Showing papers in "Composites Part A-applied Science and Manufacturing in 2001"

Thermally conducting aluminum nitride polymer-matrix composites

Abstract: Thermally conducting, but electrically insulating, polymer-matrix composites that exhibit low values of the dielectric constant and the coefficient of thermal expansion (CTE) are needed for electronic packaging. For developing such composites, this work used aluminum nitride whiskers (and/or particles) and/or silicon carbide whiskers as fillers(s) and polyvinylidene fluoride (PVDF) or epoxy as matrix. The highest thermal conductivity of 11.5 W/(m K) was attained by using PVDF, AlN whiskers and AlN particles (7 μm), such that the total filler volume fraction was 60% and the AlN whisker–particle ratio was 1:25.7. When AlN particles were used as the sole filler, the thermal conductivity was highest for the largest AlN particle size (115 μm), but the porosity increased with increasing AlN particle size. The thermal conductivity of AlN particle epoxy-matrix composite was increased by up to 97% by silane surface treatment of the particles prior to composite fabrication. The increase in thermal conductivity is due to decrease in the filler–matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 60 vol.% silane-treated AlN particles only, the thermal conductivity of epoxy-matrix composite reached 11.0 W/(m K). The dielectric constant was quite high (up to 10 at 2 MHz) for the PVDF composites. The change of the filler from AlN to SiC greatly increased the dielectric constant. Combined use of whiskers and particles in an appropriate ratio gave composites with higher thermal conductivity and low CTE than the use of whiskers alone or particles alone. However, AlN addition caused the tensile strength, modulus and ductility to decrease from the values of the neat polymer, and caused degradation after water immersion.

Investigation into the deformation of carbon nanotubes and their composites through the use of Raman spectroscopy

Abstract: The deformation micromechanics of single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) particulate nanocomposites has been studied using Raman spectroscopy. SWNTs prepared by two different methods (pulsed-laser and arc-discharge) and MWNTs have been used as reinforcement for a polymer matrix nanocomposite. The carbon nanotubes exhibit well-defined Raman peaks and Raman spectroscopy has been used to follow their deformation. SWNTs have been deformed with hydrostatic pressure in a diamond anvil pressure cell and has been found that the G′ peak position shifts to a higher wavenumber with hydrostatic compression. It has been found that for all nanocomposites samples deformed, the G′ Raman band shifts to a lower wavenumber upon application of a tensile stress indicating stress transfer from the matrix to the nanotubes and hence reinforcement by the nanotubes. The behaviour has been compared with that of high-modulus carbon fibres and has been modelled using orientation factors suggested initially by Cox. In this way it has been possible to demonstrate that the effective modulus of SWNTs dispersed in a composite could be over 1 TPa and that of the MWNTs about 0.3 TPa.

Advances in fusion bonding techniques for joining thermoplastic matrix composites: a review

Abstract: Joining composite materials is an issue because traditional joining technologies are not directly transferable to composite structures. Fusion bonding and the use of thermoplastic films as hot melt adhesives offer an alternative to mechanical fastening and thermosetting adhesive bonding. Fusion bonding technology which originated from the thermoplastic polymer industry has gain a new interest with the introduction of thermoplastic matrix composites (TPC) which are currently regarded as candidates for primary structures. The improvement of thermoplastic polymer matrices, with the introduction of recent chemistries such as PEEK, PEI and PEKEKK. exhibiting increased mechanical performance, service temperature and solvent resistance (for the semi-crystalline systems) also supported the growth of interest for fusion bonding. This review looks at the state of the art of fusion bonding technology and focuses particularly on the three most promising fusion bonding techniques: ultrasonic welding, induction welding and resistance welding. Physical mechanisms involved in the fusion bonding process for modelling purposes are discussed including heat transfer, consolidation and crystallinity aspects. Finally, the application of fusion bonding to joining dissimilar materials, namely thermosetting composites (TSC)/TPC and metal/TPC joints, is reviewed.

