One-, two- and three-dimensional transient heat transfer finite element models are developed to simulate the resistance welding process of pre-consolidated unidirectional AS4 carbon fibre reinforced Poly-ether-ether-ketone (APC-2/AS4) laminates with a metal mesh heating element, in a lap-shear configuration. The finite element models are used to investigate the effect of process and material parameters on the thermal behaviour of the coupon size welds, yielding to a better understanding of the process. The 1-D model determines: (a) the importance of including the latent heat of PEEK, and (b) the through-thickness temperature gradient away from the edges, for different tooling plate materials. The 2-D model simulates the cross-section of the process, considering the convective and irradiative heat losses from the areas of the heating element exposed to air. The 3-D model includes the heat conduction along the length of the laminates, to fully depict the thermal behaviour of the welds. Finally, the models are compared with experimental data.

Thesis (M.Tech.: Mechanical Engineering)-Dept. of Mechanical Engineering, Durban University of Technology, 2007 xiii, 101 leaves / Polymer nanocomposites may be defined as structures that are formed by infusing layered-silicate clay into a thermosetting orthermoplastic polymer matrix. The nanocomposites are normally particle-filled polymers for which at least one dimension of the dispersed particles is in nanoscale. These clay-polymer nanocomposites have thus attracted great interest in industry and academia due to their exhibition of remarkable enhancements in material properties when compared to the virgin polymer or conventional micro and macro-composites. The present work describes the synthesis, mechanical properties and morphology of nano-phased polypropylene structures. The structures were manufactured by melt- blending low weight percentages of montmorillonite (MMT) nanoclays (0.5, 1, 2, 3, 5 wt. %) and polypropylene (PP) thermoplastic. Both virgin and infused polypropylene structures were then subjected to quasi-static tensile tests, flexural tests, micro-hardness tests, impact testing, compression testing, fracture toughness analysis, dynamic mechanical analysis, tribological testing. Scanning electron microscopy studies were then conducted to analyse the fracture surfaces of pristine PP and PP nanocomposite. X-ray diffraction studies were performed on closite 15A clay and polypropylene composites containing 0.5, 1, 2, 3 and 5 wt. % closite 15A nanoclay to confirm the formation of nanocomposites on the addition of organo clays. Transmission electron miscopy studies were then performed on the PP nanocomposites to determine the formation of intercalated, exfoliated or agglomerated nanoclay structures. Analysis of test data show that the mechanical properties increase with an increase in nanoclay loading up to a threshold of 2 wt. %, thereafter the material properties degrade. At low weight nanoclay loadings the enhancement of properties is attributed to the lower percolation points created by the high aspect ratio nanoclays. The increase in properties may also be attributed to the formation of intercalated and exfoliated nanocomposite structures formed at these loadings of clay. At higher weight loading, degradation in mechanical properties may be attributed to the formation of agglomerated clay tactoids. Results of XRD, transmission electron microscopy studies and scanning electron microscopy studies of the fractured surface of tensile specimens verify these hypotheses.

Low profile additives (LPA) are thermoplastics that are incorporated to unsaturated polyester (UP) resins in order to improve the surface finish of UP/fibreglass composites, widely used in automotive applications. The effect of using LPA on the thermo-mechanical properties of resin transfer moulded UP/fibreglass composites is investigated. The flexural and shear properties are measured by three-point bending tests. The trend of these mechanical properties is identified for 0% to 40% LPA content. All the mechanical properties like flexural strength, flexural modulus and short beam strength reduce upon addition of LPA. The specimens fail by tension in the flexural test and show a mixed shear/tension failure mode in case of short beam tests. From scanning electron microscopy, morphological change of the fractured surface is observed with an LPA-rich phase. Glass transition temperature (Tg) measured by thermal mechanical analysis (TMA) and dynamic mechanical analysis (DMA) show reproducible data and compare well with each other. Tg is improved by LPA addition due to the development of a more compatible system compared to neat resin. Differential scanning calorimetry (DSC) is also performed to detect Tg, which gives unreliable results.

