Showing papers in "Composites Part A-applied Science and Manufacturing in 2006"

Recycling technologies for thermoset composite materials—current status

Abstract: The technologies for recycling thermoset composite materials are reviewed. Mechanical recycling techniques involve the use of grinding techniques to comminute the scrap material and produce recyclate products in different size ranges suitable for reuse as fillers or partial reinforcement in new composite material. Thermal recycling processes involve the use of heat to break the scrap composite down and a range of processes are described in which there are various degrees of energy and material recovery. The prospects for commercially successful composites recycling operations are considered and a new initiative within the European composites industry to stimulate recycling is described.

Biodegradable polymers/bamboo fiber biocomposite with bio-based coupling agent

Abstract: Effects of lysine-based diisocyanate (LDI) as a coupling agent on the properties of biocomposite from poly (lactic acid) (PLA), poly (butylene succinate) (PBS) and bamboo fiber (BF) were investigated. Tensile properties, water resistance, and interfacial adhesion of both PLA/BF and PBS/BF composites were improved by the addition of LDI, whereas thermal flow became somewhat difficult due to cross-linking between polymer matrix and BF. Crystallization temperature and enthalpy in both composites were increased and decreased with increasing LDI content, respectively. The heat of fusion in both composites was decreased by addition of LDI, whereas there was no significant change in melting temperature. Thermal degradation temperature of both composites was lower than those of pure polymer matrix, but the composites with LDI showed higher degradation temperature than those without LDI. Enzymatic biodegradability of PLA/BF and PBS/BF composites was investigated by Proteinase K and Lipase PS, respectively. Both composites could be quickly decomposed by enzyme and the addition of LDI delayed the degradation.

Enhanced thermal conductivity of polymer composites filled with hybrid filler

Abstract: This study aims at investigating package materials based on polymer matrix for microelectronics. The next generation package materials are expected to possess high heat dissipation capability in addition to low coefficient of thermal expansion (CTE) as the accumulated heat from high performance electronic devices should be removed for proper operation. In this study, various inorganic fillers including aluminum nitride (AlN), wollastonite, silicon carbide whisker (SiC) and boron nitride (BN) with different shape and size were used alone or in combination to prepare thermally conductive polymer composites. In case of AlN, titanate coupling agent was used for the surface treatment of fillers. The use of hybrid filler was found to be effective in increasing thermal conductivity of the composite probably due to the enhanced connectivity offered by structuring filler with high aspect ratio in hybrid filler. For given filler loading, the use of larger particle and surface treated filler resulted in composite materials with enhanced thermal conductivity. The surface treatment of filler also allowed producing the composites with lower CTE.

Physically based failure models and criteria for laminated fibre-reinforced composites with emphasis on fibre kinking. Part II: FE implementation

Abstract: 3D failure criteria for laminated fibre-reinforced composites, based on a physical model for each failure mode and considering non-linear matrix shear behaviour, are developed. Special emphasis is given to compression failure. The physical model for matrix compression failure is based on the Mohr–Coulomb criterion and also predicts the fracture angle. For fibre kinking, an initial fibre-misalignment angle is considered to trigger failure, due to further rotation during the compressive loading. The plane where the kinking takes place is predicted by the model, as well as the kink-band angle. Applications are presented that validate the model against experimental data.

Improving the properties of UD flax fibre reinforced composites by applying an alkaline fibre treatment

Abstract: Present-day industry takes an interest in environment friendly materials, due to economic and ecological reasons. The use of natural materials in composite parts fits well into this picture: plant fibres that reinforce polymer matrices can replace glass fibres in many cases, although applications are often limited to non-structural parts. The poor interface in a non-treated natural fibre reinforced composite prevents the parts to be used to their full capacity. Consequently, this study concentrates on a simple but effective fibre treatment (i.e. alkalisation) that will enable a better adhesion between flax fibres and epoxy matrix. Parameters such as time and concentration are being optimised, in order to develop a continuous process for the treatment and resin impregnation of unidirectional flax fibre epoxy composites. This paper shows a clear improvement of the mechanical properties of the resulting material: e.g. a mild treatment in a 4% NaOH solution for 45 s will increase the transverse composite strength up to 30%.

