Entangled Cross-Linked Fibres for an Application as Core Material for Sandwich Structures - Part II: Analytical Model

Entangled cross-linked carbon, aramid and glass fibres were recently produced by epoxy spraying for an application as core material for sandwich panel. The Young’s moduli in compression and tension have been previously measured and briefly summarized in this paper. To optimize the core structure, modelling of these properties has been achieved in the present paper. The cross-link fibres have a random orientation and the stiffness of the epoxy joint is modelled by a torsion spring. A parallel model is chosen for homogenisation. It was found that the experimentally estimated stiffness of these materials fits fairly well with the modelled ones.     

Mechanical behavior of entangled fibers and entangled cross-linked fibers during compression

Entangled fibrous materials have been manufactured from different fibers: metallic fibers, glass fibers, and carbon fibers. Specimens have been produced with and without cross-links between fibers. Cross-links have been achieved using epoxy spraying. The scope of this article is to analyze the mechanical behavior of these materials and to compare it with available models. The first part of this article deals with entangled fibrous materials without cross-link between fibers. Compression tests are detailed and test reproducibility is checked. In the second part, compression tests were performed on materials manufactured with cross-linked fibers. The specific mechanical behavior obtained is discussed.     

Mechanical properties of glassy carbon fibres derived from phenolic resin

Mechanical properties of glassy carbon fibres produced from a phenolic resin were determined by static tensile testing. These specimens are of special interest because they consist of an isotropic core surrounded by a sheath of oriented material of varying relative thickness. The chemistry of pyrolysis of the resin is summarized and the changes in mechanical properties of the fibres are discussed in terms of the pyrolysis mechanisms. The results are compared with hardness measurements made on discs produced from the same starting material. Scanning electron microscope studies revealed that the fibres have various types of flaws both in the surface and in the core. The effect of these flaws on the fibre strength is discussed by applying Griffith crack theory.     

Numerical simulation of entangled materials mechanical properties

A general approach to simulate the mechanical behaviour of entangled materials submitted to large deformations is described in this paper. The main part of this approach is the automatic creation of contact elements, with appropriate constitutive laws, to take into account the interactions between fibres. The construction of these elements at each increment, is based on the determination of intermediate geometries in each region where two parts of beams are sufficiently close to be likely to enter into contact. Numerical tests simulating a 90% compression of nine randomly generated samples of entangled materials are given. They allow the identification of power laws to represent the evolutions of the compressive load and of the number of contacts.     

Production of carbon fibres from a pyrolysed and graphitised liquid crystalline cellulose fibre precursor

Regenerated cellulose fibres, spun from a liquid crystalline precursor, were pyrolysed at temperatures in the range 400–2,500?°C. Raman spectroscopy and X-ray diffraction showed that the degree of graphitisation of the fibre increased with increasing temperature. Electron microscopy, however, suggested that the fibres have a skin–core structure. This observation was confirmed by micro-Raman analysis, whereupon the ratio of the intensities of the D and G bands shows that the skin consists of a graphitised structure, whereas the core consists of significantly less graphitised material. The contributions of the graphitised skin and the inner core to the potential mechanical properties of the fibres were also assessed by following the position of the 2D Raman band during tensile deformation of the fibre. The Raman band shift rate against strain was used to evaluate the fibre modulus, which suggested a modulus of ~140 GPa for the skin and 40?GPa for the core, respectively. If this incomplete graphitisation could be overcome, then there is potential to produce carbon fibres from these novel precursor materials.     

Tensile strength of glass fibres with carbon nanotube–epoxy nanocomposite coating: Effects of CNT morphology and dispersion state

The effects of carbon nanotube (CNT)–epoxy nanocomposite coating applied to glass fibre surface on tensile strength of single glass fibres are evaluated at different gauge lengths. The crack healing efficiencies obtained using two different types of CNTs with different structures, morphologies and dispersion characteristics in various concentrations are specifically studied. The results indicate that the tensile strength of single fibres increased significantly with increasing CNT content up to a certain level, depending on the type of CNTs. The crack healing efficiency was much higher for the fibres coated with straight, less entangled CNTs than those with highly entangled CNTs, indicating the CNT dispersion state in the coating played an important role. A strong correlation is established between the CNT dispersion state, the tensile properties of nanocomposite and the tensile strengths of fibres with the nanocomposite coating.     

Fracture behaviour of cross-linked collagen fibres

Mechanical properties such as stress - strain, fracture behaviour and thermal behaviour were studied for cross-linked collagen fibres. The shrinkage temperature of cross-linked fibres shows increase in temperature when compared to the control. The results on measurements of breaking strength and strain show significant change when compared with that of the control. The morphological features of the fractured ends of cross-linked fibres were indicative of certain specific patterns. A critical observation of these patterns indicate the role played by the nature of cross-linking agents on the mechanism of failure of these fibres.     

