Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Thermoplastic composites (TPCs) have shown significant advantages over thermosetting composites. They have only been put into use recently and global knowledge in TPCs is often proprietary, therefore a study into the application, processing and properties is of importance. The aim of the study is to contribute knowledge in TPCs for South African industry and academic institutions. This thesis studies continuous fibre reinforced thermoplastics (CFRTPs), focussing on the autoclave processing of commingled CFRTPs. A literature study provided background knowledge to CFRTPs regarding processing techniques and mechanics. Flexural testing and impact testing were performed on a variety of CFRTPs and thermosetting composites (TSCs). These tests were performed to further understand CFRTPs as well as to compare CFRTPs and TSCs. The flexural testing revealed that CFRTPs have comparable strength and stiffness to the TSCs that were tested. They also revealed that pre-consolidated sheets showed better and more consistent properties than sheets made from commingled fabric. The impact testing revealed that the tested CFRTPs and TSCs had similar impact resistance even though thermoplastic composites are supposed to be more impact resistant. The tests also showed that thick unreinforced thermoplastics had much higher impact resistance than the reinforced materials. Manufacturing experiments were performed to establish sound processing methods of CFRTPs. It was realised here that the high temperatures required to process the materials require specific processing consumables and tooling. The experiments began by processing flat panels in a convection oven with vacuum bagging techniques. They then progressed to autoclave processing of parts with complex geometry. An airline seat backrest was chosen as the case study in the application of CFRTPs. This application requires structural strength and stiffness and also has strict fire, smoke, toxicity and heat release (FSTH) requirements. Its geometry was sufficiently complex to demonstrate the use of commingled CFRTP material. Backrests were made from both CFRTPs and TSCs so that a comparison could be made between the two types. The backrest was modelled using finite element methods (FEM) to determine an adequate lay-up. This lay-up was then used for both the CFRTP and TSC backrests to ensure similarity between the backrests of both materials. LPET (modified polyethylene terephthalate) was the chosen thermoplastic matrix as it was more attainable than PPS (polyphenylene sulphide) CFRTPs. The backrests of both materials were manufactured in an autoclave with a vacuum bag method and then assembled using adhesives and bonding jigs. Testing revealed that the stiffness and mass of the CFRTP backrests were very similar to the epoxy backrests. This implies that commingled CFRTPs can replace the use of TSCs in similar applications. A basic cost comparison was also performed to compare the manufacture of CFRTP backrests to TSC backrests. Further work is needed to optimise processing time of these materials to make them more competitive with TSCs. The processing time of commingled materials will probably never be as quick as that of press formed pre-consolidated sheets. Their ability to be formed into more complex parts does however make their use advantageous. / AFRIKAANSE OPSOMMING: Termoplastiese saamgestelde materiale (Engels: thermoplastic composites (TPCs)) toon beduidende voordele bo termoverhardbare saamgestelde materiale. Hulle word eers sedert onlangs benut en algemene kennis in TPCs is dikwels patentregtelik, dus is ’n studie van die aanwending, prosessering en eienskappe daarvan van belang. Die doel van hierdie studie is om ’n bydrae te lewer tot die kennis van TPCs vir die Suid-Afrikaanse industrie en akademiese instellings. Hierdie tesis ondersoek kontinue veselversterkte termoplastieke (Engels: continuous fibre reinforced thermoplastics (CFRTPs)) en fokus op die outoklaafprosessering van vermengde (Engels: commingled) CFRTPs. ’n Literatuurstudie het die agtergrondkennis rakende die prosesseringstegnieke en meganika van CFRTPs verskaf. Buigtoetsing en impaktoetsing is op ’n verskeidenheid CFRTPs en termoverhardbare saamgestelde materiale (Engels: thermosetting composites (TSCs)) uitgevoer. Hierdie toetse is uitgevoer om CFRTPs beter te verstaan asook om CFRTPs en TSCs te vergelyk. Die buigtoetsing het onthul dat CFRTPs ooreenstemmende sterkte en styfheid het as die TSCs wat getoets is. Dit het ook getoon dat vooraf-gekonsolideerde plate beter en meer konsekwente eienskappe getoon het as plate wat van vermengde materiaal gemaak is. Die impaktoetsing het onthul dat die CFRTPs en TSCs wat getoets is soortgelyke impakweerstand gehad het, selfs al is termoplastiese saamgestelde materiale