This thesis presents further experimentation and modeling for bump-type gas foil bearings used in oil-free turbomachinery. The effect of shaft temperature on the measured structural force response of foil bearings is of importance for reliable high temperature applications. During actual operation with shaft rotation, the bearing structural parameters are coupled to the effects of a hydrodynamic gas film layer, thus determining the overall bearing load performance. A 38.17 mm inner diameter foil bearing, Generation II, is mounted on an affixed non-rotating hollow shaft with an outer diameter of 38.125 mm. A cartridge heater inserted into the shaft provides a controllable heat source. The clearance between the shaft and the foil bearing increases with increasing shaft temperatures (up to 188°C). A static load (ranging from 0 N to 133 N) is applied to the bearing housing, while measuring the resulting bearing displacement, which represents the compliant structure deflection. Static load versus displacement tests render the bearing static structural stiffness. As the shaft temperature increases, the static test results indicate that the bearing structural stiffness decreases by as much as 70% depending on the bearing orientation. A dynamic load test setup includes a rigid shaft support structure and a suspended electromagnetic shaker. Dynamic load (from 13 N to 31 N) test results show that the test foil bearing stiffness increases by as much as 50% with amplitude of dynamic load above a lightly loaded region, nearly doubles with frequency up to 200 Hz, and decreases by a third as shaft temperature increases. A stick slip phenomenon increases the bearing stiffness at higher frequencies for all the amplitudes of dynamic load tested. The test derived equivalent viscous damping is inversely proportional to amplitude of dynamic load, excitation frequency, and shaft temperature. Further, the estimated bearing dry friction coefficient decreases from 0.52 to 0.36 with amplitude of dynamic load and stays nearly constant as shaft temperature increases. Test results identify static and dynamic bearing parameters for increasing shaft temperature. These experimental results provide a benchmark for predictions from analytical models in current development and are essential to establish sound design practices of the compliant bearing structure.

Dynamic abnormal grain growth (DAGG) is a phenomenon by which single crystals up to centimeters in length are produced at elevated temperature during the application of strain. DAGG was previously demonstrated in commercial-purity molybdenum (Mo) materials. This is the first investigation to confirm DAGG in another material, tantalum (Ta). Previous experiments initiated and propagated DAGG using constant true-strain rate tensile tests, but this study demonstrates that DAGG can also occur under constant true-stress tensile conditions. A Mo material was tested under constant true stress, and two Ta materials were tested under constant true-strain rate. The effects of temperature, stress, strain rate, initial microstructure and texture on tensile test data and the resulting microstructures are examined. The microstructures of the Ta materials are analyzed using electron backscatter diffraction (EBSD) data to quantify the orientation, deformation, grain boundary character, and slip properties of the DAGG grains and unconsumed microstructure. The DAGG grains were found to be relatively undeformed compared to the unconsumed microstructure following DAGG and to not be oriented favorably, or unfavorably, for slip. The grain boundaries between DAGG grains in one Ta material were found to commonly have [sigma]3 character. This was likely due to a strong initial <111>-fiber texture. Previous investigations of DAGG in Mo indicated that DAGG grains commonly grow along the surface of the specimen, but this was not observed with significant frequency in Ta. Results suggest that the distance the DAGG grain boundary travels is proportional to the accumulated strain during DAGG, and the velocity of the DAGG grain boundary is proportional to the applied strain rate but is not related to the orientation of the DAGG grain or its slip properties. / text