Advisors: A. Lakhtakia
Committee: R. Messier, E. Ventsel, J. Xu, T. Kane, J. Todd
Manipulation of Optical Pulses With Chiral Sculptured Thin Films
Imagine the kind of trees that might grow at the poles of a warm alien planet. If the planet's axis were tilted with respect to the plane of its orbit, parts of the planet would be bathed in sunlight for long periods of time; in those areas its sun would wheel about the sky and not set. (The same thing happens in Earth’s polar regions, but those regions are too cold for large trees to grow.) Suppose, as on earth, that the trees tend to grow towards the sun. If they grew fast enough, or the rotation rate of the planet was slow enough, the trees would grow in helical, corkscrew-like shapes.
If the orbital path of our hypothetical planet were complex enough, perhaps skirting about a binary star system or being regularly perturbed by a large gas giant, the trees might grow in other bizarre shapes. What a strange world for a xenobiologist to explore!
This fanciful idea captures the basic concept behind the growth of sculptured thin films (STFs), which are now made routinely on our own planet, at linear scales some hundred million times smaller than a California redwood. The fabrication process for STFs is simple enough that an experienced tinkerer could make some basic ones in her basement, with an investment of perhaps $1000 in used parts.
Here, in a nutshell, is how to do it. Prepare a sturdy, sealable, metal or glass bell jar, and place at its bottom a crucible filled with the material for the film. STFs can be made from many different materials, including both metals and oxides. Make some provision for heating up the crucible, perhaps by wrapping the heating element from a toaster around it. Place a substrate-glass or silicon will do-at the top of the chamber. Affix the substrate to a motorized mount so that it can be tilted and/or rotated. Then pump the air from the chamber until you have a high vacuum. Heating the crucible will cause some of the material inside it to evaporate and land on the substrate. If the substrate is cool enough, tiny columns-nanowires- of the evaporated material will begin to form. Those nanowires will be bent and twisted depending on how the substrate tilts and rotates, respectively, during the deposition.
In this process, the crucible is analogous to the sun of our alien planet. The substrate corresponds to the surface of that planet, and the sequence of rotations and tilts mimics the the effects of the planet's motions. The difference is that in our plant analogy, only the energy and not the material for growth comes to the trees via the sunlight. Also, the nanowires that make up STFs form spontaneously. There is no blueprint, per se, that influences their formation like DNA does for plants on earth.
Why should we bother making such films? In three words: to manipulate light. The films have other applications, to be sure, including as traps and sieves for biomolecules, viruses, and bacteriums, and perhaps for cooling of microelectronic circuitry. But the most advances in STF technology have been in the area of optics, the science of producing, focussing, measuring, and in general manipulating, light.
People have been studying the optical properties of thin films for a long time. Sir Isaac Newton did it. But until now people have not had the extraordinary control over the microstructure of the films they made and used. With STF technology, the length, shape, size, and orientation of the nanowires that comprise the film can be controlled. That control, along with the ability to make the films from a large variety of materials, means that their optical properties can be tailored for a wide variety of applications. STFs can be designed to reflect, transmit, and absorb different wavelengths. They can be designed so that the infiltration of chemicals between the columns will change their reflectivity and transmissivity in a controlled way, or even produce light by fluorescence.
One of the most exciting potential applications of STFs in optics is for the manipulation of ultrashort optical pulses. Such pulses, which are of only a few femtoseconds duration (a femtosecond is a millionth of a billionth of a second) have potential applications for communications, controlling chemical reactions, surgery, and remote sensing, to name just a few areas of investigation.
By solving the equations that describe light propagation, I have been able to predict what will happen when an ultrashort light pulse hits one of these films. As expected, parts of the pulse will be reflected from and parts will be transmitted through the film. I can predict the shapes and energies of those reflected and transmitted pulses. The microstructure of STFs could be designed to shape the pulses in useful ways.
To process information in the optical realm, a device must be able to affect optical pulses differently depending on their characteristics. For instance, an STF can be designed to reflect light in a certain wavelength band. They can be designed to reflect one polarization of light. The polarization of light refers to the direction or series of directions that the electric field-which, along with the magnetic field, comprises the light-point as it propagates. And they can do both simultaneously. So an STF-based device could be used to separate incoming pulses based on their spectral content and polarization state.
