▼ Many materials (automobile tires, for example) are fabricated using fiber reinforced composite materials. A fiber reinforced composite consists of a set of stiff aligned fibers embedded in a softer matrix. In the case of automobile tires, steel fibers are embedded in a rubber matrix. The fiber and the matrix have their own material properties while the fiber reinforced composite itself has a different set of material properties, called effective properties. The main goals of this dissertation are to define the internal stress and strain fields for a fiber reinforced composite and to determine its effective material properties by focusing on the local conditions around a single fiber when a linearly graded transition zone is placed between the fiber and the matrix. Internal fields for fiber reinforced composites as well as the impact on the effective properties of the geometry and the material properties of the components are studied. Advisors/Committee Members: Kreider, Kevin.

▼ This dissertation presents the development of a semi-analytic technique developed for the determination of far field acoustic radiation in the time domain. This method solves linear, time dependent wave propagation in an unbounded medium using a numerical Laplace transform and potential theory. The end result is a robust procedure that is accurate and computationally efficient. The Transform Potential Theoretic (TPT) method is meshless and can handle arbitrary geometries. The procedure assumes the linearity of the sound field away from a bounded region surrounding the object. The TPT method depends on the sound pressure on the boundary of this region (referred to as the Kirchhoff surface). The Euler equations are linearized about a uniform mean flow. First, the problem is transformed via the Laplace transform (with appropriate initial conditions) into a reduced wave equation. By application of a dependent variable transformation, the anisotropic terms are removed and a Helmholtz-like equation with complex wave number is obtained where both single and double layer potential theory applies. This allows the calculation of the far-field acoustic pressure in the Laplace domain. Then, an inversion of the dependent variable transform is applied. Upon application of numerical inverse Laplace transform techniques, far-field acoustic pressure is then successfully obtained as a function of space and time. Using transient monopole radiation in a uniform freestream, accuracy is analyzed with excellent results. This method shows many advantages over direct simulation, including vast savings in computational time. The freestream Mach number is only a parameter in the TPT method and has no bearing on the run time, unlike direct methods. Advisors/Committee Members: Sawyer, Scott.

▼ In recent years, the demand for wireless communication systems with high data rates and improved link quality for a variety of applications has dramatically increased. To keep up with the demand, new concepts that could mitigate the channel impairments and optimally exploit the limited communication resources are necessary. One of such new concepts is the multiple input multiple output (MIMO) communication system, which has been found to be efficient in increasing the performance. To utilize the huge potential benefits of multiple antennas, it is necessary to develop new transceiver strategies. Hence, in this dissertation, we combine the concept of MIMO and code division multiple access (CDMA), and coding, into a robust communication system. We develop the analytical framework, architectures, algorithms and performance analyis. Both analytical and simulated results are presented. The performance of the MIMO-CDMA system is analyzed by taking into account advanced signal processing techniques, in the presence of mutual information coupling. The performance of the system is evaluated in terms of complexity, signal-to-noise ratio, bit error rate, outage probability and mutual information capacity. Advisors/Committee Members: Ugweje, Okechukwu C.

▼ Field emission from conducting nanofibers has a significant importance due to its possible application in electronics like flat panel displays, x-ray machines, sensors, etc.The standard theoretical model describing field emission is the Fowler-Nordheim model, which is valid for bulk material, constant applied electric field and zero temperature. A more general theoretical model is required in the realistic cases of arbitrary electromagnetic fields and arbitrary but finite temperature. This work presents an asymptotic procedure for calculating field emission from nanofibers of finite length for static and dynamic fields at arbitrary finite temperature. It investigates the behavior of a nanofiber in the presence of electrostatic and EM fields. The resultant field potentials outside the system are obtained by employing the slender-body approximation. The total external potential is used in conjunction with the the Wentzel-Krammers-Brillouin approximation to estimate the tunneling probability of the electrons in the fiber due the total external field. Unlike the standard Fowler-Nordheim method, the current density of the field emission is obtained by using quantum wire density of states. In addition, this work investigates radiative and scattering properties of conducting nanofibers for the purpose of nanoantenna applications . The results for the distributions of the induced currents are compared to the results from the solution of Hallen's integral equation and the corresponding radiation patterns are compared. The results are extended for the case of a broadside uniform array of N aligned fibers. Advisors/Committee Members: Hariharan, Subramaniya.