Performance and reliability of microelectromechanical system(MEMS) components can be enhanced dramatically through the incorporation of protective thin-filmcoatings. Current-generation MEMSdevices prepared by the lithographie-galvanoformung-abformung (LIGA) technique employ transition metals such as Ni,Cu, Fe, or alloys thereof, and hence lack stability in oxidizing, corrosive, and/or high-temperature environments. Fabrication of a superhard self-lubricating coating based on a ternary boride compound AlMgB14 described in this letter has great potential in protective coatingtechnology for LIGA microdevices. Nanoindentation tests show that the hardness of AlMgB14films prepared by pulsed laser deposition ranges from 45 GPa to 51 GPa, when deposited at room temperature and 573 K, respectively. Extremely low friction coefficients of 0.04–0.05, which are thought to result from a self-lubricating effect, have also been confirmed by nanoscratch tests on the AlMgB14films. Transmission electron microscopy studies show that the as-deposited films are amorphous, regardless of substrate temperature; however, analysis of Fourier transform infrared spectra suggests that the higher substrate temperature facilitates the formation of the B12 icosahedral framework, therefore leading to the higher hardness.

A combined experimental and modeling approach has been devised to understand the material removal mechanism during abrasion of ductile copper discs. First, single grit scratch intersection experiments are conducted at the micro-scale (with 1-30 mm depth of cut). This is followed by FEM analysis. Then a simple analytical model is developed, and the model prediction is verified against experimental observations and results from numerical simulations. A characteristic material detachment length is correlated between experimental observations and model predictions. The insights gained from this exercise may be used to develop a mechanistic model of material removal in chemical mechanical polishing (CMP) of ductile materials.

Flutter analysis of structures is usually done in frequency domain. Alternately, time-domain methods have been suggested. For frequency-domain flutter analysis, flutter derivatives are used that can be identified from section model testing in the wind tunnel. In time-domain analysis, the frequency-dependent aerodynamic self-excited forces expressed in flutter derivatives acting on the structure can be approximated in the Laplace domain by Rational functions.;The art of efficient extraction of these aeroelastic parameters requires an elastic suspension system to capture coupled displacement and aerodynamic force time histories from wind tunnel testing of section models. A novel three-degree-of-freedom (DOF) suspension system has been developed for the wind-tunnel section model study of wind-excited vibrations of flexible structures.;The extraction of flutter derivatives becomes more challenging when the number of DOF of section model increases from two to three. Since the work in the field of identifying all eighteen flutter derivatives has been limited, it has motivated the development of a new system identification method (Iterative least squares method or ILS method) to efficiently extract the flutter derivatives using a section model suspended by the three-DOF elastic suspension system. All eighteen flutter derivatives for a streamlined bridge deck and an airfoil section model were identified by using ILS approach. Flutter derivatives related to the lateral DOF were emphasized.;For time-domain flutter analysis, Rational function approximation (RFA) approach involves approximation of the experimentally obtained flutter derivatives through 'multilevel linear and nonlinear optimization' procedure. This motivated the formulation of a system identification technique (Experimental extraction of Rational function coefficients or E2RFC) to directly extract the Rational function coefficients from wind tunnel testing. The current formulation requires testing of the model at fewer numbers of velocities than in the flutter-derivative formulation leading to significant reduction in time and resources associated with extraction of flutter derivatives and eventual Rational function approximation. Successful numerical simulation using E2RFC formulation with two lag terms was performed proving the robustness of the technique. Experimental extraction of Rational function coefficients associated with one lag term formulation was made for a streamlined bridge deck section model.

