Showing papers in "Composites Part A-applied Science and Manufacturing in 1999"

Microwave processing: fundamentals and applications

Abstract: In microwave processing, energy is supplied by an electromagnetic field directly to the material. This results in rapid heating throughout the material thickness with reduced thermal gradients. Volumetric heating can also reduce processing times and save energy. The microwave field and the dielectric response of a material govern its ability to heat with microwave energy. A knowledge of electromagnetic theory and dielectric response is essential to optimize the processing of materials through microwave heating. The fundamentals of electromagnetic theory, dielectric response, and applications of microwave heating to materials processing, especially fiber composites, are reviewed in this article.

Review of applications for advanced three-dimensional fibre textile composites

Abstract: Current and future potential applications for three-dimensional (3D) fibre reinforced polymer composites made by the textile processes of weaving, braiding, stitching and knitting are reviewed. 3D textile composites have a vast range of properties that are superior to traditional 2D laminates, however to date these properties have not been exploited for many applications. The scientific, technical and economic issues impeding the more widespread use of 3D textile composites are identified. Structures that have been made to demonstrate the possible uses of 3D composites are described, and these include applications in aircraft, marine craft, automobiles, civil infrastructure and medical prosthesis.

Some mechanical properties of untreated jute fabric-reinforced polyester composites

Abstract: This research work is concerned with the evaluation of the mechanical properties—modulus, Poisson's ratio and strength—of woven jute fabric-reinforced composites. The specimens are prepared using hand lay-up techniques as per the ASTM standard. This is the first report by any single group of researchers in which tensile strength, compressive strength, flexural strength, impact strength, inplane shear strength, interlaminar shear strength and hardness are given. This work being an experimental study on untreated (`as received' jute fabric) woven jute fabric-reinforced polyester composites, demonstrates the potential of this renewable source of natural fibre for use in a number of consumable goods.

Potential application of ceramic matrix composites to aero-engine components

Abstract: The present paper describes the potential application of ceramic matrix composites to aero-engine components by reviewing the related published papers and our experience in this field. It contains the material requirements for aero-engines, trends in aero-engine materials use, Japanese projects related to ceramic matrix composites (CMCs) and potential application of CMCs to aero-engines, such as combustors, nozzle flaps, bladed disks and others. From the point of application to aero-engines, the remaining research and development issues are discussed to some extent. Material developments, particularly of the interface and fibers for high temperature, are still required and stressed.

Seawater immersion ageing of glass-fibre reinforced polymer laminates for marine applications

Abstract: Four glass-fibre reinforced polymer (GRP) materials along with castings of their matrix resins have been aged in seawater in the laboratory, some loaded under set-strain. Water uptake behaviour has been compared for the polyester, phenolic and vinylester GRPs and neat resin castings, and losses in mechanical properties determined for the GRPs. The phenolic GRP displayed anomalous uptake behaviour considered to relate to both interface and matrix absorption. Water uptake by the polyester and vinylester laminates appeared to be affected initially by suppression from fibre barrier effects then later by enhancement from interface effects. Flexural strength fell by 15–21% for the water saturated polyester and vinylester GRPs, and by 25% for the phenolic GRP. Loading at 20% of ultimate strain while under immersion exacerbated only the phenolic laminate degradation, advancing the loss in strength to 36%. Interlaminar shear strengths fell by between 12 and 21% for the GRPs at close to saturation.