They can also be designed to discriminate between pulses having different carrier phases. The ability to stay in phase is one distinction between a good marching band and a poor one. If the marchers keep perfect time (in sync with what is known as some kind of beat or carrier signal) but take their steps out of phase, their feet will not hit the ground at the same time. While we can easily see variations of phase in a marching band, with light the electromagnetic fields change so fast that not even our fastest electronic devices can measure optical phase directly. In optics, what we measure directly is the power or intensity of the light.
However, two pulses can have the same variations with time in their intensity, but different carrier phases. It turns out that if you reflect two such pulses from an STF, the differences in carrier phase will show up as variations in intensity in the reflected pulses. This phenomenon hopefully will make it easier to measure the carrier phases of ultrashort pulses. That, in turn, might make it feasible to transmit more bits of information per pulse down optical fibers, gain more control over chemical reactions, or measure more accurately the properties of our planet's atmosphere.
Advisors: J. Rose
Committee: B. Tittman, E. Ventsel, A. Segall, S. Sinha, J. Todd
Guided Elastic Waves In Structures With An Arbitrary Cross-Section
A train is one of the oldest and most important transportation methods for moving people and goods. A train accident can causes serious casualties and property damage. Many factors could lead to a train disaster and the defects in rail are one of the major problems. Detection of defects and proper maintenance action for a rail is therefore essential.
There are two kinds of typical defects in a rail head. They are shelling and transverse defects. Shelling is a horizontal plane defect generated by the sliding and/or rolling the wheel over the rail from shear reversal and is usually located just below the top surface of the rail. The transverse defects are usually generated and grown inside the rail head from the shelling region down into the head. Shelling is not fatal but transverse defects are. Conventional ultrasonic tests (the normal incident technique and the oblique incident technique) have difficulties in detecting the transverse defects under the shelling, because most of the ultrasonic energy is reflected from the shelling. For this reason, the guided wave ultrasonic technique is potentially a suitable method for detecting defects under the shelling. The cross-sectional area of the shelling is much smaller than that of the transverse defects in the guided wave propagation direction.
The basic five senses of a human are the primary tools in diagnosis. Among them, the visual test and hearing test has quite a long history. If a patient goes to the hospital, the doctor first sees the patient to observe the sickness. The doctor might then use the stethoscope to hear inside the patient. A similar procedure is applying in nondestructive evaluation. Large defects can be detected by eye-inspection and sound. However, micro cracks cannot be detected by sound; therefore ultrasonics is used to detect defects in the structures. Generally, two kinds of transducers are used to generate and detect defects with ultrasonics. One uses a piezoelectric transducer and the other used an electromagnetic acoustic transducer (EMAT). In this research, the EMAT is simulated using ABAQUS/Explicit (a commercial three dimensional finite element method (FEM) package).
The inspection technique using ultrasonics is well known because of its excellent sensitivity. However, the conventional technique (normal incident and oblique incident technique) inspects the structure point by point; therefore, becoming very tedious takeing long time. Also, this method has a difficulties in finding defects under shelling, because most of the ultrasonic energy is reflected from the shelling. On the other hand, the guided ultrasonic technique is an efficient and promising inspection technique because this wave can propagate along the structure with an excellent sensitivity.
There is still difficulty in using the guided waves because of so many modes in a structure. Because of these modes, it is difficult to understand the behavior of the guided waves, to control the modes, and to interpret the inspection results. Usually there are several modes in plates and pipes between 0 ~ 200kHz, however there are hundreds of modes in a rail in the same frequency range. Therefore, the right mode is an important factor in rail inspection along with the appropriate frequency.
It is found that the surface wave (the wave localized near the surface) is the best guided wave to keep energy in the rail head fro critical transverse crack detection. Other modes could cause confusion in interpreting the test results. With these surface guided waves, the scattering patterns from defects are also studied. The defects adapted in this study are internal notches, internal holes, side notches, transverse crack simulations, and the shelling. The lower frequency (below 60kHz) guided wave is more suitable in detecting the defects under the shelling than the higher frequency (above 175kHz) guided wave.