This dissertation studies the effect of two practical attributes of structural materials on ultrasonic inspection. The effects of the material microstructure of polycrystalline aluminum alloys on propagating ultrasonic waves and the effects of surface curvature and roughness on ultrasonic scattering and flaw detectability are both addressed.;The relationships between ultrasonic properties (velocity, attenuation, and backscattering) and the microstructure are studied both experimentally and theoretically for rolled aluminum samples with highly elongated grains. Attenuation measurements show a very small anisotropy, whereas backscattering measurements show a very high anisotropy. Existing theories and related extensions are able to explain these physical phenomena. Three approaches are proposed to simultaneously determine grain size and shape. Each approach gives reasonably accurate estimates of grain size and shape.;Three existing models are used to study how cylindrical surfaces affect transducer radiation fields and signal-to-noise ratios through a flaw detection example: the Gauss-Hermite beam model, the Born flaw signal model, and the microstructural response model. The beam model can explain the defocusing effect of cylindrical surfaces. The flaw signal model and the microstructural response model combine to yield estimates of signal-to-noise ratios and the minimum detectable inclusion sizes.;Two time-domain theories that can predict ultrasonic backscattered noise due to a periodically rough entry surface in a pulse/echo immersion test are presented. The first is an exact result and the second is a computationally efficient engineering approximation. Experimental verifications of the theories are presented at both normal incidence and oblique incidence. The predictions are compared to experiment, with good agreement between the theory and the experiment being observed in most cases. Practical implications of the theory for ultrasonic flaw detection and materials characterization are also discussed.

The High Altitude Balloon Experiments in Technology (HABET) program started at Iowa State University to carry payloads into the upper atmosphere and to simulate the activities involved with satellite programs. As HABET has evolved through the years, the payloads for HABET have become increasingly more sophisticated and expensive. The current recovery system employs a circular parachute that does not provide any means of control. This allows the payload to land in inconvenient locations such as roads or train tracks. The parachute can also land in places where the payload is not recoverable such as lakes or rivers, even causing injury to people, property or animals. The Recovery Guidance System (RGS) is a solution to these problems. The goal of the Recovery Guidance System project is to develop a fully autonomous landing system that will select its landing sites from a list of preprogrammed sites based on where the system is and where it can go. This research is designed to give the RGS project a better understanding of the aerodynamic properties of the parafoil. This research has three parts. First, a two-dimensional lifting line equation is used to determine the lift coefficient and the drag coefficient. Next, a three-dimensional model of the parafoil used in the RGS program was put into a program called PMARC to obtain the coefficients of lift and drag. Finally, the parafoil was hooked up to a dummy payload of the same size and weight as the RGS electronic package so that the coefficient of lift and drag were experimentally obtained. All of these results were then compared to each other for accuracy. The lift coefficient values were close to the predicted values ([Plus or minus]16 percent) but the drag coefficient values differed from the experimental results by a factor of three.

A scratch intersection based material removal mechanism for CMP processes is proposed in this paper. The experimentally observed deformation pattern by SEM and the trends of the measured force profiles (Che et al., 2003) reveal that, for an isolated shallow scratch, the material is mainly plowed side-way along the track of the abrasive particle with no net material removal. However, it is observed that material is detached close to the intersection zone of two scratches. Motivated by this observation, it is speculated that the deformation mechanism changes from ploughing mode to shear-segmentation mode as the abrasive particle approaches the intersection of two scratches under small indentation depth for ductile metals. The proposed mechanistic material removal rate (MRR) model yields Preston constant similar to those observed experimentally for CMP processes. The proposed model also reveals that the nature of the slurry-pad interaction mechanism, and its associated force partitioning mechanism, is important for determining the variation of MRR with particle size and concentration. It is observed that under relatively soft pads, small particles and low particle concentration, the pad undergoes local deformation, yielding an increased MRR with increasing particle size and concentration. At the other extreme, the intact walls of the surface cells and the connecting cell walls between the surface pores deform globally, resembling a beam or a plate, and a decreasing trend in MRR is observed with increasing particle size and concentration. The predicted MRR trends are compared to existing experimental observations.

Decentralized guidance of Unoccupied Air Vehicles (UAVs) is a very challenging problem. Such technology can lead to improved safety, reduced cost, and improved mission efficiency. Only a few ideas for achieving decentralized guidance exist, the most effective being the boid algorithm. Boid algorithms are rule-based guidance methods derived from observations of animal swarms. In this paper, boid rules are used to autonomously control a group of UAVs in high-level transit simulations. This paper differs from previous work in that, as an alternative to using exponentially scaled behavior weightings, the weightings are computed off-line and scheduled according to a contingency management system. The motivation for this technique is to reduce the amount of on-line computation required by the flight system. Many modifications to the basic boid algorithm are required in order to achieve a flightworthy design. These modifications include the ability to define flight areas, limit turning maneuvers in accordance with the aircraft dynamics, and produce intelligent waypoint paths. The use of a contingency management system is also a major modification to the boid algorithm. A Simple Genetic Algorithm is used to partially optimize the behavior weightings of the boid algorithm. While a full optimization of all contingencies is not performed due to computation requirements, the framework for such a process is developed. Wolfram's Matlab software is used to develop and simulate the boid guidance algorithm. The algorithm is interfaced with Cloud Cap Technology's Piccolo autopilot system for Hardware-in-the-Loop simulations. These high-fidelity simulations prove this technology is both feasible and practical. They also prove the boid guidance system developed herein is suitable for comprehensive flight testing.