Prediction of in situ strengths and matrix cracking in composites under transverse tension and in-plane shear

Abstract: A criterion for matrix failure of laminated composite plies in transverse tension and in-plane shear is developed by examining the mechanics of transverse matrix crack growth. Matrix cracks are assumed to initiate from manufacturing defects and can propagate within planes parallel to the fiber direction and normal to the ply mid-plane. Fracture mechanics models of cracks in unidirectional laminates, embedded plies and outer plies are used to determine the onset and direction of propagation of crack growth. The models for each ply configuration relate ply thickness and ply toughness to the corresponding in situ ply strength. Calculated results for several materials are shown to correlate well with experimental results.

Mechanical properties of biodegradable composites reinforced with bagasse fibre before and after alkali treatments

Abstract: Biodegradable composites reinforced with bagasse fibre before and after alkali treatments were prepared, and mechanical properties were investigated. Mechanical properties of the composites made from alkali treated fibres were superior to the untreated fibres. Composites of 1% NaOH solution treated fibres showed maximum improvement. Approximately 13% improvement in tensile strength, 14% in flexural strength and 30% in impact strength had been found, respectively. After alkali treatment, increase in strength and aspect ratio of the fibre contributed to the enhancement in the mechanical properties of the composites. SEM observations on the fracture surface of composites showed that the surface modification of the fibre occurred and improved fibre–matrix adhesion.

Residual stresses in thermoplastic composites—A study of the literature—Part I: Formation of residual stresses

Abstract: Continuous fibre reinforced thermoplastic composites are increasingly applied in aircraft structures. These high-performance thermoplastic matrices need processing at high temperatures; therefore, thermal residual stresses arise due to the mismatch in coefficients of thermal expansion between the fibres and the thermoplastic matrix. Since residual stresses are inherently present in virtually all composite materials and influence the properties of the composite structures significantly, it is of utmost importance that the residual thermal stresses are taken into account in both design and analytic modelling of composite structures. In order to understand the effects of residual stresses and find ways to decrease their magnitude or use them to our advantage, the factors responsible for the residual stress build-up need to be understood. This first part of a literature review discusses these factors, focusing on thermoplastic composites, and in particular the material properties of their constituents and processing parameters governing stress build-up.

The effect of fiber surface treatments on the tensile and water sorption properties of polypropylene–luffa fiber composites

Abstract: The effects of coupling agents on the mechanical, morphological, and water sorption properties of luffa fiber (LF)/polypropylene(PP) composites were studied. In order to enhance the interfacial interactions between the PP matrix and the luffa fiber, three different types of coupling agents, (3-aminopropyl)-triethoxysilane (AS), 3-(trimethoxysilyl)-1-propanethiol (MS), and maleic anhydride grafted polypropylene (MAPP) were used. The PP composites containing 2–15 wt% of LF were prepared in a torque rheometer. The tensile properties of the untreated and treated composites were determined as a function of filler loading. Tensile strength and Young's modulus increased with employment of the coupling agents accompanied by a decrease in water absorption with treatment due to the better adhesion between the fiber and the matrix. The maximum improvement in the mechanical properties was obtained for the MS treated LF composites. The interfacial interactions improved the filler compatibility, mechanical properties, and water resistance of composites. The improvement in the interfacial interaction was also confirmed by the Pukanszky model. Good agreement was obtained between experimental data and the model prediction. Morphological studies demonstrated that better adhesion between the fiber and the matrix was achieved especially for the MS and AS treated LF composites. Atomic force microscope (AFM) studies also showed that the surface roughness of LFs decreased with the employment of silane-coupling agents.

Mechanisms generating residual stresses and distortion during manufacture of polymer-matrix composite structures

Abstract: The mechanisms causing residual stresses and distortions are considered, and the way they develop during the cure discussed. Experimental results are presented through the cure cycle on curvature of unsymmetric laminates, spring-in of curved sections and stresses in flat plates due to tool–part interaction. It is shown that thermal stresses after vitrification are the main cause of distortion of unsymmetric plates, whereas thermal stresses and chemical shrinkage, while the material is in the rubbery state, both contribute to spring-in. Large stresses can develop due to tool–part interaction, even before gelation.