veronderstel om meer impakweerstand te toon. Die toetse het ook getoon dat dik onversterkte termoplastieke veel hoër impakweerstand gehad het as die versterkte materiale. Vervaardigingseksperimente is uitgevoer om betroubare prosesseringsmetodes vir CFRTPs vas te stel. Daar is besef dat die hoër temperature wat vereis word om die materiale te prosesseer ook spesifieke prosesseringsverbruiksware en -gereedskap benodig. Die eksperimente het begin met die prosessering van reguit panele in ’n konveksie-oond met vakuumsaktegnieke. Daar is toe aanbeweeg na die outoklaafprosessering van onderdele met komplekse geometrie. Die rugleuning van ’n vliegtuigsitplek is gekies as die gevallestudie in die gebruik van CFRTPs. Hierdie toepassing vereis strukturele sterkte en styfheid en is ook onderhewig aan streng vereistes t.o.v. brand, rook, toksisiteit en hittevrystellimg (Engels FSTH). Die geometrie daarvan was kompleks genoeg om die gebruik van vermengde CFRTP-materiaal te demonstreer. Rugleunings is gemaak van beide CFRTPs en TSCs sodat ’n vergelyking tussen die twee tipes gemaak kon word. Die rugleuning is gemodelleer deur eindige element metodes (EEM) te gebruik om ’n aanvaarbare oplegging te bepaal. Hierdie oplegging is toe gebruik vir beide die CFRTP en TSC rugleunings om die gelykvormigheid tussen die rugleunings van beide materiale te verseker. LPET (Engels: modified polyethylene terephthalate) was die gekose termoplastiese matriks aangesien dit meer verkrygbaar was as PPS (Engels: polyphenylene sulphide) CFRTPs. Die rugleunings van beide materiale is vervaardig in ’n outoklaaf met ’n vakuumsakmetode en toe geintegreer deur die gebruik van kleefstowwe en setmate. Toetsing het getoon dat die styfheid en massa van die CFRTP rugleunings baie soortgelyk was aan die epoksie rugleunings. Dit impliseer dat vermengde CFRTP die plek van TSCs in soortgelyke gebruike kan inneem. ’n Basiese kostevergelyking is ook gedoen om die vervaardiging van CFRTP-rugleunings teenoor TSC-rugleunings te vergelyk.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: In the primary circuit components of high temperature reactors, various unwanted particles have been found. These particles include, for example, graphite and silver- 110 (110mAg). The silver-110 (110mAg) particles are radioactive, with a half-life of 253 days. The presence of radioactive particles in the primary circuit components constitutes an unwanted maintenance problem from a radiation hazard point of view. The development of a method to remove these particles from the helium stream is therefore needed. This thesis consideres two possible methods of removing silver from the helium stream, namely laser ablation of microparticles and electrostatic precipitation. These methods require the generation of silver ions or charged particles, the deflection of these particles in a helium gas stream passing through an electric field and the subsequent plate-out of these particles onto deflection electrodes. To find a suitable method of generating ions, various methods to generate the silver ions were investigated and evaluated. These methods include existing ion sources, thermionic, field and photoelectric emission and laser ablation of microparticles. No existing ion sources could be found which could be utilised in helium at high pressure. From calculations it was concluded that thermionic, field and photoelectric emission could also not be used to raise the energy of the emitted electrons sufficiently to ionize silver in a helium flight path. These methods were found not to be feasible ion sources in helium at high pressures. However, laser ablation of microparticles was found to constitute a feasible technology. Laser ablation was successfully utilised by Nichols et al. (2000) to deflect silver nanoparticles in an electric field across a two bar helium stream. An apparatus, similar to the one developed by Nichols et al. (2000), was designed and built. The apparatus included a silver insertion mechanism and tests with this apparatus were called the microparticle tests. To determine the efficacy of the silver insertion mechanism, the microparticle tests were done without the use of a laser. It was found that a laser was not necessary as microparticles collected on both the deflection electrodes. Dielectrophoresis was proposed as a possible explanation for the deflection and the plate-out of the mcroparticles. To theoretically model the deflection of the silver particles, two models were proposed, namely the deterministic and the stochastic deflection models. The latter describes the deflection of atoms, ions and polarized particles by using probability theory. From this model it was found that the Brownian motion force is far larger that the force created by the polarizibility of the atom due to an electric field. The deterministic