Vapor grown carbon fiber composites with epoxy and poly(phenylene sulfide) matrices

Abstract: Vapor grown carbon fibers (VGCF, Pyrograf III™ from Applied Sciences, Inc.), with 100–300 nm diameters and ∽10–100 μm lengths, were formulated in various fiber volume fractions into epoxy (thermoset) and into poly(phenylene sulfide) (thermoplastic) composites. Increases in stiffness were observed as with previous VGCF/organic matrix composites. Large increases in flexural strengths were achieved in both systems demonstrating for the first time that discontinuous randomly oriented Pyrograf III™ can give strength increases and has substantial potential as a reinforcement in composites. Here-to-fore, addition of VGCF caused strength decreases. Voids, residual thermal strains (as the fiber surface area is ∽35 times greater than 7 μm-diameter PAN fiber), or uncertainties about fiber strength, fiber–matrix bonding and the degree of fiber dispersion, could cause losses of strength. Thermal conductivity properties of VGCF/ABS (acrylonitrile–butadiene–styrene from GE Plastics) and VGCF/epoxy composites with various fiber volume fractions were measured. Thermal conductivity increased with an increase in fiber volume fraction. However, these increases were not significant enough to make these VGCF fiber/organic matrix composites candidates for thermally conductive materials.

Tensile properties and failure mechanisms of 3D woven GRP composites

Abstract: Tensile tests were performed on glass reinforced polymer (GRP) composites with three-dimensional (3D) orthogonal, normal layered interlock, and offset layered interlock woven fibre architectures. The mechanical properties and failure mechanisms under tensile loading were similar for the three composites. Cracks formed at low strains within the resin-rich channels between the fibre tows and around the through-thickness binder yarns in the composites, although this damage did not alter the tensile properties. At higher applied tensile stresses the elastic modulus was reduced by 20–30% due to inelastic tow straightening and cracking around the most heavily crimped in-plane tows. Further softening occurred at higher strains by inelastic straightening of all the tows. Composite failure occurred within a localised region and the discrete tow rupture events that have caused tow lock-up and pullout mechanisms in other 3D woven composites were not observed.

The use of neural networks for the prediction of fatigue lives of composite materials

Abstract: Constant-stress fatigue data for five carbon-fibre-reinforced plastics and one glass-reinforced plastic laminate have been used to evaluate possible artificial neural network architectures for the prediction of fatigue lives and to develop network training methods. It has been found that artificial neural networks can be trained to model constant-stress fatigue behaviour at least as well as other current life-prediction methods and can provide accurate (and conservative) representations of the stress/R-ratio/median-life surfaces for carbon-fibre composites from quite small experimental data-bases. Although their predictive ability for minimum life is less satisfactory than that for median life, and is non-conservative, the procedures developed in this work could nevertheless be used in design with little further modification. Some success has been achieved in modelling fatigue under block-loading conditions, but this problem is more difficult and requires much more effort before ANNs could be used with confidence for variable-stress conditions.

Mode I interlaminar fracture toughness properties of advanced textile fibreglass composites

Abstract: The Mode I interlaminar fracture toughness properties of vinyl ester-based composites reinforced with fibreglass manufactured by the advanced textile technologies of braiding, knitting, stitching and through-the-thickness weaving are assessed in comparison to a variety of traditional composites made from fibreglass such as unidirectional or woven rovings. The interlaminar fracture toughness ( G Ic ) of braided and knitted composites are higher than traditional composites by factors of more than two and four, respectively. Toughening in these textile composites was caused by extensive crack branching as the interlaminar crack was forced to follow a tortuous path through the complex fibre architectures. The G Ic values of the composites reinforced in the through-thickness direction by weaving or stitching were higher than traditional composites by factors of nearly two and three, respectively, with the main toughening mechanism being crack bridging by the through-thickness binder yarns/stitches. A review of Mode I interlaminar fracture data collected from papers shows that advanced textile techniques are capable of manufacturing composites with substantially improved delamination resistance.

Mechanical behaviour of cellular core for structural sandwich panels

Abstract: This paper deals with the analysis of the mechanical properties of the core materials for sandwich panels. In this work, the core is firstly a honeycomb and secondly tubular structure. This kind of core materials are extensively used, notably in automotive construction (structural components, load floors...). For this study, three approaches are developed: a finite element analysis, an analytical study and experimental tests. Structural members made up of two stiffs, strong skins separated by a lightweight core (foam, honeycomb, tube...) are known as sandwich panels. The separation of the skins by the core increases the inertia of the sandwich panel, the flexure and shear stiffness. This increase is obtained with a little increase in weight, producing an efficient structure to resist bending and buckling loads. A new analytical method to analyse sandwich panels core will be presented. These approaches (theoretical and experimental) are used to determine elastic properties and ultimate stress. A parameter study is carried out to determine elastic properties as a function of geometrical and mechanical characteristics of basic material. Both theoretical and experimental results are discussed and a good correlation between them is obtained.

