Preferred Fiber Orientation Research Articles

Effects of coarse aggregates on 3D printability and mechanical properties of ultra high performance fiber reinforced concrete

3D printing of ultra high performance fiber reinforced concrete (UHPFRC) suffers from high shrinkage and poor interlayer properties. To mitigate these problems, this study develops new mixtures of UHPFRC materials containing coarse aggregates (CA), and critically examines their suitability for 3D printing (3DP). Totally 90 mould-cast and 3DP cylinder and beam specimens with different CA sizes (5–15 mm) and CA-binder ratios (0.3–0.5) were tested under compression and bending in three directions. The internal distribution and volume fractions of steel fibers and pores and the crack trajectories were characterized and analysed by micro X-ray CT scanned 3D images with 37 μm voxel resolution. The results show that adding more and bigger CAs into UHPFRC reduced the flowability but enhanced the buildability with desired extrudability achieved by adjusting 3D printing velocity. Although the compressive strength of the 3DP cylinders was 15–42 % lower than that of the mould-cast ones due to higher porosities, over 100 MPa strength was still achieved for all the 3DP cylinders with less than 10 % anisotropy. The CT images did not show evident interlayers and interlayer delamination under compression, indicating the new 3DP mixtures and printing parameters may have highly promoted cement hydration. The flexural strength of 3DP beams was 3–34 % higher than that of the cast ones, because most fibres were oriented in the printing or beam axis direction as represented by overall orientation indices calculated from CT images, thereby providing significant crack-bridging effects. The developed new mixtures are therefore well suited for 3D printing, particularly for fabrication of structural members with preferred fibre orientation, such as beams, slabs and shells.

Piezoresistive modeling of orthotropic 3D-printing composite line extrusions with non-affine reorientation of fibers

This work introduces a micromechanical semi-analytical model to estimate piezoresistive constants in 3D-printable short-fiber composites, extending the Mori–Tanaka scheme with two innovative components. Firstly, the model incorporates non-affine reorientation of fibers, controlled by a parameter η, which significantly impacts texture coefficients. This parameter introduces an additional layer of complexity, modifying texture coefficients beyond traditional affine fiber reorientations. Observational data illustrate a linear relationship between piezoresistivities and η, leading to the development of a statistical model that calculates η based on the fiber aspect ratio. Secondly, the model accounts for preferential fiber orientation in the undeformed state, resulting in an orthotropic material model. This anisotropy is quantified using an Orientation Distribution Function (ODF) derived from X-ray scans, which is parameterized to maintain symmetry across three orthogonal planes. Initial investigations indicate that the inherent anisotropy introduced by the printing process substantially affects the initial resistivity of 3D-printed composites. Utilizing a Gaussian transversely isotropic model, we find that neglecting this anisotropy could lead to significant errors in understanding and predicting piezoresistive behavior. The model’s incorporation into advanced nanocomposite frameworks offers preliminary reconciliation with experimental data. Additionally, our results emphasize the necessity to refine wavy fiber models by incorporating non-affine rotations, particularly crucial for fibers with small aspect ratios, thereby enhancing predictive accuracy. The findings lay a robust foundation for future research, emphasizing the need for comprehensive experimental validation and setting the stage for nuanced applications in piezoresistive composite materials.

Using carbon fiber tape to tailor the coefficient of thermal expansion in 3D-Printed composite tooling

In this work, we investigated the effect of plain woven carbon fiber tape embedded in each layer of an additively manufactured part on the coefficient of thermal expansion (CTE) and compared it to conventionally printed parts. Current advancements in Additive Manufacturing enable cost-efficient 3D printing of composite tools. However, these tools do not yet offer a low CTE comparable to Invar, necessary for producing aerospace-quality composite parts. Using the novel Advanced Tape Layer Additive Manufacturing process, the tape is placed on top of the bead immediately after extruding the short fiber-reinforced material. The samples are compared to Material Extrusion specimens from a Large Format Additive Manufacturing System. A lower CTE was achieved within the printing plane. Micro-computed tomography images correlate the preferential orientation of short fibers with measured CTE values. The CTE modification can match the part CTE to the tool CTE and therefore optimize the quality of manufactured parts.