deflection model describes the deflection of larger particles in a continuum. From this model it was found that a silver microparticle with a radius of 3 mm in a helium stream with bulk velocity of 0.0198 m/s would deflect 4.6 mm per helium flight path length of 140 mm. From these calculation it was found that the apparatus which had been built was not long enough to deflect and plate-out all the silver microparticles. The dielectrophoresis force on nanoparticles cannot be calculated, as the theory of dielectrophoresis is only valid for particles with diameter larger than 1 mm. Changes were therefore made to the apparatus to generate nanoparticles as their mobility is larger than that of microparticles. The nanoparticles were created by means of an arc discharge in helium; therefore tests with this modified apparatus were called the arc discharge test. The nanoparticles so created, deflected and deposited on both deflection electrodes. With the use of an atomic force microscope some of the particles could be classified as microparticles. According to the deterministic deflection model they should not have deflected. Combined with the fact that oxygen was in the plasma, due to the oxidation of the electrodes, a hypothesis of bipolar charging was thus proposed. The deterministic deflection model was used and supplemented with field and diffusion charging calculations, to support this hypothesis. A reasonable correlation between the theoretical model and this experimental results was obtained. Based on the arc discharge test, electrostatic precipitation was proposed as the indicated means of scrubbing silver and other particles such as graphite from a helium stream. It is recommended that a new apparatus be built and that the deterministic deflection model be used to predict the deflection of the particles. With this apparatus the uncertainties of breakdown voltage, the effect of thermionic emission and the size of the particles, all of which have been identified as being important, can then also be determined. / AFRIKAANSE OPSOMMING: In die primêre komponente van die kringloop van hoë temperatuurreaktors, was verskillende ongewenste partikels soos grafiet en silwer-110 (110mAg) teenwoordg. Silwer- 110 (110mAg) is radioaktief met ’n halfleeftyd van 253 dae. Vanuit ’n radiasie-risiko oogpunt word daar onderhoudsimplikasies geskep deur die teenwoordigheid van radioaktiewe partikels in die primêre komponente. Die ontwikkeling van ’n metode om hierdie partikels uit die heliumstroom the verwyder was dus nodig. Hierdie tesis ondersoek twee moontlike metodes van verwydering van silwer uit die heliumstroom, naamlik laser-ablasie van mikropartikels en elektrostatiese presipitasie. Hierdie metodes benodig die generasie van silwer ione of gelaaide partikels, die defleksie daarvan in ’n heliumstroom wat deur ’n elektriese veld vloei en die platering van die partikels op defleksie elektrodes. Om ’n geskikte metode the vind wat ione genereer was, verskillende metodes om die silwer ione te verkry, ondersoek en geëvalueer. Hierdie metodes sluit in bestaande ioonbronne, termioniese, veld en fotoëlektriese emissie en laser-ablasie van mikropartikels. Geen ioonbronne was gevind wat gebruik kan word in helium by hoë druk nie. Die gevolgtrekking is gemaak vanaf berekeninge dat termioniese, veld en fotoëlektriese emissie ook nie gebruik kan word om die energie van die voortgebronge elektrone genoeg te verhoog om silwer in ’n heliumstroom te ioniseer nie. Daar was gevind dat hierdie metodes nie geskik is as ioonbronne in helium by hoë druk nie. Daarenteen was laser-ablasie van mikropartikels gevind om ’n geskikte tegnologie voor te stel. Laser-ablasie van mikropartikels was suksesvol deur Nichols et al. (2000) gebruik om silwer nanopartikels te deflekteer in ’n elektriese veld oor helium van twee bar. ’n Apparaat soortgelyk aan Nichols et al. (2000) se eksperiment, was dus ontwerp en gebou. Die apparaat het ’n silwer insitmeganisme bevat en toetse met hierdie apparaat was die mikropartikel toetse genoem. Om die effektiwiteit van die insitmeganisme te bepaal, was toetse gedoen sonder opstelling van die laser. Daar was eksperimenteel gevind dat die laser nie nodig was nie, omdat mikropartikels op beide defleksie elektrodes geplateer het. Dielektroforese was voorgestel as ’n moontlike verduideliking vir deflektering en platering vir die silwer mikropartikels. Om die defleksie van silwer partikels teoreties te moduleer was twee modelle voorgestel, naamlik deterministiese en stogastiese defleksiemodelle. Laasgenoemde beskryf die defleksie van atome, ione en gepolariseerde partikels deur gebruik te maak van waarskynlikheidsteorie. Die stogastiese defleksiemodel dui aan dat die Brownian bewegingskrag veel groter is as die krag wat geskep word deur die polarisasie van ’n atoom as gevolg van ’n