A. James Clark School of Engineering

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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.

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Start date: 1990End date: 1999

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    Microwave Nonlinearities in Photodiodes
    (1994) Williams, Keith Jake; Dagenais, Mario; Electrical & Computer Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
    The nonlinearities in p-i-n photodiodes have been measured and numerically modeled. Harmonic distortion, response reduction, and sinusoidal output distortion measurements were made with two singlefrequency offset-phased-locked Nd: YAG lasers, which provided a source dynamic range greater than 130 dB, a 1 MHz to 50 GHz frequency range, and optical powers up to 10 mW. A semi-classical approach was used to solve the carrier transport in a one-dimensional p-i-n photodiode structure. This required the simultaneous solution of three coupled nonlinear differential equations: Poisson's equation and the hole and electron continuity equations. Space-charge electric fields, loading in the external circuit, and absorption in undepleted regions next to the intrinsic region all contributed to the nonlinear behavior described by these equations. Numerical simulations were performed to investigate and isolate the various nonlinear mechanisms. It was found that for intrinsic region electric fields below 50 kV/cm, the nonlinearities were influenced primarily by the space-charge electric-field-induced change in hole and electron velocities. Between 50 and 100kV/cm, the nonlinearities were found to be influenced primarily by changes in electron velocity for frequencies above 5 GHz and by p-region absorption below 1 GHz. Above 100 kV/cm, only p-region absorption could explain the observed nonlinear behavior, where only 8 to 14 nm of undepleted absorbing material next to the intrinsic region was necessary to model the observed second harmonic distortions of -60 dBc at 1 mA. Simulations were performed at high power densities to explain the observed response reductions and time distortions. A radially inward component of electron velocity was discovered, and under certain conditions, was estimated to have the same magnitude as the axial velocity. The model was extended to predict that maximum photodiode currents of 50 mA should be possible before a sharp increase in nonlinear output occurs. For capacitively-limited devices, the space-charge-induced nonlinearities were found to be independent of the intrinsic region length, while external circuit loading was determined to cause higher nonlinearities in shorter devices. Simulations indicate that second harmonic improvements of 40 to 60 dB may be possible if the photodiode can be fabricated without undepleted absorbing regions next to the intrinsic region.
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    Predicting the Ignition Time and Burning Rate of Thermoplastics in the Cone Calorimeter
    (1995) Hopkins, Donald Jr.; Quintiere, James G.; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
    Ignition and burning rate data are developed for Nylon 6/6, Polyethylene, and Polypropylene in a Cone Calorimeter heating assembly. The objective is to examine a testing protocol that leads to the prediction of ignition and burning rate for thermoplastics from Cone data. The flame heat flux is not measured, but is inferred from Cone data. The constancy of the flame heat flux for thermoplastics in the Cone calorimeter is due to the geometry of the flame. The burning rate model is shown to yield good accuracy in comparison to measured transient values. Ignition and burning rate data are developed for Redwood and Red Oak in a Cone Calorimeter heating assembly. Measurements of the flame plus external heat flux are presented. The data is intended to be used for future work to develop a testing protocol and burning rate model for charring materials.
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    Stress-Controlled Versus Strain-Controlled Triaxial Testing of Sand
    (1994) Alqutri, Samir Ahmed; Goodings, Deborah J.; Civil Engineering; University of Maryland (College Park, Md); Digital Repository at the University of Maryland
    The purpose of this research was to compare the strength characterizations of Mystic White Silica Sands using stress-controlled loading versus strain-controlled loading in a standard compression triaxial tests. To this end one hundred sixty-six tests were conducted involving two types of quartz sand, one fine MWSS45 and one medium coarse MWSS18 , tested at three low to intermediate confining stresses of 14 kN/m2, 28 kN/m2 and 55 kN/m2 with only one specimen diameter size of 71.1 mm. Of the one hundred sixty-six tests, eighty-six were stress-controlled tests and eighty were strain-controlled tests. All specimens were dry, but both loose and dense specimens were tested. The results were evaluated individually and as group. Comparison of the two types of loading tests were evaluated for repeatability, stress-strain characteristics and strength parameters. The plots show that stress-controlled loading in general gives more reproducible results with smoother. steeper stress-strain plot s and a larger average deviator stresses at failure than strain-controlled loading at all three levels of confining stresses for both sands. This results in somewhat larger values of Φ' . Stress-controlled specimens were stiffer and failed with a clear cut failure surface while strain-controlled specimens mostly barreled.