Heat transfer analysis in a horizontal anisotropic porous channel with Bi-viscous Bingham nanofluid and temperature-dependent Brownian diffusion

PurposeThe purpose of this article is to present a mathematical model for the fully developed flow of Bi-viscous Bingham nanofluid through a uniform-width anisotropic porous channel. The model incorporates a generalized Brinkman-Darcy formulation for the porous layers while considering the motion of nanoparticles influenced by both Brownian diffusion and thermophoresis effects.Design/methodology/approachThe similarity transformations derived through Lie group analysis are used to reduce the system from nonlinear partial differential equations to nonlinear ordinary differential equations. The finite difference method-based numerical routine bvp4c is employed to collect and graphically present the outcomes for velocity, temperature, and nanoparticle concentration profiles. The flow pattern is analyzed through streamlined plots. Furthermore, skin friction, heat, and mass transmission rates are investigated and presented via line plots.FindingsIt is observed that in anisotropic porous media, the temperature profile is stronger than in isotropic porous media. The thermal anisotropic parameter enhances the concentration profile while reducing the temperature.Practical implicationsAnisotropy arises in various industrial and natural systems due to factors such as preferred orientation or asymmetric geometry of fibers or grains. Hence, this study has applications in oil extraction processes, certain fibrous and biological materials, geological formations, and dendritic zones formed during the solidification of binary alloys.Originality/value1. The permeability and thermal conductivity are not constant; instead, they have different values in the x and y directions. 2. This study considers the dependency of thermophoresis on nanoparticle volume fraction and Brownian diffusion on the temperature in both the fluid flow equations and boundary conditions. 3. A novel similarity transformation is derived using Lie group analysis instead of using an existing transformation already available in the literature.

Betholletia excelsa Fruit: Unveiling Toughening Mechanisms and Biomimetic Potential for Advanced Materials.

Dry fruits and nutshells are biological capsules of outstanding toughness and strength with biomimetic potential to boost fiber-reinforced composites and protective structures. The strategies behind the Betholletia excelsa fruit mechanical performance were investigated with C-ring and compression tests. This last test was monitored with shearography and simulated with a finite element model. Microtomography and digital and scanning electron microscopy evaluated crack development. The fruit geometry, the preferential orientation of fibers involved in foam-like sclereid cells, promoted anisotropic properties but efficient energy dissipating mechanisms in different directions. For instance, the mesocarp cut parallel to its latitudinal section sustained higher forces (26.0 ± 2.8 kN) and showed higher deformation and slower crack propagation. The main toughening mechanisms are fiber deflection and fiber bridging and pullout, observed when fiber bundles are orthogonal to the crack path. Additionally, the debonding of fiber bundles oriented parallel to the crack path and intercellular cracks through sclereid and fiber cells created a tortuous path.

Design of PGM-Free Cathode Catalyst Layers for PEMFC Applications: The Impact of Electronic Conductivity

The impact of the electronic resistance of platinum-group-metal-free cathode catalyst layers (PGM-free CCLs) for proton-exchange-membrane fuel cells (PEMFCs) was systematically investigated. Here we selected two different PGM-free catalysts (having high and low electronic conductivity) and integrated them into CCLs without and with conductive carbon fiber additives. To investigate the impact of the electronic resistivity () of PGM-free catalysts and CCLs, their through-plane values were quantified by an in situ electrochemical impedance spectroscopy (EIS) approach based on a one-dimensional transmission line model. The results indicate that the electronic conductivity of PGM-free CCLs can be increased by adding carbon additive, resulting in a significant improvement of the fuel cell performance (by ∼60 mV at 1 A cm−2 in H2/O2 configuration). Ex situ four-point probe measurements of the in-plane values of some of the CCLs were found to differ vastly from the through-plane values. This difference is attributed to the anisotropic morphology of the CCLs, caused by preferential fiber orientation and/or cracks in the CCLs. In the end, we suggest guidelines for the design and evaluation of PGM-free CCLs and for assessing and improving their electronic resistance.