The flexural behaviour of aramid fibre hybrid composite materials

In this paper the elastic-plastic model of flexural behaviour of beams is applied to hybrid composites containing aramid fibres. In the hybrids, as in the parent aramid-fibre-reinforced composite, the neutral axis is shifted toward the tensile face. The shift depends on the quantity and placement of the aramid fibre. The analysis and experimental work reported here relate to two fundamental sandwich hybrids, one with aramid fibres in the skins and carbon or glass fibres in the core, and the other with aramid fibre in the core and carbon or glass fibres in the skins. The flexural behaviour of the hybrids is discussed in terms of the effect of the placement of the aramid layer and of the relative thickness of the skin on the ultimate stresses, the elastic-plastic behaviour and the mode of failure.     

Preparation and electrochemical properties of multiwalled carbon nanotubes-nickel oxide porous composite for supercapacitors

Porous nickel oxide/multiwalled carbon nanotubes (NiO/MWNTs) composite material was synthesized using sodium dodecyl phenyl sulfate as a soft template and urea as hydrolysis-controlling agent. Scanning electron microscopy (SEM) results show that the as-prepared nickel oxide nanoflakes aggregate to form a submicron ball shape with a porous structure, and the MWNTs with entangled and cross-linked morphology are well dispersed in the porous nickel oxide. The composite shows an excellent cycle performance at a high current of 2 A g⁻¹ and keeps a capacitance retention of about 89% over 200 charge/discharge cycles. A specific capacitance approximate to 206 F g⁻¹ has been achieved with NiO/MWNTs (10 wt.%) in 2 M KOH electrolyte. The electrical conductivity and the active sites for redox reaction of nickel oxide are significantly improved due to the connection of nickel nanoflakes by the long entangled MWNTs.     

Some properties of a condensed polynuclear aromatic resin (COPNA) as a binder for carbon fibre composites

A new thermosetting resin consisting of condensed aromatic nuclei cross-linked with methylene bridges was prepared from a mixture of pyrene, phenanthrene and 1,4-benzenedimethanol by heating above 393 K. This resin, named COPNA, adheres well to carbon fibres, and the carbon fibre/resin composite (CFRP) prepared by using this resin as a binder exhibits no remarkable changes in mechanical properties after heating at 523 K for 10 h and 573 K for 2h. The CFRP specimens were converted into carbon fibre/carbon composite (CFRC) by further heating up to 1 273 K without any trouble.     

Finite element modelling of thermally bonded bicomponent fibre nonwovens: Tensile behaviour

Nonwovens are polymer-based engineered textiles with a random microstructure and hence require a numerical model to predict their mechanical performance. This paper focuses on finite element (FE) modelling the elastic–plastic mechanical response of polymer-based core/sheath type thermally bonded bicomponent fibre nonwoven materials. The nonwoven fabric is treated as an assembly of two regions having distinct mechanical properties: fibre matrix and bond points. The fibre matrix is composed of randomly oriented core/sheath type fibres acting as load-transfer link between bond points. Random orientation of individual fibres is introduced into the model in terms of the orientation distribution function (ODF) in order to determine the material’s anisotropy. The ODF is obtained by analysing the data acquired with scanning electron microscopy (SEM) and X-ray micro computed tomography (CT). On the other hand, bond points are treated as a deformable bicomponent composite material composed of the sheath material as matrix and the core material as fibres having random orientations. An algorithm is developed to calculate the anisotropic material properties of these regions based on properties of fibres and manufacturing parameters such as the planar density, core/sheath ratio and fibre diameter. Having distinct anisotropic mechanical properties for two regions, the fabric is modelled with shell elements with thicknesses identical to those of the bond points and fibre matrix. Finally, nonwoven specimens are subjected to tensile tests along different loading directions with respect to the machine direction of the fabric. The force–displacement curves obtained in these tests are compared with the results of FE simulations.     

Electrical conductivity of polyethylene-carbon-fibre composites mixed with carbon black

The study of electrical conductivity of high-density polyethylene-carbon-fibre composites mixed with different concentrations of carbon black is reported. The influence of the mixing procedure of the additives and material preparation is examined with regard to the conductivity values. The use of these two filler types in polyethylene composites combines the conducting features of both. Thus, while fibres provide charge transport over large distances (several millimetres), carbon black particles improve the interfibre contacts. Results are discussed with reference to simple electrical models. It is shown that for composites in which the segregated carbon black-polyethylene component lies above the percolation threshold the electrical interfibre contacts are activated through carbon black particle bridges, leading to a conductivity rise. This effect is more relevant in the case of shorter fibres. Processing of the material involving fibre orientation, such as in injection-moulding, decreases drastically the conductivity level reached.     

