Showing papers in "Composites Part A-applied Science and Manufacturing in 2002"

Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase

Abstract: The knowledge of the behaviour of flax fibres is of crucial importance for their use as a reinforcement for composites materials. Flax fibres were tested under tensile loading and in repeated loading–unloading experiments. We have shown that fibre stiffness increases with the strain. This phenomenon is attributed to the orientation of the fibrils with the axis of the fibre when a strain occurs. By using micro-mechanical equations, the Young's modulus of a flax fibre is estimated by taking into account the composition of the fibre and the evolution of the orientation of the fibrils during a tensile test. A good agreement is found between experimental and calculated results. The origin of the large spread observed in the mechanical characteristics is analysed here.

Effects of environmental aging on the mechanical properties of bamboo–glass fiber reinforced polymer matrix hybrid composites

Abstract: Short bamboo fiber reinforced polypropylene composites (BFRP) and short bamboo–glass fiber reinforced polypropylene hybrid composites (BGRP) were fabricated using a compression molding method. Maleic anhydride polypropylene (MAPP) was used as a compatibilizer to improve the adhesion between the reinforcements and the matrix material. By incorporating up to 20% (by mass) glass fiber, the tensile and flexural modulus of BGRP were increased by 12.5 and 10%, respectively; and the tensile and flexural strength were increased by 7 and 25%, respectively, compared to those of BFRP. Sorption behavior and effects of environmental aging on tensile properties of both BFRP and BGRP systems were studied by immersing samples in water for up to 1200 h at 25°C. Compared to BFRP, a 4% drop in saturated moisture level is seen in BGRP. After aging in water for 1200 h, reduction in tensile strength and modulus for BGRP is nearly two times less than that of BFRP. Use of MAPP as coupling agent in the polypropylene matrix results in decreased saturated moisture absorption level and enhanced mechanical properties for both BFRP and BGRP systems. Thus it is shown that the durability of bamboo fiber reinforced polypropylene can be enhanced by hybridization with small amount of glass fibers.

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 5. Injection moulded long and short fibre PP

Abstract: We present results of a step by step comparison of the mechanical performance of injection moulded ‘long’ (LF-PP) and ‘short’ (SF-PP) glass fibre-polypropylene compounds. The study allows direct comparison of the mechanical performance of long and short fibre systems in the same resin at the same fibre diameter, and the effect of fibre diameter in short fibre compounds. Furthermore, the comparison of these three systems has been made over the 0–40 wt% fibre content range. At the same fibre diameter and fibre content LF-PP gives significant improvements in room temperature tensile and flexural strength, notched and unnotched impact resistance. The improvement in impact resistance is higher still at lower test temperature. LF-PP also gives increasingly higher modulus over SF-PP as the strain is increased. The effect of lowering the fibre diameter in SF-PP has been shown to increase both strength and unnotched impact, but not to the levels obtained with LF-PP at higher fibre diameter. Notched impact and modulus of SF-PP were relatively unaffected by reduction of the fibre diameter. The relative mechanical data are shown to conform well to available models. The results are discussed in terms of the relevant micro-mechanical parameters of these materials.

Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part I. Development and investigation of surface treatments

Abstract: Natural fibres have long been used as cost-cutting fillers in the plastics industry. Nowadays, they are considered to be a potential replacement of glass fibres for use in composite materials. However, although natural fibres have many advantages, the most important being their low cost and low density, they are not totally free of problems. A serious problem of natural fibres is their strong polar character, which creates many problems of incompatibility with most thermoplastic matrices (especially polyolefins). Surface treatments, although having a negative impact on economics, are potentially able to overcome the problem of incompatibility. The present study focuses on the development, optimisation and characterisation of two such treatments; acetylation and stearation. The two treatments were applied on two grades of flax fibres (green and dew retted flax), the results are discussed in terms of process variables, such as temperature, time of treatment, recycling of reactants, etc. Three characterisation techniques were applied on the treated and untreated fibres; X-ray diffraction, scanning electron microscopy, and inverse gas chromatography. It was found that both treatments result in a removal of non-crystalline constituents of the fibres, and alter the characteristics of the surface topography. It was also found that both treatments change the fibre surface free energy, with acetylation increasing it and stearation decreasing it.

Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites

Abstract: The local microstructure can be altered significantly by various fibre surface modifications, causing property differences between the interphase region and the bulk matrix. By using tapping mode phase imaging and nanoindentation tests based on the atomic force microscope (AFM), a comparative study of the sized fibre surface topography and modulus as well as the local mechanical property variation in the interphase of E-glass fibre reinforced epoxy resin and E-glass fibre reinforced modified polypropylene (PPm) matrix composites was conducted. The phase imaging AFM was found a highly useful tool for probing the interphase with much detailed information. Nanoindentation experiments indicated the chemical interaction during processing caused by a gradient profile in the modulus across the interphase region of γ-aminopropyltriethoxy silane (γ-APS) and polyurethane (PU)-sized glass fibre reinforced epoxy composite. The interphase with γ-APS/PU sizing is much softer than the PPm matrix, while the interphase with the γ-APS/PP sizing is apparently harder than the matrix, in which the modulus was constant and independent of distance away from the fibre surface. The interphase thickness varied between less than 100 and ≈300 nm depending on the type of sizing and matrix materials. Based on a careful analysis of ‘boundary effect’, nanoindentation with sufficient small indentation force was found to enable measuring of actual interphase properties within 100 nm region close to the fibre surface. Special emphasis is placed on the effects of interphase modulus on mechanical properties and fracture behaviour. The interphase with higher modulus and transcrystalline microstructure provided simultaneous increase in the tensile strength and the impact toughness of the composites.

Carbon-nanofibre-reinforced poly(ether ether ketone) composites

Abstract: Poly(ether ether ketone) nanocomposites containing vapour-grown carbon nanofibres (CNF) were produced using standard polymer processing techniques. Evaluation of the mechanical composite properties revealed a linear increase in tensile stiffness and strength with nanofibre loading fractions up to 15 wt% while matrix ductility was maintained up to 10 wt%. Electron microscopy confirmed the homogeneous dispersion and alignment of nanofibres. An interpretation of the composite performance by short-fibre theory resulted in rather low intrinsic stiffness properties of the vapour-grown CNF. Differential scanning calorimetry showed that an interaction between matrix and the nanoscale filler could occur during processing. Such changes in polymer morphology due to the presence of a nanoscale filler need to be considered when evaluating the mechanical properties of such nanocomposites.

Thermal conductivity of polystyrene–aluminum nitride composite

Abstract: The thermal conductivity of polymer composites having a matrix of polystyrene (PS) containing aluminum nitride (AlN) reinforcement has been investigated under a special dispersion state of filler in the composites: aluminum nitride filler particles surrounding polystyrene matrix particles. Data for the thermal conductivity of the composites are discussed as a function of composition parameters (aluminum nitride concentration, polystyrene particle size) and temperature. It is found that the thermal conductivity of composites is higher for a polystyrene particle size of 2 mm than that for a particle size of 0.15 mm. The thermal conductivity of the composite is five times that of pure polystyrene at about 20% volume fraction of AlN for the composite containing 2 mm polystyrene particle size. The relationship between thermal conductivity of composites and AlN filler concentrations has been compared with the predictions of two theoretical models from the literature.

A study of fibre and interface parameters affecting the fatigue behaviour of natural fibre composites

Abstract: The tension–tension fatigue behaviour of different natural fibre reinforced plastics was investigated. The composites used were made of flax and jute yarns and wovens as reinforcements for epoxy resins, polyester resins and polypropylene. Fibre type, textile architecture, interphase properties, fibre properties and content were found to affect the fatigue behaviour strongly as illustrated with damping versus applied maximum load curves. It was found that natural fibre reinforced plastics with higher fibre strength and modulus, stronger fibre–matrix adhesion or higher fibre fractions possess higher critical loads for damage initiation and higher failure loads. In addition, damage propagation rates were reduced. Furthermore, unidirectional composites were less sensitive to fatigue induced damage than woven reinforced ones.