Mechanical properties of short-flax-fibre reinforced compounds

Abstract: The mechanical properties of flax/polypropylene compounds, manufactured both with a batch kneading and an extrusion process were determined and compared with the properties of Natural fibre Mat Thermoplastic (NMT) composites. The fibre length and width distributions of the fibres from the compounds were determined and used to model the expected properties of the materials, which led to reasonable predictions of the interfacial shear stress. It was found that, given their mechanical properties, flax fibres are quite effective in improving strength and stiffness of a compound and effective compatibilisation of the fibre/matrix interphase can be easily reached. The most important factor limiting the properties of the compounds lies in the intricate structure of the fibres themselves, after the interfacial strength is optimised, the internal fibre structure becomes the weakest point.

Improvement of plant based natural fibers for toughening green composites—Effect of load application during mercerization of ramie fibers

Abstract: Effect of mercerization to tensile properties of a ramie fiber was explored. Load application technique during mercerization has been employed in order to improve mechanical properties of the fiber. A chemical treatment apparatus with tensile loading portion for applying monofilaments was newly developed. The ramie fiber was alkali-treated by 15% NaOH solution with applied loads of 0.049 and 0.098 N. The results showed that tensile strength of the treated ramie fiber was improved, 4–18% higher than that of the untreated ramie fiber, while Young’s modulus of the treated fibers decreased. It should be noted that fracture strains of the treated ramie fiber drastically increased to 0.045–0.072, that is, twice to three times higher than those of the untreated ramie fiber. It was considered that such property improvements upon mercerization were correlated with change of morphological and chemical structures in microfibrils of the fiber. Finally, the plastic deformation behavior and fracture mechanism of the mercerized fibers under tensile loading process was explained using a schematic model.

Mechanical properties of corrugated composites for candidate materials of flexible wing structures

Abstract: Corrugated-form composites are expected to be very flexible in the corrugation direction and stiff in the direction perpendicular to the corrugation. In this study, the corrugated composites manufactured from carbon fiber plain woven fabrics draw attention as a candidate material for flexible structural components, e.g. morphing wings. In-plane stiffness and strength of the original corrugated composites are evaluated through the tensile and bending tests in both in-plane longitudinal and transverse directions. A simple analytical model for the initial stiffness of the corrugated composites is developed, and the predictions are compared with the experimental results. Moreover, some improvements, installing of stiff rod and flexible rubber, are attempted for the creation of smooth aerodynamic surface and the improvement of stiffness. Mechanical properties of the modified corrugated composites are also evaluated and compared with those of the original corrugated composites. The applicability of the corrugated composites to the flexible wing structures are discussed based on the specific stiffness, longitudinal-to-transverse stiffness ratio, etc.

Predicting the elastic modulus of natural fibre reinforced thermoplastics

Abstract: Natural fibre reinforced thermoplastics (NFRT) are increasingly used in a variety of commercial applications, but there has been little theoretical modeling of structure/property relationships in these materials. In this study, micromechanical models available in the short fibre composites literature were used to predict the stiffness of some commercially important natural fibre composite formulations. Also included are equations that correct the Young’s modulus of natural fibres for changes in moisture content and density that occur as a result of processing. Hemp fibres, hardwood fibres, rice hulls, and E-glass fibres were blended into high-density polyethylene in mass fractions of 10–60-wt%. The Young’s modulus of these composites was compared to theoretical values generated by the rule of mixtures, Halpin–Tsai, Nairn’s generalized shear-lag analysis and Mendels et al. stress transfer (micromechanical) models. Based on a sum of errors squared criterion, the Halpin–Tsai equation was found to predict the experimental data most accurately for the NFRT created for this study.

