Showing papers by "Missouri University of Science and Technology published in 1993"

Active Vibration Control of Laminated Composite Plates Using Piezoelectric Devices: A Finite Element Approach

Abstract: A finite element formulation is presented for modeling the behavior of laminated composites with integrated piezoelectric sensors and actuators. This model is valid for both con tinuous and segmented piezoelectric elements that can be either surface bonded or embedded in the laminated plate. The present model takes into account the mass and the stiffness of the piezoelectric patches. The formulation is based on the first-order shear deformation theory, which is applicable for both thin and moderately thick plates. An additional feature of the present model is that it does not introduce the voltage as an additional degree of freedom. The charge/current generated by the sensor and the response of the plate to an actuator voltage can be computed independently. These features are then coupled with a constant-gain negative-velocity/positive-position feedback control algorithm to actively control the transient response of the plate in a closed loop. Numerical results are presented which indicate the increase in...

Hydrothermal synthesis and sintering of ultrafine CeO2 powders

Abstract: Undoped CeO2 and Y2O3-doped CeO2 powders, with particle sizes of almost-equal-to 10-15 nm, were prepared under hydrothermal conditions of 10 MPa at 300-degrees-C for 4 h. The compacted powders were sintered freely in air or in O2 at constant heating rates of 1-10-degrees-C/min up to 1350-degrees-C. The undoped CeO2 started to sinter at almost-equal-to 800-900-degrees-C and reached a maximum density of 0.95 of the theoretical at 1200-degrees-C, after which the density decreased slightly. Isothermal sintering at 1150-degrees-C produced a sample with a relative density of almost-equal-to 0.98 and an average grain size of almost-equal-to 100 nm. The samples sintered above 1200-degrees-C exhibited microcracking. The decrease in density and the microcracking above 1200-degrees-C are attributed to a redox reaction leading to the formation of oxygen vacancies and the evolution Of O2 gas. Doping with Y2O3 produced an increase in the temperature at which measurable sintering commenced and an increase in the sintering rate, compared with the undoped CeO2. Sintered samples of the doped CeO2 showed no microcracks. The CeO2 doped with up to 3 mol % Y2O3 was sintered to almost full density and with a grain size of almost-equal-to 200 nm at 1400-degrees-C.

Chemical durability of Y2O3-Al2O3-SiO2 glasses for the in vivo delivery of beta radiation.

Abstract: Microspheres made from Y2O3-Al2O3-SiO2 (YAS) glasses, which contain radioactive Y-90, are currently being used to treat liver cancer in humans, where their chemical durability is of prime importance. In deionized water or saline at 37 degrees C, the weight percent Yttrium (Y) dissolved from eight different YAS glasses ranged from only 0.02-0.13% of the total Y present and their dissolution rate was barely measurable, < or = 1.0 x 10(-9) g/cm2-min. The most chemically durable YAS glass was 17Y2O3-19Al2O3-64SiO2, mol%. The small amount of Y released from microspheres, 25-35 microns diameter, of this glass after corrosion in saline or deionized water at 37 degrees C was essentially the same as for bulk glass samples. Based on their excellent chemical durability, it is concluded that YAS glass microspheres are suitable for in vivo use.

Modeling of solute redistribution in the mushy zone during solidification of aluminum-copper alloys

Abstract: A mathematical model has been established to predict the formation of macrosegregation for a unidirectional solidification of aluminum-copper alloys cooled from the bottom. The model, based on the continuum formulation, allows the calculation of transient distributions of temperature, velocity, and species in the solidifying alloy caused by thermosolutal convection and shrinkage-induced fluid flow. Positive segregation in the casting near the bottom (inverse segregation) is found, which is accompanied by a moving negative-segregated mushy zone. The effects of shrinkage-induced fluid flow and solute diffusion on the formation of macrosegregation are examined. It is found that the redistribution of solute in the solidifying alloy is caused by the flow of solute-rich liquid in the mushy zone due to solidification shrinkage. A higher heat-extraction rate at the bottom increases the solidification rate, decreasing the size of the mushy zone, reducing the flow of solute-rich liquid in the mushy zone and, as a result, lessening the severity of inverse segregation. Comparisons between the theoretical predictions from the present study and previous modeling results and available experimental data are made, and good agreements are obtained.

