The generation of electricity to meet an ever-growing demand has become a defining characteristic of the modern world for both developed and developing nations alike. This, coupled with the intensifying concern with pollution and its effects on the environment has put immense pressure on how quickly and efficiently power is produced. Being the most prevalent source of electricity generation, coal fired power plants have been subject to increasing scrutiny and study in an effort to improve the efficiency at which they operate. Hence, coal fired power plants are being run at increased temperatures and pressures such as those observed in Super-critical and Ultra-super-critical plants. This has by extension put excessive demand on materials used in these plants specifically within the boiler and superheater pipe sections where the most extreme thermodynamic conditions are experienced. The most commonly used materials for these applications are in the family of ferritic/martensitic 9-12% Cr steels chosen for their superior material properties especially during long-term exposure as coal fired power plants typically operate for over 20 years before being decommissioned. One of the lesser understood aspects of 9-12%Cr steels is with regard to their long-term material properties specifically that of creep degradation and deformation. This has been partially due to the reliance of creep life predictions in the past being based on accelerated creep testing and empirically based modelling. With the relatively recent revelations of empirically based modelling shown to be inaccurate when extrapolated to the long-term, a need has been identified amongst researchers to develop more accurate models based on physical relationships and material microstructure. Moreover, the insight obtained from modern experimental techniques and technologies as well as ever-expanding computing capabilities provide an opportunity to produce microstructurally based models with a high degree of complexity. Thus motivated, the focus of this dissertation was to develop a physically based dislocation creep model using the Continuum Damage Mechanics (CDM) approach. A dislocation CDM model was developed and implemented in the current work for uniaxial creep loading using the numerical modelling software Matlabᵀᴹ. The CDM approach was built upon fundamental dislocation theory as well as other microstructural considerations pertaining to dislocation creep including subgrain coarsening, M₂₃C₆ precipitate coarsening and stress redistribution. The CDM model was found to require calibration in order to be applied to specific 9- 12% Cr steels which was implemented using a parameter optimisation routine. The results obtained were compared with experimentally obtained, long-term creep-time and microstructural data for the 11% Cr steel CB8 and the 9% Cr steel P92. The CDM creep-time predictions were found to vary in accuracy depending upon the experimental data against which the model was calibrated. Upon further investigation, it was hypothesised that the discrepancy observed was due to the formation of the Modified Z-phase in some of the long term creep data but not in others which was based primarily on the differing creep exposure times of the various samples. The CDM creep-time predictions for P92 were found to be accurate when compared with experimental results regardless of creep exposure times. The apparent difference in the approximation of the creep deformation for the two steels was concluded as being due to the formation of the Modified Z-phase in CB8 but not in P92 as Modified Zphase formation is intrinsically linked with the Cr content of the steel.

Solid particle erosion tests of glass, stainless steel, WC-Co and sintered alumina, have been performed with seven erodents using a range of particle diameters D (63 μm - 1000 μm), velocities V (33 m.s⁻¹ - 131 m.s⁻¹ ) and impact angles α (30° - 90°). The seven erodents are steel shot, glass beads, silica, alumina, tungsten carbide, silicon carbide and diamond· particles. In addition, the target materials have been subjected to cavitation erosion using a conventional ultrasonic horn in distilled water. Systematic studies of the influence of the impact variables on the erosion rate have been made. Scanning electron microscopy of the eroded surfaces and the erodents after impact has been performed. Empirical correlations between erosion rate and the parameters of erosion and the erodents were obtained and are discussed in terms of the modes and mechanisms of erosion. A semi-quantitative theoretical model has been developed to explain the empirical correlations for brittle and ductile materials. The mode of erosion of glass impacted by irregularly shaped particles is associated with the formation and interaction of lateral cracks over all impact velocities and angles used in this study. The erosion of glass by spherical particles is determined by particle size, impingement velocity and angle. An erosion map, in which the erosion of glass is mapped against velocity and particle size, has been constructed to categorise the types of damage observed in glass for impingement angles between 90° and 30°. The erosion. behaviour of 304 stainless steel is associated with cutting or ploughing and plastic accumulation processes. The erosion of WC-Co is associated with a combination of ductile and brittle modes of erosion. The erosion of alumina is brittle and associated with intergranular spallation and grain-crushing. An analysis of the results reveals that for the brittle materials, glass and alumina, the erosion rate is determined by kinetic energy, particle size and the relative hardness and toughness of the erodents. However, for ductile materials, the shape and kinetic energy of erodents are the most important factors determining the erosion rate. There is no significant effect of hardness and toughness of erodents on erosion. Surprisingly, the erosion resistance of the softer 304 stainless steel is better than that of alumina and WC-Co when hard erodents are used at impact angle greater than 40°. On the other hand the erosion resistance of the harder WC-Co and alumina is better than that of 304 stainless steel for softer erodents like silica erodents. Glass always exhibits poor erosion resistance. In cavitation erosion, stainless steel exhibits better cavitation erosion resistance than glass, alumina and WC-Co. The cavitation erosion resistance of WC-Co is dependent upon the cobalt content. An attempt to rationalise the results in terms of mechanisms has been made. Both solid particle and cavitation erosion rate for the as received glass is higher than that for the tempered glass due to introduction of residual compressive stresses into the surface by the tempering process. Particularly, it reveals that compressive stresses are more efficient in preventing the formation and propagation of Hertzian cracks. These findings will assist in the choice and design of materials that undergo both particle and cavitation erosion under specified conditions.

