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The high temperature fatigue crack growth behaviour of the newly developed, metastable titanium-based alloy, TIMETAL 21S, was investigated in an inert and an oxidizing environment. The investigation adopted a two pronged approached, namely, to initially establish the pure microstructural behaviour under oxidizing and inert environments at various elevated temperatures, and consequently, to establish the environmental effects on the fatigue crack growth behaviour in the various environments at high temperature. The effect of the oxidizing environment on the metastable alloy and on the mechanical and chemical events occurring at the fatigue crack were studied by using optical and scanning electron microscopy, including ED X analysis, x-ray diffraction, and Auger Electron Spectroscopy (AES) . For the microstructural investigation, the TIMETAL 21S samples were exposed for 5 hours to a pure argon and argon + 20% O2 environment at 300°C to 750°C in increments of 50°C. The results showed that in the oxidizing environment a more homogeneous nucleation of the alpha phase had occurred at higher temperatures and that the oxide Ti02, in addition to the alpha case, had predominantly formed on the exposed surfaces. AES analysis showed that dissolution of the oxygen into the alloy occurred even at low temperatures. An LEFM approach was used to investigate fatigue crack growth rate (FCGR) of C(T) specimens at 375°C, 450°C, 550°C and 620°C in the argon and argon + 20% oxygen environment. The crack growth rates were monitored using load-line compliance and the beachmarking method - a method by which beach marks were impressed on the fracture surface to track the progressing crack. The results showed that the crack growth rates were lower in the oxidizing environment and was influenced by a synergistic effect of the temperature, stress intensity at the crack tip and the environment. In addition to the phenomena of crack tip shielding (a process whereby the effective crack tip driving force experienced at the crack tip was locally reduced), other mechanisms such as slip character modification and secondary cracking ahead of the crack tip, leading to crack tip blunting and branching, had to be incorporated to fully explain the crack growth behaviour. The tests conducted in the inert environment effectively excluded the effect of oxygen on the crack growth behaviour and substantiated that various mechanisms ultimately determined the FCGR in TIMETAL 21S at elevated temperatures. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 1995.

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M.Ing. / This dissertation investigates the fatigue and monotonic tensile properties of cast aluminium alloy wheels in various post cast conditions. It was found that monotonic tensile properties could be used in the original universal slopes method of Manson to predict the fatigue properties as a conservative first approximation for A356 cast aluminium alloy wheels. Using finite element analysis and the predicted fatigue properties the fatigue life of A356 aluminium alloy wheels could be determined. Further work is required on the surface effect of paint on the wheel and residual stress on the surface of the wheel.

This dissertation involves the experimental investigation of commercially pure titanium (CP Ti) which was subjected to laser forming and mechanical forming processes. Commercially pure titanium grade 2 was formed to a radius of curvature of approximately 120 mm using three forming procedures, i.e. i) laser forming; ii) mechanical forming (stretched forming) and iii) a combined forming process (laser-mechanical forming). Fatigue testing revealed, for all the forming processes, that samples produced by laser forming performed the best at high load settings. However, mechanically formed specimens performed the best at low load settings, whereas the laser-mechanical process resulted in midway performance between laser and mechanical processing. Considering microstructure vs fatigue; impact vs fatigue; and residual stress vs fatigue; at high load settings it is evident that the microstructure is the dominant contributor to crack initiation and growth. Crack morphology of fatigue samples revealed that secondary cracks (parallel to main crack front) followed the grain boundaries of the Widmanstätten microstructure, whereas irregular secondary cracks grew parallel and through the twinning planes and along the grain boundaries of the equiaxed microstructure. Laser forming resulted in microstructural changes from equiaxed grains to a Widmanstätten structure due to fast cooling rates. Excessive twinning is developed within the equiaxed microstructure after the mechanical forming procedure. This is due to cold working / strain hardening. The combined process shows a combination of equiaxed grains and Widmanstätten microstructure. Residual stress relieved for all forming processes revealed an increase in the magnitude of the residual stress compared to the parent plate and that the maximum values were obtained at the inner radius of curvature (i.e. 118.4 mm). Laser forming revealed the highest values in residual stress whereas the other two processes i.e. mechanical and laser-mechanical forming exhibited an increase midway between the parent plate and laser forming. The second most influential factor with regards to fatigue was the magnitude of the residual stress, especially at medium to low load settings. When considering theoretical models to predict fatigue life it was found that the Goodman model showed the closest relation to the actual fatigue data when considering the entire theoretical curve. Vickers microhardness profiling was applied to the thickness of the samples for the parent plate and all forming processes. No significant hardening occurred due to the forming processes and differences in hardness were considered negligible. Charpy impact testing revealed that the laser formed specimens exhibited the most brittle behaviour when compared to the parent plate results. Mechanical formed specimens showed a slight increase in brittleness compared to parent plate whereas the combined process yielded results midway between the laser and mechanically formed specimens. Mathematical equations are formulated and presented for predicting the fatigue life of CP Ti grade 2 for the parent plate and the three forming processes. This study proved that the laser forming process can be successfully used as a production stage in the forming of CP Ti grade 2.