Experimental and numerical study of the effect of cure cycle, tool surface, geometry, and lay-up on the dimensional fidelity of autoclave-processed composite parts

Abstract: Resin cure shrinkage and anisotropic thermal expansion cause process induced residual stresses in polymer composites. When relieved, the residual stresses cause reduction in enclosed angles of composite laminates; a phenomenon often called spring-in. Spring-in compromises the dimensional fidelity of composite parts and is often accounted for when designing the tool the part is made on. Spring-in is often estimated using past experience or simple analytical formulas that ignores many process parameters affecting the spring-in. This paper presents an experimental study that shows that spring-in can be strongly affected by a number of factors such as cure cycle, tool surface, part geometry, and lay-up. The paper also shows that by developing material models that accurately represent the stress transfer between the part and the tool at the tool-part interface, and by implementing a large deformation solution technique, the experimental results observed in this study can be predicted using finite element based process models.

Electrical relaxations in polymeric particulate composites of epoxy resin and metal particles

Abstract: Composites of epoxy resins and nickel particles in various amounts were prepared and their dielectric spectra were measured in the frequency range 5 Hz–13 MHz and temperature interval from ambient to 140°C. The formalism of electric modulus proved to be efficient in analysing and interpreting obtained data. For these composites two relaxation processes are revealed in the frequency range and temperature interval of the measurements. One is an interfacial dielectric relaxation (Maxwell–Wagner–Sillars), MWS and the other is a conductivity relaxation. They both follow the Cole–Davidson approach with the exponent γ reflecting a distribution of relaxation times with the characteristics of each process. AC conductivity of these composites is frequency and temperature dependant, it generally follows the exponential law σ ac ∼ ω s and reveals a conductivity relaxation process, in the low frequencies.

Arthropod cuticle: A natural composite shell system

Abstract: The cuticle of arthropods (jointed-limb animals), and especially of insects is, by biological standards, a relatively simple composite. It is a single external layer of material forming the skeleton and many sense organs. The fibrous phase is crystalline chitin making nanofibrils of about 3 nm diameter, a few hundreds of nanometers long and a modulus probably in excess of 150 GPa. At least two surfaces of the nanofibril can have silk-like protein attached through specific H-bonds; the rest of the protein is globular. The protein matrix stiffens through dehydration controlled by the introduction of hydrophobic phenolics. Crustacea add up to 40% calcium salts. The stiffness of cuticle can range from tens of GPa to 1 kPa. It can be hardened by the addition of Zn or Mn. It can form springs and change its stiffness and plasticity under the control of the animal.

Mixed mode delamination of polymer composite laminates reinforced through the thickness by z-fibers

Abstract: The mixed mode delamination behavior of through-thickness reinforced carbon–epoxy laminates was investigated using two different test specimens, a T-stiffener and a mixed-mode bending (MMB) specimen. Small quantities of titanium or carbon z-fibers (short rods) substantially improve delamination resistance in both types of specimen. Reinforcement raises the ultimate strength of the MMB specimen by a factor of three. However, the failure sequence and therefore the ultimate load in the T-stiffeners depend strongly on the test configuration. No change in ultimate load is seen in some cases but up to 40% improvement is observed in others. Improved delamination resistance results from crack bridging by the z-fibers, which reduces the driving force for crack growth. Mode I crack displacement is suppressed more effectively than mode II displacement, resulting in purely mode II cracking in what without z-fibers would be a mixed mode or primarily mode I loading situation. This important consequence of so-called large scale bridging effects confirms recent theoretical results for delamination specimens. The mechanisms of bridging and crack propagation are described here in detail.