A smart repair system for polymer matrix composites

Abstract: The widespread use of polymer composites is still caveated with concerns about the loss in structural performance that can result from impact damage. Such events give rise to delaminations which may not be easily detectable by eye. This paper describes a technique for ‘smart’ repair of delaminations in polymer composites. This involves the filling of hollow fibres with a resin, which is released into the damaged area when the fibre is fractured. A two-part epoxy resin is used as the repair medium, the two components being diluted with solvent and infiltrated into different plies of a composite based on ‘Hollex’ S2-glass fibre. Compression strength after impact tests were used as a measure of the effectiveness of the repair technique, and a potential improvement was noted after application of heat and vacuum to the damaged composite. Resin release from the fibres was noted by microscopy. A more comprehensive study is required to verify the improvement in post-impact strength, and the use of larger internal diameter fibres would enhance the amount of resin released.

Effects of environmental conditions on mechanical and physical properties of flax fibers

Abstract: The environmental degradation behaviour of flax fibers and their mechanical properties were investigated Upgraded Duralin flax fibers, which have been treated by a novel treatment process for improved moisture and rot sensitivity, were studied Results showed that upgraded Duralin flax fibers absorbed less moisture than untreated Green flax fibers, whereas the mechanical properties of the upgraded fibers were retained with moisture absorption, if not improved In addition electrochemical studies were conducted on these fibers These data agreed well with conventional moisture absorption data Zeta (ζ)-potential measurements at different pH-levels showed differences for Duralin fibers, which can be attributed to differences in morphological features

Self-activated healing of delamination damage in woven composites

Abstract: A study of the healing of delamination damage in woven E-glass/epoxy composites is performed. With the ultimate goal of self-healing in mind, two types of healing processes are studied. In the first a catalyzed monomer is manually injected into the delamination. In the second a self-activated material is created by embedding the catalyst directly into the matrix of the composite, then manually injecting the monomer. Healing efficiencies relative to the virgin fracture toughness of up to 67% are obtained when the catalyzed monomer is injected and about 19% for the self-activated materials. Scanning electron microscopy is used to analyze the fracture surfaces and provide physical evidence of repair.

The mechanical properties of vinylester resin matrix composites reinforced with alkali-treated jute fibres

Abstract: Jute fibres were subjected to alkali treatment with 5% NaOH solution for 0, 2, 4, 6 and 8 h at 30°C. The modulus of the jute fibres improved by 12, 68 and 79% after 4, 6 and 8 h of treatment, respectively. The tenacity of the fibres improved by 46% after 6 and 8 h treatment and the % breaking strain was reduced by 23% after 8 h treatment. For 35% composites with 4 h-treated fibres, the flexural strength improved from 199.1 to 238.9 MPa by 20%, modulus improved from 11.89 to 14.69 GPa by 23% and laminar shear strength increased from 0.238 to 0.283 MPa by 19%. On plotting different values of slopes obtained from the rates of improvement of flexural strength and modulus, against NaOH treatment time, two different failure modes were apparent before and after 4 h of NaOH treatment. In the first region between 0 and 4 h, fibre pull out was predominant whereas in the second region between 6 and 8 h, transverse fracture occurred with minimum fibre pull out. This observation was well supported by the SEM investigations of the fracture surfaces.

Textile composites: modelling strategies

Abstract: Textile materials are characterised by the distinct hierarchy of structure, which should be represented by a model of textile geometry and mechanical behaviour. In spite of a profound investigation of textile materials and a number of theoretical models existing in the textile literature for different structures, a model covering all structures typical for composite reinforcements is not available. Hence the challenge addressed in the present work is to take full advantage of the hierarchical principle of textile modelling, creating a truly integrated modelling and design tool for textile composites. It allows handling of complex textile structure computations in computer time counted by minutes instead of hours of the same non-linear, non-conservative behaviour of yarns in compression and bending. The architecture of the code implementing the model corresponds to the hierarchical structure of textile materials. The model of the textile geometry serves as a base for meso-mechanical and permeability models for composites, which provide therefore simulation tools for analysis of composite processing and properties.

