In contrast to a homogeneous isotropic elastic medium with only two elastic constants, a linear elastic micropolar solid is characterized by six such constants. To evaluate these constants it is necessary to solve selected boundary-value problems in terms of measurable surface tractions and displacements. Representative problems are tension and torsion of a circular cylinder and cylindrical bending of a rectangular plate. Analytical solutions to these static problems have been obtained, and to complement these solutions a combination of experiments was planned and a novel man-made micropolar solid was developed. The solid consisted of a composite material with aluminum shot uniformly distributed throughout an epoxy matrix. The shot, with an elastic modulus 20 times that of the matrix, was selected to represent rigid microelements embedded in a deformable medium and thereby model a micropolar solid. Static torsion tests revealed that the micropolar effects were masked by material inhomogeneity. It was concluded that, to within the resolution of the measurements, the special material developed here behaved as a classical elastic solid. Detection of possible micropolar phenomena will demand either higher resolution static measurements or a series of dynamic tests.

The paper presents the experimental unbalance response of two slightly different rigid rotors supported by aerodynamic foil bearings. Impulse (Pelton) turbines manufactured directly in the mass of the rotors (on the outer surface) entrain both rotors at rotation speeds comprised between 50 krpm and 100 krpm. The displacements in the two foil bearings are measured during coast down and are depicted as waterfall plots. They show typical nonlinear behavior, i.e., subsynchronous vibrations accompanying the synchronous component. The measurements clearly show that the subsynchronous components bifurcate or jump at typical rotation speeds (mostly rational fractions of the rotation speed). The nonlinear behavior of the rigid rotor supported on foil bearings is also emphasized by varying the added unbalance: with increasing unbalance the vibration spectrum becomes gradually more diverse as new subsynchronous vibrations appear. The experimental results are compared with very simplified theoretical predictions based on the assumption that the air film in the two bearings is infinitely stiff compared to the foil structure. The latter is characterized by a cubic stiffness and a structural damping coefficient. The comparisons show only a rough qualitative agreement.

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The paper deals with the experimental analysis of the torque and of the lift-off velocity of a foil thrust bearing. The geometric characteristics of the foil thrust bearing follow the design recently proposed by Dykas et al. (2009, “Design, Fabrication, and Performance of Foil Gas Thrust Bearings for Microturbomachinery Applications,” ASME J. Eng. Gas Turbines Power, 131(1), p. 012301-1). A dedicated test rig was developed and enables the measurement of the speed, the torque, and temperatures under the foils. The measurements underlined the importance of managing heat transfer in a foil thrust bearing. Results are presented for mild static loads ranging from 5 to 60 N and rotation speeds comprised between 20 and 35 krpm. The value of the start-up torque was validated by comparisons with results obtained with a rapid camera.

This paper deals with the experimental analysis of the dynamic characteristics of a foil thrust bearing (FTB) designed according to specifications given by NASA scientists in 2009 (Dykas et al., 2009, “Design, Fabrication, and Performance of Foil Gas Thrust Bearings for Microturbomachinery Applications,” ASME J. Eng. Gas Turbines Power, 131(1), p. 012301). The present work details the new configuration of the same test rig that was used to test start-up characteristics of the aforementioned bearing (Balducchi et al., 2013, “Experimental Analysis of the Start-Up Torque of a Mildly Loaded Foil Thrust Bearing,” ASME J. Tribol., 135(3), p. 031703). The rig has been reconfigured to test dynamic characteristics. The dynamic characteristics of the bump foil structure were measured for static loads comprised between 30 N and 150 N while measurements for the FTB were performed at 35 krpm for 30 N, 60 N, and 90 N. Excitation frequencies were comprised between 150 Hz and 750 Hz. Results showed that the dynamic stiffness of the FTB increase with excitation frequency while the equivalent damping decreases. Both stiffness and damping increase with the static load but are smaller at 35 krpm compared to 0 rpm.