This study is dedicated to thermoplastic composite joints obtained by an electrical resistance welding procedure. This welding process consists in joining two substrates with an electrical resistor which acts as a heating element melting the polymer substrates. The substrates considered herein are 2mm thick 7-layer hybrid composites, with the following stacking sequence ([0°/90°]G, [0°/90°]C, [45°]C, [0°/90°]C, [45°]C, [0°/90°]C, [0°/90°]G), where G and C denote plies with PPS matrix reinforced by continuous glass or carbon bres, respectively. The heating element is a stainless metallic grid surrounded by two PPS amorphous lms. For a better understanding of the the time evolution of the temperature eld in the welded zone, a heat transfer model was developed in nite element code Abaqus®. The prediction capabilities of the numerical tool were validated by comparing the numerical results with thermocouple measurements. The thermal properties required by the nite element model, viz. the specic heat and the thermal conductivities, were identied from DSC tests and from an inverse identication procedure, respectively. The inverse identication procedure is based on a Levenberg-Marquart algorithm applied to the analysis of specic experiments instrumented with thermocouples and an infra-red camera. Thermal or/and mechanical analyses of anisotropic composite laminates can lead to high computational costs even for linear analyses. Proper Generalized Decomposition constitutes a promising tool to reduce computational costs for multi-dimensional problems such as multi-parametric problems typical of manufacturing process simulations and/or problems with dierent length scales typical of composite laminates. To demonstrate its capabilities and its eciency {including in terms of computation costs for small size problems- PGD technique is applied to the solution of an axisymmetric heat transfer problem. Specimens were manufactured (with a laboratory welding machine designed and built during this study) with dierent processing parameters - eating element geometry, intensity of the electrical current, time evolution of the pressure. DCB specimens were tested to characterize the mechanical toughness under mode I. The analysis with the compliance method of the tests results exhibits two non-negligible energy dissipation mechanisms, related to crack creation and localized plastic deformation, respectively. An original model developed within the internal variable thermodynamics framework is proposed and used to describe the R-curves representative of the ductile behaviour of the DCB specimens. A rst sensitivity analysis of the processing parameters on the joint fracture toughness exhibits the key role of the pressure applied onto the joint during the cooling phase of the welding process. / Este estudo é dedicado a juntas de compósitos termoplásticos soldadas pelo processo de soldagem por resistência elétrica. Este processo consiste em unir dois substratos com um resistor elétrico que atua como um elemento de aquecimento que funde o polímero dos substratos. Os substratos considerados neste trabalho são laminados compósitos híbridos, constituídos de 7 camadas que totalizam 2 mm de espessura, com a seguinte sequencia de empilhamento ([0°/90°]G, [0°/90°]C, [45°]C, [0°/90°]C, [45°]C, [0°/90°]C, [0°/90°]G), onde G e C denotam camadas de PPS reforçadas com fibra de vidro ou carbono, respectivamente. O elemento de aquecimento utilizado é uma malha metálica de aço inoxidável entre dois filmes de PPS (amorfos). Para um melhor entendimento do histórico do campo de temperatura na região soldada, um modelo de transferência de calor foi desenvolvido no pacote de elementos nitos Abaqus®. As capacidades de predição de temperatura do modelo computacional foram validadas a partir da comparação com resultados experimentais de termopares. As propriedades térmicas do modelo em elementos nitos, viz. o calor específico e as condutividades térmicas, foram identificadas a partir de ensaios DSC e de um procedimento de identificação inverso, respectivamente. O procedimento de identificação inversa foi baseado no algoritmo de Levenberg-Marquart aplicado na análise de experimentos específicos intrumentados com termopares e com uma câmera infra-vermelha. A análise térmica ou/e mecânica de laminados compósitos anisotropos podem apresentar elevados cusos computacionais, mesmo para análises lineares. A técnica PGD (Proper Generalized Decomposition) é uma ferramenta promissora na redução de custos computacionais de problemas multidimensionais, tópicos de simulação do processo de manufatura, e/ou problemas multi-escalas, tópico de laminados compósitos. Para demonstrar sua capacidade e sua eficiência, a técnica PGD é aplicada na solução de um problema axissimétrico de transferência de calor. Corpos-de-prova foram fabricados (com a máquina de soldagem laboratorial desenvolvida e construída durante este estudo) com diferentes parâmetros de processamento - geometria do elemento de aquecimento, intensidade da corrente elétrica, histórico de pressão. Corpos-de-prova DCB foram testados para caracterizar a resistência mecânica à propagação de trinca em modo I. A análise com o método da exibilidade dos resultados mostram dois mecanismos predominantes de dissipação de energia, correlatos com a criação da trinca e a localização de deformação plástica, respectivamente. Um modelo original desenvolvido baseado nas variáveis internas termodinâmicas é proposto e usado para descrição das curvas-R representativas do comportamento dúctil dos corpos-de-prova DCB. Uma primeira análise de sensibilidade da resistência à fratura ao variar os parâmetros de processamento mostra que a pressão aplicada na junta durante a etapa de resfriamento desempenha papel fundamental na resistência final da junta.