Hopping conductivity in polymer matrix–metal particles composites

Abstract: Charge transport properties, such as the temperature dependent dc conductivity and the frequency dependent conductance, of polymer matrix–metal particles composites, are investigated in the present study. Dc and ac conductivity is examined with varying parameters the filler content, temperature and the frequency in the case of ac field. The examined systems, though they are characterized as dielectrics, exhibit considerable conductivity, which alters by several orders of magnitude with temperature and frequency. The temperature and frequency dependence of conductivity gives evidence for the charge carriers transport mechanism via the occurred agreement of experimental results with the employed hopping models (variable range hopping model and random free-energy barrier model).

Compressive characteristics of A356/fly ash cenosphere composites synthesized by pressure infiltration technique

Abstract: Loose beds of hollow fly ash particles (cenospheres) were pressure infiltrated with A356 alloy melt to fabricate metal-matrix syntactic foam, using applied pressure up to 275 kPa. The volume fractions of cenospheres in the composites were in the range of 20–65%. The processing variables included melt temperature, gas pressure and particles size of fly ash. The effect of these processing variables on the microstructure and compressive properties of the synthesized composites is characterized. Compressive tests performed on these metal-matrix composites containing different volume fractions of hollow fly ash particles showed that their yield stress, Young's modulus, and plateau stress increase with an increase in the density. Variations in the compressive properties of the composites in the present study were compared with other foam materials.

Influence of chemical treatments on surface properties and adhesion of flax fibre-polyester resin

Abstract: Vegetal fibres among which flax fibres are often used in reinforced composite materials have exhibited numerous advantages such as high mechanical properties, low density and biodegradability. It is well known that the mechanical performances of a composite material strongly depend on the nature and orientation of the fibres and the nature of the matrix but also on the quality of the adhesion between the two components. In order to be incorporated in composites, individual flax fibres require further chemical treatments even after long time dew retting in the field. In this study we focussed on the influence of the chemical treatment on the surface tension of flax fibres investigated by wettability measurements. The chemical composition of the modified surfaces was characterized by infrared microspectroscopy and finally on the adherence of resin microdrops was probed by a microbond test. The results obtained from classical sodium hydroxyl plus acetic anhydride based treatments and with formic acid treatment exhibits a general increase of the flax fibre/unsaturated polyester adhesion. However, no agreement was found between the calculated reversible adhesion energy, W A , and the experimentally measured interfacial shear strength. It was pointed out that the complex composite nature of flax fibres has to be taken into account to get a better understanding of the adhesion in the multiscale complex composite system built up in flax reinforced polymers.

Properties of hemp fibre reinforced concrete composites

Abstract: This research is concerned with the mechanical and physical properties of hemp fibre reinforced concrete (HFRC). An experimental program was developed based on the statistical method of fractional factors design. The variables for the experimental study were: (1) mixing method; (2) fibre content by weight; (3) aggregate size; and (4) fibre length. Their effects on the compressive and flexural performance of HFRC composites were investigated. The specific gravity and water absorption ratio of HFRC were also studied. The results indicate that the compressive and flexural properties can be modelled using a simple empirical linear expression based on statistical analysis and regression, and that hemp fibre content (by weight) is the critical factor affecting the compressive and flexural properties of HFRC.

Preparation, morphology and thermal/mechanical properties of epoxy/nanoclay composite

Abstract: A novel approach assisted with solvent was developed to disperse clay into epoxy matrix. The dispersion of clay was examined by means of optical microscopy (OM), wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). Batches of cured samples containing 1–3 wt.% silane-modified clay (SMC) were prepared and their thermal/mechanical properties were studied by dynamic mechanical analysis (DMA), tensile and fracture tests. Improvements on storage modulus, Young’s modulus and fracture toughness were achieved with incorporation of SMC clay. The fracture surfaces of the nanocomposites were imaged by scanning electron microscopy (SEM) to investigate the toughening mechanisms.

