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Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The South African manufacturing industry plays a pivotal role in the growth of its local economy. Modern manufacturing requirements include the ability to respond quickly to product variability, fluctuations in product demand and new process technologies. The reconfigurable manufacturing paradigm has been proposed to meet the demands of the new manufacturing requirements. In order to assess the feasibility of incorporating automated, reconfigurable manufacturing technologies into the production process of thermoplastic fibre-reinforced composite parts, a system, based on the thermoforming process, that implements these technologies was developed and evaluated. The assessment uses a seat pan for commercial aircraft as case study. Aspects that were addressed include the architecture, configuration and control of the system. The architecture and configuration addressed the sheet cutting, fixturing, reinforcing, heating, forming, quality assurance and transportation. The control, implemented using agents and based on the ADACOR holonic reference architecture, addresses the cell control requirements of the thermoforming process. An evaluation of the system’s reconfigurability and throughput is performed using KUKA Sim Pro. The evaluation of the system’s throughput is compared to the predicted throughput of the conventional technique for manufacturing thermoplastic fibre reinforced composite parts in a thermoforming process. The evaluation of the system’s performance show that the system designed in this thesis for the manufacture of a thermoplastic fibre-reinforced composite seat pan sports a significant advantage in terms of throughput rate, which demonstrates its technical feasibility. The evaluation of the system’s reconfigurability show that, through its ability to handle new hardware and product changes, it exhibits the reconfigurability characteristics of modularity, convertibility, integrability and scalability. / AFRIKAANSE OPSOMMING: Die Suid-Afrikaanse vervaardigingsbedryf speel 'n sentrale rol in die groei van die plaaslike ekonomie. Moderne vervaardiging vereistes sluit in die vermoë om vinnig te reageer op die produk veranderlikheid, skommelinge in die produk aanvraag en nuwe proses tegnologieë. Die herkonfigureerbare vervaardiging paradigma is voorgestel om te voldoen aan die nuwe produksie vereistes. Ten einde die uitvoerbaarheid van die integrasie van outomatiese, herkonfigureerbare vervaardiging-tegnologieë in die produksieproses van veselversterkte saamgestelde onderdele te evalueer, is 'n stelsel, gebaseer op die termo-vormingsproses, wat sulke tegnologieë implementeer, ontwikkel. Die assessering gebruik 'n sitplek pan vir kommersiële vliegtuie as gevallestudie. Aspekte wat aangespreek is sluit in die argitektuur, konfigurasie en beheer van die vervaardigingstelsel. Die argitektuur en konfigurasie spreek aan die sny, setmate, versterking, verwarming, vorm, gehalteversekering en vervoer van n veselversterkte saamgestelde sitplek pan in 'n termo-vormingsproses. Die beheer, geïmplementeer deur die gebruik van agente en gebaseer op die ADACOR holoniese verwysing argitektuur, spreek die selbeheervereistes van die termo-vormingsproses aan. 'n Evaluering van die stelsel se herkonfigureerbaarheid en deurvoer word gedoen met die behulp van KUKA Sim Pro. Die evaluering van die stelsel se deurvoer word vergelyk met die deurvoer van die konvensionele vervaardigingsproses vir termoplastiese vessel-versterkte saamgestelde onderdele in 'n termo-vormingsproses. Die evaluering van die stelsel se prestasie toon dat die stelsel wat in hierdie tesis ontwerp is vir die vervaardiging van 'n termoplastiese vessel-versterkte saamgestelde sitplek pan, hou 'n beduidende voordeel, in terme van deurvloeikoers, in wat die stelsel se tegniese haalbaarheid toon. Die evaluering van die stelsel se herkonfigureerbaarheid wys dat, deur middel van sy vermoë om nuwe hardeware en produk veranderinge te hanteer, die stelsel herkonfigureerbare einskappe van modulariteit, inwisselbaarheid, integreerbaarheid en skaalbaarheid vertoon.