This research provides a new modeling technique to simulate EMAT loading and can suggest guide lines for a new inspection technique for finding defects in the rail head under shelling. Furthermore, the research area can be extended to various types of defects, different location of the defects, different loading position, and welding areas.
Advisors: S. Hayek, K. Lee
Committee: M. Urquidi-Macdonald, J. Cusumano, R. Melton, J. Todd
Active Control of the Attitude motion and Structural Vibration of A Flexible Satellite by Jet Thrusters
In the early stages of space exploration, spacecraft were small, mechanically simple and essentially inflexible, hence elastic deformations of satellite structures were relatively insignificant. In a modern space vehicle, a spacecraft consists of lightweight, flexible, deployable members in the form of solar panels, antennas, and booms.
The structures of lightweight materials tend to be flexible, and flexible structures will deform during maneuvers, causing complication of satellite motion due to interaction between the satellite motion and structural motion. Hence, the development of a general formulation for dynamics and control of flexible spacecraft is one of the major tasks in spacecraft industry today.
In developing a general formulation of flexible satellites, the Lagrangian approach is used for deriving the equations of motion of flexible satellites in a tree-type geometry. Since the Lagrangian formulation for flexible satellites in a tree-type geometry is general one, it can be easily expanded to complicated structures such a large space structure(LSS).
The formulation is applied to the INSAT-II satellite, and the equations of librational motion and structural motion of are obtained in a set of second-order nonlinear differential equations.
Since the librational motion and structural vibration of flexible appendages are coupled with one another, the librational and structural motions of the INSAT-II are complicated. Uncontrolled dynamics shows that the librational and structural motions are oscillatory and undamped motions. Hence, a control system for stability and performance improvement is necessary.
For simultaneous librational motion and flexible mode control, a control system design is synthesized by using feedback linearization control, thrust determination, thrust management, and pulse-width pulse-frequency modulation.
Feedback linearization control determines the theoretical control input command to actuators. As actuators, jet thrusters are used for simultaneous librational motion and flexible mode control. T thrust determination and thrust management algorithms are designed for jet thrusters to realize the theoretical control input command from feedback linearization. After the thrust determination and thrust management algorithms, the theoretical control input command is converted into continuous-time non-negative jet thrust input command.
For actual jet thrusters, continuous-time thrust input control from thrust are modulated into on-off pulsed jet thrust input control by pulse-width pulse frequency modulation, since actual jet thrusters are working only with on-off pulsed signal command.
For the control system having the flexible satellite dynamics, feedback linearization, thrust determination, thrust management, and pulse-width pulse-frequency modulation, numerical simulations are carried out for several initial conditions.
Numerical simulations show that the equations of motion of the INSAT-II satellite are accurate and the designed control system is working well for simultaneous librational motion and flexible mode control. Hence, it shows simultaneous librational motion and flexible mode control is possible with jet thrusters.
For future works, different flexible satellite models, different control theory, and different control objectives will be considered. As a more realistic model, a flexible satellite model with jet thrusters and momentum exchange devices. As to different control theory, PID control, sliding mode control, and other available control will be considered. As to control objectives, the pin-pointing maneuver as well as the slewing motion control of flexible structures will be considered.
Advisors: J. Cumano
Committee: F. Costanzo, C. Lissenden, L. Berlyand, L. Friedman, J. Todd
Coupled Field Damage Structural Dynamics
Fracture or damage has been one of the most common ways in which a machine or a structure loses its integrity and functions. We believe that damage growth in a structure under varying load is controlled jointly by several factors such as structural dynamics, initial value of damage and its spatial distribution. Interaction between structural dynamics and damage evolution can not be avoided during the whole process of damage evolution. We will study the coupled field damage dynamics from a dynamics point of view while concentration will be emphasized the interaction between them.