Functional composites for sensors, actuators and transducers

Abstract: Following the trend in structural applications, composite structures are being used more commonly in transducer applications to improve acoustic, mechanical and electrical performance of piezoelectric devices. Functional composite transducers for actuators and sensors generally consist of an active ceramic phase incorporated with a passive polymer phase, each of which has a phase transition associated with it. In this paper, several polymer–piezoelectric ceramic composite transducers, mostly designed for sensing hydrostatic waves, are discussed based on the connectivity of the constituent phases. Also discussed are some recent examples of metal–ceramic composites, and single element ceramic transducers with modified shapes for improved performance. A comparison of these designs is given based on their hydrophone figure of merit ( d h · g h ).

Radial flow permeability measurement. Part A: Theory

Abstract: This paper deals with permeability measurement in the context of Resin Transfer Moulding (RTM). A new approach to two-dimensional radial flow permeability measurement with constant inlet pressure is proposed. It allows principal permeability to be measured even if the experimental axes are not aligned with the principal direction. This part of the paper looks at the underlying theory of the new approach while part B reports on validation experiments. Formulae are derived which allow to calculate principal permeability and the orientation of the principal axes from flow front measurements in three directions. Transient effects of the developing flow front caused by the circular inlet are discussed and its influence on the measured permeability is illustrated. Numerical studies are performed which show that the shape of the flow front is dependent only on the size of the inlet diameter and the degree of anisotropy. This leads to the development of a formula for estimating the minimum required mould size for permeability measurement.

Experimental study on clamping effects on the tensile strength of composite plates with a bolt-filled hole

Abstract: An experimental study was performed to assess the effects of clamp-up on the net-tension failure of laminated composite plates with bolt-filled holes. Graphite/epoxy prepreg of T800/3900-2 was selected for fabricating the laminates for the tests. The tensile strength and failure response of specimens with an open hole and a bolt-filled hole were evaluated. Both 100% bypass load (no bolt bearing load) and no bypass load (100% bolt bearing load) were considered during the experiments. X-radiographs were taken for specimens after pre-loading at different stress levels for the purpose of characterizing the failure modes and damage progression inside the composite. Experimental results showed that the bolt clamping force can significantly reduce the notch tensile strength of composite laminates which are prone to fiber-matrix splitting and delamination. A reduction in failure load of up to 20% was observed. Higher clamping pressure resulted in higher reductions of notch strength. However, for bolted joints which failed in a net-tension mode, clamping improved the joint strength regardless of the ply orientation.

The draping of woven fabric preforms and prepregs for production of polymer composite components

Abstract: Drape trials were conducted on a simple shape and a number of aircraft parts. Predictions from computer drape simulation using the pin-jointed net model were compared with draping test results. The pin-jointed net model was evaluated. Yarn slippage, and how it is affected by the material properties and part geometry, were investigated. The criteria for optimal draping, and the means of manipulation of the drape simulation are discussed. The different methods of draping constraint are compared and their most useful applications are indicated. Some useful guidance for the application of drape simulation is given.

Influence of surface treatment of glass fibres on the dynamic mechanical properties of epoxy resin composites

Abstract: In this study the influence of the interface of differently treated glass fibres in epoxy resin composites is investigated by dynamic mechanical analysis. These results are compared with mechanical measurements of adhesion. It is found that the flexural storage modulus E′ of the composites with improved interfacial bonding is increased over the whole investigated temperature range. Decrease of the magnitude of tan δ at the α-relaxation associates with an improvement of interfacial bonding too. The behaviour differences can only be attributed to interfacial phenomena because all other parameters were kept constant. The dynamic mechanical analysis is an additional possibility for quantifying interfacial interactions.