A new approach for the simulation of skeletal muscles using the tool of statistical mechanics

The structure of a skeletal muscle is dominated by its hierarchical architecture in which thousands of muscle fibres are arranged within a connective tissue network. The single muscle fibres consist of many force‐producing cells, known as sarcomeres. These micro biological engines are part of a motor unit and contribute to the contraction of the whole muscle. There are a lot of questions concerning the optimisation of muscle strength and agility. Standard experimental investigations are not sufficient to answer these questions because they do not supply enough information. Additionally, these methods are limited because not enough material for testing is accessible. To overcome these problems, numerical testing tools can be an adequate alternative. From the mechanical point of view the material behaviour of muscles is highly non‐linear. They undergo large deformations in space, thereby changing their shape significantly, so that geometrical nonlinearity has to be considered. Many authors use continuum‐based approaches in combination with the finite element method to describe such material behaviour. However, models of this kind require realistic constitutive relations between stress and strain which are difficult to determine in an inhomogeneous material. Furthermore, biomechanical information cannot be fully exploited in these models. The present approach is crucially based on the use of the finite element method. The material behaviour of the muscle is split into a so‐called active and a passive part. To describe the passive part special unit cells consisting of one tetrahedral element and six truss elements have been derived. Embedded into these unit cells are further truss elements which represent bundles of muscle fibres. Depending on the discretisation it is possible to simulate the material behaviour of e.g. artery walls characterised by oriented fibres or soft tissue including only non‐oriented fibres. In summary, the present concept has the advantage that a three‐dimensional model is developed which allows us take into account many physiological processes at the micro level.     

Ballistic resistance of the carbon nanotube fibres reinforced composites – Numerical study

The numerical investigations were carried out to determine the ballistic resistance of the carbon nanotube (CNT) fibres reinforced composites. In this paper, the fundamental studies of the reinforcement characteristics are presented. It includes the single fibre mechanical and geometric properties as well as fibres distribution and volume (mass) concentration. The continuum matrix material includes a certain amount of fibres made of CNTs. An impact of the projectile with the sharp nose on the metal matrix composite plate was analysed. The computer simulations were performed with the finite element method implemented in LS-DYNA code. The plane formulation allows analysing extremely dense meshes. The obtained results presented the significant role of the carbon nanotube fibres.     

The environmental fatigue behaviour of carbon fibre reinforced polyether ether ketone

The fatigue behaviour of carbon fibre/PEEK composite is compared with that of carbon/ epoxy material of similar construction, particularly in respect of the effect of hygrothermal conditioning treatments. Laminates of both materials were of 0/90 lay-up, and they were tested in repeated tension at 0° and at 45° to the major fibre axis. The superior toughness of the polyether ether ketone and its better adhesion to the carbon fibres results in composites of substantially greater toughness than that of the carbon/epoxy material, and this is reflected in the fatigue behaviour of the carbon fibre/PEEK. The tougher PEEK matrix inhibits the development of local fibre damage and fatigue crack growth, permitting a 0/90 composite with compliant XAS fibres to perform as well in fatigue as an epoxy laminate with stiffer HTS fibres. Hygrothermal treatments have no effect on the fatigue response of either material in the 0/90 orientation. The fatigue response of a cross-plied carbon/PEEK laminate in the ±45° orientation is much better than that of equivalent carbon/epoxy composites, again because the superior properties of the thermoplastic matrix.     

Characterisation of Bionate polycarbonate polyurethanes for orthopaedic applications

Two polycarbonate polyurethanes, Bionate 75D and Bionate 80A, have been characterized for application in biomimetic joint replacement systems. Procedures involved measurement of the effects of compounding and moulding on molecular weight, melt rheometry, and mechanical testing using conditioned and aged specimens. The effects of compounding with hydroxyapatite and carbon fibres were also evaluated. With Bionate 75D moulding reduces the molecular weight by 30%. Passing the material through a twin screw extruder without filler has similar molecular weight reduction effects to injection moulding. Inclusion of carbon fibre has little additional effect on molecular weight, although moulding of the fibre filled compound causes some further degradation, and Mw is almost halved compared with the original value. Inclusion of hydroxyapatite reduces Mw in a moulded component to less than a quarter of the original value and some form of chemical interaction between the polymer and filler is presumed. The apparent melt viscosity of the Bionate 75D was reduced by the addition of both carbon fibres and hydroxyapatite and this is thought to arise from reduction in molecular weight during the compounding process and the development of shear planes at the polymer-filler interface. The polymer glass transition temperatures are shifted to slightly higher values by the inclusion of filler. The tensile test results show the reinforcing effect of the carbon fibres, but poor wetting and pull out of the fibres was evident. Water absorption results suggest that the materials stabilise after 2 weeks, but the tensile results indicate that property change occurred between 1 month and 5 months of exposure. However the shape of the stress-strain curves is not altered, but with extended water exposure is translated to lower stress levels.     