Using a continuum damage mechanics definition of damage variable, a coupled field damage/structural dynamics model was developed in this work to study qualitatively the coupled field dynamics of a mechanical structure with slowly evolving damage in a dynamical systems viewpoint. The dynamical aspect of damage evolution which is of great importance to the understanding of the phenomenon of damage evolution has seldom if ever studied in the literature. It will be able to provide us with a fresh viewpoint on the phenomenon of fracture and damage. The microstructural configurational continuum damage variable is defined through crack density parameter and the coupled field damage/structural dynamics model is developed via Hamilton's principle. For solid materials containing a population of microcracks, the coupled field dynamics model are specified by generalizing the Grifith energy release rate and the well known Paris law.
Finite difference method is used to integrate numerically the partial differential equations of the coupled field dynamics model of a bar. Averaging procedure was used to obtain analytically the damage growth rate function expressed in terms of strain level and current damage state. Conditions for the averaged growth rate to work are illustrated. A simply supported bar with a variety of initial damages and loading conditions is studied to understand the coupled field dynamics in time-spatial space. The impact of spatial distribution of strain which is determined from displacement response on the damage distribution is illustrated using constant initial damage condition. The complexity damage evolution dynamics was demonstrated by comparing damage evolution under different loading frequencies and starting with random initial damage condition. Initial damage uncertainty is accounted for by using random initial damage. Using the coupled field dynamics model, the relationship between initial damage uncertainty and variability of final failure time was studied statistically.
Advisors: J. Cumano
Committee: G. Gray, F. Costanzo, E. Mockensturm, D. Swanson, J. Todd
Enhanced Damage Tracking Via State Variable Normalization
Machinery condition monitoring and failure prediction technology has been developing over sixty years. Condition based maintenance methodologies should reduce product maintenance costs by performing maintenance only when it is needed. However, the key to these performances is that we need to accurately monitor damage processes in systems and predict their remaining useful life. Additionally, we want this monitoring to be performed while the machinery is operating.
In the last ten years, the phase space warping (PSW) method has been developed as a predictive maintenance method. The machinery condition monitoring problem is considered from the perspective of dynamical systems theory and the damaged system is viewed as a hierarchical system, which includes a fast subsystem and a slow subsystem. The small distortion in the state space of the fast subsystem caused by the slowly evolving damage shows its ability to be a tracker of the drifting damage. The drifting damage includes material failure, damage, or any other drifting parameter of the system. However, the distortion fluctuates with variance of the initial conditions. Aimed at improving the PSW method by reduce these variance and conceptually understanding the practical issues, such as sensitivity and theoretical solution, the effort of this dissertation focused on developing the theoretical solution of the distortion.
Aimed at improving the PSW method, the efforts presented in this dissertation addressed above problems from perspective of theoretical solution of the system. We focus our work on the hierarchical system whose fast subsystem is linear for simplification. Based on this solution, the theoretical solution of the distortion as a function of fast variable and slow variable is developed. Hence a new tracking function is designed to be independent of fast variable. Experimental implementation for the hierarchical system with linear fast subsystem is discussed with the focus on linear stochastically driving system. To compensate the noise and model error, a scalar tracking metric is defined as the weighted average of the new tracking function and serves as a tracker of the slow variable. To study the multiple damage system, another tracker based on the application of multivariate analysis on the small distortion is designed.
These two new trackers were applied to three experiments whose fast subsystem is linear and the slow subsystem is directly measurable so that the estimated slow variables from the trackers can be compared to the real slow variables. Since the PSW method is essentially a time domain residual method in the context of Fault Detection and Identification, the main challenge of time domain residual methods is studied. In additional, these two trackers are compared to each other and to the methods in time-frequency domain.
Other than condition monitoring and failure prediction, the method demonstrated in this dissertation can also be used to verify and validate continuum damage theories. Continuum damage models are widely investigated in scientific and engineering literature. However, it is difficult to experimental validation of such models. The improved phase space warping methods could be used to verify and validate continuum damage theories comparing the tracking results with the damage model predictions.