The effect of fibre content on the mechanical properties of glass fibre mat/polypropylene composites

Abstract: Glass fibre mat was prepared by the fibre mat-manufacturing machine developed in our laboratory. Glass fibre mat reinforced polypropylene (PP) composites were fabricated with the variation of glass fibre content. Tensile, flexural and high rate impact test was conducted to investigate the effect of glass fibre content on the mechanical properties of the glass fibre mat/PP composite. Deformation and fracture behaviour of the glass fibre mat/PP composites was investigated to study the relationship with the mechanical property data. The tensile and flexural modulus increased with the increment of glass fibre content. However, the tensile and flexural strengths exhibited maximum values and showed a decrease at the higher glass fibre content than this point. The impact absorption energy also exhibited a similar result with the tensile and flexural property data.

Optical coherence tomography of glass reinforced polymer composites

Abstract: Optical coherence tomography (OCT) is a nondestructive and noncontact technique to image microstructure within scattering media. The application of OCT to highly scattering materials such as polymer composites is especially challenging. In this work, OCT is evaluated as a technique to image fiber tows and voids in two materials: an epoxy E-glass-reinforced composite and a vinyl-ester E-glass-reinforced composite. Features detected using OCT are compared with optical microscopy. Fiber architecture and voids of glass-reinforced polymer composites can be successfully imaged using OCT. The quality of the OCT image is strongly affected by the refractive index mismatch between the fibers and reinforcement. The largest sources of noise in the images arise from fiber lens effects, interference from within the sample, and a very large reflection at the surface.

Characterization of nanoscale property variations in polymer composite systems: 1. Experimental results

Abstract: A technique utilizing the indenting capabilities of the atomic force microscope is used to evaluate local changes in the response of polymer composite systems near the fiber-matrix interface. Room temperature and elevated temperature indentation response is measured for several model composite systems. Results of indentation studies are compared to finite element model predictions to understand the influence of interphase properties on the measured responses. For sized fiber systems, unexpected property variations are observed, leading to the discovery of a possible interphase formation mechanism in these systems. q 1998 Published by Elsevier Science Ltd. All rights reserved.

Micro/minicomposites: a useful approach to the design and development of non-oxide CMCs

Abstract: Micro (one single filament) and mini (one single fiber tow) non-oxide composites (C/C; C/SiC and SiC/SiC) with simple (PyC or BN) or complex interphases [C (B) or (PyC-SiC)n multilayers] are fabricated in a short time by CVD/CVI. The fiber/matrix interfacial zone is characterized by AES and TEM. Tensile tests are used to assess the mechanical properties and the Weibull statistical parameters of both the fiber and matrix, as well as the fiber–matrix interfacial parameters (τi; ld; Gic). The tensile stress–strain behavior has been modelled. The tensile curves exhibit the same features as those previously reported for real nD-composites. Lifetime at high temperatures in air is characterized through static/cyclic fatigue tests and modelled. It is improved by replacing conventional pyrocarbon by highly engineered interphases. The micro/mini composite approach is used in the optimization of processing conditions and to derive parameters necessary for the modelling of the thermomechanical and chemical behavior of composites with more complex fiber architectures.

Investigation on diffusion bonding characteristics of SiC particulate reinforced aluminium metal matrix composites (Al/SiCp-MMC)

Abstract: Joining characteristics of SiC particulate reinforced aluminium metal matrix composites (Al/SiCp-MMC) were investigated by vacuum diffusion bonding process. The joining performances of the similar and dissimilar composites were studied, and the influences of SiCp volume percentage and the insert alloy layer on bonding quality and properties of the bonded joints were also estimated. The experimental results indicate that the strength of vacuum diffusion bonded joints decreases with increasing SiCp volume percentage, and obtaining satisfactory bonding quality in the diffusion bonded joints of the dissimilar Al/SiCp-MMC is much more difficult than that of the similar Al/SiCp-MMC. Moreover, the results still manifest that the diffusion bonding either for the similar or for the dissimilar Al/SiCp-MMC, the suitable insert alloy layer can improve evidently the joining quality of joints, and the strength of diffusion bonded joints corresponding to using the insert alloy layer is apparently higher than that of no insert layer.