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    Physics-of-Failure Methodology for Accelerated Thermal Cycling of LCC Solder Joints
    (1995) Rothman, Timothy Paul; Dasgupta, Abhijit; Mechanical Engineering; University of Maryland (College Park, Md); Digital Repository at the University of Maryland
    This research presents a case study were existing physics-of-failure models and Bayesian statistical methods are used in conjunction to quantify the test-time compression achieved during accelerated temperature cycling tests on leadless solder joints. Different combinations of substrate materials and package styles are evaluated with physics-of-failure models and calculable information is obtained from a relatively small population of test specimens under accelerated stresses, because the critical variables are identified, and their influences on the stress magnitude are quantified. Bayesian statistical analysis is employed to obtain an acceleration transform, determine the confidence on the calculations, and determine which outliers are contaminating the database. In addition to accelerating the stress levels, the total test time is further minimized by tailoring the stress drivers in each sample such that multiple stress levels can be achieved under a single loading, which eliminates the need for repeating the test at multiple load levels. This research presents the details of how the models and statistical methods are applied, the results of evaluating different material combinations and package styles, problems encountered during the test, and a summary of the acceleration transforms obtained from the test. Analytical predicative models for life predictions are essential and will obviously result in significant savings of cost and time. The methods used in this are general enough to be applied to screening, qualification, and reliability enhancement tests of a wide range of new or existing electronics assemblies.
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    Data Acquisition Interface of a VLSI Cochlea Model
    (1993) Edwards, Thomas G.; Shamma, Shihab; Electrical Engineering; Digital Repository at the University; University of Maryland (College Park, Md)
    Computer models of cochlear processing take exceedingly long times to run, even for short data sets. A data acquisition system was developed for a new switched-capacitor VLSI cochlea model chip, in order to rapidly perform cochleaI processing on digitzed speech samples. The system is capable of processing very long speech samples. Processing is in near-real-time, though it, takes about 2 minutes per second of speech to write the large amount of data to a hard drive. Software has also been developed to convert the output data into a form readable by the ESPS digital signal processing package from Entropic Speech, Inc.
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    The Influence of Variable Flow Velocity on Unsteady Airfoil Behavior
    (1991) van der Wall, Berend G.; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    The importance of unsteady aerodynamics for prediction of rotor dynamics is unquestioned today. The purpose of unsteady aerodynamic models is to represent the effect of unsteady airfoil motion on the lift, moment and drag characteristics of a blade section. This includes unsteady motion (arbitrary motion) of the airfoil in angle of attack (pitch) and vertical movement (plunge), as well as the effects of an airfoil traveling through a vertical gust field. However, the additional degrees of freedom, namely the fore-aft motion and the unsteady freestream variations commonly are acknowledged, but neglected in virtually all analyses. Since the effect of unsteady freestream results in a stretching and compressing of the shed wake vorticity distribution behind an airfoil, it will have an effect on the airfoil characteristics. The subject of this thesis is to provide a review of the analytic and experimental work done in the area of unsteady freestream and unsteady fore-aft motion, to clarify the limits of the various theories, and to show the differences between them. This will be limited to the attached flow regime since all theories are based on the small disturbance assumption in incompressible flow. As far as possible the theories are compared with experimental data, however most of the available experimental data are confined to stalled flow conditions and are not useful here. In addition to the theories, a semiempirical mathematical model will be used based on the aerodynamics of indicial functions. The purpose is to show the differences of using the theories of unsteady airfoil motion in a constant flow, and those accounting for unsteady freestream flow. This will help to justify whether it is necessary to include the unsteady freestream effect in comprehensive rotor codes. Finally, a generalisation of Isaacs unsteady aerodynamic theory for an airfoil undergoing a frequency spectra in pitch and plunge in a freestream oscillating with the fundamental frequency is presented here for the first time. Therein the axis of rotation of the airfoil is a free parameter.