Advisors: C. Bakis
Committee: F. Costanzo, C. Lissenden, E. Smith, K. Wang, J. Todd
Flexible Matrix Composites: Dynamic Characterization, Modeling, And Potential For Driveshaft Applications
Flexible matrix composites (FMCs), as the name implies, are composite materials where rigid load carrying fibers are embedded in the soft, flexible polymeric materials such as elastomers. The mechanical properties of FMCs are highly directional dependent and can therefore be tailored to achieve very unique characteristics that are difficult to obtain using the conventional rigid matrix composites (RMCs). In the current investigation, the potential of using an FMC to construct a driveshaft for the helicopter tail rotor is examined. Due to the tailboom motion, helicopter tail rotor driveline must be capable to sustain certain amount of misalignment. To achieve that, the existing tail rotor driveline use multi-segment shafting and flex couplings. By taking advantage of the highly anisotropic properties of FMC materials and proper laminate design, it is possible to construct a flexurally-soft shaft that can accommodate large misalignment. At the same time, the shaft must be torsionally-stiff to effectively transmit power. With this FMC driveshaft, the tail rotor driveline can be significantly simplified. As a consequence, the maintenance and weight can be reduced. However, because of the viscoelastic nature of the elastomeric matrix material, an FMC driveshaft rotating under a misaligned condition could potentially suffer from self-heating problem caused by internal damping. Therefore, a model that is capable of predicting the self-heating behavior is needed for shaft design and material selection. In the current approach, a thermo-mechanical self-heating model for a rotating misaligned composite shaft is developed. The inputs to the model are the basic lamina properties of the FMC materials which are frequency and temperature dependent. A lab-scale misaligned rotating shaft test stand is built for model validation. Good agreement has been achieved between the model predicted and experimentally measured shaft temperature increases. Based on the model, it was determined that the product of the shaft longitudinal damping loss factor and the shaft longitudinal modulus is the determining factor for self-heating. Therefore, in spite of the loss factor of the FMC shaft being almost 10 times larger than that of an RMC shaft, the self-heating can still be less than that of an RMC shaft. Fatigue tests on FMC shafts are also performed on the current test stand and based on the preliminary test results it is found FMC shaft are strongly fatigue resistant. The conclusion of this work to-date is that FMC materials show great promise for helicopter driveshaft applications; less heat generation, and good fatigue life.
Advisors: R. Engel
Committee: R. German, J. Rose, D. Green, E. Lehtihet, J. Todd
Die Compaction Simulation: Simplifying The Application of a Complex Constitutive Model Using Numerical And Physical Experiments
Powder Metallurgy is the science of manufacturing parts out of metal powders. The die compaction process is a shaping technique in powder metallurgy in which metal powder is pressed in a rigid tool cavity called the die to form a medium density weak part with loose particle bonding. To impart strength and increase density, the part is subjected to a thermal process called sintering in which the particles bond together to form the final desired part. The increase in density due to bonding of particles leads to shrinkage in this stage. During compaction, friction between the powder and the die causes the applied pressure to reduce along the die wall leading to a pressure gradient which results in a compacted part with density gradient. During the subsequent sintering of a part with density gradients, the shrinkage is non uniform and can lead to dimensional distortion. To understand the distortion it is necessary to determine the density and the density gradients formed as a result of compaction. Computer modeling of the process provides a cost-effective technique to determine the properties of the part after compaction.
Numerical modeling of a process involves mathematically quantifying the physics of the process, including the material behavior when subjected to mechanical loads. During the initial stages of compaction, the powder particles are rearranged; this is followed by permanent deformation of the particles under higher pressure as they pack together. The behavior of the particle system as the loose powder is shaped into a weak solid part is described by a material model with parameters that characterize various aspects of the response, e.g., the cohesion between particles, inter particle friction and strength of the powder material. Thus, modeling the process requires both the selection of an appropriate material model as well as the determination of each of the model’s parameters. The recommended testing procedures to determine these parameters can be expensive making it difficult for industrial applications. This research looks at ways of simplifying the application of a well defined complex material model. A test procedure involving simple physical and numerical experiments has been developed to quantify the material model for commonly used metal powders. The model and the test procedure predict density and density gradient results that are in good agreement with physical measurements of density fields in compacted parts.
This page was last updated on October 8, 2007.