elektriese veld. Die deterministiese defleksiemodel beskryf die defleksie van groter partikels in ’n kontiuum. Met hierdie model was gevind dat silwer mikropartikels met ’n radius van 3 mmin ’n heliumstroom van snelheid van 0.0198 m/s, 4.6 mm sal deflekteer per 140 mm van heliumstroom lengte. Dit bewys dat die apparaat wat gebou was, se lengte onvoldoende was om al die silwer mikropartikels te deflekteer en te laat neerslaan. Die dielektroforese krag van nanopartikels kan nie uitgewerk word nie, omdat die dielektroforese model slegs geldig is vir partikels groter as 1 mm. Veranderings was dus aan die apparaat gemaak om nanopartikels te genereer omdat hul mobiliteit hoër is as die van mikropartikels. Die nanopartikels was geskep deur gebruik van ’n boogontlading in helium; daarom was toetse met hierdie gemodifiseerde apparaat die boogontladingstoets genoem. Die nanopartikels wat so geskep was, het gedeflekteer en het op beide elektrodes neergeslaan. Met die gebruik van ’n atomiese krag mikroskoop was dit gevind dat sommige van hierdie partikels mikropartikels was. Volgens die deterministiese defleksiemodel moes hul nie gedeflekteer het nie. Gekombineerd met die feit dat daar, weens oksidasie van die elektrodes, suurstof in die plasma was, was ’n hipotese van bipolêre lading voorgestel. Die deterministiese defleksiemodel is saam met die veld- en diffusielading gebruik om hierdie hipotese te staaf. ’n Redelike korrelasie tussen die teoretiese en eksperimentele data was gevind. Gebaseer op die boogontladingstoets, was elektrostatiese presipitasie voorgestel as ’n metode om silwer en ander partikels soos grafiet uit ’n heliumstroom te verwyder. Daar word voorgestel dat ’n nuwe apparaat gebou word en dat die deterministiese defleksiemodel gebruik word vir die bepaling van defleksie van die partikels. Deur die nuwe apparaat te gebruik kan die onsekerhede van deurslagspanning, effek van termioniese emissie en grootte van die partikels wat geidentifiseer is as belangrik, ook bepaal word.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / Cooling tower rain zone performance characteristics such as the loss coefficient and the Merkel number are evaluated and simulated. To this end the influence of drop diameter and drop deformation on the velocity, path length and cooling of single water drops are investigated. Experimental drop size and pressure drop data over a counterflow rain zone are presented and the effect of drop deformation on the pressure drop is investigated using the experimental data and CFD. Using the experimental drop size data and CFD, the performance uncertainty produced by using the Rosin-Rammler drop distribution function as opposed to the discrete drop distribution data is investigated. CFD models are developed to investigate the feasibility of modelling rain zones by assuming a constant drop diameter and to establish which diameter definition is the most representative of a particular polydisperse drop distribution. These models were used to validate the correlations for the rain zone performance characteristics proposed in literature.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2008. / A solution is sought for the measurement of human deep tendon reflexes as part of a comprehensive patient condition monitoring system for use in a telemedicine context. This study focused on the development, testing and performance evaluation of a prototype compact patellar tendon reflex measurement system that is able to provide a quantitative reflex evaluation for use by medical practitioners and in a telemedicine environment. A prototype system was developed that makes use of Xsens MTx orientation sensors, force-sensing resistors and an electromyogram (EMG) to measure the reflex response. Suitable parameters identified for analysis included the change in pitch, angular velocity and acceleration of the lower leg, the EMG response, the tendon impact, and various latencies associated with these measurements. Other information considered included the age, mass, and physical dimensions of the test subject. Clinical testing was performed to collect data to evaluate the system performance. Subjective reflex evaluations were conducted by three doctors according to a standard reflex grading scale using video recordings of the tests. Self-organizing maps and multi-layer feed-forward (MLFF) artificial neural networks (ANNs) were used to analyze the collected data with the aim of pattern identification, data classification and reflex grading prediction. It was found that the MLFF network delivered the correct reflex grading with an accuracy of 85%, which was of the same order as the rate of differences between the subjective reflex evaluations performed by the doctors (80%). Furthermore, analysis of the data suggested that certain parameters were not necessary for the autonomous evaluation, such as EMG data and the tendon impact. The use of ANNs to analyze a reflex measurement as proposed by this study offers an accurate, repeatable and concise representation of the reflex that is familiar to doctors and suitable for use in a general clinical setting or for telemedicine purposes.