Changes in pretreated 5182, 6061 and 7075 aluminum surfaces on exposure to moisture for short times was studied. The pretreatment used was the standard ASTM method for FPL etching of aluminum. The moisture treatment used was either immersion in water at 81° C or exposure to water vapor at 81° C. The experimental techniques used to analyze the pretreated aluminum surfaces before and after exposure to moisture were ESCA or XPS, AES, high resolution SEM, and specular reflectance FTIR. There was a change in the surface topography on exposure of the aluminum surfaces to water as determined using high resolution SEM. Stoichiometric calculations based on XPS analysis were made to estimate the amount of excess water present on the surface. Water was present on the surface before exposure to moisture, for all three alloys. The amount of water present on the surface was found to decrease with increasing times of exposure to water for all three alloys. This result was consistent with the model that pseudoboehmite formed on the surface was being converted into boehmite at longer times of exposure to water. The thickness of the oxide layer was found to increase with time of exposure to water based on ESCA results. The same conclusion was reached by depth profiling the oxide layer using AES. The rate of increase in the concentration of pseudoboehmite on the surfaces as calculated from FTIR data went in the order 7075 < 5182 < 6061. The activation energy for the third step (transport of soluble species to the surface) in the conversion of surface Al₂O₃ to AlOOH was calculated from FTIR results to be 3.5 kcal mol⁻¹. / Master of Science

Fatigue tests on compact tension specimens of 90-10 copper nickel were conducted in 3.5% NaCl solutions. Anodic or cathodic currents were applied during testing. Anodic currents decreased and cathodic currents increased the fatigue life. Both anodic and cathodic currents changed the fracture mode from predominantly transgranular to intergranular. Constant extension rate tests were performed on similar CT specimens in environments of 3.5% NaCl solution and 3.5% NaCl solution titrated to pH 1.0 with various levels of applied current. The environment had little influence on the monotonic failure of 90-10 copper nickel. Polarization studies were conducted to determine the effects of welding and pH on the corrosion behavior of 90-10 copper nickel. The rate of corrosion was less in the weld and the heat affected zone than in the base metal. As the pH of the environment was lowered, the corrosion rate of 90-10 copper nickel increased due to the retardation of film formation and repassivation. These studies showed that three different mechanisms of corrosion fatigue were likely: localized anodic dissolution, surface film rupture and hydrogen embrittlement. The dominance of one mechanism over the other two depends on the applied current. No evidence of susceptibility to stress corrosion cracking was found, therefore, a true corrosion fatigue process is operative in 90-10 copper nickel. / Master of Science / incomplete_metadata

A study has been carried out to investigate the growth and closure behavior of short cracks in 2024-T351 aluminum alloy and four different conditions of 4340 steel using through thickness cracks of straight fronts. The experiments were carried out to study the influence of stress level, stress ratio, yield strength and prior austenitic grain A sizes in notched and unnotched specimens. The stereoimaging technique was developed and adapted to obtain crack closing and opening points, and also near tip displacement fields. Experimental results are presented with a general discussion. It was found that long cracks showed good correlation when analyzed in terms of effective stress intensity range. However,correlations were poor for short cracks. lt was found that short cracks show less closure behavior than long cracks. The estimates of initial crack lengths based on linear elastic data were made. These estimates differed significantly from the actual initial crack lengths for completely reversed cycling tests. Suggestions have been made to the equivalent initial flaw size approach and conclusions have been drawn. / Ph. D.

The increasing use of alpha/beta Ti-Al alloys in critical aircraft gas turbine engine and airframe applications necessitates the further development of physically-based constitutive models that account for their complex microdeformation mechanisms. Alpha/beta Ti-Al alloys are dual-phase in nature consisting of a mixture of hcp (alpha) and bcc (beta) crystal structures, which through variation in alloying elements and/or processing techniques can be produced in a wide range of microstructural compositions and morphologies. A constitutive model for these materials should address the various sources of material anisotropy and heterogeneity at both the micro and macroscales. The main sources of anisotropy in these materials are the low symmetry of the hcp phase, the texture, the relative strengths of different slip systems, non-planar dislocation core structures, phase distributions, and dislocation substructure evolution. The focus of this work is the development of a 3-D crystal plasticity model for duplex Ti-6Al-4V (Ti-64), an (alpha+beta) alloy. The model is used to study the process of attachment fatigue. Attachment fatigue is a boundary layer phenomenon in which most of the plastic deformation and damage accumulation occurs at depths on the order of tens of microns and encompasses regions of only a few grains into the depth of the material. The use of computational micromechanics-based crystal plasticity models to study attachment fatigue is a relatively new approach. This approach has the potential to offer additional insight to classical homogeneous plasticity models, since the length scales over which relative slip and crack initiation occur during this process is on the order of microstructural dimensions. Emphasis is placed on understanding the effects that texture, slip strength anisotropy, and phase distribution have on the surface and subsurface deformation fields during attachment fatigue. The deformation fields are quantified in terms of cumulative effective plastic strain distributions, plastic strain maps, and plastic strain-based critical plane multiaxial fatigue parameters.