Investigation of the performance of needle-punched nonwoven fabrics using Triumfetta cordifolia and thermoplastic fibers, compared to other commercial bast fibers used in preformed biosourced composites

In an attempt to increase biodiversity in natural plant fiber nonwovens, new sources of natural fibers must be discovered. Nonwoven fabric is a promising commercial product for upgrading the new bast fiber Triumfetta cordifolia (TCF) and giving it an opportunity to be used in composite nonwoven applications. Two types of TCF nonwoven mats blended with polylactide fibers for one and polypropylene fibers for the other at a mass ratio of 50 : 50 were manufactured using carding and needling technology. The aim of the present work is to compare the different types of TCF-based nonwovens with other nonwovens based on commercial bast fibers, namely flax and hemp, known for their use in automotive interiors. The nonwoven fabrics were characterized in terms of weight per unit area, thickness, tensile strength and flexural rigidity. In addition, morphological aspects of fiber organization, density and distribution within the nonwoven reinforcement were observed using Scanning Electron Microscopy. The results revealed great variability in terms of surface density and thickness. Increasing the surface mass of nonwovens led to an increase in mechanical performance in terms of strength and stiffness, while retaining anisotropy in terms of fiber orientation, which has a significant effect on mechanical behavior due to the preferential fiber orientation generated during carding. In addition, the type of thermoplastic polymer fiber in the nonwoven mat has an influence on the characteristics evaluated. The results obtained showed that TCFs are good candidates, given their competitive performance and availability compared with flax and hemp fibers, and that they can be used in the same composite applications. Such a non-woven mattress based on TCFs from the tropical African region, manufactured using carding and needling technology, could open up opportunities to create new value-added products that can benefit these countries from an economic and ecological point of view.

On the tensile response of formed fiber networks with low areal density

Fiber networks are ubiquitous. The networks of interest to this study are distinguished from the widely studied planar random fiber networks (RFN) by their low areal density (grammage), preferential orientation, and fiber curl. Oriented networks are created through a high speed manufacturing process called forming in paper making. A dilute suspension of elastic fibers in water impinges on a moving wire to create a formed network. A discrete element method (DEM) based forming model is developed that incorporates the qualitative effects of fluid shear (preferred fiber orientation) and viscous drag during drainage (fiber conformation to the wire topography). Virtual tensile tests performed on thus formed networks reveal that the tensile response is initially softer due to non-affine deformation, and anisotropic due to preferential orientation. Consequently, an orientation dependent power law with a non-unique exponent emerges for the scaling of modulus and strength with the network density. Tensile test simulations reveal the re-alignment of fibers along the loading direction, and strong strain localization resulting in a few fibers (∼1%) carrying the imposed strain energy. Fiber curl and orientation are observed to govern the degree of non-affine response, stiffness, and strain localization, suggesting that controlling the degree of anisotropy and fiber curl offers the possibility to design networks for specific stiffness and specific strength.