Analytical prediction of large mass impact damage in composite laminates

Abstract: Analytical models are suggested for prediction of impact damage initiation and growth during quasi-static response caused by large mass impactors. Comparisons with experiments are presented for different layups, geometries and boundary conditions. The critical load for delamination growth is found almost insensitive to geometry and boundary conditions. The critical energy for delamination growth is separated in bending, shear and indentation contributions. Further growth depends on the number of delaminations developing, and is in thin laminates limited by the early occurrence of penetration. Observed delamination sizes are compared with a suggested upper bound and predictions based on the observed number of delaminations.

Analyses of fabric tensile behaviour: determination of the biaxial tension–strain surfaces and their use in forming simulations

Abstract: The forming of fibre fabric reinforcements without a matrix is possible because of their very specific mechanical behaviour. The lack of some rigidities is due to possible motions between the fibres. For the fabrics used as reinforcement in the R.T.M. process and composed of warp and weft yarns made with untwisted fibres, the tension stiffness is very preponderant compared to the others. The tensile behaviour of such a fabric is biaxial, i.e. the tension-deformation states in warp or weft directions depend on the other direction because of the interweaving. It is given by the knowledge of two surfaces relating the warp and weft tensions to the two strains in these directions (or that of a single surface if the fabric is balanced). In the present paper, three complementary methods are investigated in order to determine these surfaces. A biaxial tensile device on a cross-shaped specimen is first used. 3D finite element simulations of the unit woven cell are then presented. This mesoscopic study permits to understand some phenomena at the elementary woven cell level. Finally a simplified model, which is consistent with the geometry of the plain weave woven mesh is presented. The agreement of the two last methods with experimental results is shown. From these tensile behaviour surfaces, a dynamic explicit approach for the simulations of a fabric sheet forming process is presented. The interests of the method are both its good numerical efficiency, particularly due to the direct use of the biaxial tension surfaces, and its proximity with fabric physics.

Mechanical properties of wood flake–polyethylene composites. Part I: effects of processing methods and matrix melt flow behaviour

Abstract: The structure–property relationship of wood flake–high-density polyethylene (HDPE) composites was studied in relation to the matrix agent melt flow behaviour and processing technique. The flake distribution and flake wetting were optimised to obtain acceptable mechanical properties in these composites using two processing techniques, namely twin-screw compounding and mechanical blending. The microstructure of the composites revealed that the twin-screw compounded composites based on medium melt flow index (MMFI) HDPE always achieved better flake wetting and distribution, and therefore had higher mechanical properties, than those mechanically blended composites or twin-screw compounded composites with low MFI (LMFI) HDPE. For 50:50 wt% composites the overall flake wetting, depending on processing technique and matrix flow behaviour, is ranked as compounded MMFI>compounded LMFI>blended MMFI>blended LMFI. However, the uniformity of flake distribution of the composites follows a somewhat different pattern, i.e. compounded MMFI>blended MMFI>compounded LMFI>blended LMFI. Evidence shows that the medium MFI HDPE penetrates into lumens of wood fibres in wood flakes. This phenomenon combined with flake wetting and flake distribution had a profound effect on the mechanical properties, in particular the impact strength.

Simulation of the mechanical properties of fibrous composites by the bridging micromechanics model

Abstract: The overall thermal–mechanical properties of a fibrous composite out of an elastic deformation range can be simply simulated using a recently developed micromechanics model, the Bridging Model. Only the in situ constituent fiber and matrix properties of the composite and the fiber volume fraction are required in the simulation. This general yet easy-to-implement micromechanics model is reviewed and summarized in the present paper. Application of the model to predict various properties of unidirectional laminae and multidirectional laminates, including thermoelastic behavior, elasto-plastic response, ultimate failure strength, strength at elevated temperature, and fatigue strength and S–N curve, is demonstrated. It is suggested that use of the bridging model, appropriately calibrated with experimental data, can therefore inform composite design by identifying suitable constituent materials, their contents, and their geometrical arrangements. Some technical issues regarding applications of the bridging model are also addressed.