Effect of resin and fibre properties on impact and compression after impact performance of CFRP

Abstract: The effect of resin and fibre properties on composite impact, compression after impact (CAI) and mode II energy release rate (GIIC) performance has been studied. Impact events were instrumented to record values of Pc, the critical load for initiation of impact damage. Impact response of the laminates was strongly influenced by the fracture toughness of the resin. In contrast, use of high strength and high stiffness fibres did not improve the resistance to impact. The differences in impact and CAI response of the laminates were largely a consequence of the impact damage created at the damage threshold, Pc, rather than of the differences in delamination growth. As a strong correlation was found between GIIC values measured by delamination tests, and those calculated from measurements of Pc, it is suggested that instrumented impact testing may be a more convenient way of determining GIIC in CFRP laminates than delamination tests.

Processing and characterization of aligned vapor grown carbon fiber reinforced polypropylene

Abstract: This investigation describes a method for aligning vapor grown carbon nano-fibers suspended in a polymer flow during extrusion to produce an improved thermoplastic composite. A twin-screw extruder was used to shear mix and extrude fiber/polypropylene mixtures through a converging-annular die that generates fiber alignment along the flow direction. The degree of fiber alignment was quantified by using X-ray diffraction. It was shown that fiber alignment could be improved by increasing the residence time in the die channel. Tensile specimens were fabricated by molding the extruded strands and the strength properties of the aligned samples increased with fiber content. The tensile strength improved with greater fiber orientation, however, more fiber alignment had little affect on the modulus. The addition of vapor grown carbon nano-fiber also increased the thermal conductivity and decreased the electrical resistivity.

Delamination detection in CFRP laminates with embedded small-diameter fiber Bragg grating sensors

Abstract: Newly developed small-diameter fiber Bragg grating (FBG) sensors were applied for the detection of the delamination in carbon fiber reinforced plastic (CFRP) cross-ply laminates Since the diameter of the FBG sensor was very small, the sensor was easily embedded into the 0° ply along the reinforcing carbon fibers The reflection spectra from the FBG sensor were measured at various delamination lengths through four-point bending test The form of the spectrum changed sensitively, as the delamination length increased Moreover, the spectrum was calculated theoretically for confirmation of the measured spectrum The calculated result reproduced the change in the measured spectrum very well Then, we proposed the intensity ratio in the spectrum as an effective indicator for the prediction of the delamination length This indicator has a potential to be applied to other laminate configurations

Ablation, mechanical and thermal conductivity properties of vapor grown carbon fiber/phenolic matrix composites

Abstract: The ablation, mechanical and thermal properties of vapor grown carbon fiber (VGCF) (Pyrograf III™ Applied Sciences, Inc.)/phenolic resin (SC-1008, Borden Chemical, Inc.) composites were evaluated to determine the potential of using this material in solid rocket motor nozzles. Composite specimens with varying VGCF loadings (30–50% wt.) including one sample with ex-rayon carbon fiber plies were prepared and exposed to a plasma torch for 20 s with a heat flux of 16.5 MW/m 2 at approximately 1650°C. Low erosion rates and little char formation were observed, confirming that these materials were promising for rocket motor nozzle materials. When fiber loadings increased, mechanical properties and ablative properties improved. The VGCF composites had low thermal conductivities (approximately 0.56 W/m-K) indicating they were good insulating materials. If a 65% fiber loading in VGCF composite could be achieved, then ablative properties are projected to be comparable to or better than the composite material currently used on the Space Shuttle Reusable Solid Rocket Motor (RSRM).

Carbon composites based on multiaxial multiply stitched preforms. Part 1. Geometry of the preform

Abstract: Experimental investigation of the internal geometry of a multiaxial multiply carbon reinforcement, stitched by warp-knitting, reveals the general features of this promising class of textile reinforcements. The uniform placement of the fibres is disturbed by the stitching, which creates resin-rich zones in the composite. The shape of the stitching loops, change of the stitching yarn thickness along the loop and statistical characteristics of spacing of the stitching has been studied. These features are covered by a descriptive empirical model of the internal geometry of multiaxial multiply fabrics. The model is implemented in the textile modelling software WiseTex, which serves as a preprocessor for meso-mechanics and permeability modelling.