Finite element modeling of meniscal tears using continuum damage mechanics and digital image correlation

Meniscal tears are a common, painful, and debilitating knee injury with limited treatment options. Computational models that predict meniscal tears may help advance injury prevention and repair, but first these models must be validated using experimental data. Here we simulated meniscal tears with finite element analysis using continuum damage mechanics (CDM) in a transversely isotropic hyperelastic material. Finite element models were built to recreate the coupon geometry and loading conditions of forty uniaxial tensile experiments of human meniscus that were pulled to failure either parallel or perpendicular to the preferred fiber orientation. Two damage criteria were evaluated for all experiments: von Mises stress and maximum normal Lagrange strain. After we successfully fit all models to experimental force–displacement curves (grip-to-grip), we compared model predicted strains in the tear region at ultimate tensile strength to the strains measured experimentally with digital image correlation (DIC). In general, the damage models underpredicted the strains measured in the tear region, but models using von Mises stress damage criterion had better overall predictions and more accurately simulated experimental tear patterns. For the first time, this study has used DIC to expose strengths and weaknesses of using CDM to model failure behavior in soft fibrous tissue.

Steered fiber orientation: correlating orientation and residual tensile strength parameters of SFRC

Adding steel fibers to concrete improves the post-cracking tensile strength of the composite material due to fibers bridging the cracks. The residual performance of the material is influenced by fiber type, content and orientation with respect to the crack plane. The latter is a main issue in fiber-reinforced concrete elements, since it significantly influences the structural behavior. The aim of this research is to develop a tailor-made composite material and casting method to orient fibers in order to optimize the performance of the material for structural applications. To this aim, a mechanized concreting device that induces such preferred fiber orientation is designed and fabricated. It uses vibration and a series of narrow channels to guide and orient fibers. A composite with oriented fibers is produced using a hybrid system of macro and micro fibers and high-performance concrete. From the same concrete batch, specimens are cast both with and without the fiber orientation device, obtaining different levels of fiber orientation. Three-point bending tests are performed to measure and compare the residual tensile strength capacities with standard specimens cast according to EN 14651. Elements with favorable fiber orientation show a significant increase in residual tensile strength with respect to the standard beams. Finally, computed tomography and an electromagnetic induction method are employed to better assess the orientation and distribution of fibers in the beams. Their results are in good agreement and enable to link the residual tensile strength parameters with fiber orientation.

Three-dimensional analysis of load-dependent changes in the orientation of dermal collagen fibers in human skin: A pilot study

The availability of quantitative structural data on the orientation of collagen fibers is of crucial importance for understanding the behavior of connective tissues. These fibers can be visualized using a variety of imaging techniques, including second harmonic generation (SHG) microscopy. However, characterization of the collagen network requires the accurate extraction of parameters from imaging data. To this end, several automated processes have been developed to identify the preferred orientation of collagen fibers. Common methods include fast Fourier transforms and curvelet transforms, but these tools are mostly used to infer a single preferred orientation.The purpose of this pilot study was to develop an easy procedure for comprehensively comparing multiple human skin samples with the goal of analyzing load-dependent changes via SHG microscopy. We created a 3D model based upon 2D image stacks that provide fiber orientation data perpendicular and parallel to the plane of the epidermis. The SHG images were analyzed by CurveAlign to obtain angle histogram plots containing information about the multiple fiber orientations in each single image. Subsequently, contour plots of the angle histogram intensities were created to provide a useful visual plotting method to clearly show the anomalies in the angle histograms in all samples.Our results provided additional details on how the collagen network carries a load. In fact, analysis of SHG images indicated that increased stretch was accompanied by an increase in the alignment of fibers in the loading direction. Moreover, these images demonstrated that more than one type of preferred orientation is present. In particular, the 3D network of fibers appears to have two preferred orientations in the planes both perpendicular and parallel to the plane of the epidermis.