Recent developments in fibers and interphases for high temperature ceramic matrix composites

Abstract: The long term durability of CMCs is limited by two factors: (1) creep and rupture of the fibers, and (2) environmental degradation of the constituents, primarily in the nonoxide composites. Oxide CMCs are limited by the creep resistance of the fiber at this stage of development and by interphase concepts that are not yet mature. Nonoxide fibers have been developed with excellent creep resistance relative to oxide fibers, but oxidation of the interphase and the interface, particularly at intermediate temperatures, causes embrittlement of these composites. This effect is particularly severe when matrix cracks are present and under cyclic loading conditions.

Analysis of oxidation-assisted stress-rupture of continuous fiber-reinforced ceramic matrix composites at intermediate temperatures

Abstract: A model is presented to estimate the reliability and time-to-failure of an unidirectional continuous fiber-reinforced ceramic composite when subjected to stresses beyond the matrix cracking stress. The particular case of oxidation-assisted stress-rupture at intermediate temperatures is considered. The effects of stress and temperature on the reliability of the model composite are examined. Model predictions are presented for the specific case of CG-Nicalon™/SiC CFCCs with carbonaceous fiber coatings.

Effect of off-axis ply orientation on 0°-fibre microbuckling

Abstract: The aim of the present work is to study both experimentally and theoretically the compression failure mechanisms in multi-directional composite laminates, and especially the effect of the off-axis ply orientation on fibre microbuckling in the 0°-plies. The critical mechanism in the compressive fracture of unidirectional polymer matrix composites is plastic microbuckling/kinking. In multi-directional composites with internal 0°-plies, catastrophic failure also initiates by kinking of 0°-plies at the free-edges or manufacturing defects, followed by delamination. When 0°-plies are located at the outside, or in the case of cross-ply laminates, failure rather tends to occur by out-of-plane buckling of the 0°-plies. T800/924C carbon-fibre–epoxy laminates with a [(±θ/02)2]s lay-up are used here to study the effect of the supporting ply angle θ on the stress initiation of 0°-fibre microbuckling. Experimental data on the compressive strength of laminates with θ equal to 30, 45, 60 or 75° are compared to theoretical predictions obtained from a fibre kinking model that incorporates interlaminar shear stresses developed at the free edges at (0/θ) interfaces. Initial misalignment of the fibres and non-linear shear behaviour of the matrix are also included in the analysis.

High-temperature oxidation behavior of SiC-coated carbon fiber-reinforced carbon matrix composites

Abstract: The high-temperature oxidation behavior of bare and SiC-coated carbon fiber-reinforced carbon matrix (C/C) composites was examined in the temperature range 900–3000 K. In the course of the measurements, the main focus was placed upon the effect of oxygen partial pressure on the oxidation behavior and the transition temperature between the passive and active oxidation regimes. To understand the oxidation behavior of SiC-coated C/C composites quantitatively, a morphological characterization of coating cracks was carried out, and then, in the oxygen diffusion controlling temperature range, an analytical model was developed for the prediction of weight loss due to oxidation of SiC-coated C/C composites. The oxidation rates derived from this model were in fairly good agreement with the experiment results.

Curing simulation of thermoset composites

Abstract: This paper deals with the modelling and simulation of resin flow, heat transfer and the curing of multilayer thermoset composite laminates during processing in an autoclave. Darcy's Law and Stokes’ slow-flow equations are used for the flow model and, for approximately isothermal flows, a similarity solution is developed. This permits the decoupling of the velocity and thermal fields. A two-dimensional convection–diffusion heat equation with an internal heat generation term is then solved numerically, together with the equation for the rate of cure, using a finite difference scheme on a moving grid. The simulations are performed with varying composite thicknesses, and a comparison of numerical results with known experimental data confirms the approximate validity of the model.