Buckling and Post-buckling of Composite Plates and Shells Subjected to Elevated Temperature

Abstract: Effects of temperature on buckling and post-buckling behavior of reinforced and unstiffened composite plates or cylindrical shells are considered. First, equilibrium equations are formulated for a shell subjected to the simultaneous action of a thermal field and an axial loading. These equations are used to predict a general form of the algebraic equations describing the post-buckling response of a shell. Conditions for the snap-through of a shell subjected to thermomechanical loading are formulated. As an example, the theory is applied to prediction of post-buckling response of flat large-aspect-ratio panels reinforced in the direction of their short edges. 19 refs.

Factors controlling the level and determination of D-amino acids in the urine and plasma of laboratory rodents

Abstract: Unambiguous methodologies were developed for the accurate and reproducible determination of specific D-amino acids in the physiological fluids of common laboratory rodents. Depending on the strain of rodent and the type of amino acid examined, excreted D-amino acids ranged from the low percent levels to over 40 percent of the total specific amino acid level. Relative plasma levels tended to be considerably lower, typically an order of magnitude less. A number of factors were found to alter the relative amounts of excreted D-amino acids. This included: diet, age, pregnancy, advanced cancer, and antibiotics. The two factors that seemed to result in substantially lower levels of excreted D-amino acids were fasting and young age. Pregnancy was the only factor that consistently resulted in higher relative D-amino acid excretion. Much of the observed data are believed to be related to the efficiency with which the kidney reabsorbs L-amino acids. No claims are made as to the meaning and/or importance of free D-amino acids in regards to pathology, age, clinical usefulness and so forth. However, a knowledge of normal D-amino acid levels and dynamics is necessary before it is possible to identify perturbations caused by either natural or pathological conditions. The techniques are now available that should allow these topics to be addressed properly.

Thermally stimulated polarization and dc conduction in iron phosphate glasses

Abstract: Thermally stimulated polarization (TSPC) and depolarization current (TSDC) techniques were used to study electrical polarization and conduction mechanisms in iron phosphate and sodium-iron phosphate glasses. TSDC measurements from 120 to 350 K show two current peaks, P1, attributed to the polarization caused by intrinsic dipolar defects, and P2, due to space-charge relaxation. The electrical conductivity was examined on the basis that the activation energy for electronic conduction is lower than that for ionic conduction. The dc conductivity depends upon iron oxide content and distance between iron ions, which suggests electronic conduction. The difference in activation energy between TSDC peaks and dc conductivity is discussed. Infrared absorption spectra indicate that iron ions can act as a network former and/or modifier depending upon the Fe(II)/Fe(III) ratio in the glass.

Analysis and Optimization of Thermal Stratification and Self-Pressurization Effects in Liquid Hydrogen Storage Systems—Part 1: Model Development

Abstract: This paper reports on analyses and optimization studies of problemes associated with liquid hydrogen thermal stratification and self-pressurization in cryogenic vessels. Three different pressure rise models were employed to calculate the self-pressurization and boil-off rates. These are a homogenous model, a surface-evaporation model, and a thermal stratification model. The first two uodels are based on the assumption that no temperature gradients exist in the tank, while the third one takes the temperature distribution into account. Employing the thermal stratification model, temperature gradients and their effect on the pressure rise rates in liquid hydrogen tank are analyzed

Dollar movements and inflation: a cointegration analysis

Abstract: Many studies have concluded that changes in US dollar values affect US prices. However, since exchange rates may reflect underlying economic conditions, this result may be spurious if those conditions are ignored. Recent advances in time series analysis permitted investigation of both the short and long term interactions between the US price level and the yen/dollar exchange rate, taking into account the effects of changes in domestic economic activity, such as income, interest rates and monetary policy. Examination of these interactions helps to determine to what extent the yen/dollar exchange rate impacts the domestic price level. The finding is that there is no long term relation between the two variables. In addition, the results suggest that in the short run, monetary authorities respond to changes in the consumer price index and Japanese exchange rates; however, the price level does not respond to change in the exchange rate.