Uniaxial tensile testing has been undertaken on a range of aluminium base alloys. Material investigated included commercial binary Al-Mg (5182), ternary Al-Mg-Si (6061) and quaternary Al-Cu-Mn-Si (2014) as well as experimental alloys containing 2at.% additions of Ag, Mg and Zn to commercially pure AI (1070). In addition, composite materials based on both alloys 2014 and 6061, containing 10%, 15% and 20% additions of Ah03 particulate, as well as 20% SiC particulate in the case of 6061, were also tested. Microstructures of materials were varied by prior heat treatments but, for comparison, all materials, were initially tested in the solution treated and quenched condition. Mechanical testing was undertaken at room temperature throughout the course of the work, and at strain rates such that serrated tensile test curves were manifest. The evolution of microstructural features of the deformation was evaluated utilising both optical and electron microscopy. Surface deformation features, including the formation of both type A and type B deformation markings, was examined on pre-polished specimen gauge lengths at various levels of tensile strain. The planarity of slip line traces was correlated with the evolution of related deformation structures in dynamic experiments in a high voltage transmission electron microscope (HVEM). In addition, the formation of slip lines on the surface of the HVEM microtensile specimens compared favourably with those formed on the surfaces of macroscopic tensile specimens. Microscale heterogeneities in the deformation observed during in-situ dynamic HVEM experiments on poly crystalline material correlated with the extent of serrated flow manifest in bulk specimens. All materials deformed in the HVEM displayed inhomogeneous dislocation motion consistent with the macroscopically observable discontinuities. The alloys tested were microstructurally distinguishable during dynamic experiments depending primarily on whether or not they had been deliberately alloyed with magnesium. The alloys containing Mg exhibited the activation of parallel slip traces together with minimal cross-slip in any single micro-yield event. In contrast, the alloys which did not contain Mg exhibited the simultaneous activation of various intersecting slip systems and were characterised by extensive cross-slip during similar yield events. On the basis of these observations, the magnitude of serrations and extent of serrated flow in the alloys has been discussed. The extent to which the different alloys were able to undergo dynamic recovery affected both the evolution of the dislocation structure observed in the conventional transmission electron microscope ( CTEM) as well as the final fracture mode. The existence of a characteristic shear fracture mode was consistently observed to follow tensile deformation which had been dominated by unstable plastic flow. The ready occurrence of dynamic recovery and the associated formation of dislocation cell structures allowed for more fully developed plastic instability during the final stages of tensile deformation and a lower likelihood of final failure by premature shear. Finally, the addition of particulate reinforcement to 2014 and 6061 had different effects that were accounted for by the difference in strength between the two monolithic materials. In the case of the weaker 6061, all particulate additions had a strengthening effect whereas in 2014, increasing the volume percent of reinforcement progressively weakened the composite. Serrated flow properties of both alloys were affected by the addition of the particulate reinforcement. The homogeneity of particle distribution as well as the size of the particulate inclusions affected both the tensile properties and final fracture of the composites.

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No description available.

