The effect of microstructural defect evolution during elastic fatigue loading on subsequent mechanical properties

Evolution of mechanical properties due to fatigue loading is often overlooked in the design process. Material properties in the as-received condition from a vendor are characterized and then these properties become the basis of design considerations. The effect of expected fatigue loading during operation and its potential to degrade mechanical performance is frequently accounted for with appropriate safety factors. Most notably, the aerospace industry does not have the luxury of large safety factors due to the detrimental effect of weight on cost and performance. Therefore, it becomes necessary to quantify not only the change in mechanical properties as a function of its fatigue life, but also understand what microstructural mechanisms are responsible for any property changes in order to optimize component design. Research into material evolution at both the macro and micro length scale has become even more relevant in the past few years with the advent of reusable launch systems. This research investigates the microstructural evolution during elastic fatigue loading and the mechanisms driving subsequent mechanical property change. Two different material systems are analyzed, pure $\alpha$-iron and 7075-T6 aluminum alloy. A unique experimental approach was implemented in which multiple sub-tensile specimens are obtained from the interior of a fatigue specimen. Pre-fatigued sub-tensile specimens are tested under quasi-static and dynamic strain rates. Subsurface material for microscopy characterization is also obtained from the fatigue specimens. It was found that the evolution of mechanical properties is dependent on both the subsequent loading strain rate and the amount of dislocation reversibility (controlled by the fatigue stress ratio) in each fatigue loading cycle. In-depth microscopy characterization revealed that microscopic fatigue-induced voids are the primary factor in subsequent strength degradation. Results from the research advance our current understanding of microstructural defect accumulation during fatigue loading and their effect on subsequent mechanical properties. Additionally, the dependence of post-fatigue material behavior on both strain rate and dislocation reversibility highlight the need to investigate additional materials and different fatigue conditions. Lastly, several analysis techniques utilizing statistical analysis and machine learning have been applied to microscopy datasets in order to identify underlying microscopic mechanisms that contribute to changes in subsequent mechanical response

Dc track edge interactions

Includes bibliographical references.We have developed an experimental method for investigating the interaction between two dc track edges by studying the track edge noise. We conclude that two edges do not interact when they are several micrometers apart, but the noise reduces nearly to zero when their separation is less than about half a micrometer. There is a transition region that exists between these two limits. The net track edge noise power from two dc edges is quantized, implying that in our experiment track edges interact around the complete revolution of the disk or not at all.This work was supported in part by NSF Grant No. ECS-880470 and NSF Presidential Young Investigator Award (Indeck) ECS-89-5714

Exploiting Truly Fast Hardware in Data Mining

In many data mining applications, the size of the database is not only extremely large, it is also growing rapidly. Even for relatively simple searches, the time required to move the data off magnetic media, cross the system bus into main memory, copy into processor cache, and then execute code to perform a search is prohibitive. We are proposing that a significant portion of the data mining task (i.e., the portion that examines the bulk of the raw data) be implemented in fast hardware, close to the magnetic media on which it is stored. Furthermore, this hardware can be replicated allowing mining tasks to be performed in parallel, thus providing further speed up for the overall mining application. In this paper, we describe a general framework under which this can be accomplished, and identify a number of important research questions that must be addressed for it to be practical

The Mercury System: Exploiting Truly Fast Hardware in Data Mining

In many data mining applications, the size of the database is not only extremely large, it is also growing rapidly. Even for relatively simple searches, the time required to move the data off magnetic media, cross the system bus into main memory, copy into processor cache, and then execute code to perform a search is prohibitive. We are building a system in which a significant portion of the data mining task (i.e., the portion that examines the bulk of the raw data) is implemented in fast hardware, close to the magnetic media on which it is stored. Furthermore, this hardware can be replicated allowing mining tasks to be performed in parallel, thus providing further speedup for the overall mining application. In this paper, we describe a general framework under which this can be accomplished and provide initial performance results for a set of applications

Citizen science reveals widespread negative effects of roads on amphibian distributions

Landscape structure is important for shaping the abundance and distribution of amphibians, but prior studies of landscape effects have been species or ecosystem-specific. Using a large-scale, citizen science-generated database, we examined the effects of habitat composition, road disturbance, and habitat split (i.e. the isolation of wetland from forest by intervening land use) on the distribution and richness of frogs and toads in the eastern and central United States. Undergraduates from nine biology and environmental science courses collated occupancy data and characterized landscape structure at 1617 sampling locations from the North American Amphibian Monitoring Program. Our analysis revealed that anuran species richness and individual species distributions were consistently constrained by both road density and traffic volume. In contrast, developed land around wetlands had small, or even positive effects on anuran species richness and distributions after controlling for road effects. Effects of upland habitat composition varied among species, and habitat split had only weak effects on species richness or individual species distributions. Mechanisms underlying road effects on amphibians involve direct mortality, behavioral barriers to movement, and reduction in the quality of roadside habitats. Our results suggest that the negative effects of roads on amphibians occur across broad geographic regions, affecting even common species, and they underscore the importance of developing effective strategies to mitigate the impacts of roads on amphibian populations

Strategies used by high-school students in solving a rule induction task

The present study examined problem solving strategies used by high school students while working with a contrived task. The task was to determine the nature of a pattern formed by a sequence of playing cards. The subjects alternated between making a conjecture as to what rule determines which cards fit into the different positions of the sequence and then collecting more information about that sequence. Because these steps parallel key elements of "the scientific method" as it is typically characterized in precollege education, this investigation also provided the opportunity to examine the extent to which the component skills necessary to implement that method have become part of the subjects' working repertoire of skills that are brought to bear on a problem.The subjects were typically found to make plausible conjectures in that the rules they generated were consistent with the available facts. The subjects usually did not entertain rival hypotheses and then try to distinguish which was best. Component parts of compound rules were usually not isolated for examination. Cards expected not to follow the sequence generally were not checked to see if in fact they did not fit the sequence. When prompted to do so, however, the subjects were able to generate alternative explanations and design methods for determining which was superior. This suggests that our current approach to public education does not prompt students to incorporate scientific method process skills for developing knowledge into their everyday life.U of I OnlyETDs are only available to UIUC Users without author permissio