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Creep degradation of steam carrying vessels in the power generation industry is a concern that needs to be constantly monitored. The Weldcore® process has been earmarked as a potential method of creep sampling which will allow for thick-walled sections to be analysed. A component of the process involves plugging the resultant hole after removing a creep sample using a novel welding technique called friction hydro pillar processing. At the commencement of this study, insufficient data was available to warrant safe industrial application of the process. This research was conducted to evaluate the performance of 10CrMo910 friction hydro pillar process welds. The effects of downward force, stud taper angle, hole taper angle and hole base diameter on process response, defect population, static properties and dynamic performance were evaluated. The variation of downward force showed that higher forces produce significantly smaller defects and higher fatigue life. The occurrence of defects was linked to process parameters and geometry thereby identifying the correct parameters for safe use in the power generation industry. Flash formation was identified as an early indicator of weld defects and can assist with quality control in industrial applications. Methods of standardising the plunge depth and forge force were developed to identify the correct magnitudes for different geometries, without the need for testing. Defects were shown to populate specific regions of the weld and produce variations in fatigue life. Crack initiation sites were detected which will aid in identifying areas of focus in further research and development. Temperature measurements were linked to the occurrence of defects and crack initiation sites and have been identified as a method of identifying defective welds. The effects of process parameters and stud and hole taper angles on energy inputs and near interface temperatures were statistically evaluated. Downward force was shown to have the largest effect on energy input rates, total energy input and temperatures at the 11.5mm and 20.5mm positions. Smaller hole and stud taper angles produced lower energy inputs and were identified as more energy efficient than the larger taper angles. A regression model was also developed to predict the fatigue life of welds and can assist with critical process related decision making. A range of hole base diameters were identified which produced welds with low defect populations and fatigue performance similar to that of the parent plate. Larger hole base diameters were shown to produce significant defects along the hole bottom fillet, in the weld nugget and along the bond line. Temperature measurements of the larger diameter welds showed a delay in response and are attributed to a delayed contact of plasticised stud material with the sidewall. Welds with hole base diameters larger than 11mm produced unrepeatable and defective welds, and also required higher energy inputs making smaller diameters more desirable. Analysis of all welds in this study revealed that clearance and interfacial pressures characterise the quality of friction hydro pillar process welds, therefore models were developed to aid in critical decision making with respect to downward force and geometry. This study has successfully evaluated the effects of process parameters and geometry on the properties of friction hydro pillar process welds and thereby has increased understanding of the process.

A multi-input multi-output system to implement on-line process monitoring and intelligent control of complex curvature friction stir welding was proposed. An extra rotation axis was added to the existing three translation axes to perform friction stir welding of complex curvature other than straight welding line. A clamping system was designed for locating and holding the workpieces to bear the large force involved in the process between the welding tool and workpieces. Process parameters (feed rate, spindle speed, tilt angle and plunge depth), and process conditions (parent material and curvature), were used as factors for the orthogonal array experiments to collect sensor data of force, torque and tool temperature using multiple sensors and telemetry system. Using statistic analysis of the experimental data, sensitive signal features were selected to train the feed-forward neural networks, which were used for mapping the relationships between process parameters, process conditions and sensor data. A fuzzy controller with initial input/output membership functions and fuzzy rules generated on-line from the trained neural network was applied to perceive process condition changes and make adjustment of process parameters to maintain tool/workpiece contact and energy input. Input/output scaling factors of the fuzzy controller were tuned on-line to improve output response to the amount and trend of control variable deviation from the reference value. Simulation results showed that the presented neuro-fuzzy control scheme has adaptability to process conditions such as parent material and curvature changes, and that the control variables were well regulated. The presented neuro-fuzzy control scheme can be also expected to be applied in other multi-input multi-output machining processes.

Heat generation during the FTSW process plays and important role in determining the characteristics of the weld. In order to obtain temperature fields, a transient temperature heat analysis is required. An area is the maximum temperatures reached within the base material during the FTSW process. These temperatures will be measured during experimentation, and compared to the welding simulation done using FEA. From the literature search it appeared that no heat transfer analysis had been done using finite element methods.

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The static friction of steel under boundary lubricated conditions was investigated both experimentally and theoretically. The theoretical model was developed using the assumption that during the application of a tangential load to a friction couple, the real area of contact grows in a viscoelastic manner until a critical shear stress is reached. Using this model, it was possible to distinguish the effect of static and dynamic contact time on area growth and thus to show why the traditional "time dependence of static friction" theories have limited validity. The model predicts that µs, the static friction coefficient, is a function of the rate parameter θ, and that a relaxation time can be assigned to a given interface. Subsequent experimental work using steel surfaces in vacuum as well as steel surfaces lubricated by various surface films showed that surface conditions play a large role in determining the exact µs - θ relationship for a given friction couple. Over the range of θ investigated the static friction coefficient of steel is constant if certain surface films are present; for other films the static friction coefficient vs θ curve shows an upper and lower asymptote. In the latter case a relaxation time was assigned to each boundary lubricant. For given asymptotes these relaxation times can be used to predict whether the film will be a useful lubricant at a particular θ. A subsequent investigation showed that the relaxation times are strongly affected by temperature. Since raising the substratum temperature results in smaller relaxation times, it is obvious that a particular lubricant may become ineffective as the substratum temperature changes. Both the experimental and theoretical work clearly demonstrate that the static friction of steel can be significantly modified by the application of appropriate boundary lubricants. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

The study of frictional drag force reduction has been the subject of much research for many years. One of the significant developments in the field of frictional drag force reduction has been the discovery of the effectiveness of dilute polymer solutions in reducing drag. By using dilute high-polymer solutions frictional drag forces in pipe flow have been reduced by as much as 70 per cent.