Infiltration processing of fibre reinforced composites: governing phenomena

Abstract: All three classes of fibre reinforced composite materials (polymer, metal and ceramic matrix) may be produced by flow of liquid matrix into the open spaces left within pores of a fibre preform. Even though several specific issues arise from the nature of each composite matrix class, governing phenomena apply to all infiltration processes, and include in particular: (i) capillary phenomena, (ii) transport phenomena, and (iii) the mechanics of potential fibre preform deformation. These phenomena and their governing laws are reviewed for the case of isothermal infiltration with no phase transformations. Four basic functional quantities, which need to be known to model the processes, are identified, and addressed in turn. The paper concludes with some examples of modelling methodologies and comparison with experimental data.

High performance oxide fibers for metal and ceramic composites

Abstract: A family of oxide fibers, Nextel™ 610 Ceramic Oxide Fiber, Nextel™ 720 Ceramic Oxide Fiber and a new fiber, Nextel™ 650 Ceramic Oxide Fiber, has been developed specifically for the reinforcement of metal and ceramic matrix composites. This paper summarizes room and high temperature properties for these fibers. The strength of both single filaments and multi-filament rovings of Nextel 610, 650 and 720 fibers was determined between 25 and 250 mm gauge length. Weibull analysis was used to compare the statistical fracture distribution and gauge length dependence of strength. Fiber fracture statistics were in accord with Weibull theory; the effect of diameter variability on the statistical analysis was found to be small. Fractographic analysis on Nextel 610 fiber was used to identify primary fracture-causing defects; defect size was correlated with Griffith fracture predictions. High temperature single filament strength measurements were performed on Nextel 610, 650 and 720 fibers between 800 and 1400°C. High temperature strength varied inversely with strain rate. In combination with tensile creep tests at 1100 and 1200°C, these were used to compare the elevated temperature capability of each fiber and determine maximum use temperatures. The development of crystalline yttrium aluminum garnet fibers that demonstrate further improvements in creep performance relative to Nextel 720 fibers is also discussed.

General unit cells for micromechanical analyses of unidirectional composites

Abstract: Two typical idealised packing systems have been employed for unidirectionally fibre reinforced composites, viz. square and hexagonal ones. A systematic approach has been adopted and it involves the use of only the translational symmetry transformations. There are a number of important advantages resulting from this. The unit cells so derived are capable of accommodating fibres of irregular cross-sections and imperfections asymmetrically distributed around fibres such as microcracks and local debonding in the system, provided the regularity of the packing and imperfections is present. Furthermore, all the unit cells established can be subjected to arbitrary combinations of macroscopic stresses or strains unlike most available unit cells in the literature which can only deal with individual macroscopic stress or strain components. Boundary conditions for these unit cells have been derived from appropriate considerations of the conditions of symmetry transformations. Applications of macroscopic stresses or strains as the loads to the unit cells have been described in such a way that they can be implemented in a straightforward manner and the effective properties of the composite can be evaluated following a standard procedure.

Hygroscopic aspects of epoxy/carbon fiber composite laminates in aircraft environments

Abstract: In this study, various hygroscopic effects of such parameters as hygrothermal temperature, matrix volume ratio (Vm), void volume ratio (Vv), specimen thickness, lay-up sequence and internal stress were investigated for epoxy/carbon fiber composite laminates. The specimen thickness and lay-up sequence had little effect on the through-the-thickness water absorption behavior of composite laminates, but the other parameters affected the moisture absorption rate and equilibrium water uptake in different ways and intensities. The glass transition temperature of composite laminates was strongly affected and linearly decreased by the quantity of equilibrium water uptake. A characteristic length of moisture migration through the unidirectional laminates was proposed as a function of fiber angle to the exposed laminate surface. In this approach, the fibers imbedded in the matrix were assumed to act as a barrier to the penetrating water molecules, and the developed model was well compared with the experimental results.