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    Networks for Fast and Efficient Unicast and Multicast Communications
    (1992) Lee, Ching-Yi; Oruç, A. Yavuz; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
    This dissertation presents new results on networks for high-speed unicast and multicast communications which play key roles in communication networks and parallel computer systems. Specifically, (1) we present past parallel algorithms for routing any one-to-one assignment over Beneš network, we propose new multicasting networks that can efficiently realize any one-to-many assignments, and we give an explicit construction of linear-size expanders with very large expansion coefficients. Our parallel routing algorithms for Beneš networks are realized on two different topologies. Using these algorisms, we show that any unicast assignment that involves )(k) pairs of inputs and outputs can be routed through and n-input Beneš network in O(log2 k+lg n) time without pipelining and O(lg k) time with pipelining if the topology is complete, and in O(lg4k+lg2k lg n) time without pipelining and O(lg3 k+lg k lg n) time with pipelining if the topology is extended perfect shuffle. These improve the best-known routing time complexities of parallel algorithms of Lev et al. and Nassimi and Sahni by a factor of O(lg n). Our multicasting networks uses a very simple self-routing scheme which requires no separate computer model for routing. Including the routing cost, it can be constructed with O(n lg2 n) bit-level constant fanin logic gates, O(lg2 n) bit-level depth, and can realize any multicast assignment in O(lg3 n) bit-level time. These complexities match or are better than those of multicasting networks with the same cost that were reported in the literature. In addition to its attractive routing scheme, our multicasting network is input-initiated and can pipeline multicast assignments through itself. With pipelining, the average routing time for O(lg2 n) multicast assignments can be reduced to O(lg n) which is the best among those of the multicasting networks previously reported in the literature. Our linear-size expanders are explicitly constructed by following a traditional design and analysis technique. We construct a family of linear-size with density 33 and expansion coefficient 0.868. This expansion coefficient is the larges among the linear-size expanders that were similarly constructed. Using these expanders, we also report a family of explicitly constructed superconcentrators with density 208.
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    A MANUFACTURING PROCESS AND MATERIALS DESIGN ADVISOR
    (1996) Kunchithapatham, Arun; Magrab, Edward B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    A computer assisted tool, called the DESIGN ADVISOR, has been developed to help obtain a compatible set of candidate materials and manufacturing processes in a fast and straightforward manner. Information for seventeen manufacturing processes includes animations, written descriptions, still pictures and geometric design rules; information for forty-two materials includes written information and data. The DESIGN ADVISOR determines candidate manufacturing processes based on user-specified levels of one or more of seven manufacturing attributes; namely, surface condition, dimensional accuracy, complexity of shape, size, production run or production rate, and cost. The determination of candidate materials is based on the user-specified levels of one or more of eleven material attributes; namely, yield strength/density, fracture toughness/density, elastic modulus/density, high temperature strength/density, density, magnetic properties, electrical resistivity, thermal distortion, thermal insulation, solvent resistance, and cost. It also determines the suitability of candidate manufacturing processes with the candidate materials, and ranks the suitability of each candidate material within each candidate manufacturing process.