The role of fibre/matrix interactions on the dynamic mechanical properties of chemically modified banana fibre/polyester composites

Abstract: The role of fibre/matrix interactions in chemically modified banana fibre composites were investigated using dynamic mechanical analysis and compared with those of untreated fibre composites The dynamic modulus value and damping parameter, used to quantify interfacial interaction in composites were investigated with special reference to the effect of temperature and frequency Increased dynamic modulus values and low damping value show the improved interactions between the fibre and the matrix The damping peaks were found to be dependent on the nature of chemical treatment Both storage modulus and damping values measured experimentally are consistent and point to the effectiveness of silane A174 coupling agent (γ-methacryloxypropyl trimethoxy silane) for improving fibre–matrix adhesion Activation energy values for the transitions of the composites were determined from Arrhenius plots Cole–cole plots were made to evaluate the heterogeneity of the system

Heat release of polymer composites in fire

Abstract: The relationship between heat release rate and other fire reaction properties of fibre reinforced polymer composite materials is investigated. The heat release rate and fire reaction properties of thermoset matrix composites reinforced with combustible fibres (aramid, extended-chain polyethylene) or non-combustible fibres (glass, carbon) were determined over a range of heat flux levels using the oxygen consumption cone calorimeter technique. The fire reaction properties that were measured were time-to-ignition, smoke density, carbon monoxide yield, carbon dioxide yield, mass loss rate and total mass loss. It is discovered that these reaction properties (apart from ignition time) are linearly related to the heat release rate for composites containing non-combustible fibres. When the reinforcement is combustible, however, the heat release rate only appears to be related to the carbon monoxide yield, mass loss rate and (in some cases) smoke density. This study clearly shows the importance of the relationship between heat release rate with smoke density and carbon monoxide yield, the two reaction properties that influence the survival of humans in fire.

Tensile mechanical properties, morphological aspects and chemical characterization of piassava (Attalea funifera) fibers

Abstract: The tensile mechanical properties of piassava fibers, as well as their chemical composition and morphological aspects, are reported. The values obtained showed that piassava has a mechanical behavior and chemical composition comparable to that of coir fibers. The difficulties related with a reliable way of measuring the true elastic modulus of these slender fibers are discussed and a simple correction of the experimental data is presented. As main characteristic surface features piassava fibers present a well arranged pattern of silicon rich star-like protrusions. Its chemical composition reveals that piassava are lignin rich fibers, 48.4 wt%. X-ray diffraction showed that cellulose I is their main crystalline constituent. Their thermal degradation begins at 225 °C, and the whole thermal degradation behavior of piassava fibers has many aspects, like the initial water loss and the content of residues, close to that shown by pure lignin.

Thermal behavior of single-walled carbon nanotube polymer–matrix composites

Abstract: Single-walled carbon nanotube (SWNT)–poly(vinylidene fluoride) (PVDF) composites were fabricated by dispersion of SWNT in an aqueous surfactant solution, followed by mixing with PVDF powder, filtration and hot pressing. The thermal properties of the composites at various SWNT volume fraction up to 49% were investigated. The coefficient of thermal expansion (CTE) was decreased with increase of the SWNT content. The thermal conductivity increased with temperature in the temperature range from 25 to 150 °C. The thermal conductivity was enhanced, but not up to the level required by heat sink applications. The melting point was not affected significantly by the addition of SWNT, but the degree of crystallinity was increased and the decomposition temperature of the matrix was decreased. The large number of junctions among SWNT largely offsets the benefit of the high thermal conductivity of SWNT. In addition, the impurity and defects in SWNT are believed to limit the thermal conductivity of the composites. Lastly, the reduced thermal stability of the composite compared to the matrix might result from the presence of the metal catalyst contained in the SWNT.

Advanced numerical simulation of liquid composite molding for process analysis and optimization