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Air-cooled steam condensers are used in arid regions where adequate cooling water is not available or very expensive. In this thesis the effect of steam-side and air-side effects on the condenser performance, steam distribution and critical dephlegmator length is investigated for air-cooled steam condensers as found in power plants. Solutions are found so that no backflow is present in the condenser. Both single and two-row condensers are investigated. The tube inlet loss coefficients have the largest impact on the critical dephlegmator tube length in both the single and two-row condensers. The critical dephlegmator tube lengths were determined for different dividing header inlet geometries and it was found that a step at the inlet to the dividing header resulted in the shortest tubes. Different ambient conditions were found to affect the inlet steam temperature, the steam flow distribution, heat rejection distribution and the critical dephlegmator length for the single and two-row condensers. There were differences in the steam mass flow distributions for the single and two-row condensers with opposite trends being present in parts of the condenser. The single-row condenser’s critical dephlegmator tube lengths were shorter than those of the two-row condenser for the same ambient conditions. Areas of potential backflow change with different ambient conditions and also differ between a single and two-row condenser. The two-row condenser always have an area of potential backflow for the first row at the first condenser fan unit. / AFRIKAANSE OPSOMMING: Droë lug-verkoelde stoom kondensors word gebruik in droë gebiede waar genoegsame verkoelingswater nie beskikbaar is nie of baie duur is. In hierdie tesis word die effek van stoomkant en lugkant effekte op die vermoë van die kondensor, die stoomvloeiverdeling en kritiese deflegmator lengte ondersoek vir lug-verkoelde stoom kondensors soos gevind in kragstasies. Dit word opgelos sodat daar geen terugvloei in enige van die buise is nie. ʼn Enkel- en dubbelry kondensor word ondersoek. Die inlaatverlieskoëffisiënte van die buise het die grootste impak op die lengte van die kritiese deflegmator buise in beide die enkel- en dubbelry kondensors. Die kritiese deflegmator buis lengtes is bereken vir verskillende verdeelingspyp inlaat geometrië en dit is gevind dat ʼn trap by die inlaat van die verdeelingspyp die kortste buise lewer. Dit is gesien dat verskillende omgewingskondisies die inlaat stoom temperatuur, die stoomvloeiverdeling, die warmteoordrag verdeling en die kritiese lengte van die deflegmator buise vir die enkel- en dubbelry kondensor. Daar was verskille tussen die stoomvloeiverdelings vir die enkel- en dubbelry met teenoorgestelde neigings in dele van die kondensor. Die kritiese deflegmator buis lengte vir die enkelry kondensor was korter as die vir die dubbelry kondensor vir dieselfde omgewingskondisies. Die areas in die kondensor waar terugvloei moontlik kan plaasvind in die kondensor verander met ongewingskondisies en verskil vir die enkel- en dubbelry kondensers. Die dubbelry kondensor het altyd ʼn area van moontlike terugvloei vir die eerste buisry by die eerste kondensor waaiereenheid.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / Electronic engine control systems utilise tables, or maps, of data to determine the set-points of the various actuators on the engine and to calculate the values of variables that cannot be directly measured. To ensure accurate control of the engine processes the values in these maps have to be accurately calibrated for the particular engine being controlled. Due to the complex interaction of the various systems in the engine it is becoming more and more difficult for human calibration engineers to be able to take all the effects of changes to a particular parameter into account. This problem is made worse by increasingly strict emissions regulations and performance demands from the customer. The process of calibrating the maps in an Electronic Control Unit (ECU) is also very resource intensive since it involves taking a test engine installed in an engine test cell to every operating point on the various maps and adjusting the map values until the desired response is achieved. The aim of this project was to develop a solution to this problem in the form of a simulation based calibration system. The proposed system would use an accurate model of the engine to simulate the effect of various map values on the engine