Water sorption in oil palm fiber reinforced phenol formaldehyde composites

Abstract: Water sorption kinetics in oil palm fiber reinforced phenol formaldehyde (PF) composites and oil palm/glass hybrid fiber reinforced PF composites was investigated. The influence of fiber loading, relative volume fractions of fibers in hybrid composites and fiber surface modifications on the kinetic and thermodynamic parameters of water sorption by the composites were also studied. Water sorption at four different temperatures was analysed and compared. Composite with 10 wt% fiber loading exhibits maximum water uptake. Hybridisation of the oil palm fiber with glass considerably decreased the water sorption by the composite. The concentration dependency of the diffusion coefficient was analysed and discussed.

Bias extension measurements on cross-plied unidirectional prepreg

Abstract: The behaviour of woven reinforcements has been extensively studied under conditions of bias extension, from both experimental and analytical viewpoints. The work reported here covers the measurement of the deformation characteristics of cross-plied unidirectional prepreg under conditions of bias extension. The results show that the global behaviour of cross-plied unidirectional prepreg can be predicted with reasonable accuracy by the pin-jointed-net assumption developed for woven cloth, but that the detailed behaviour is very different from that of woven cloth.

Prediction of low velocity impact damage in carbon–epoxy laminates

Abstract: It is well known that composite laminates are easily damaged by low velocity impact. This event causes internal delaminations that can drastically reduce the compressive strength of laminates. In this study, numerical and experimental analyses for predicting the damage in carbon–epoxy laminates, subjected to low velocity impact, were performed. Two different laminates (04,904)s and (02,±452,902)s were tested using a drop weight testing machine. Damage characterisation was carried out using X-rays radiography and the deply technique. The developed numerical model is based on a special shell finite element that guarantees interlaminar shear stresses continuity between different oriented layers, which was considered fundamental to predict delaminations. In order to predict the occurrence of matrix failure and the delaminated areas, a new failure criterion based on experimental observations and on other developed criteria, is included. A good agreement between experimental and numerical analysis for shape and orientation of delaminations was obtained. For delaminated areas, reasonable agreement was obtained.

Selection of composite materials and manufacturing routes for cost-effective performance

Abstract: A study has been conducted to estimate the costs of manufacture of a simple component in a number of different composite materials and by different manufacturing routes. The materials and routes selected span the range of composites from those appropriate for general engineering applications to aerospace. A simple methodology is introduced for a comparison on the basis of cost-performance efficiency. It is demonstrated that more economic solutions may often be realised by choice of ‘expensive’ carbon rather than ‘cheaper’ E-glass as the reinforcing fibre.

Cell-wall hardness and Young's modulus of melamine-modified spruce wood by nano-indentation

Abstract: Samples of spruce wood were infiltrated with a melamine–formaldehyde resin. After curing of the resin, a melamine concentration of 24% (v/v) was measured in the secondary cell walls of melamine treated wood. Nano-indentation tests revealed an average Young's modulus of 16.1 GPa and a hardness of 0.24 GPa for untreated secondary cell walls. In the melamine treated cell walls, an increase in the Young's modulus of 33% to 21.4 GPa was observed. With 115%, i.e. 0.52 GPa, the increase in longitudinal hardness due to melamine–formaldehyde treatment was even more pronounced. This proves clearly that melamine treatment of wood improves mechanical properties of cell walls. Thus, treatment of wood with melamine–formaldehyde resin shows a considerable potential to improve mechanical properties, as desired for applications where large stresses normal to grain arise.

