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SUMMARY GTD-111 DS is a directionally solidified superalloy currently used in turbine engines. To accurately predict the life of engine components it is essential to examine and characterize the microstructural evolution of the material and its effects on material properties. The as-cast microstructure of GTD-111 is highly inhomogeneous as a result of coring. The current post-casting heat treatments do not effectively eliminate the inhomogeneity. This inhomogeneity affects properties including tensile strength, fracture toughness, fracture path, and corrosion behavior, primarily in terms of the number of grains per specimen. The goal of this work was to link microstructural features to these properties. Quantitative fractography was used to determine that the path of cracks during failure of tensile specimens is influenced by the presence of carbides, which are located in the interdendritic regions of the material as dictated by segregation. The solvus temperature of the precipitate phase, Ni3(Al, Ti), was determined to be 1200C using traditional metallography, differential thermal analysis, and dilatometry. A heat-treatment was designed to homogenize the microstructure for tensile testing that isolates the carbide by dissolving all of the eutectic Ni3(Al, Ti) precipitate phase, which is also found in the interdendritic areas. High temperature oxidation/sulfidation tests were conducted to investigate the corrosion processes involved when GTD-111 DS is utilized in steam and gas combustion turbine engines. The kinetics of corrosion in both oxidizing and sulfidizing atmospheres were determined using thermogravimetric analysis. Additionally, metallography of these samples after TGA revealed a correlation between the presence of grain boundaries and sulfur attack, which led to catastrophic failure of the material under stress-free conditions in a sulfur bearing environment. In summary, this work correlates the inhomogeneous microstructure of GTD-111 DS to tensile fracture, and the corrosion process in turbine engines.

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Limited experimentation has been conducted on fractography of fresh versus dry bones. The present project examined the presence of select fractographic features on wet and dry bone specimens over a time interval of 15 months. The experimental remains consisted of a total of 81 white-tailed deer (Odocoileus virginianus) long bones as a proxy for human long bones. A subsample of 15 long bones that were defleshed of most external soft tissue was subjected to blunt force trauma every 30 days for a total of three months. After these three months passed, a subsample of 15 bones was subjected to blunt force trauma every 90 days for the remaining 12 months. Following fracturing, the long bones were macerated and the fractures on the long bones were inspected, and observations were recorded and photographed. The author hypothesized that the presence (or absence) of fractographic features including hackle patterns, bone mirror, cantilever curls, and arrest ridges, on the fractured long bones would differ on fresh versus dry bones. Therefore, the difference in fractographic features found on the fresh versus dry bones would allow greater separation of perimortem from postmortem fractures. Other fracture characteristics such as fracture angle, fracture surface texture, fracture jaggedness, number of fragments produced, and type of fracture produced were also observed as part of the data collected in this research to potentially confirm the findings and results of previous studies conducted on differentiating between perimortem and postmortem trauma on bone. This study disproved the hypothesis by concluding that the presence (or absence) of fractographic features is not greatly affected by time exposure and therefore, does not aid in distinguishing between fresh bone and dry bone fractures. Fractographic features were present and absent on bone specimens during all postmortem intervals. The only statistically significant difference discovered was that bone hackle patterns are more commonly observed than cantilever curls on bones with a later PMI. Other general trends observed were that the number of bones showing bone hackle patterns increased over time and the number of bones showing bone mirror decreased over time. In addition, the results of the study revealed that the only fracture characteristic that showed a slightly significant difference with time of exposure was the fracture surface texture produced. The probability of a bone showing intermediate fracture surfaces is statistically significantly higher than a bone showing rough fracture surfaces when the represented PMI is fresh. The probability of a bone showing intermediate fracture surfaces is statistically significantly higher than a bone showing smooth fracture surfaces when the represented PMI is dry. The present study showed that the fracture characteristics including fracture angle, fracture type, number of fragments produced, and fracture jaggedness were not greatly influenced by exposure of time but, certain patterns and trends were recognized. The number of bones showing sharp fracture angles increased over time, while the number of bones showing intermediate fracture angles stayed stagnant. Bones showing comminuted fractures also increased with the progression of drying time. The average number of fragments produced were high during both fresh and dry PMIs and low throughout the transitional postmortem intervals. Bones showing jagged, intermediate, and not jagged broken ends increased with the progression of time however, not jagged broken ends only began to appear in the sample starting at a PMI of 90 days.

