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M.Ing. / In the design and analysis of components and structures, detailed information on the material behaviour and its properties is required. When a material is loaded dynamically, such as in metal punching, the material properties may not be the same as when loaded statically. This is known as the strain rate sensitivity of a material, which implies that properties such as the yield strength, tensile strength and ductility may vary with the rate at which the material is loaded. South Africa is one of the large stainless steel producing countries. Seventy percent of the known chromium ore reserves are found in the Bushveld Igneous Complex in the Northern Province and Mpumalanga. To compete on the global stainless steel market it is essential that the South African producers have all the relevant product information directly available. Considerable research has been performed on mild steel at different strain rates and temperatures[1]. Work has also been done on some austenitic stainless steels. Very little, or no work has been done in this regard on ferritic and martensitic stainless steels and on the proprietary alloy 3CR12[2]. The aim of this thesis is to investigate the strain rate sensitivity of Types 304, 430 and 316 stainless steel, 3CR12 corrosion resistant steel and mild steel at different temperatures. To achieve this, tensile tests are performed. at strain rates between 10's -1 to approximately 100s -1 and at temperatures ranging from -40°C to 140°C. Shear tests are also performed at various strain rates, to investigate the effect that material behaviour has on a typical metal working process. The results obtained show that all the materials tested are strain rate sensitive. The strain rate sensitivity varies as a function of the material tested and the testing temperature. Constitutive models which take into account the strain rate sensitivity at room temperature for all the materials are also presented. These models describe the behaviour of the material fairly accurately. Three dimensional plots are also presented which depict how the yield strength, tensile strength and elongation vary as a function of both strain rate and temperature. These plots clearly show material trends for the strain rates and temperatures tested.

No description available.

Evaluation and characterisation of thermal barrier coatings (TBCs) have been conducted correlating microstructure with physical and mechanical properties, to further understand TBC failure mechanisms and performances in this thesis. A modified four-point bending test was employed to investigate the interfacial toughness of atmospheric plasma sprayed TBCs. The delamination of the TBCs occurred mainly within the topcoat. The energy release rate increased from ~50 J/m-2 for as-sprayed conditions to ~120 J/m-2 after annealing at 1150 ºC for 200 hours with a loading phase angle about 42º. Micro X-ray tomography revealed how various types of imperfections developed near the interface and the 3D interface was characterised. Stress measurements by photoluminescence piezospectroscopy (PLPS) and analytical solutions were combined to investigate the local stress around spherically symmetrical portions of a TGO layer formed on Fecralloy. Spherical indenters were used to create curvature with different curvature radii and depths on alloys. The effect of curvature radius on stress was found to be more significant than the depth of local curved area. TGO stress as a function of oxidation time at the curved areas was also discussed. Electron beam physical vapour deposited (EBPVD) TBCs with a β-(Ni,Pt)Al bond coat on CMSX4 substrate were investigated by micro X-ray computed tomography (XCT). The 3D microstructures evolution and damage accumulation were studied. 3D interfacial roughness was calculated and compared to scanning electron microscope image analysis. The calculated interfacial roughness did not change much even after 200 thermal cycles, indicating there was not obvious rumpling in this TBCs sample. Commercial simple and Pt-modified aluminide coatings were studied and compared. Both coatings consisted mainly of β-NiAl phase. Thermogravimetric analysis (TGA) tests indicated that the Pt-modified aluminide coating was much more resistive for oxidation than simple aluminide coating. Instrumented indentation was used to measure the mechanical properties, showing the coatings had similar young’s modulus around 130 GPa while Pt-modified aluminide coating was more ductile and had a higher fracture toughness than simple aluminide coating. The Raman spectra of yttria-stabilised zirconia (YSZ) in the temperature range of 25-1100 ºC were investigated. The peak shift and broadening were carefully analysed. The thermal mismatch stress was found to have a negligible effect on the Raman shift. The dependence can be used to monitor the temperature in YSZ without contact.

The processing of materials has become a specialist field and the industry will continue to grow due to rising costs in labour and raw materials which has forced many automotive industry suppliers to invest heavily in this field. In order to be relevant and competitive in today’s industrial world, companies in South Africa are now forced to dedicate billions of rands in profits to research and development. Metals like titanium are finding favour with automotive and aviation companies in pursuit of savings in fuel consumption. This saving is achieved by reducing weight on aircraft and automobiles yet still meeting acceptable and improved structural integrity. In-depth research into the behaviour of various materials under varying loading conditions is therefore essential. The study on the processing of commercially pure grade 2 titanium alloy plates focuses on the development of process parameters for bending the material using a 4kW Nd: YAG laser to an approximate radius of curvature of 120mm. The resulting mechanical properties of laser formed plates are then compared to those obtained from mechanically formed samples. The titanium parent material was used to benchmark the performance of formed samples. The effect of process parameters on the mechanical properties and structural integrity also formed part of this study. To obtain the bending parameters for laser forming, various combinations of processing speeds and laser powers were used. The line energy is dependent on the power and scanning velocity parameters and these are shown in table 1. The laser power, line energy and scanning velocity were the main parameters controlled in this study and the beam diameter remained unchanged. Residual stress analysis, micro-hardness and fatigue life testing were carried out to analyse mechanical properties and the structural integrity of the plate samples. Microstructural analysis was also done to observe changes in the material as a result of the forming processes. From the results it is evident that laser forming is beneficial to the hardness of titanium but detrimental to the fatigue life at higher line energies. Residual stress analysis showed the amount of stress within the study samples increased with each forming operation. This information was vital in the analysis of the fatigue life of titanium. A fatigue life prediction model was developed from this study and it shed some light on the behaviour of titanium in fatigue testing. The model could be used to predict fatigue life when no fatigue data is available for commercially pure grade 2 titanium alloy plates. In conclusion, this study helped establish parameters that could be used to bend titanium while the analysis of mechanical properties showed the limits of working with this alloy.