State-of-the-art aircraft engine manufactures aim to reduce theirenvironmental impact steadily. Thereby they attempt to increase engineefficiency, use new renewable fuel sources and most importantly aim toreduce component weight. While Titanium, Aluminum and continuousfiber reinforced thermosetting composites and superalloys prevail in thecurrent material selection, the present work desires to raise awareness fora novel group of materials; short carbon fiber reinforced thermoplasticcomposites (SCFRTPs). In this kind of composite short fibers givedimensional stability and strength while the thermoplastic matrix ensuresthe physical properties, even at temperatures up to 300°C.Even though in some applications these materials offer great potential tosave weight and cost, it is not clear if their properties suffice to be used indemanding areas of the aero engine and if they are still able provide costand weight reductions there.The present work therefore investigated potential aero-engine componentsthat could be replaced by SCFRTPs. With literature, manufacturer data andmaterial and process modelling approaches, it is shown that SCFRTPsmechanical and physical properties suffice for the selected component.Further it is shown that cost reductions up to 77% and weight savings upto 67% compared to the Ti-6Al-4V baseline component are possible.

No description available.

No description available.

This study aimed at understanding the adhesion between wood fibers and thermoplastics. Direct applications include the development of better wood fiber composites and better paper coatings. The objectives of the study were two-fold. First, to quantify the effects of surface treatments on the surface properties, and, second, to determine if adhesion can be described in terms of surface properties. A model consisting of amorphous cellulose spherical beads was used to eliminate the effects of morphology, composition, and fiber size and orientation; adhesion was studied only in terms of surface properties. The surface of the cellulose beads was modified by blending, by coating or by chemical surface reaction. Surface modification by blending was achieved by dissolving cellulose along with another polymer (cellulose propionate) and by beading the mixed solution. An alternative consisted of coating the beads with a thin layer of polymer, such as poly(4-vinyl-pyridine-co-styrene). Finally the surface was also modified by grafting functional groups or polymer segments onto the hydroxyl groups of cellulose. A thin layer of cellulose derivative, such as cellulose trifluoroethoxyacetate or cellulose laurate was produced on the bead surface. Polystyrene and polypropylene segments were grafted onto cellulose to create an interphase. The surface properties of the cellulose beads were then fully characterized. The surface composition was analyzed by X-ray Photoelectron Spectroscopy (XPS), and the morphology was investigated through Scanning Electron Microscopy. Inverse Gas Chromatography (IGC) was used to measure the surfaces' energetics. Two kinds of probes were used: alkanes to measure the dispersive component of adhesion, and acid/base probes to quantify the specific properties. Composites having variable bead content were made by injection molding. The adhesion between the cellulose beads and the thermoplastics were measured by tensile testing and by DMTA using the modified Nielsen Model with the damping factor (tan δ). The surface energy of cellulose was found to depend mostly on the presence and concentration of free hydroxyl groups on the surface. For low degrees of substitution (DS), how these OH groups are replaced by modification, whether by fatty acid type substituents or by fluorine-containing groups, is essentially irrelevant for surface characteristics. The dispersive component of the surface energy (γs^D) declined with DS, almost irrespective of substituent type. The acidic character of the cellulose surface is attributed mainly to the presence of hydroxy groups. It was furthermore established that, while wetting is a necessary condition, it is in itself insufficient for achieving good adhesion and adequate composite strength characteristics. The mechanical properties of polyethylene, polypropylene and polystyrene all decreased as cellulose beads were added in increasing amounts. It was found that improved cellulose fiber-reinforced composite performance requires the development of an interphase, such as by grafting polymer segments onto the cellulose surface which are capable of entanglement with the thermoplastic polymer in the melt. Maleic anhydride/polypropylene copolymers were found to be efficient coupling agents that better transmit stress when their molecular size increased. Adsorption of poly(4-vinylpyridine-co-styrene) [basic] by the cellulose beads [acidic] resulted in completely coated surfaces. However, strength differences between composites, with coated and uncoated beads, were insignificant probably owing to the large bead sizes used. / Ph. D.