The thesis aims to study the effects of the mixing of immiscible polymers on the frictional properties of rubber compounds. The novelty of this work is to consider the rubber as a heterogeneous material in which the microscopic inhomogeneities are the domains generated from the mixing of immiscible compounds. Systematic experimental tests are performed to investigate the frictional properties of two different groups of tyre compounds, both provided by Pirelli Tyre S.p.A. The first group is represented by the homogeneous compounds realized by using Natural Rubber (NR) or Styrene Butadiene Rubber (SBR) and different amounts of filler and Sulphur. The second group considers heterogeneous compounds, generated by mixing in different percentages the homogeneous compounds, to obtain compounds characterized by microscopic domains of NR and SBR. The experimental outcomes proved that the presence of domains increases the friction coefficients. A Dynamic Mechanical Analysis (DMA) is performed to correlate the dynamic properties of the specimens with the friction coefficient. The results of the DMA of the homogeneous compounds agree with the frictional properties while the heterogeneous compounds show intermediate dynamical properties in contrast with the frictional results. The DMA is not able to recognize the microscopic domains of the heterogeneous compounds interpreting the system as a uniform material, suggesting that more complex dynamics arise during the sliding. The dependence of friction on the composition of the material is also investigated by using a numerical approach. For this purpose, a straightforward model is chosen to investigate numerically the frictional behaviour of a rubber material starting from the microscopic properties. The model is discretized as a chain of blocks connected by springs and dampers. Playing with the microscopic parameters of the model, such as the elastic modulus and the damping coefficient, it is possible to link the macroscopic frictional response of the bulk to the microscopic characteristics that locally describes the interactions between the blocks. Firstly, the frictional properties of compounds characterized by i) uniformly distributed viscoelastic or elastic elements and ii) a combination of purely elastic and viscoelastic elements randomly distributed is compared. The numerical outcomes reveal an increase of the frictional properties for samples realized by mixing elastic and viscoelastic elements pointing out that the presence of different domains due to the mixing of two immiscible materials, affects the macroscopic frictional response. Secondly, a comparison between the experiments and the numerical simulations is performed to verify if the 1D model can correctly predict the observed experimental behaviour. The 1D model, in its simplicity, is unable to predict the increase of frictional properties observed experimentally testing the heterogeneous compounds, confirming that more complex interactions influence the friction as suggested by the DMA.

Due to new environmental regulations, the demand for brake systems producing low polluting emissions and keeping high performance, is increasing. Therefore, a reduction in the toxic substances contained in brake friction materials is required that still meets all applicable safety standards and retaining properties such as low wear, high temperatures resistance, friction coefficient stability and consistency. Among the various ingredients used in formulations, copper has recently become the subject of different regulations on brake pad materials and will be reduced, or even prohibited, in the coming years. Recent studies have found that brake pads are major contributors to the deposit of copper in rivers and lakes, and this has a toxic impact on the environment. This metal is currently added in brake material formulations because of its good physical properties and its contribution to the formation of a uniform and stable friction layer. Its characteristics make it a constituent that is not easy to replace and an adequate substitute covering all the roles of copper has not been found as yet. The present research aims at developing, producing and testing new formulations for brake pads without copper. A relatively wide range of friction materials was investigated, in order to understand the role that selected constituents have in friction and wear behaviour. In the first place, an investigation on the role of copper in friction materials was performed. The study was followed by the selection of a suitable replacement for this constituent in the formulation. To do so, different friction components were studied. Additionally, other aspects relating to friction materials, such as the deterioration of the binder, were subject of investigation. The novel formulations, produced starting from commercially available compositions, were ranked in terms of wear and friction behaviour by means of a pin-on-disc tribometer. Cylindrical specimens were produced directly from powders, so that constituents could be easily modified based on the test outcome. This is a very effective method to study the role of individual constituents in the mixture, considering the relatively small amount of each specific composition to be prepared and the ease of processing it. In order to identify the principal wear mechanisms and their dependence on material properties and test conditions, the worn materials were analysed via scanning electron microscopy (SEM) techniques and Energy Dispersive X-Ray Spectroscopy (EDXS). Part of the acquired knowledge from the first part of the work was used in the initial stages of the ECOPADS project (the project started during the doctoral period) to develop and manufacture real brake pads that were tested on brake dynamometers and evaluated in terms of both performance and emissions.

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