The material detachment mechanisms of ductile metal surfaces are studied experimentally during dry grinding operation in a simulated experiment with near single grit contact with the surface. The spectra of the cutting and thrust forces are recorded and analyzed. It is found that the thrust force changes its direction from a compressive to a tensile mode. The ratio between the thrust and cutting force is consistently found to be greater than 1. In the grinding process, the chip is found to be much shorter and thicker than those predicted by traditional continuum cutting theories. From the analysis of chip dimensions and cutting forces, we speculate that the cutting process during a grinding operation comprises of three phases as follows: (i) lifting up of the surface ahead of the abrasive particle, (ii) segmentation through shear instability, and finally (iii) chip tearing from the surface. Accordingly, the heating cycle is much longer with a lower mean temperature, compared to those of macro machining. In addition, the proposed deformation field leads to loss of constraints ahead of the cutting grits, and possibly reducing the thrust to cutting force ratio. This suggests that forming took place prior to material detachment in grinding.

Annual variation of midlatitude precipitation and its maintenance through divergent water vapor flux were explored by the use of hydrological variables from three reanalyses [(NCEP–NCAR, ECMWF Re-Analysis (ERA), and Goddard Earth Observing System (GEOS-1)] and two global precipitation datasets [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and Global Precipitation Climatology Project (GPCP)]. Two annual variation patterns of midlatitude precipitation were identified:

1. Tropical–midlatitude precipitation contrast: Midlatitude precipitation along storm tracks over the oceans attains its maximum in winter and its minimum in summer opposite to that over the tropical continents.

2. Land–ocean precipitation contrast: The annual precipitation variation between the oceans and the continent masses exhibits a pronounced seesaw.

The annual variation of precipitation along storm tracks of both hemispheres follows that of the convergence of transient water vapor flux. On the other hand, the land–ocean precipitation contrast in the Northern Hemisphere midlatitudes is primarily maintained by the annual seesaw between the divergence of stationary water vapor flux over the western oceans and the convergence of this water vapor flux over the eastern oceans during winter. The pattern is reversed during the summer. This divergence–convergence exchange of stationary water vapor flux is coupled with the annual evolution of upper-level ridges over continents and troughs over the oceans.

Flutter is a dynamic instability that aerodynamic vehicles encounter in atmospheric flight. The interaction between structural elastic, structural inertial and aerodynamic forces may cause the flexible vehicle to undergo divergent oscillations, at which point flutter is encountered. Undesirable effects of this behavior include difficult controllability, structural fatigue and even catastrophic structural failure. This point of instability is dependant on many factors including the structural properties, structural geometry, aerodynamic shape and the flight condition. Since these factors may influence the flutter point in a sensitive manner investigation of uncertainty in these properties is warranted. A modern method to investigate system uncertainty is with the use of robust stability, namely [mu] analysis. These modern technologies are used to analyze the uncertainty in structural properties (mass and stiffness properties) of a wing in flight and the effect these uncertainties have on the flutter point. Recent use of these robust stability techniques on the flutter problem have focused on uncertainty in the natural structural modal frequencies. The uncertainties in the modal frequencies are also typically assumed independent. Uncertainties in the natural structural mode shapes have not been explored in complete detail. By including uncertainty in the structural mode shapes the robust flutter margins will be much less conservative. A complete structural uncertainty model for robust flutter prediction is constructed. Robust flutter margins are found for a fictitious wing with uncertainties in wing mass and stiffness properties, using the structured singular value ([mu]). Since the robust flutter margins include uncertainty in the structural mode shapes, as well as the structural mode frequencies, they are least conservative estimates. The uncertainties in many structural properties on the wing are investigated and the effect that they have on the flutter point is determined. The formulation presented herein can be applied to a wide array of problems concerning the sensitivity of the flutter solution.