fibers to less than 10% for pyrocarbons. Dimensional changes of highly anisotropic carbon fibers appear to be sensitive to irradiation temperature, as slightly anisotropic pyrocarbons are, whereas temperature seems to have little influence on the behavior of isotropic glassy carbons over the range from 600 to 1350°C. Dimensional changes for graphite-filled matrix materials were roughly isotropic on the average and did not seem to be strongly temperature dependent for the lower fluences investigated. Increased graphite filler lowered volumetric dimensional changes of the matrix in agreement with a rule-of-mixtures relationship between change components for the filler and the less-stable binder phases. Instabilities of all of the poorly crystalline materials were generally greater than those for more crystalline carbons under the same conditions, including highly orientated graphites that approximate single-crystal behavior.

Bending Behavior and Tensile Strength of Carbon Fibers

The strength of anisotropic polycrystalline carbon fibers was investigated. Theoretical estimates of the strength of graphite are far higher than observed values for carbonized rayon fibers, and we have searched for an explanation. Internal ``grain boundaries'' in the fibers can account for the reduced strength inasmuch as the fracture appears to be intergranular. The ``grains'' are believed to be axially oriented fibrils, ap-approximately 500-Å across. Experimental evidence for some inelastic behavior in carbon fibers at room temperature was obtained from bending experiments on single filaments. The results were interpreted in terms of local ``yielding'' or rupture of cross-link bonds between adjacent fibrils. The stress at which such ``yielding'' takes place in bending is approximately equal to the failure stress in a tensile test. Although a Griffith model of completely brittle fracture in tensile tests was considered, the possibility that local ``yielding'' might precede tensile fracture seemed more likely. Consideration of the brittle failure model and the localized plastic flow model both lead to the same conclusion: The tensile strength of carbonized rayon fibers is limited by the fibrillar microstructure and cannot be expected to approach the theoretical estimates for graphite single crystals.