High cycle fatigue of AA6082 and AA6063 aluminum extrusions

The high cycle fatigue behavior of hollow extruded AA6082 and AA6063 aluminum extrusions has been studied. Hollow extruded aluminum profiles can be processed into intricate shapes, and may be suitable replacements for fatigue critical automotive applications requiring reduced weight. There are several features inherent in hollow aluminum extrusions, such as seam welds, charge welds, microstructural variations and die lines. The effects of such extrusion variables on high cycle fatigue properties were studied by taking specimens from an actual car bumper extrusion. It appears that extrusion die lines create large anisotropy differences in fatigue properties, while welds themselves have little effect on fatigue lives. Removal of die lines greatly increased fatigue properties of AA6082 specimens taken transverse to the extrusion direction. Without die lines, anisotropy in fatigue properties between AA6082 specimens taken longitudinal and transverse to the extrusion direction, was significantly reduced, and properties associated with the orientation of the microstructure appears to be isotropic. A fibrous microstructure for AA6082 specimens showed great improvements in fatigue behavior. The effects of elevated temperatures and exposure of specimens to NaCl solutions was also studied. Exposure to the salt solution greatly reduced the fatigue lives of specimens, while elevated temperatures showed more moderate reductions in fatigue lives.

Problems in the classical calculus of variations

This dissertation concerns convergence analysis for nonparametric problems in the calculus of variations and sufficient conditions for weak local minimizer of a functional for both nonparametric and parametric problems. Newton's method in infinite-dimensional space is proved to be well-defined and converges quadratically to a weak local minimizer of a functional subject to certain boundary conditions. Sufficient conditions for global converges are proposed and a well-defined algorithm based on those conditions is presented and proved to converge. Finite element discretization is employed to achieve an implementable line-search-based quasi-Newton algorithm and a proof of convergence of the discretization of the algorithm is included. This work also proposes sufficient conditions for weak local minimizer without using the language of conjugate points. The form of new conditions is consistent with the ones in finite-dimensional case. It is believed that the new form of sufficient conditions will lead to simpler approaches to verify an extremal as local minimizer for well-known problems in calculus of variations.

Paleoenvironmental reconstructions from cave sediments of the Moravian Karst, Czech Republic / by Pavel Sroubek.

This thesis presents a paleoclimatic/paleoenvironmental study conducted on clastic cave sediments of the Moravian Karst, Czech Republic. The study is based on environmental magnetic techniques, yet a wide range of other scientific methods was used to obtain a clearer picture of the Quaternary climate. My thesis also presents an overview of the significance of cave deposits for paleoclimatic reconstructions, explains basic environmental magnetic techniques and offers background information on the study area – a famous karst region in Central Europe with a rich history. In Kulna Cave magnetic susceptibility variations and in particular variations in pedogenic susceptibility yield a detailed record of the palaeoenvironmental conditions during the Last Glacial Stage. The Kulna long-term climatic trends agree with the deep-sea SPECMAP record, while the short-term oscillations correlate with rapid changes in the North Atlantic sea surface temperatures. Kulna Cave sediments reflect the intensity of pedogenesis controlled by short-term warmer events and precipitation over the mid-continent and provide a link between continental European climate and sea surface temperatures in the North Atlantic during the Last Glacial Stage. Given the number of independent climate proxies determined from the entrance facies of the cave and their high resolution, Kulna is an extremely important site for studying Late Pleistocene climate. In the interior of Spiralka Cave, a five meter high section of fine grained sediments deposited during floods yields information on the climatic and environmental conditions of the last millenium. In the upper 1.5 meters of this profile, mineral magnetic and other non-magnetic data indicate that susceptibility variations are controlled by the concentration of magnetite and its magnetic grain size. Comparison of our susceptibility record to the instrumental record of winter temperature anomalies shows a remarkable correlation. This correlation is explained by coupling of the flooding events, cultivation of land and pedogenetic processes in the cave catchment area. A combination of mineral magnetic and geochemical proxies yields a detail picture of the rapidly evolving climate of the near past and tracks both natural and human induced environmental changes taking place in the broader region.

Evaluation of flood risk in response to climate variability

Standard procedures for forecasting flood risk (Bulletin 17B) assume annual maximum flood (AMF) series are stationary, meaning the distribution of flood flows is not significantly affected by climatic trends/cycles, or anthropogenic activities within the watershed. Historical flood events are therefore considered representative of future flood occurrences, and the risk associated with a given flood magnitude is modeled as constant over time. However, in light of increasing evidence to the contrary, this assumption should be reconsidered, especially as the existence of nonstationarity in AMF series can have significant impacts on planning and management of water resources and relevant infrastructure. Research presented in this thesis quantifies the degree of nonstationarity evident in AMF series for unimpaired watersheds throughout the contiguous U.S., identifies meteorological, climatic, and anthropogenic causes of this nonstationarity, and proposes an extension of the Bulletin 17B methodology which yields forecasts of flood risk that reflect climatic influences on flood magnitude. To appropriately forecast flood risk, it is necessary to consider the driving causes of nonstationarity in AMF series. Herein, large-scale climate patterns—including El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), and Atlantic Multidecadal Oscillation (AMO)—are identified as influencing factors on flood magnitude at numerous stations across the U.S. Strong relationships between flood magnitude and associated precipitation series were also observed for the majority of sites analyzed in the Upper Midwest and Northeastern regions of the U.S. Although relationships between flood magnitude and associated temperature series are not apparent, results do indicate that temperature is highly correlated with the timing of flood peaks. Despite consideration of watersheds classified as unimpaired, analyses also suggest that identified change-points in AMF series are due to dam construction, and other types of regulation and diversion. Although not explored herein, trends in AMF series are also likely to be partially explained by changes in land use and land cover over time. Results obtained herein suggest that improved forecasts of flood risk may be obtained using a simple modification of the Bulletin 17B framework, wherein the mean and standard deviation of the log-transformed flows are modeled as functions of climate indices associated with oceanic-atmospheric patterns (e.g. AMO, ENSO, NAO, and PDO) with lead times between 3 and 9 months. Herein, one-year ahead forecasts of the mean and standard deviation, and subsequently flood risk, are obtained by applying site specific multivariate regression models, which reflect the phase and intensity of a given climate pattern, as well as possible impacts of coupling of the climate cycles. These forecasts of flood risk are compared with forecasts derived using the existing Bulletin 17B model; large differences in the one-year ahead forecasts are observed in some locations. The increased knowledge of the inherent structure of AMF series and an improved understanding of physical and/or climatic causes of nonstationarity gained from this research should serve as insight for the formulation of a physical-casual based statistical model, incorporating both climatic variations and human impacts, for flood risk over longer planning horizons (e.g., 10-, 50, 100-years) necessary for water resources design, planning, and management.