Cartilage tissue engineering (TE) is a promising osteoarthritis therapy whereby cell-seeded constructs are generated in vitro for use in restoring degenerated cartilage in patients. While cartilage TE technology has exhibited growing clinical success, it continues to be encumbered by the utilization of high cost and laborious protocols, such as the need for frequent replenishment of culture media (every other day) during the duration of standard in vitro cultivation phases (2-8 weeks). This constitutes a significant time/cost burden for researchers and clinical technicians. Interestingly, the adoption of this convention is based on traditional cell culture protocols, rather than on a fundamental understanding of the stability of culture media constituents in current cartilage TE culture systems, leading one to consider that current TE replenishment protocols may be far from optimized. In the current study, we hypothesize that larger media volumes can be used to: 1) mitigate the depletion of constituents and accumulation of waste products in tissue constructs over time and accordingly, 2) reduce the media replenishment frequency required to generate engineered cartilage with functional mechanical properties and composition. Bovine chondrocyte-seeded agarose constructs (Ø4mm×2mm) were cultivated for 7 weeks in chondrogenic media of increasing cumulative media volumes (3mL, 6mL, 9mL, 18mL, and 54mL) and replenishment frequencies, including the conventionally utilized thrice-weekly and lowered frequencies of weekly, biweekly, and replenishment-free. The stability of influential media constituents (glucose, ascorbic acid, insulin), waste product accumulation (assessed via pH), and the properties of constructs were monitored over time. Results demonstrated that concentrations of growth-promoting media constituents and pH decreased over culture duration but this decrease can be mitigated by the use of larger replenished media volumes. For all replenishment frequencies, tissue construct mechanical properties and sulfate glycosaminoglycan (sGAG) content generally increased with replenished media volumes. For weekly, biweekly, and replenishment-free frequencies, the generation of constructs with native properties required the higher replenished media volumes per replenishment but did not require the use of higher cumulative media volumes. These results suggest that functional engineered cartilage can be generated with lower media replenishment frequencies or replenishment-free conditions. These protocols may be adopted in clinical and research-grade TE platforms to reduce labor costs and contamination risk. / 2023-09-24T00:00:00Z

Stress corrosion cracking (SCC) is a corrosion phenomenon which continues to plague the power generating industry especially in low pressure (LP) steam turbine blades operating in the phase transition zone. An investigation has therefore been conducted to examine the effect of heat treatment condition on the microstructure, mechanical properties and SCC properties of one such LP turbine blade material, FV520B, used in the steam turbines of coal-fired power stations in South Africa. The three stage heat treatment cycle of the FV520B turbine blades consists of homogenisation at 1020°C for 30 minutes, solution treatment at 790°C for two hours and precipitation hardening at 545°C for six hours. In this study, the precipitation hardening temperature was varied in the range 430-600°C to investigate how this variation would affect the material and SCC properties. Hardness and tensile testing were performed to obtain mechanical properties while the investigative techniques used to characterise the microstructures were light microscopy, dilatometry, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Stress corrosion susceptibility for the different heat treatment conditions was quantified using U-bend specimens while crack growth rates and threshold stress intensities for SCC (KISCC) were measured using fatigue precracked wedge open loaded (WOL) specimens. Both SCC tests were conducted in a 3.5% NaCl environment maintained at 90°C. XRD results revealed the presence of reverted austenite in the higher tempered specimens due to the precipitation hardening temperature being close to the Ac1 temperature for the material. The presence of reverted austenite was shown to adversely affect mechanical strength and hardness which decreased with increasing precipitation hardening temperature. Light and electron microscopy (SEM and TEM) revealed the presence of Cr-rich precipitates along the prior austenite grain boundaries in all tested heat treatment conditions. The propensity, quantity and size of the Cr-rich precipitates increased as the specimen temper temperature increased. SCC susceptibility was shown to be dependent upon yield strength and decreased as precipitation hardening temperature increased with specimens in the overaged condition showing no cracking after more than 5000 hours in the test environment. WOL testing only produced cracking in the three highest strength specimens after 2000 hours. Crack growth rates and threshold stress intensities were found to be dependent on yield strength and decreased with increasing precipitation hardening temperature. Analysis of fracture surfaces revealed crack propagation along prior austenite grain boundaries in all test heat treatment conditions indicating intergranular stress corrosion cracking (IGSCC) as the dominant cracking mechanism.