Non-orthogonal constitutive equation for woven fabric reinforced thermoplastic composites

Abstract: One of the ultimate objectives of this study was to investigate the feasibility of shaping preconsolidated woven FRT (fabric reinforced thermoplastics) using stamp thermo-hydroforming, a new forming method for composite manufacturing. A new constitutive model has been developed based on a homogenization method by considering the microstructures of composites including both the mechanical and structural properties of fabric reinforcement. In particular, the current model aims to account for the effect of the fiber strength difference and orientation on anisotropy and also to simulate shear deformation without significant length change, common in FRT composite forming. For validation purposes, the model was implemented in an explicit dynamic finite element code and tested for in-plane simple shear, pure shear, uniaxial tension, and draping behavior of woven composites.

Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part II. The effect of surface treatments on the interface

Abstract: Natural fibres have attracted much attention recently for use as reinforcing agents in composite materials. However, even though natural fibres possess many advantages over glass fibres, such as lower density, lower cost and recycleability, they are not totally free of problems. Natural fibres are comprised mostly of cellulose, a highly hydrophilic macromolecule with strong polarity and, as a result, problems of compatibility with very apolar matrices (e.g. polyolefins) almost certainly arise. Surface treatments, although having a negative impact on economics, may improve the compatibility and strengthen the interface in natural fibre composite materials. In Part I of the present study two such surface treatments, acetylation and stearation, have been developed and applied to flax fibres. In this second part, the effect of these treatments upon the interface of flax fibre/polypropylene composites is assessed by means of fragmentation tests. It has been found that both treatments led to improvement of the stress transfer efficiency at the interface, and both applied treatments were optimised, accordingly.

Influence of the filament winding tension on physical and mechanical properties of reinforced composites

Abstract: In this experimental investigation the influence of the applied tow tension during filament winding on the physical and mechanical properties of glass-fibre reinforced polymeric composite tubulars, was studied. Pressure retaining tubular products used in the transportation/storage of fluids are generally subjected to a variety of loading conditions during their service life; thus tubular specimens were tested under different biaxial loading ratios. The stress/strain response was recorded and functional and structural failure envelopes were developed. These envelopes indicate the leakage and final failure characteristics of the components, respectively. The mechanical properties were analysed in conjunction with the measured physical properties: ‘fibre volume fraction’ and ‘effective wall thickness’. Experimental findings demonstrate that the component strength depends on the degree of fibre tensioning. Under fibre-dominated loading conditions, higher winding tension leads to an improved resistance against failure of tubular components, whereas under matrix-dominated loading failure is delayed by reduced fibre tensioning.

Impact fatigue behaviour of vinylester resin matrix composites reinforced with alkali treated jute fibres

Abstract: An impact fatigue study has been made for the first time on 35% jute/vinylester composites containing both untreated and alkali treated fibres. Longer alkali treatment removed the hemicellulose and improved the crystallinity and gave better fibre dispersion. The flexural strength properties of the composites made from treated fibre were superior. 4 h alkali treated jute fibres gave the optimum combination of improved interfacial bonding and fibre strength properties. However this was not reflected in their impact fatigue behaviour. On the contrary, the composites reinforced with 8 h alkali treated fibres displayed superior impact fatigue properties. Here, the fibres suffered catastrophic fracture with microfibrillar pull-out at some places and improved the fatigue resistance property of the composites as evident from SEM micrographs.

Interfacial strength in thermoplastic composites - at last an industry friendly measurement method?

Abstract: Many elegant techniques have been developed for the quantification of composite micro-mechanical parameters in recent years. Unfortunately, most of these techniques have found little enthusiastic support in the industrial product development environment, where they are viewed as time consuming, complex, inefficient, labour intensive, and in many cases unproven or inapplicable in 'real' systems. Despite this reaction, there is a real need for a 'user-friendly' micro-mechanics to aid the composites industry to move to the next level of development. A method for deriving values for tau (the interfacial shear strength) and etao (a fibre orientation factor) from a simple combination of the composite tensile stress-strain curve and the fibre length distribution has been available for some time. Despite the recent wealth of activity in the development of micro-mechanical test techniques, there has been little follow-up on this older technique. In this paper we explore this analysis by its application to injection moulded glass-fibre-reinforced thermoplastic composites produced using three matrices (polypropylene, polyamide 6,6 and polybutyleneterephthalate) and containing different levels of glass-fibre. We furthermore show how the analysis can be extended to obtain another important micro-mechanics parameter, sigmauf, the fibre stress at composite failure. Values of tau and etao obtained using this improved version of the original model are presented and discussed.