This thesis looks into the examination of polymer composite wreckage from the perspective of the aircraft accident investigator. It develops an understanding of the process of wreckage examination as well as identifying the potential for visual and macroscopic interpretation of polymer composite aircraft wreckage. The in-field examination of aircraft wreckage, and subsequent interpretations of material failures, can be a significant part of an aircraft accident investigation. As the use of composite materials in aircraft construction increases, the understanding of how macroscopic failure characteristics of composite materials may aid the field investigator is becoming of increasing importance. The first phase of this research project was to explore how investigation practitioners conduct wreckage examinations. Four accident investigation case studies were examined. The analysis of the case studies provided a framework of the wreckage examination process. Subsequently, a literature survey was conducted to establish the current level of knowledge on the visual and macroscopic interpretation of polymer composite failures. Relevant literature was identified and a compendium of visual and macroscopic characteristics was created. Two full-scale polymer composite wing structures were loaded statically, in an upward bending direction, until each wing structure fractured and separated. The wing structures were subsequently examined for the existence of failure characteristics. The examination revealed that whilst characteristics were present, the fragmentation of the structure destroyed valuable evidence. A hypothetical accident scenario utilising the fractured wing structures was developed, which UK government accident investigators subsequently investigated. This provided refinement to the investigative framework and suggested further guidance on the interpretation of polymer composite failures by accident investigators.

This thesis looks into the examination of polymer composite wreckage from the perspective of the aircraft accident investigator. It develops an understanding of the process of wreckage examination as well as identifying the potential for visual and macroscopic interpretation of polymer composite aircraft wreckage. The in-field examination of aircraft wreckage, and subsequent interpretations of material failures, can be a significant part of an aircraft accident investigation. As the use of composite materials in aircraft construction increases, the understanding of how macroscopic failure characteristics of composite materials may aid the field investigator is becoming of increasing importance. The first phase of this research project was to explore how investigation practitioners conduct wreckage examinations. Four accident investigation case studies were examined. The analysis of the case studies provided a framework of the wreckage examination process. Subsequently, a literature survey was conducted to establish the current level of knowledge on the visual and macroscopic interpretation of polymer composite failures. Relevant literature was identified and a compendium of visual and macroscopic characteristics was created. Two full-scale polymer composite wing structures were loaded statically, in an upward bending direction, until each wing structure fractured and separated. The wing structures were subsequently examined for the existence of failure characteristics. The examination revealed that whilst characteristics were present, the fragmentation of the structure destroyed valuable evidence. A hypothetical accident scenario utilising the fractured wing structures was developed, which UK government accident investigators subsequently investigated. This provided refinement to the investigative framework and suggested further guidance on the interpretation of polymer composite failures by accident investigators.

The effects of systematic variations in the size and volume fraction of reinforcing phase on the mechanical properties of and fracture processes in silicon carbide particlereinforced aluminium matrix composites have been studied. Tensile tests to failure have been performed to determine the mechanical properties of the composites. A simple model has been proposed for this behaviour. The micromechanisms of fracture have been investigated by a combination of fractographic and dynamic techniques. Matched fracture halves have been obtained from the composites and the fracture processes elucidated. Fracture proceeded by a ductile void nucleation, growth and coalescence mechanism. Void nucleation occurred at the reinforcing phase, with a change in nucleation mechanism on varying the micrstructural parameters. A simple critical stress criterion has been proposed for the nucleation process. Support for this proposal has been obtained by the study of sections through the failed tensile specimens. In situ scanning electron microscopy fracture studies have been performed. These revealed void nucleation before the onset of macroscopic cracking. Crack propagation has been shown to occur by the concurrent formation of microcracks ahead of the crack tip and failure of the joining matrix ligaments. The magnitude of matrix deformation has been shown to determine the extent of microcracking. Acoustic emissions have been monitored during tensile straining. Void nucleation events have been recorded from the onset of plastic deformation and continuing throughout the plastic régime until final failure. The suppression of void coalescence by the constaint imposed on matrix flow by rigidly-bonded interfaces has been proposed to account for the extended void growth in materials containing fractured particles. The importance of the local values of the microstructural parameters on the far-field strain at nucleation has been shown.