Electrospinning production of polymer nanofibers containing Ag nanoparticles or carbon nanotubes

Recent years’ electrospinning technology for fabrication of nanofibers of polymers with incorporation of nanoparticles made noticeable progress in different fields, including biomedicine due to their biocompatibility, adhesiveness, antibacterial properties, and sterile nature. In this study, the electrospinning production of antibacterial polyvinyl alcohol (PVA) nanofibers containing silver nanoparticles (AgNPs) was considered. The synthesis of the AgNPs was performed from the polymer-encapsulated AgNO3 precursor material in water with Ag atom reduction by an ultrasonication treatment. The effect of the AgNO3 concentration and the sonication time on the size and quantity of the obtained AgNPs on the PVA scaffold was studied. Absorption spectra were exploited for the control of the appearance of AgNPs in the suspension. From the peak position of the plasmon band observed in the absorption spectrum of the composite the average diameter of the as-prepared AgNPs was estimated as 55–60 nm. As it follows from scanning electron microscopy images the diameter of PVA:AgNPs nanofibers is about 250 nm. Nanofibers formed by polyvinylpyrrolidone (PVP) with incorporation of single-walled carbon nanotubes (SWNTs) were fabricated from the PVP alcohol solution containing SWNTs. The diameter of the nanofibers was in the range of 1–3 μm. Using a dielectric substrate and the collector of a special shape, a mat of fibers with preferred fiber orientation was prepared. Our estimation shows that about 90% of the fibers are oriented.

Airflow-induced P(VDF-TrFE) fiber arrays for enhanced piezoelectric energy harvesting

Piezoelectricity, flexibility, light weight, and biocompatibility of piezoelectric polymer fibers are the desired attributes for energy harvesting and sensing in wearable and biomedical applications. However, the relatively insufficient piezoelectric performance of piezoelectric polymers remains an issue. Here, we demonstrate a considerable increase in P(VDF-TrFE) fiber alignment via electrospinning with a rapidly rotating collector, which substantially enhanced the piezoelectric performance of the fiber mat over a large area. Considering the relationship between the airflow induced near the collector surface and the rotating speed, the collectors with different geometries were systematically compared in terms of the degree of alignment, fiber morphology, and the resulting crystalline electroactive phases of the fibers produced by each collector. We found that the strong airflow induced by the rapid rotation of the modified drum collector contributes to the preferential fiber orientation by pulling and stretching over a large area, which led to an increase in the crystalline electroactive β-phase content responsible for piezoelectricity. As a result, a maximum voltage of 116.6 V and maximum output power of 13.6 µW were achieved using a flexible piezoelectric device comprising a large-area, highly aligned P(VDF-TrFE) fiber mat produced from a modified drum collector at a significantly high speed. This work provides a facile but powerful solution for the wide use of piezoelectric polymer fibers.

Self‐compacting high performance fiber concrete for foundations: Part 2—Fiber orientation and distribution

AbstractAn investigation was executed into the applicability of self‐compacting high performance fiber concrete in foundations. The applied concrete has a concrete cube strength of about 110 MPa and contains 60 kg/m3 hooked‐end steel fibers (LF = 30 mm, DF = 0.38 mm). This publication consists of two parts: (1) Experimental assessment and verification of design rules and (2) assessment of fiber orientation and distribution. In the first part, experiments are described which were carried out to determine the pre‐ and post‐cracking strength properties, the shear resistance of short beams, the anchorage length of rebars, and the shear capacity of foundation slabs supported on piles. The test results were used for a verification and extension of design rules for fiber reinforced concrete (FRC) found in the fib Model Code 2010. The application of the FRC developed can lead to substantial savings in concrete and reinforcing steel. In the present second part, cross‐sections of two slabs and a beam of this testing series have been analyzed with regard to fiber orientation and distribution. An image analysis, executed on 111 concrete areas, indicates that a preferred fiber orientation could not be identified throughout the assessed elements. The data supports the conclusion of a good fiber distribution as well; a strong correlation was obtained for the data set of full images between measured and theoretical relation of fiber orientation and fiber density.