Hybrid composites based on polyethylene and carbon fibres Part 3: Impact resistant structural composites through damage management

Damage tolerance and impact resistance have become key parameters for composite materials in structural applications. In this paper a toughening concept for structural composites based on the hybridization of carbon fibres with high performance polyethylene (HP-PE) fibres is presented. Impact behaviour of hybrid HP-PE/carbon laminates was studied using a falling weight impact test. The effect of the addition of HP-PE fibres as well as the effect of the adhesion level of these fibres on the impact resistance of hybrid HP-PE/carbon structures was investigated. Hybridization results in structural composites exhibiting a significantly better resistance to impact damage than all-carbon laminates due to a change in energy absorption mode. After hybridization more energy is stored in the HP-PE component and consequently less energy is available for damage in the structural carbon component, resulting in a reduction in impact damage and improved post-impact properties.     

Fibre-reinforced geopolymer concrete with ambient curing for in situ applications

Geopolymer concrete is proven to have excellent engineering properties with a reduced carbon footprint. It not only reduces the greenhouse gas emissions (compared to Portland cement-based concrete) but also utilises a large amount of industrial waste materials such as fly ash and slag. Due to these positive attributes, it is becoming an increasingly popular construction material. Previous studies on geopolymer concrete report that heat curing plays an important role in gaining higher compressive strength values (as opposed to ambient curing), and hence the application of this material could be limited to precast members. Therefore, this research was aimed at investigating the effect of heat curing by comparing the mechanical properties such as compressive strength and ductility of ambient cured and heat cured geopolymer concrete samples. It is worth noting that there was marginal strength change due to heat curing. In Australia, fibre-reinforced geopolymer concrete is being used in precast panels in underground constructions. Commercially available geopolymer cement and synthetic fibres are effectively being used to produce elements that are more durable than what is currently used in industry. As a result, this research investigated the effects of polypropylene fibres in geopolymer concrete using 0.05 and 0.15 % fibres (by weight). The addition of polypropylene fibres enhances the compressive strength and the ductility of geopolymer concrete.     

Electrochemical coating for prevention of carbon fibre release from polymer composites: Thermogravimetric analysis of organophosphorus coatings on carbon fibres

Fire retardant coatings of phosphorus compounds were formed on graphite fibres by a new electrochemical technique. Thus, tetrakis (hydroxymethyl)-phosphonium sulphate, ammonium polyphosphate, titanium di(dioctylpyrophosphate) oxyacetate, di(dioctylphosphato)ethylene titanate, and propargyltriphenyl-phosphonium bromide, were electrodeposited or electropolymerized on commercial graphite fibres used for polymer reinforcement. The effect of these coatings on the thermal oxidative behaviour of the coated carbon fibres, epoxy resin, and composites prepared from them was studied by thermogravimetric analysis, and compared with that of polyimide coatings. Generally, the coated fibres showed higher decomposition temperature than the untreated carbon fibres. The fire retardant phosphorus compounds promoted the formation of char from the matrix resin, and accelerated the decomposition of char. Organophosphorus titanate coatings left an incombustible, white residual layer of titanium dioxide. The polyphosphate coating caused the decomposition of the fibres in the epoxy composite to occur at a reduced temperature compared to that in the absence of the matrix resin. A synergistic interaction between the polyphosphate and the amine-cured, epoxy resin to catalyse the decomposition of carbon fibres is inferred from this. Polyimide precursor coatings lowered the oxidation temperature of the carbon fibres, both as neat coatings and in the presence of epoxy matrix resin, thus reducing the temperature of survival of the fibres under combustion conditions. The results confirm the potential of this novel approach of forming precursor coatings on carbon fibres to minimize the release of conductive fibre fragments from carbon fibre-reinforced polymer composites exposed to fire.     

Amorphous carbon fibres from linear low density polyethylene

Carbon fibres with good mechanical properties have been produced from linear low density polyethylene (LLDPE). The melt-spun LLDPE fibres were made infusible by treatment with chlorosulphonic acid. The cross-linked fibres were pyrolysed at temperatures between 600 and 1100 °C under tension, in a nitrogen atmosphere, within 5 min. Carbon fibres prepared at 900 °C had a tensile strength of 1.15 GPa and a Young's modulus of 60 GPa. The elongation at break was extremely high, up to 3%. The carbon yield of the process was 72 to 75%.