Failure of cellular foams under multiaxial loading

Abstract: A thorough investigation of the mechanical behavior of a closed-cell cellular foam (Divinycell) under multiaxial stress conditions was undertaken. Two types of Divinycell, H100 and H250, with densities of 100 and 250 kg/m3, respectively, were investigated. The uniaxial tensile, compressive and shear stress–strain curves along the in-plane and the through-the-thickness directions of both materials were obtained. The materials showed quite different stress–strain behavior in tension and compression. The H100 material showed a nearly isotropic behavior, while the H250 material showed orthotropic behavior with a higher stiffness along the through-the-thickness than the in-plane direction. A series of biaxial tests were conducted, including: (i) constrained strip specimens in tension and compression with the strip axis along the through-the-thickness and in-plane directions; (ii) constrained thin-wall ring specimens in compression and torsion; (iii) thin-wall tube specimens in tension and torsion; and (iv) thin-wall tube specimens under axial tension, torsion and internal pressure. From these tests, biaxial strength results in the stress plane of the through-the-thickness and in-plane directions for different values of applied shear were obtained. Failure envelopes were constructed by the Tsai–Wu failure criterion based on the strength values in uniaxial tension, compression and shear. The experimental results were described well by the Tsai–Wu failure criterion.

Thermal and electrical properties of magnetite filled polymers

Abstract: By the addition of metal-oxide particles to plastics the electrical and thermal conductivity of polymers can be increased significantly. Such particle filled polymers can substitute metals and metal oxides in applications like radio frequency interference shielding. Furthermore, particle filled polymers with higher thermal conductivity than unfilled ones become more and more important in applications with decreasing geometric dimensions and increasing output of power, like in computer chips. Therefore, thermal and electrical properties of polypropylene and polyamide with metal-oxide particle filler (magnetite, Fe 3 O 4 ) are investigated. Different particle sizes of magnetite and types of additives were added in various proportions to a standard polypropylene and polyamide in an extrusion process. Samples were prepared by injection molding to investigate thermal and electrical properties systematically. The thermal conductivity increases from 0.22 to 0.93 W/(m K) for a filler content of 44 vol% of magnetite, whereas the electrical resistivity decreases more than seven orders of magnitude from an insulator (0% of magnetite) to 10 kΩ m (47 vol% of magnetite). The experimental results of thermal and electrical conductivity were correlated to the amount and particle sizes of magnetite filler. Electrical resistivity shows a significant drop at the theoretical percolation threshold (∼0.33) and for filler contents exceeding 33 vol% the magnetite particles have point contacts and are surrounded by the polymer matrix.

Electromechanical modeling of unidirectional CFRP composites under tensile loading condition

Abstract: The present paper addresses the correlation between mechanical damage and the change in electrical resistance of CFRP under tensile loading. A linear relation between the strain and the electrical resistance of single carbon fibers was obtained experimentally, and the electrical behavior of CFRP under tensile loading was investigated. At stresses approaching the failure stress, the composite resistance rises non-linearly, which is attributed to damage in the form of broken fibers. These experiments lead to the concept of electrical ineffective length over which a broken fiber does not carry electric current, in analogy to the well-established mechanical ineffective length over which a broken fiber carries reduced stress. Based on this concept, a DC circuit model consisting of a serial array of discrete parallel cells of length equal to the electrical ineffective length is proposed to explain the resistance evolution in the composite. An analytical model for fiber damage evolution within the electrical ineffective length is constructed using the Global Load Sharing model and the Weibull fiber strength distribution, The model successfully explains the experimental results on the resistance change of CFRP under tensile loading with an electrical ineffective length of 5 mm. (C) 2001 Elsevier Science Ltd. All rights reserved.