Abstract Salt water environments are very harsh on materials that are used within them. Many issues are caused by either corrosion and/or internal degradation to the materials themselves. Composites are better suited for this environment due to their high strength to weight ratios and their corrosion resistance, but very little is known about the fracture mechanics of composites. The goal of this study is to gain a better understanding for the behavior of a composite boat hull under a shear loading, similar to the force water applies on the hull as the boat moves through the water; then attempt to strengthen the composite sandwich panel against the shear loading. A parametric study was conducted to investigate monotonic in-plane shear loading for composite sandwich panels used in commercial naval vessels. In order to model a conventional composite boat hull, test specimens were composite sandwich panels made of a Divinycell H100 foam core with four layers of fiberglass on both sides of the core. Specimens were tested under a monotonic loading with a rate of 0.2 in/min, and tested until complete failure using the standard test. Seawater specimens were manufactured in the same manner as the original test specimens, but then were submersed in either filtered seawater or the ocean. The differences between the filtered pieces and the ocean allowed us to determine if any changes found in the composite sandwich panels were related to environment conditions or if the changes were related to the saltwater interaction itself. To create these different environments the seawater specimens were taken to the Avila pier where 36 specimens were placed in a tub that was fed filtered saltwater, while 30 specimens were placed in a plastic mesh with weights and lowered to a depth of approximately 30 ft. in the ocean. Three specimens were then removed at monthly intervals from both filtered and ocean environments. Shear Keys were created as a method to strengthen the composite sandwich panels against the shear force that the previous specimens had been tested to. Eight Shear Keys were then placed into groves cut into the foam core (four on each side) and the four fiberglass layers were laid on top. Testing showed that the seawater did have an initial effect on the composite sandwich panels. The filtered pieces showed a decrease in yield strength and stiffness the longer they were subjected to the seawater. The raw unfiltered pieces placed in the ocean saw an even higher decrease in their yield strength and decrease in stiffness. However, for both the unfiltered and raw specimens there was an increase in the ultimate strength and fracture point of the specimens. The effects of the sea water seemed to taper off after the 3rd month however. The Shear Key specimens were tested with a 4mm and an 8mm Shear Key. The 8mm Shear Keys showed a decrease in shear strength, which was primarily due to removing too much material from the core and weakening the specimen. It was concluded that the decrease in area created a force concentration at the deepest part of the Shear Key causing the premature failure. The 4mm Shear Key showed an increase in the yield strength, ultimate strength, and fracture point. A finite model was built to simulate the original test specimen along with the 4mm and 8mm Shear Key cases, and the results were compared to the experimental results. The numerical results showed that it was possible to relate the experimental results to the linear or elastic portion of the plots. There was a difference between the maximum displacement of the model and the actual specimens, but this was attributed to potential inaccurate comparison of the loading on the model compared to the actual specimens. The correlation between the model itself and the experimental data was close enough to conclude that it could be used for predicting baseline trends. Further investigation of the specimens should include looking into the effects of a cyclic shear loading on the specimens. This combined with the seawater element used in this thesis would provide further insight to the initial degradation seen in the seawater specimens, and could potentially provide a closer relation to current hull failures. In addition to including a cyclic loading another numerical model should be created. A model that could be constrained both locally and globally would provide more accurate results. The FEM should also include the ability to run a crushable foam core model within the solver which would also increase the accuracy of the numerical solution.

No description available.

The narrow band gap semiconductor iron antimonide, FeSb2, has been grown as bulk single crystals, and investigated for its electronic and magnetic properties at low temperatures. The electronic behavior of FeSb2 is associated with strong electron correlations, similar to the characteristic behavior observed in FeSi, another strongly correlated d-electron correlated semiconductor. Recent studies found an enhancement of the Seebeck coefficient and thermopower, two key parameters in thermoelectric performance, at temperatures below 77K, making FeSb2 potentially useful in cryogenic Peltier-cooling applications below liquid nitrogen temperatures. Crystals with sizes of 1.5-5.0 mm are grown using three different synthesis approaches: chemical vapor-phase transport (CVT), using either chlorine (Cl2(g)) or iodine (I2(g)) as transporting agents, and molten flux growth using excess antimony (Sb). Single crystal and powder x-ray diffraction experiments on select samples provided for structural phase identification and confirmed the absence of secondary impurity phases. FeSb2 crystallizes in the FeS2-marcasite structure type, featuring FeSb6 edge-sharing octahedra forming chains along the c-axis and corner-sharing in the a-b plane. The crystal surfaces were characterized using SEM-EDS and AFM measurements, revealing the characteristic morphological features resulting from CVT growth, as well as helping to identify surface contamination. Magnetic susceptibility measurements show weak temperature induced paramagnetism above 50K, with the diamagnetic-to-paramagnetic crossover above 100K. Temperature-dependent electronic transport properties, ρ(T), showed semiconducting behavior and the formation of a resistivity plateau between 10K-40K, corresponding to a secondary transport gap, εg, of about 3.9-7.2meV, similar in magnitude and in accordance to previous studies on FeSb2. Hybridization between iron (Fe) 3d and antimony (Sb) 5p and 5s valence states appears to be responsible for complex electronic behavior at low temperatures, with the formation of a small secondary gap. Earlier studies have shown a significant enhancement of the Seebeck coefficient (thermopower) at the onset of this gap (10K-12K). Thermopower measurements in our sample also show peak values around this temperature, but the maximum values are several orders of magnitude lower than previous reports, while magnetic and electronic properties are in good agreement. / A Thesis submitted to the Interdisciplinary Program in Materials Science in partial fulfillment of the requirements for the degree of Master of Science. / Summer Semester, 2012. / May 22, 2012. / Correlated Systems, FeSb2, FeSi, Peltier cooling, Seebeck Coefficient, Thermoelectrics / Includes bibliographical references. / Theo Siegrist, Professor Directing Thesis; Eric Hellstrom, Committee Member; James Brooks, Committee Member.