response. This data would then be used to find the map values that would enable the engine to deliver a desired torque curve. In the case where it is not practical to use engine simulation the system would be able to process a database created by testing an actual engine. This testing could also be automated. To achieve this aim the AutoCal program was created. This program can manage a commercial engine simulation code to create a database of the effect of various calibration values on the engine response. It can then evaluate the created database subject to user defined operating constraints and find calibration values that will deliver a desired torque curve. It can also be used to evaluate and process databases created by engine testing. To provide the data required for the development and testing of the AutoCal program, a naturally-aspirated engine was turbo-charged and tested at various operating points. The resulting data was used to calibrate and validate a model of the engine created and simulated with the WAVE software package from Ricardo. The project was focused on finding calibration values for the maps used to control the turbo-charger wastegate and ignition timing of the test engine. Work was limited to the full load operating region and fixed Air/Fuel Ratio (AFR) values were used. The project showed that simulation based calibration can be used to calibrate control system maps once an accurate model of the engine being controlled has been created. Very useful insight was gained into the process of building, testing and modelling a turbo-charged internal-combustion engine and calibrating modern electronic engine control systems. The end result is a useful engineering tool with the following functions: ² Automatically simulating the effect of various control inputs on engine performance. ² Determining the correct calibration settings to deliver the desired performance subject to user-definable constraints. This can be done using results from simulation or physical engine testing in the case of simulation tools not being available. ² Providing the data required when calibrating the engine model used during simulation by processing and displaying the outputs of the simulation program compared to test data. ² Plotting any test or simulation results in a format configured by the user. Using this tool facilitates a more structured and less resource intensive approach to engine control system calibration. / Imported from http://etd.sun.ac.za April 2010. / np2010

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2005. / Vehicle Noise, Vibration and Harshness (NVH) is now a more important component of the vehicle design process than ever. Road noise is one of the key criteria used by potential buyers (albeit subconsciously) to choose what they perceive as the best vehicle. Road rumble is a key concern for vehicle manufacturers. Light Utility Vehicles (LUVs) are especially sensitive to a low frequency booming noise due to the fundamental acoustic mode that exists in the vehicle cabin. An investigation into this booming noise in an LUV is documented. The noise is identified and quantified after which the source of the noise in the vehicle cabin is identified using NVH techniques such as Acoustic Modal Analysis (AMA), Experimental Modal Analysis (EMA) and Transfer Path Analysis (TPA). The cabin’s fundamental acoustic mode lay at 100 Hz. Finally the source of the vibrations in the vehicle leading to the booming noise in the cabin is identified, along with its transfer path to the cabin. Solutions for the specific vehicle’s booming noise are proposed, two of which are tested with some success. Solutions to the problems associated with the fundamental acoustic mode of LUVs are also proposed and discussed.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / Elderly patients suffering from aortic valvular dysfunction are often denied aortic valve replacement due to the fact that they are classified as too old and fragile to handle the physical stress of open-heart surgery and cardio-pulmonary bypass. There exists a need for an alternative solution which places less physical stress on the body. The development of a percutaneous aortic heart valve (PAHV), which may be implanted through a minimally invasive procedure, will provide a solution to old and fragile patients who otherwise have a very limited life expectancy. The development of such a device entails a costly and time-consuming process which involves a number of phases, including a prototype development phase, an in-vitro testing phase, an animal trial phase and a human trial phase. This thesis focuses on the design and analysis of the stent component for a PAHV, suitable for implantation in sheep (animal trial phase). The process of developing a first prototype, involved an analysis of the stent design