The purpose of this investigation was to examine the effects of thermal history during cooling from the melt on the degree of crystallinity, morphology, and mechanical properties of (polyphenylenesulfide) PPS/carbon fiber composites. Three thermal treatments were employed in this study: isothermal crystallization from the melt at 140,160,180,200, and 220°C, quenching from 315° C and then annealing at 160 and 200° C, and nonisothennal crystallization from the melt at rates varying from 0040 C/minute to 68° C/second. The effect of varying the thermal history of the sample on the degree of crystallinity developed in the matrix polymer was determined using differential scanning calorimetry. The effect of thermal history on and the resulting matrix morphology was examined by scanning electron microscopy. The subsequent effects of the degree of crystallinity and the morphology on the mechanical behavior of the samples were monitored by transverse tensile tests and flexural tests. In all cases, the transverse tensile and flexural moduli increased as the amount of crystallinity in the samples increased. However, samples with greater amounts of crystallinity did not always yield higher transverse tensile or flexural strengths. Upon examination of the composite samples by electron microscopy, it was observed that trends in the values of the transverse tensile and flexural strengths could be correlated with structural changes in the matrix. This paper is concerned with the simulation of the development of crystallinity and morphology (both amount of crystallinity and the size of spherulites) which arise during the cooling of a slab of a semicrystalline polymer reinforced with long continuous carbon fibers. This situation is commonly found during the processing of semicrystalline thermoplastic composites. Whereas published attempts at simulating this process have treated the composite material as a continuum and thereby used mass averaged physical properties (such as thermal conductivity, density, and specific heat), we use a quasi-continuum approach in which locally we consider the properties of the matrix and fiber separately. Once a temperature distribution is calculated using the continuum approach, the fmite element method is applied locally at various points in the slab to calculate the amount of crystallinity and the size of the developing spherulites. This is done by using the Avrami equation and the Hoffman and Lauritzen radial growth equation. The amount of crystallinity and the spherulite size are predicted as a function of fiber spacing and packing geometry, and the predictions are found to be in good agreement with experimental results obtained on polyphenylenesulfide/carbon fiber composites. The advantages of our approach over the continuum approach is that a relatively accurate prediction of the spherulite size is possible due to constraints imposed by the fiber on the spherulitic growth. / Ph. D.

The overall objectives of this work were to improve a dual extrusion process (DEP) which is used to blend thermotropic liquid crystalline polymers (TLCPs) with thermoplastics, determine the mechanism by which TLCP morphology is developed in the DEP and to determine the optimal properties possible in composite materials generated from the blends. The DEP consists of two single screw extruders within which the TLCP and matrix material are plasticated separately. The two continuous polymer streams are joined and then mixed in a series of static mixing elements. Composite materials were formed from pelletized pregenerated strands by processing at temperatures below the melting point of the TLCP. The DEP was improved by the addition of a gear pump to the TLCP stream, a multiple port phase distribution system, static mixing design, minimization of residence time, die design, and introduction of thermal control over the entire strand production process. The TLCP material was introduced into the matrix phase by means of a multiple port phase distribution system which injected 12 individual TLCP streams parallel to the flow direction of the matrix stream. This design resulted in improvements in the axial continuity of the TLCP phase during mixing and improved radial mixing as compared with a simple T-injection system. Both Kenics and Koch static mixer designs were evaluated in this investigation and it was found that the use of either could produce similar mechanical property enhancement in the resulting blends provided that an excessive number of elements were not used. Furthermore, it was found that the most stable strand materials were formed when the die was designed with respect to the flow exiting the static mixer elements. For example, a dual strand die with each capillary having an L/D ratio of 1 produced the most stable strands when used with the Kenics mixing elements. Finally, it was found that drawing the molten blend strand in a vertical drawing chimney provided a favorable thermal environment and resulted in much higher draw ratios and high mechanical properties of the strand. The other objectives of this work including the development of morphology and composite materials produced from pregenerated strands is presented in two manuscripts formatted for submission to appropriate journals. Detailed abstracts dealing with these two topics are included therein. / Master of Science