A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials

Abstract: Cellulose fibres have long been used in the plastics industry as cost-cutting materials. Nowadays they are recognised as a potential replacement for glass fibres for use as reinforcing agents in composite materials. They have a number of certain advantages over glass fibres, such as low cost, high strength-to-weight ratio, biodegradability and ease of processing. In this study crystallisation from the melt of two different isotactic polypropylene matrices (iPP) in the presence of flax ( Linum usitatissinum ) fibres of four different types (green flax, dew retted flax, Duralin ® treated flax and stearic acid sized flax) was examined. The effect of processing parameters such as crystallisation temperature and cooling rate was investigated using hot stage optical microscopy. Differential scanning calorimetry (DSC) was used to investigate the inner morphology of the transcrystalline (TC) layer. Scanning electron microscopy (SEM) and X-ray diffraction were used in an attempt to identify the origin of the TC layer in connection with the structural characteristics of the fibres. The effect of transcrystallinity upon the mechanical properties of the interface was assessed using the single fibre fragmentation test. It was found that the interfacial adhesion is improved by the presence of a TC layer.

Interfacial mechanics of push-out tests: theory and experiments

Abstract: The thermo-mechanical characterization of interfaces in composite systems (PMC/MMC/IMC/CMC) is one of the challenging problems in composite mechanics and engineering. Each system has its own distinguishing features; however, in MMCs and IMCs the study is rendered more complex due to the evolving chemical species (both temporally and spatially), and the multi-axial state of residual stresses. Before MMCs or IMCs can be used in actual applications, the role of interfaces in not only the strengthening but also toughening mechanisms needs to be clearly understood. For evaluating the interfacial mechanical properties of interfaces, thin slice push-out test has emerged as the de-facto standard. Though, conceptually the testing procedure is simple, interpretation of the test results is not. It is essential to conduct very careful experiments, make precise meso- and macroscopic chemical/structural/mechanical observations and perform a thorough theoretical/numerical simulation before the test data can be used in a quantitative manner. In this paper, a comprehensive analysis of the push-out test is presented based on the theoretical/numerical and experimental research work of the authors’ group during the past few years. In this work, thin slice push-out tests were conducted primarily on Titanium Matrix Composites at various test temperatures (room and elevated) with different processing conditions (temperature and time). Different composite systems with Titanium based matrices (Ti–6Al–4V, Timetal 21S, Ti–15Nb–3Al) uniaxially reinforced with Silicon Carbide fibers (SCS-6) were chosen for the study. Effect of the evolution of interfacial chemistry and architecture (in matrix, coating and reaction zone) on both shear strength τs and frictional strength τf were studied. A novel finite element analysis based on nonlinear finite element method was implemented, in which not only the initiation but propagation of interfacial cracks are simulated. In the analysis, both shear stress and fracture energy based criteria are used to model the initiation of (closed) cracks. Quantitative values of τs, τf, GI and GII are then extracted based on the experimental data and the numerical simulation. A critical review of stress and energy based interface-modeling approaches and their applicability to various boundary value problems are made.

Nanoscale characterisation of interphase in silane treated glass fibre composites

Abstract: The properties of the interphase formed between a glass fibre and a polymer resin have been characterised based on novel techniques, including the nanoindentation and nanoscratch tests and the thermal capacity jump measurement. The variation of interphase thickness affected by differing silane coupling agents is specifically evaluated. The nanoindentation test gives an interphase thickness of approximately 1 μm with large variations between specimens, and is not sensitive enough to identify the effect of different silane agents. The effective interphase thickness measured from the nanoscratch test varies between 0.8 and 1.5 μm depending on the type and concentration of silane agent. The higher is the silane agent concentration, the larger is the interphase thickness. These values are consistent with those measured based on the heat capacity changes in terms of general trend, although the latter technique tends to present slightly higher values. The foregoing observations strongly support the usefulness of the techniques for interphase characterisation.