Effect of the Fibre Orientation Distribution on the Mechanical and Preforming Behaviour of Nonwoven Preform Made of Recycled Carbon Fibres

Recycling carbon-fibre-reinforced plastic (CFRP) and recovering high-cost carbon fibre (CF) is a preoccupation of scientific and industrial committees due to the environmental and economic concerns. A commercialised nonwoven mat, made of recycled carbon fibre and manufactured using carding and needle-punching technology, can promote second-life opportunities for carbon fibre. This paper aims to evaluate the mechanical and preforming behaviour of this nonwoven material. We focus on the influence that the fibre orientation distribution in the nonwoven material has on its mechanical and preforming behaviour at the preform scale, as well as the tensile properties at composite scale. The anisotropy index induced by fibre orientation is evaluated by analysing SEM micrographs using the fast Fourier transform (FFT) method. Then, the anisotropy in the tensile, bending, and preforming behaviour of the preform is inspected, as well as in the tensile behaviour of the composite. Additionally, we evaluate the impact of the stacking order of multi-layers of the nonwoven material, associated with its preferred fibre orientation (nonwoven anisotropy), on its compaction behaviour. The nonwoven anisotropy, in terms of fibre orientation, induces a strong effect on the preform mechanical and preforming behaviour, as well as the tensile behaviour of the composite. The tensile behaviour of the nonwoven material is governed by the inter-fibre cohesion, which depends on the fibre orientation. The low inter-fibre cohesion, which characterises this nonwoven material, leads to poor resistance to tearing. This type of defect rapidly occurs during preforming, even at too-low membrane tension. Otherwise, the increase in nonwoven layer numbers leads to a decrease in the impact of the nonwoven anisotropy behaviour under compaction load.

An study on the influence of collagen fiber directions in TAVs performance using FEM

Transcatheter Aortic Valve Implantation (TAVI) or Replacement (TAVR) is a promising treatment for aortic valve stenosis, consisting of a procedure to replace a damaged native aortic valve by a bioprosthetic one. This replacement valve control the flow of blood using leaflets that are similar to the ones of a native aortic valve. Commonly manufactured using bovine or porcine pericardium, it is a tissue histologically composed of collagen fibers embedded into a nearly-isotropic matrix, where their distribution makes the pericardium behave as an anisotropic hyperelastic material. Because of such complicated behavior, bioprosthetic pericardium valves are, as expected, sensitive to the distribution and orientation of these fibers in such device.Therefore, the objective of this work is a thorough systematic study on the influence of these fibers’ distribution. First, a Finite Element model of a bioprosthetic valve is generated; then, a material routine to accurately describe the behavior of pericardium is implemented in a commercial software package; in addition, a dedicated algorithm to specify the direction of fibers is developed. Finally, a systematic study on the influence that fiber orientations have on the overall behavior of the TAV is performed.As a result of this study, two extreme behaviors are highlighted depending on the preferential orientation of collagen fibers; namely, one with fibers in circumferential direction and the opposite with fibers in an axial orientation. Then, it is concluded that the behavior of fibers in circumferential direction is very sensitive to small variations of the orientation angle, whereas such orientation is not as determining when the aim is to achieve a behavior near to the one corresponding with axial orientation.

Non-Invasive in Vivo Quantification of Directional Dependent Variation in Mechanical Properties for Human Skin.

Skin is the body’s largest organ, and it shows non-linear and anisotropic behavior under the deformation. This behavior of the skin is due to the waviness and preferred orientation (in a particular direction) of collagen fibers. This preferred orientation of collagen fibers results in natural pre-tension and anisotropy of the skin. The knowledge of natural skin pre-tension and anisotropy is essential during incisions and surgery. The available suction-based devices quantify the anisotropy through the displacement field and cannot measure the stress-strain relation in particular directions. Therefore, in the current study, an in vivo full-field measurement suction apparatus was developed to measure the stress and strain of skin in all planar directions through a single experiment. First, this apparatus was tested on silicone substrates of known properties, and then it was used to test the skin of 12 human forearms. Further, to check the effect of hand stability on the measurements, the obtained results of the skin were compared with the results of a standard test performed in the same skin using a steady setup. The consistency between these two results confirms that the stability of the hand does not influence the measurements of skin properties. Furthermore, using the developed apparatus, the skin’s anisotropy and its relation with the Kraissl’s lines orientation was quantified by measuring the toe and linear moduli at an interval of one degree. The minimum and maximum values of the toe and linear moduli were 0.52 ± 0.09 and 0.59 ± 0.11 MPa, and 3.09 ± 0.47 and 5.52 ± 1.13 MPa, respectively. Also, the direction of maximum moduli was found almost similar to Kraissl’s lines’ orientation. These results confirm the contribution of skin pre-tension on the anisotropy of the skin. The present apparatus mimics the tissue expansion procedure, where observation of the test may be helpful in the selection of size and shape of the expander.