The early history of the resin transfer moulding process for aerospace applications.

Abstract: The resin transfer moulding (RTM) process has been the subject of a great deal of practical and theoretical development for aerospace applications since the early 1980s. This article looks at the very early developments of RTM in an aerospace setting. This development took place over a few years at the start of the 1950s. By 1956 almost all the features of RTM for aerospace applications had been introduced in a series of six patents. This achievement was made without any of the theoretical infrastructure now considered critical and was the work of a small group within a single company. The developed technology dropped from view in the general aerospace composites community and had to be redeveloped 25 years after the last patent was applied for.

Weak link scaling analysis of high-strength carbon fibre

Abstract: The strengths of polyacrylonitrile-based carbon fibres were obtained for lengths ranging from 1 to 500 mm. These were analysed according to the two-parameter Weibull distribution and the accuracy of using weak-link scaling was assessed. Extrapolation using weak link scaling over the orders of magnitude required for models of composite strength was found to give large errors, highlighting the inadequacies of the two parameter Weibull expression for modelling purposes.

Fatigue life prediction for GRP subjected to variable amplitude loading

Abstract: A method of predicting lifetime to failure for any glass fibre reinforced composite system subjected to a complex load–time history has been developed. The prediction first requires the generation of a model to characterize the general fatigue response of the particular composite system. Once the models are derived they can be used to predict lifetimes to failure for any load–time history using a modified Miner’s damage summation rule and rainflow analysis. Variable amplitude fatigue testing of different GRP materials, using the same load–time history, allowed a comparison to be made between predicted and actual lifetimes to failure and was useful in verifying the accuracy and robustness of the life prediction methodology employed. The results from the life prediction models for three very different GRP systems showed themselves to be accurate predictors of fatigue behaviour for the variable amplitude loading condition investigated.

An empirical fatigue-life model for high-performance fibre composites with and without impact damage

Abstract: This article follows earlier work on the development of a life-prediction method for carbon-fibre/epoxy laminates. For comparison with the behaviour of a number of different CFRP laminates already studied, further constant-life fatigue data have now been obtained for a further CFRP composite and a GRP laminate of similar construction – a 16-ply [(±45,0 2 ) 2 ] S lay-up. Fatigue tests have been carried out on these materials in both the virgin condition and after damage by low-velocity impacts. Following analysis of these new data and a re-examination of the older data base, the constant-life model has been appropriately modified. It now offers a prediction procedure for the fatigue response of composite materials in the virgin and impact-damaged conditions which requires, in the first instance, only the tensile and compressive strengths of the composite in question. The model is equally applicable to both GRP and CFRP, despite the fact that the fatigue response of a GRP laminate is different from that of an equivalent CFRP material.

Experimental and theoretical study on the strain rate and temperature dependence of mechanical behaviour of Kevlar fibre

Abstract: The tensile impact experiments on Kevlar 49 fibre bundles were carried out at strain rates 140, 440, 1350 s −1 and at temperatures −60, −20, 15, 50 and 90°C. It was found that the tensile mechanical properties of Kevlar 49 fibre bundles depend both upon the strain rate and the temperature. A bimodal Weibull distribution statistical model of the strain rate and temperature dependence of fibres, and a test method of determining mechanical properties and Weibull parameters of fibres from the fibre bundle test were adopted to characterize the combined effects of strain rate and temperature on Kevlar 49 fibre strength distribution. The simulated results are in good agreement with the experimental data, which proves that the bimodal Weibull distribution function is suitable to represent the statistical strength distribution of Kevlar 49 fibre which has peculiar ‘skin–core’ physical structure, and that the test method is valid and reliable.