A homogenization procedure for the numerical analysis of woven fabric composites

Abstract: The paper presents a procedure for the numerical evaluation of the mechanical properties of woven fabric laminates. Woven fabrics usually present orthogonal interlaced yarns (warp and weft) and distribution of the fibers in the yarns and of the yarns in the composite may be considered regular. This allows us to apply the homogenization theory for periodic media both to the yarn and to the fabric. Three-dimensional finite element models are used in two steps to predict both the stiffness and the strength of woven fabric laminates. The model includes all the important parameters that influence the mechanical behavior: the lamina thickness, the yarn orientation, the fiber volume fraction and the mechanical characteristics of the components. The capabilities of the numerical model were verified studying the elastic behavior of a woven fabric laminate available in the literature and the ultimate strength of a glass fabric laminate experimentally investigated. The procedure, that can be implemented into commercial finite element codes, appears to be an efficient tool for the design of textile composites.

Asymptotic expansion homogenization for heterogeneous media: computational issues and applications

Abstract: Developments in asymptotic expansion homogenization (AEH) are overviewed in the context of engineering multi-scale problems. The problems of multi-scales presently considered are those linking continuum level descriptions at two different length scales. Concurrent research in the literature is first described. A recipe of the AEH approach is then presented that can be used for future developments in many areas of material and geometric non-linear continuum mechanics. Then, a derivation is outlined using the finite element method that is useful for engineering applications that leads to coupled hierarchical partial differential equations in elasticity. The approach provides causal relationships between macro and micro scales wherein procedures for homogenization of properties and localization of small-scale response are built-in. A brief discussion of a physical paradox is introduced in the estimation of micro-stresses that tends to be a barrier in the understanding of the method. Computational issues are highlighted and illustrative applications in linear elasticity are then presented for composites containing microstructures with complex geometries.

Challenges for composites into the next millennium : a reinforcement perspective

Abstract: This paper summarises the current level of technology within the manufacturing processes of filament winding, fibre placement, pultrusion and advanced textile preforming. It also examines the current problems within each of these manufacturing techniques and the areas of predicted future development.

Fracture and impact behaviours of hollow micro-sphere/epoxy resin composites

Abstract: Fracture and impact behaviours of hollow-glass micro-sphere/epoxy resin composites are studied in terms of fracture toughness, fractography, flexural properties and impact force. Volume fraction of micro-spheres for the composites was varied up to 0.65. The addition of micro-spheres did not enhance the specific fracture toughness of the composites despite the presence of a pinning mechanism at relatively low volume fractions. Performance in reducing the impact force was enhanced as the content of micro-spheres increased, but at the expense of other properties such as specific fracture toughness and specific flexural strength, while specific flexural modulus marginally increased at some high volume fractions of micro-spheres.

Fabrication process and electrical behavior of novel pressure-sensitive composites

Abstract: A novel route was developed to fabricate a new pressure-sensitive composite by dispersing homogeneously conductive carbon particles in an insulating silicone rubber matrix. The composites showed a gradual change in electrical resistivity with applied pressure within percolation threshold region at a constant temperature. This type of gradual fall of resistivity with applied pressure is very important to fabricate pressure sensors. Various amounts of carbon particles were dispersed in a rubber matrix to understand the effect of volume fraction of conductive filler with applying external pressure on resistivity. A quantitative general effective media (GEM) theory was used to understand the resistivity of carbon–rubber composites system over a large range of volume fraction of carbon with applied pressure. The use of two different sizes of silicon rubber particles showed a significant effect in gradual fall of resistivity with applied pressure in the narrow range of percolation threshold. However, a large variation in resistivity from 1st measuring to 10th measuring was observed. A significant improvement in successive measuring of resistivity variation from 1st measuring to 10th measuring was observed when composites were fabricated in hexane solvent media. Finally, nano-sized Al2O3 was dispersed to control the resistivity variation upon successive measurement and to improve the mechanical properties of the composites. The material was suggested to use as unique materials as pressure sensors in practical applications mainly for robots.

Sizing up the interphase: An insider's guide to the science of sizing

Abstract: Sizing is a surface coating of organic materials applied to nearly all types of man-made fibres during their manufacture. In the case of glass fibres, sizing is probably the key component influencing the success or failure of most reinforcement products. This is due to the major role played by the sizing in the price, processiblity, and performance of that product. Due to its physical location on the fibre surface, sizing is also a critical component in the formation and properties of the fibre-matrix interphase. Therefore, any attempt to understand the science of the composite interphase must encompass an understanding of the science of sizing. In this paper we will review the role of sizings from fibre manufacture through to performance of composite parts. The review is illustrated by practical examples of sizing development and results from more fundamental studies of sizing application and absorption.