The processing of materials has become a specialist field and the industry will continue to grow due to rising costs in labour and raw materials which has forced many automotive industry suppliers to invest heavily in this field. In order to be relevant and competitive in today’s industrial world, companies in South Africa are now forced to dedicate billions of rands in profits to research and development. Metals like titanium are finding favour with automotive and aviation companies in pursuit of savings in fuel consumption. This saving is achieved by reducing weight on aircraft and automobiles yet still meeting acceptable and improved structural integrity. In-depth research into the behaviour of various materials under varying loading conditions is therefore essential. The study on the processing of commercially pure grade 2 titanium alloy plates focuses on the development of process parameters for bending the material using a 4kW Nd: YAG laser to an approximate radius of curvature of 120mm. The resulting mechanical properties of laser formed plates are then compared to those obtained from mechanically formed samples. The titanium parent material was used to benchmark the performance of formed samples. The effect of process parameters on the mechanical properties and structural integrity also formed part of this study. To obtain the bending parameters for laser forming, various combinations of processing speeds and laser powers were used. The line energy is dependent on the power and scanning velocity parameters and these are shown in table 1. The laser power, line energy and scanning velocity were the main parameters controlled in this study and the beam diameter remained unchanged. Residual stress analysis, micro-hardness and fatigue life testing were carried out to analyse mechanical properties and the structural integrity of the plate samples. Microstructural analysis was also done to observe changes in the material as a result of the forming processes. From the results it is evident that laser forming is beneficial to the hardness of titanium but detrimental to the fatigue life at higher line energies. Residual stress analysis showed the amount of stress within the study samples increased with each forming operation. This information was vital in the analysis of the fatigue life of titanium. A fatigue life prediction model was developed from this study and it shed some light on the behaviour of titanium in fatigue testing. The model could be used to predict fatigue life when no fatigue data is available for commercially pure grade 2 titanium alloy plates. In conclusion, this study helped establish parameters that could be used to bend titanium while the analysis of mechanical properties showed the limits of working with this alloy.

The dynamic response of various geological materials has been investigated through a series of plate impact experiments. The materials involved were supplied from various mines by De Beers and Rio Tinto and were generically termed: sandstone, scilified siltstone, kimberlite, quartz/feldspathic gneiss, biotite schist, amphibolite, amphibolitic gneiss, basalt and iron ore. Investigations into compressional, shear and tensional behaviour were carried out. This project was part of a larger international study to develop models for the explosive loading of rock in a mining environment. This model is known as the Hybrid Stress Blasting Model, or HSBM. For this model to be accurate and relevant to the mining process it is essential to have dynamic data on the various rock types concerned. This was the purpose of the current project. As the material data are destined for use in a computer modelling programme it was essential to attempt to develop prediction methodologies to avoid the need for expensive dynamic characterisation of any new materials encountered in the mining environment. Much of the static data provided with the materials from De Beers proved of little use in predicting behaviour, although crucially it was not possible to determine sufficient dynamic tensile strengths in this investigation to make comparisons with the De Beers data. More success was found in predicting the slope of the Hugoniot with the elastic impedance of the material (for the non-porous linear Hugoniot materials). A fairly strong trend was found, which was backed up with data from the literature. Additionally some effort at further analysis using mineral data was undertaken. Attempts at predicting the HEL were also partially successful. While no specific quantitative prediction method was found, it was noted that the HEL did seem to scale with grain size, in that the large grained materials had a lower value of the HEL (below 2 GPa) compared with the finer grained materials (around 4 GPa and above).

Restoring both anterior and posterior teeth with resin-composite materials is now an established clinical procedure with almost universal acceptance. The clinical performance of these restorations in the patient’s mouth is determined by a number of factors including the clinical techniques involved in their placement, the patient’s oral habits, and the physical and mechanical properties of the restorative materials themselves. These materials are being increasingly used in load-bearing areas of the posterior dentition and are therefore inevitably subject to masticatory forces of varying magnitude. The success of different resin-composites in different applications is understood through their clinical performance and laboratory-based experimental evaluation.My research was divided into two parts; the first part was concerned with the examination of different types of contemporary restorative resin-composites and in the second part, I compared different methods of examination. In the first part, I investigated and compared different sets of varied types of resin-composites, such as flowable resin-composites, bulk-fill resin-composites and conventional resin-composites. Using different sets of these materials, I examined a number of properties that affect their clinical performance and durability.In the second part, I studied and compared the conventional (macroscopic) methods of investigation with nanoindentation method. Both methods were applied to examine and characterise different properties for some types of resin-composites.The flowable and the bulk-fill resin-composites exhibited satisfactory results comparable with conventional resin-composites. The properties investigated included strength properties, modulus of elasticity, hardness and viscoelastic time-dependent creep deformation. The results obtained by nanoindentation confirmed that this method of examination is a valuable experimental tool to investigate and characterise some mechanical properties of resin-composites.