Abstract: Composite manufacturing processes based on Liquid Composite Molding (LCM) have progressed much in recent years. Focusing on the filling and cure stages during LCM, this paper aims to sum up the main contributions in terms of process simulation and optimization that can be devised to improve the efficiency of mold virtual prototyping. Early investigations have led to successful 2D models (Fracchia CA, Castro J, Tucker III CL. A finite element/control volume simulation of resin transfer mold filling. Proceedings of the American society for composites, fourth technical conference, Lancaster, PA: Technomic Publishing Co., Inc; 1989; 157–66; Bruschke MV, Advani SG. A finite-element control volume approach to mold filling in anisotropic porous-media. Polym Compos 1990; 11(6); Trochu F, Gauvin R, Gao DM. Numerical analysis of the resin transfer molding process by the finite element method. Adv Polym Technol 1993; 12(4): 329–42.) particularly appropriate to handle thin parts. Unfortunately, these models give poor predictions of total filling time in the case of flexible injection processes such as in Vacuum Assisted Resin Infusion (VARI) or RTM-Light. Even for injections in rigid and closed molds such as in Resin Transfer Molding (RTM), thin shell approximation is not always applicable, for example in the case of multi-layer reinforcements with flow-enhancing skin, for thick parts or when the top and bottom halves of the mold are not at the same temperature. To further enrich the current 2D modeling capability, a new approach is proposed in this paper to predict thickness variations during the injection. However, as RTM molded parts tend to become thicker, several issues arise for which the numerical simulation must consider the transport phenomena along the transverse direction. A new modeling strategy is proposed to simulate 3D composite shells with multi-layer reinforcements. In full 3D analyses, the major concern is computer time. Several enhancements can be considered to speed up calculations in RTM flow simulations. The first one is connected with appropriate domain discretization. The second consists of using prismatic finite elements instead of tetrahedrons to reduce the number of degrees of freedom in 3D analyses. The third improvement concerns the different levels of coupling that can be implemented between in-plane and transverse calculations. A mesh refinement technique is combined to an extrusion algorithm to generate new non-conforming prismatic finite elements. This new element gives accurate and much faster mold filling results than standard 3D conformal finite element. In order to provide tradeoff between speed and accuracy, different levels of coupling can be considered between finite element in-plane calculations and through-thickness finite difference approximation. This flexibility will help process engineers to initiate mold design with simplified and faster analyses, while keeping the most complex and fully coupled simulations for the final optimization stage. A new optimization procedure based on void content minimization has also been devised. Based on capillary considerations a processability window can be defined to achieve optimum filling of composite parts made by resin injection.

Properties and failure mechanisms of z-pinned laminates in monotonic and cyclic tension

Abstract: The effects of through-thickness reinforcement of carbon/epoxy laminates with thin pins on the in-plane tensile properties, tensile fatigue life and failure mechanisms are investigated. Tensile studies in the 0 fibre direction are performed on unidirectional and quasiisotropic laminates reinforced with different volume contents and sizes of fibrous composite z-pins. Microstructural analysis reveals that z-pinning causes several types of damage, including out-of-plane fibre crimping, in-plane fibre distortion, mild dilution of the in-plane fibre volume fraction due to laminate swelling, and clusters of broken fibres. In unidirectional composites, resin pockets form around pins and coalesce into continuous resin channels at higher z-pin contents. Young’s modulus falls only a few percent at most, due partly to in-plane fibre dilution and partly to fibre waviness. Monotonic tensile strength is degraded more significantly, falling linearly with both pin content and pin diameter. Comparison with prior data shows that the rate of degradation is evidently a strong function of the particular pin insertion method used. Failure mechanisms include fibre rupture, presumably affected by broken fibres, and, in unidirectional laminates, longitudinal splitting cracks emanating from resin pockets. Whereas non-pinned laminates show very modest fatigue effects, the pinned laminates exhibit strong fatigue effects, with strength falling by as much as 33% at 10 6 cycles. The slope of the fatigue life (S–N) curve tends to increase in magnitude with pin content and density. Limited evidence and prior literature suggest that the dominant fatigue mechanism may be progressive softening and fibre damage in misaligned segments of in-plane fibres. 2005 Elsevier Ltd. All rights reserved.

Induction welding of thermoplastic composites—an overview

Abstract: This paper presents a comprehensive overview of the process of induction welding of thermoplastic composites. The main objective is to provide a deeper insight into the nature of the induction welding process and to summarise the investigative effort that was put into it by a large group of researchers. The main focus is put on the types of heat generation mechanisms during the induction heating process and the parameters that govern the welding process (frequency, power, pressure, residence time), as well as on the secondary phenomena that can influence the quality of the weld. An overview of the experimental procedure is also presented, with an emphasis on the experimental set-up. Finally, a brief overview of the modelling of the heat generation mechanisms and the induction welding process is presented.