Effects of plasma oxidation on the surface and interfacial properties of ultra-high modulus carbon fibres

Abstract: An ultra-high modulus (UHM) carbon fibre was submitted to an oxygen plasma treatment. The effects of this treatment on the physical and chemical properties of the carbon surfaces were investigated by using surface characterisation techniques. SEM and STM studies were performed in order to determine the changes in the surface morphology. Observations on the nanometre scale lead to the conclusion that the plasma oxidation “cleaned” the original surfaces of carbonaceous impurities. XPS analysis of the treated fibres revealed a very significant increase of oxygen content. Single-fibre epoxy composites were prepared from as-received and plasma-treated fibres, and fragmentation tests were performed in order to characterise fibre/matrix interfacial adhesion. Raman spectroscopy has been used to map the strain along the fibre during tensile loading of the matrix, and the distribution of interfacial shear stress has been obtained. The quality of the interface improved dramatically after the surface treatment, supporting the ability of cold plasma oxidation to enhance the adhesion of UHM carbon to epoxy matrices. It is concluded that the increase of the oxygen surface content and the removing of the outermost layers may contribute in a co-operative way to the improvement on fibre/matrix adhesion.

Natural fibre reinforced sheet moulding compound

Abstract: This paper describes a newly developed system, which enables utilisation of short flax fibres for SMC (Sheet Moulding Compound) production. It is shown that by using an evenly distributed layer of short dried flax fibres, after controlled impregnation and maturation, a homogeneous flow of the prepreg in the mould is obtained, and accordingly a flax fibre reinforced SMC can be produced. Mechanical data indicates that for applications designed with respect to stiffness, flax fibre reinforced SMC materials compete with glass fibre SMC, especially when the fibre length exceeds 25 mm.

Analysis of vacuum bag resin transfer molding process

Abstract: An analytical model is developed to analyze the resin flow through a deformable fiber preform during vacuum bag resin transfer molding (VBRTM) process. The force balance between the resin and the fiber preform is used to account for the swelling of fiber preform inside a flexible vacuum bag. Mold filling through multiple resin inlets is analyzed under different vacuum conditions. The formation of dry spots is demonstrated in the presence of residual air. Molding of a three-dimensional ship hull with lateral and longitudinal stiffeners is simulated to demonstrate the applicability of the model.

Interfacial properties of polymer composites measured by push-out and fragmentation tests

Abstract: The interfacial properties for E-glass/epoxy composites were measured using push-out tests and single fiber fragmentation tests. Theoretical models for both stress-based and energy-based criteria were used to interpret the experimental results. Fibers treated with γ-aminopropyl-triethoxysilane (γ-APS) showed higher bond strength (∼1.7 times higher) and interfacial toughness (∼1.9 times higher) than those of unsized E-glass based composites. However, the average interfacial toughness obtained from fragmentation tests was about six times higher than that obtained from push-out tests. Considering the analytical frameworks employed to interpret the values measured in the present work, the fragmentation test is a more appropriate method to obtain interfacial energy for polymeric composites, but both methods are appropriate for relative measurements of interface strength.

Modelling fabric reinforced composites under impact loads

Abstract: The paper describes recent progress on the materials modelling and numerical simulation of the in-plane response of fibre reinforced composite structures. A continuum damage mechanics model for fabric reinforced composites under in-plane loads is presented. It is based on methods developed for UD ply materials (Compos. Sci. Technol., 43 (1992) 257), which are generalised here to fabric reinforcements. The model contains elastic damage in the fibre directions, with an elastic–plastic model for inelastic shear effects. Test data on a glass fabric/epoxy laminate show the importance of inelastic effects in shear. A strategy is described for determining model parameters from the test data. The fabric model is being implemented in an explicit FE code for use in crash and impact studies and preliminary results are presented on a plate impact simulation.