The work presented in this thesis concerns the problem of detecting and characterising the effect of fibre-matrix debonds in a fibre-reinforced composite, on ultrasonic backscattering. Theoretical and experimental investigations were conducted into this problem. Three mathematical models were examined. The first assumed that the debond was a thin crack with non-contacting faces surrounding the fibre. The second modelled the debond by allowing tangential slip between the matrix and fibre defined by an effective shear modulus, K. For the third model, the debond was approximated by a thin visco-elastic layer separating the matrix and fibre. The results of the modelling suggested that for an incident longitudinal wave, the first model acts as an air-filled void with a sharp resonance present in the low ka region. The second and third models both show the backscattering to be attenuated. Experimental investigations were carried out on scale models of a single fibre embedded in an araldite matrix. Steel or copper wires were used for the reinforcing fibre. The agreement between theory and experiment for a well-bonded wire was excellent. The effect on the longitudinal backscattered wave of the wire immediately after debonding was to attenuate heavily the resonances in the backscatter form function. However, after approximately an hour the scatter is seen to relax, closely resembling that from a well-bonded wire. In view of the only partial ability of longitudinal incident waves to detect debonds, shear wave (SH) incidence was investigated. It was shown that shear waves were far more sensitive to the presence of fibre-matrix debonds.

Fibre reinforced plastics are widely used for energy dissipating parts. Due to their superior strength to density ratio they provide a high performance and are ideal for lightweight design for crashworthiness. For this, it is essential that the mechanical behaviour of fibre reinforced composites can be predicted correctly by simulation. However, due to the complex inner structure, this is still a challenging task, in particular in case of highly nonlinear crash loading. In this work, a new purely virtual method is developed, which derives the complex fibre structure of a filament wound tube by a chain of numerical simulations. Thereby a finite element simulation of the fibre placement, taking into account the occurring physical effects, constitutes the fundamental base. Based on the results of the manufacturing simulation, a 3D fibre architecture is generated and compared to the real existing structure. The fibre structure, combined with an automatic matrix implementation algorithm, subsequently provides a finite element model of the composite on meso-scale. Using micro-scale analysis, effective material properties for the roving structure, based on filament-matrix interaction, are derived. Incorporation of the effective properties in a USER MATERIAL model completes the finite element model generation. The mesoscale model is subsequently used to analyse the filament wound tube in terms of quasi-static and crash loading. Finally, the obtained results are compared to experimental observations.

Damaged cartilage can cause severe pain and restricted mobility, with few long term treatments available. The developing field of tissue engineering offers an alternative to the currently used full joint replacement. Restoring damaged cartilage through tissue engineering would enable an active lifestyle to be recovered and retained, without restrictions to joint mobility. This is increasingly important as the prevalence of osteoarthritis rises. Tissue engineering requires biomaterial scaffolds that mimic the function of the tissue while cells develop, and so the scaffold must provide the appropriate biological, chemical and mechanical stimuli. In this work, methods were developed to enable the design of scaffolds that mimic the microstructure and mechanical properties of articular cartilage. Electrospinning was investigated as a method to mimic the nanoscale collagen fibres within cartilage extracellular matrix. A parametric study was conducted to determine how changes to a gelatin solution affect the mechanical properties of the non-woven fibrous mesh. The solution properties had a clear impact on the morphology of the fibres, but the effect on the mesh mechanical properties was convoluted. The results demonstrated the need for greater understanding of the 3D morphology of electrospun meshes, to establish how these may be altered in order to design scaffolds with desirable mechanical properties. The fracture mechanics of soft materials are complex, and are generally overlooked when designing tissue engineering scaffolds. The complexities have led to a lack of standardised testing, making comparisons between studies impractical. In this work, fracture testing methods were compared, using a viscoelastic polymer to mimic some of the complexities of soft tissue mechanics. Mode III trouser tear tests and mode I pure shear tests were found to provide reliable measurements. Due to the ease of testing small samples, trouser tear testing was concluded to be the most advantageous for determining the fracture resistance of soft tissue engineering scaffolds. Finally, electrospun meshes were combined with hydrogels to create biomimetic scaffolds, which were characterised using tensile and trouser tear fracture tests. Fibre-reinforcement was shown to enhance the mechanical properties of a weak hydrogel, but diminished those of a strong, tough polyacrylamide (PAAm)-alginate hydrogel. The PAAm-alginate hydrogel exhibited mechanical properties close to those of natural articular cartilage, but without the microstructure that would enhance its suitability for use as a cartilage tissue engineering scaffold. An alternative method for reinforcing PAAm-alginate was proposed, which shows promise for producing a biocompatible scaffold that mimics both the mechanics and the microstructure of articular cartilage. Ultimately, this thesis aimed to improve the design of biomimetic scaffolds for cartilage tissue engineering, and advance mechanical characterisation techniques within this field.