requirements. This analysis was followed by a concept generation phase as well as comprehensive finite element (FE) analyses of the most promising concepts. The objective of the FE analyses was to determine the effects of a variation in strut width on the performance characteristics of the concepts. Based on the results of the FE analyses, final geometries were selected for each of the two most promising concepts. Subsequent to the selection of the final geometries, a number of prototypes were manufactured. The prototypes were subjected to an electro-polishing process. An experimental analysis was also conducted on the prototypes to evaluate the accuracy of the (FE) simulations as well as the actual performance of the stent prototypes. The results of the FE analyses and experimental analyses indicated that strut width had a substantial influence on the parameters that were defined to characterise stent performance. The results of the analyses also highlighted the advantages and disadvantages of each concept and aided in identifying the concept that would be most suitable for the required application. Limitations of the study were identified and recommendations were made to assist the continued research and development of the device.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2008. / Suspension seats are commonly used for earth moving machinery to isolate vehicle operators from vibrations transmitted to the vehicle body. To provide the required stiffness and damping for these seats, air-springs are typically used in conjunction with dampers. However, to eliminate the need for additional dampers, air-springs can be used in conjunction with auxiliary air volumes to provide both spring stiffness and damping. The damping is introduced through the flow restriction connecting the two air volumes. In this study, simplified models of an air-spring were derived followed by a model including the addition of an auxiliary volume. Subsequent to simulations, tests were performed on an experimental apparatus to validate the models. The air-spring models were shown to predict the behaviour of the experimental apparatus. The air-spring and auxiliary volume model followed the trend predicted by literature but showed approximately 27 % lower transmissibility amplitude and 21 % lower system natural frequency than obtained by tests when using large flow restriction diameters. This inaccuracy was assumed to be introduced by the simplified mass transfer equations defining the flow restriction between air-spring and auxiliary volume. The models however showed correlation when the auxiliary volume size was decreased by two thirds of the volume actually used for the experiment. This design of a prototype air-spring and auxiliary volume is presented for a suspension seat used in articulated or rigid frame dump trucks. The goal of this study was to design a suspension seat for this application and to obtain a SEAT value below 1,1. The design was optimised by varying auxiliary volume size, flow diameter and load. A SEAT value of less than 0,9 was achieved.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / Vibrating a tillage tool is an effective way of reducing the draft force required to pull it through the soil. The degree of draft force reduction is dependent on the combination of operating parameters and soil conditions. It is thus necessary to optimize the vibratory implement for different conditions. Numerical modelling is more flexible than experimental testing and analytical models, and less costly than experimental testing. The Discrete Element Method (DEM) was specifically developed for granular materials such as soils and can be used to model a vibrating tillage tool for its design and optimization. The goal was thus to evaluate the ability of DEM to model a vibratory subsoiler and to investigate the cause of the draft force reduction. The DEM model was evaluated against data obtained from field testing done with a full scale single tine vibratory subsoiler. Soil testing was also done for material characterization and for the calibration of DEM material properties. The subsoiler was simulated using a commercial code, PFC3D. The effect on the simulation results of particle diameter, different bonding models and damping models was investigated. The final simulations were evaluated against the experimental results in terms of the draft force and material behaviour. The cause of the draft force reduction due to vibration was also investigated with the aid of the DEM model. From the results it was concluded that DEM is able to model the vibratory subsoiler for its design and optimization. The DEM model also provided valuable insight into the cause of the draft force reduction such as the increased peak stresses due to vibration and the increase in particle kinetic energy.