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    New Methods for the Detection and Interception of Unknown, Frequency-Hopped Waveforms
    (1990) Snelling, William Edward; Geraniotis, Evaggelos; Electrical & Computer Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    Three new methods for the detection and interception of frequency-hopped waveforms are presented. The first method extends the optimal, fixed-block detection method based on the likelihood ratio to a sequential one based on the Sequential Probability Ratio Test (SPRT). The second method is structured around a compressive receiver and is highly efficient yet easily implemented. The third method is based on the new concept of Amplitude Distribution Function (ADF) and results in a detector that is an extension of the radiometer. The first method presents a detector structured to make a decision sequentially, that is, as each data element is collected. Initially, a purely sequential test is derived and shown to require fewer data for a decision. A truncated sequential method is also derived and shown to reduce the data needed for a decision while operating under poor signal-to-noise ratios (SNRs). A detailed performance analysis is presented along with numerical and Monte Carlo analyses of the detectors. The second method assumes stationary, colored Gaussian interference and presents a detailed model of the compressive receiver. A locally optimal detector is developed via the likelihood ratio theory and yields a reference to which previous ad hoc schemes are compared. A simplified, suboptimal scheme is developed that trades off duty cycle for performance, and a technique for estimating hop frequency is developed. The performance of the optimal and suboptimal detectors is quantified. For the suboptimal scheme, the trade-off with duty cycle is studied. The reliability of the hop frequency estimator is bounded and traded off against duty cycle. In the third method, a precise definition of the ADF is given, from which follows a convolutional relationship between the ADFs of signal and additive noise. A technique is given for deconvolving the ADF, with which signal and noise components can be separated. A detection statistic characterized, yielding a framework on which to synthesize a detector. The detector's performance is analyzed and compared with the radiometer.
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    FINITE ELEMENT ANALYSES OF PARTIALLY REINFORCED MASONRY SHEAR WALLS
    (1996) Love, Aaron Ray; Chang, Peter; Civil & Environmental Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    Partially reinforced shear walls are used in regions of relatively low seismic risk. Nevertheless, these structures should be capable to resist some lateral motion. The purpose of this paper is to examine the behavior of in-plane cyclic load tests of typical partially-grouted masonry shear walls and the capability of FEM/I in simulating their response. FEM/I is a nonlinear finite element program originally developed for reinforced masonry shear walls with a uniform grid of orthogonal reinforcement at close spacing. FEM/I has successfully simulated the response of fully grouted uniformly distributed reinforced masonry walls [Ewing, 1987]. FEM/I uses a smeared property approach in which the reinforcing steel and masonry composite is modeled as a single material. The applicability of FEM/I to partially grouted partially reinforced masonry shear walls is measured by comparing FEM/I force-displacement cycles, peak lateral forces, strains, energy dissipation and crack patterns with those generated from the experimental tests conducted at the National Institute of Standards and Technology. Partially reinforced shear walls can be modeled in FEM/I by smearing the steel over the blocks which are grouted and reinforced. The ungrouted blocks can be modeled as reinforced blocks with a reinforcement ratio of zero. This approach was shown to be adequate when the displacements and cracks were small. As the cracks increase in size, the smeared property assumption can no longer adequately represent the wall• s geometry and it's property. The result is a poor prediction of both local and global behavior at large displacements. The ratio of the lateral loads at the first major event (FME) demonstrate a good relationship in the forces generated by FEM/I for each wall with the exception Wall I. Results from Walls 3, 5, 9 and 11 exhibit FEM/I was able to predict the lateral load adequately up to the FME. The ratio of the lateral load up to the FME ranges from 0.9 - 1.3. After the occurrence of the FME, FEM/I overpredicts the lateral load considerably. In each of the finite element analyses, FEM/I overestimated the peak strength of the masonry specimens. The FEM/I models for Wall 11 and Wall 3 produced the best prediction of the peak strength. The difference for these two walls in the FEM/I predicted maximum lateral load and experimental data were 31 o/o and 41 o/o, respectively. Individual force displacement cycles are plotted at the various stages in the displacement history. FEM/I performs fairly well in predicting the force displacement response of the experiment. Walls 3, 5, 9 and 11 exhibit a good force displacement relationship for the first half of their displacement history until the development of major cracks. Wall 7 corresponded well with the experiment during the initial stages (Cycles 1 -17) of its displacement history. FEM/I did not produce good results in representing the cracking pattern generated by the experimental study. The inability of modeling the crack pattern is also shown in the differences in the plots for the amount of energy dissipated. FEM/I did reasonably well in the prediction of yielding in the vertical reinforcement. Local stress and strain of masonry predicted by FEM/I did not match the experimental data.