Performance amplification and process restructuring in computer-based writing

Abstract: This research compared composing on a word processor with writing in longhand to explore whether the computer-based tool amplifies performance and restructures attentional allocation to writing processes. Performance was assessed in terms of the quality of the resulting documents, based on subjective ratings and text analysis, and the fluency of language production. The allocation of attentional resources was monitored in terms of the degree of cognitive effort (secondary task reaction times) and processing time (directed retrospective reports) devoted to planning ideas, translating ideas into text, and reviewing ideas and text. In Experiment 1, word processing increased the attentional investment in and nature of planning and reviewing, without improving either the quality or fluency of writing. In Experiment 2 these restructuring effects were again observed both for writers who reported modest experience composing on a computer and to an even greater degree for those who reported extensive experience. Only participants with extensive word processing experience matched the quality and fluency of those who wrote in longhand.

Investigation of spin-orbit effects in the excitation of noble gases by spin-polarized electrons

Abstract: We have measured the integrated Stokes parameters of the light emitted following the impact excitation by polarized electrons of the ${\mathit{np}}^{5}$ (n+1)p[5/2${]}_{3}$ states in Ne (n=2), Ar (n=3), Kr (n=4), Xe (n=5), and the ${\mathit{np}}^{5}$ (n+1)p[5/2${]}_{2}$ state in Kr. The near-threshold linear-polarization fractions ${\mathrm{\ensuremath{\eta}}}_{1}$ were consistent with zero for all of the J=3 states measured, providing no evidence of Mott scattering. For J=2 excitation, ${\mathrm{\ensuremath{\eta}}}_{1}$ shows the clear influence of the intermediately coupled nature of the state. At threshold, the measured circular polarization ${\mathrm{\ensuremath{\eta}}}_{2}$ and linear polarization ${\mathrm{\ensuremath{\eta}}}_{3}$ for the J=3 states agree well with the LS-coupled threshold predictions. These results provide the basis for optical measurements of electron polarization.

Genetic neuro-scheduler for job shop scheduling

Abstract: This paper describes a hybrid approach between two new techniques, Genetic Algorithms and Artificial Neural Networks, for generating Job Shop Schedules (JSS) in a discrete manufacturing environment based on non-linear multi-criteria objective function. Genetic Algorithm (GA) is used as a search technique for an optimal schedule via a uniform randomly generated population of gene strings which represent alternative feasible schedules. GA propagates this specific gene population through a number of cycles or generations by implementing natural genetic mechanism (i.e. reproduction operator and crossover operator). It is important to design an appropriate format of genes for JSS problems. Specifically, gene strings should have a structure that imposes the most common restrictive constraint; a precedence constraint. The other is an Artificial Neural Network, which uses its highly connected-neuron network to perform as a multi-criteria evaluator. The basic idea is a neural network evaluator which maps a complex set of scheduling criteria (i.e. flowtime, lateness) to evaluate values provided by experienced experts. Once, the network is fully trained, it will be used as an evaluator to access the fitness or performance of those stimulated gene strings. The proposed approach was prototyped and implemented on JSS problems based on different model sizes; namely small, medium, and large. The results are compared to the Shortest Proceesing Time heuristic used extensively in industry.

Measurements in Buoyancy-Assisting Separated Flow Behind a Vertical Backward-Facing Step

Abstract: The effect of buoyancy on the flow and the heat-transfer characteristics was investigated experimentally and numerically, using the results of measurements of velocity and temperature distributions in buoyancy-assisting laminar mixed convection boundary layer flow over a vertical two-dimensional heated backward-facing step. As a result of measurements, heat transfer (Nusselt number) and reattachment length behaviors were obtained as a function of the buoyancy level. It was found that an increase in the buoyancy force causes a decrease in the size of the recirculation region behind the step and decreases in the reattachment length. An increase of the buoyancy force also causes the local Nusselt number to increase, and the location of its maximum value to move closer to the step. The results of measurements agreed well with numerical predictions. 14 refs.