Bayesian inversion for anisotropic hydraulic phase-field fracture

In this work, we employ a Bayesian inversion framework to fluid-filled phase-field fracture. We develop a robust and efficient numerical algorithm for hydraulic phase-field fracture toward transversely isotropic and orthotropy anisotropic fracture. In the fluid-driven coupled problem, three primary fields for pressure, displacements, and the crack phase-field are solved while direction-dependent responses (due to the preferred fiber orientation) are enforced via penalty-like parameters. A new crack driving state function is introduced by avoiding the compressible part of anisotropic energy to be degraded. Next, we use a successful extension of the anisotropic hydraulic phase-field fracture as a departure point for Bayesian inversion to estimate material parameters. To this end, we employ the delayed rejection adaptive Metropolis–Hastings (DRAM) algorithm to identify the parameters. The focus is on uncertainties arising from different variables, including elasticity modulus, Biot’s coefficient, Biot’s modulus, dynamic fluid viscosity and Griffith’s energy release rate in the case of the isotopic hydraulic fracture while in the anisotropic setting, we will have additional penalty-like parameters to be identified. Several numerical examples substantiate our algorithmic developments.

Mechanical Analysis of a Thermo-Hydroforming Fiber-Reinforced Composite Using a Preferred Fiber Orientation Model and Considering Viscoelastic Properties

Abstract Thermo-hydroforming (THF) process is a single-step process for thermoplastic composite forming, which has a great advantage in terms of the process time and mass production potential as compared to conventional processes. However, with THF processes, wrinkles and deformations are easily generated due to the process characteristics and process parameters. In this study, the matrix material was examined by considering viscoelasticity and changes in formability according to the forming speed. A numerical analysis of the THF composites was performed based on the preferred fiber orientation (PFO) analysis model, which considers the viscoelasticity of the matrix. The deformation change and molding possibility were examined according to various forming speeds. The viscoelastic PFO model showed better analysis efficiency and stability than the primitive PFO model. This analysis will help improve the process of forming thermoplastic composites.

Mechanical and Thermal Behavior of Fibrous Carbon Materials.

The ability of various commercial fibrous carbon materials to withstand stress and conduct heat has been evaluated through experimental and analytical studies. The combined effects of different micro/macro-structural characteristics were discussed and compared. Large differences in mechanical behavior were observed between the different groups or subgroups of fibrous materials, due to the different types of fibers and the mechanical and/or chemical bonds between them. The application of the Mooney–Rivlin model made it possible to determine the elastic modulus of soft felts, with a few exceptions, which were studied in-depth. The possible use of two different mechanical test methods allowed a comparison of the results in terms of elastic modulus obtained under different deformation regimes. The effective thermal conductivity of the same fibrous materials was also studied and found to be much lower than that of a single carbon fiber due to the high porosity, and varied with the bulk density and the fiber organization involving more or less thermal contact resistances. The thermal conductivity of most materials is highly anisotropic, with higher values in the direction of preferential fiber orientation. Finally, the combination of compression and transient thermal conductivity measurement techniques allowed the heat conduction properties of the commercial fibrous carbons to be investigated experimentally when compressed. It was observed that thermal conductivity is strongly affected under compression, especially perpendicular to the main fiber orientation.