Complex permittivity and microwave absorption properties of a composite dielectric absorber

Abstract: Two types of composite samples were prepared using dielectric particulates such as BaTiO3, polyaniline and conducting carbon in polyurethane matrix. One of the composite samples contains synthesized BaTiO3 and polyaniline, while the other sample using, the commercial ingredients. Structural properties of both synthesized and commercial BaTiO3 and polyaniline have been investigated. Complex permittivity ( e r ′ - j e r ″ ) and microwave absorption properties of the prepared composites were studied in X-band (8.2–13.5 GHz). An optimized composite sample with synthesized BaTiO3 and polyaniline has shown a maximum reflection loss of −25 dB (>99% power absorption) at 11.2 GHz with a bandwidth (full frequency width at half of the maximum response) of 2.7 GHz in a sample thickness of 2.5 mm. The measured absorption values have been validated by theoretical calculations. Materials can find applications in suppression of electromagnetic interference (EMI) and reduction of radar signature.

The mechanical properties of unidirectional all-polypropylene composites

Abstract: The creation of highly oriented, co-extruded polypropylene (PP) tapes allows the production of recyclable ‘all-polypropylene’ composites, with a large temperature processing window (>30 °C) and a high volume fraction of highly oriented PP (>90%). These composites show little deviation of mechanical properties with compaction temperature. This paper introduces all-polypropylene composites and reports the tensile and compressive properties of unidirectional composites. These composites show good retention of tape properties despite the relatively high temperatures used in composite manufacture.

Manufacturing carbon nanofibers toughened polyester/glass fiber composites using vacuum assisted resin transfer molding for enhancing the mode-I delamination resistance

Abstract: Polymer composite materials reinforced by continuous fibers have excellent in-plane strength but are usually weak against delamination. This paper presents an experimental study of using carbon nanofibers (CNF) to improve the interlaminar fracture properties of polyester/glass fiber composites. Surfactant-treated CNF were dispersed in polyester resin and then the CNF-resin suspension was infused to impregnate a glass fiber preform using vacuum assisted resin transfer molding (VARTM). The manufacturability of using VARTM for thick and large CNF toughened composite parts has been experimentally investigated. The influence of CNF concentration on the CNF filtration in the glass fiber preform, the resin viscosity, and the micro-void formation has been examined. By choosing appropriate manufacturing parameters, we were able to use VARTM process to infuse the surfactant-treated CNF/resin matrix into the glass fiber preform and successfully manufactured the CNF toughened polyester/glass fiber composite specimens for mode-I delamination tests. The critical energy release rates of mode-I delamination ( G IC ) were characterized for several composite specimens with 1 wt% CNF concentrations and for those with pure resin. Significant improvement in the G IC was consistently observed as 1 wt% CNF were added to toughen the polyester resin. Microscopy pictures showed that the fracture surfaces of the 1 wt% CNF toughened polyester/glass fiber composite samples were more complex than the fracture surfaces of regular polyester/glass fiber composites.

A comprehensive closed form micromechanics model for estimating the elastic modulus of nanotube-reinforced composites

Abstract: This paper describes an approximate, yet comprehensive, closed form micromechanics model for estimating the effective elastic modulus of carbon nanotube-reinforced composites. The model incorporates the typically observed nanotube curvature, the nanotube length, and both 1D and 3D random arrangement of the nanotubes. The analytical results obtained from the closed form micromechanics model for nanoscale representative volume elements and results from an equivalent finite element model for effective reinforcing modulus of the nanotube reveal that the reinforcing modulus is strongly dependent on the waviness, wherein, even a slight change in the nanotube curvature can induce a prominent change in the effective reinforcement provided. The micromechanics model is also seen to produce reasonable agreement with experimental data for the effective tensile modulus of composites reinforced with multi-walled nanotubes (MWNTs) and having different MWNT volume fractions.