For the next generation of aircraft, the use of fibre-reinforced polymer composites and the design of

L’utilisation de composites à matrice thermoplastique renforcée par fibres courtes (TRFC) connait une forte croissance pour une large gamme d’applications industrielles pour des conditions de chargement extrêmes (e.g. pare-chocs d’automobiles). Il est donc indispensable de développer des modèles de comportement des TRFC tenant compte des spécificités du matériau pour une large gamme de vitesse de déformation. Toutefois, le comportement de ces composites est complexe. Cette complexité est due, en premier lieu, au comportement viscoélastique (VE)-viscoplastique (VP) de la matrice avec une sensibilité à la pression. A cela s’ajoute les caractéristiques complexes du renfort en termes de distributions d’orientation des fibres courtes. De plus, le comportement de ces composites est affecté par des phénomènes d’endommagement coexistants (e.g. endommagement de la matrice et décohésion l’interface fibre/matrice). Dans ce travail, un modèle permettant la prise en compte de l’ensemble de ces phénomènes est proposé. Sa formulation est basée sur la décomposition du matériau en un milieu matriciel et plusieurs milieux de fibres, sur la base d’une décomposition additive du potentiel thermodynamique. Cette approche permet une implémentation simplifiée avec une résolution successive (mais non indépendante) du comportement de chaque milieu. Un avantage immédiat est la possibilité de prendre en compte tout type de comportement matriciel et tout type d’orientation. L’interface fibre/matrice, siège de la transmission de l’effort est modélisée par un transfert par cisaillement, avec sur une hypothèse locale d’iso-déformation dans la direction de la fibre. L’endommagement ductile de la matrice est pris en compte par un modèle d’endommagement anisotrope. La dégradation de l’interface fibre/matrice est décrite par un modèle de décohésion initiée en pointe de fibres. Un critère de rupture se basant sur le taux maximal de vide crée par décohésion est enfin introduit. La caractérisation du modèle est basée sur des campagnes d’essais quasi-statiques et dynamiques pour le cas de polypropylène pur et renforcé par fibres courtes de verre, à différents angles de chargement par rapport à la direction d’injection. Ces essais sont complétés par des observations au microtomographe permettant la caractérisation des distributions d’orientation locale des fibres. Des observations au MEB ont enfin permis de constater une éventuelle influence de la vitesse de sollicitation sur les mécanismes d’endommagement. / Short fibre-reinforced composites are commonly used in a variety of engineering applications, including automotive and aerospace industry. Today, their use is progressively extended to parts possibly subjected to severe loading conditions (e.g. crash...), characterised by high strain rates. Therefore, an efficient modelling that takes into account material’s specificities at a large strain rate range is needed. A constitutive model of viscous behaviour of short-fibre reinforced composites (SFRC) where complex distributions of fibre orientations are taken into account is proposed in this work. The approach considered for the computation of composite macroscopic behavior is based on an additive decomposition of the state potential. The SFRC is assimilated to an assembly of several fibre media embedded in a polymeric matrix medium. One of the main assets of this approach is the possibility to model reinforcement with complex distributions of fibre orientations. Moreover, this decomposition allows the implementation of complex behaviour laws coupled with damage models. The polymeric matrix behaviour is typically strain-rate sensitive, i.e. viscoelastic-viscoplastic. This property has to be taken into account when the modelling of the composite behaviour over a large range of strain rate is intended. Therefore, a viscoelastic constitutive model, based on generalised Maxwell model, and a viscoplastic correction scheme, based on an overstress approach, are implemented for matrix material. The developed constitutive model is then coupled to two damage laws. The first one is introduced in the framework of Continuum Damage Mechanics in order to model the anisotropic ductile damage behaviour of the matrix material. The second one deals with fibre/matrix interfacial degradation through an interfacial debonding law. In order to identify the parameters involved in the present model, experimental tests are performed (case of polypropylene reinforced with short glass fibres). Microcomputed tomography is used for the characterisation of the fibres distribution of orientation. The efficiency of the proposed model is demonstrated by comparisons between numerical and experimental responses in different loading conditions, including dynamic loadings.