Showing papers by "Wright-Patterson Air Force Base published in 1989"
TL;DR: In this paper, a great deal of effort has been made to improve these ductile properties through chemistry modification and microstructure control, and some success has met with some success.
Abstract: Titanium-aluminide alloys based on TiAl have an excellent potential to become one of the most important aerospace materials because of their low density, high melting temperature, good elevated-temperature strength and modulus retention, high resistance to oxidation and hydrogen absorption, and excellent creep properties. The chief roadblock to their application is poor ductility at low to intermediate temperatures that results in low fracture toughness and a fast fatigue-crack growth rate. During the last several years, a great deal of effort has been made to improve these ductile properties. These endeavors have met with some success through chemistry modification and microstructure control.
536 citations
TL;DR: A gas-phase vial equilibration technique is presented for determining the liquid: air and tissue:air partition coefficients for low-molecular-weight volatile chemicals and an approach is described for predicting the tissue solubilities of untested compounds based on oil:air and saline:air coefficients using regression analyses.
470 citations
TL;DR: In this paper, a numerical study is presented for unsteady laminar flow past a NACA 0015 airfoil that is pitched, at a nominally constant rate, from zero incidence to a very high angle of attack.
Abstract: A numerical study is presented for unsteady laminar flow past a NACA 0015 airfoil that is pitched, at a nominally constant rate, from zero incidence to a very high angle of attack. The flowfield simulation is obtained by solving the full two-dimensional compressible Navier-Stokes equations on a moving grid employing an implicit approximate-factorization algorithm. An evaluation of the accuracy of the computed solutions is presented, and the numerical results are shown to be of sufficient quality to merit physical interpretation. The highly unsteady flowfield structure is described and is found to be in qualitative agreement with available experimental observations. A discussion is provided for the effects of pitch rate and pitch axis location on the induced vortical structures and on the airfoil aerodynamic forces.
236 citations
TL;DR: The existing control-volume finitedifference approach is modified so it can be applied to the numerical performance of Stefan problems, and the model is tested by applying it to a three-dimensional freezing problem and the numerical results are in agreement with those existing in the literature.
163 citations
TL;DR: The propagation of Lamb waves in plates has been the subject of numerous investigations since their postulation by Lamb in 1916 [1,2] as discussed by the authors, and theoretical analyses have been reported in plates of cubic [3,4], transversely isotropic [5,6], and orthotropic [7,9] media.
Abstract: The propagation of Lamb waves in plates has been the subject of numerous investigations since their postulation by Lamb in 1916 [1,2]. Most of the work in existence deals with various aspects of these guided waves in plates of isotropic materials. Comparatively speaking a limited number of results has appeared in which Lamb or horizontaly polarized SH wave propagation in anisotropic plates has been considered in any detail. For Lamb waves, theoretical analyses have been reported in plates of cubic [3,4], transversely isotropic [5,6], and orthotropic [7,9] media.
154 citations
TL;DR: The crystal structure analysis and IR study of the 1-methyl-3-ethylimidazolium chloride (MEICl) has been undertaken as part of an investigation of the ion-ion interactions in room temperature melts, where the mole fraction of AlCl 3 is less than 0.5 as mentioned in this paper.
140 citations
TL;DR: Kinetics in the pregnant rat exposed to TCE by inhalation, by bolus gavage, or by oral ingestion in drinking water are described and the fetus is clearly at risk both to parent TCE and its TCA metabolite.
131 citations
TL;DR: The formulation of this generalization includes a unified presentation of the optimality conditions, the Lagrangian multipliers, and the resizing and scaling algorithms in terms of the sensitivity derivatives of the constraint and objective functions.
Abstract: This paper presents a generalization of what is frequently referred to in the literature as the optimality criteria approach in structural optimization. This generalization includes a unified presentation of the optimality conditions, the Lagrangian multipliers, and the resizing and scaling algorithms in terms of the sensitivity derivatives of the constraint and objective functions. The by-product of this generalization is the derivation of a set of simple nondimensional parameters which provides significant insight into the behavior of the structure as well as the optimization algorithm. A number of important issues, such as, active and passive variables, constraints and three types of linking are discussed in the context of the present derivation of the optimality criteria approach. The formulation as presented in this paper brings multidisciplinary optimization within the purview of this extremely efficient optimality criteria approach.
94 citations
01 Sep 1989-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the status of RS alloy exploration for the three lightweight metals is reviewed and suggestions made on possible future directions for alloy development for aluminium, magnesium, and titanium, as well as their properties can be rationalized in terms of the alloying behavior of these systems.
Abstract: Rapid solidification (RS) of the light metals aluminium, magnesium, and titanium can lead to enhanced mechanical properties which can be rationalized in terms of the alloying behavior of these systems. The status of RS alloy exploration for the three lightweight metals is reviewed and suggestions made on possible future directions for alloy development.
69 citations
TL;DR: The effect broadens the band-edge photoreflectance by 5--10 meV, and artifically lowers the estimates for the critical-point energy, ${E}_{\mathrm{CP}}$, obtained through the customary third-derivative functional fit to the data.
Abstract: Photoreflectance from semi-insulating GaAs, and GaAs/GaAs interfaces, is discussed in terms of its behavior with temperature, doping, epilayer thickness, and laser intensity. Semi-insulating substrates show an exciton-related band-edge signal below 200 K and an impurity-related photoreflectance above 400 K. At intermediate temperatures the band-edge signal from thin GaAs epilayers contains a contribution from the epilayer-substrate interface. The interface effect depends on the epilayer's thickness, doping, and carrier mobility. The effect broadens the band-edge photoreflectance by 5--10 meV, and artifically lowers the estimates for the critical-point energy, ${E}_{\mathrm{CP}}$, obtained through the customary third-derivative functional fit to the data.
64 citations
TL;DR: In this paper, a sufficient condition for the stability robustness of linear systems with time-varying norm bounded state space uncertainty is extended to include the structure of the uncertainty.
Abstract: A recent sufficient condition for the stability robustness of linear systems with time-varying norm bounded state space uncertainty is extended to include the structure of the uncertainty. Our new result requires that the nominal eigenvalues lie to the left of a vertical line in the complex plane which is determined by a norm involving the structure of the uncertainty and the nominal closed-loop eigenvector matrix. Therefore, this robustness result is especially well suited to the design of control systems using eigenstructure assignment. When the uncertainty is time-invariant, our norm is also an upper bound on the incremental eigenvalue perturbations. We also consider the use of Perron weightings to reduce conservatism and the extension of the results to discrete time systems.
TL;DR: Observations suggest that significant differences in proteoglycan metabolism exist between articular cartilage and intervertebral disc.
TL;DR: In this paper, the interlaminar Mode II critical strain energy release rates of unidirec tional graphite/epoxy, graphite polyetheretherketone (PEEK), and graphite with interleaf material were characterized using the End Notch Flexural (ENF) and Center Notchflexural (CNF) test specimens.
Abstract: The interlaminar Mode II critical strain energy release rates of unidirec tional graphite/epoxy, graphite/polyetheretherketone (PEEK), and graphite/epoxy with interleaf material were characterized using the End Notch Flexural (ENF) and Center Notch Flexural (CNF) test specimens. The CNF test method was developed specifically for this study. Compliance and strain energy release rate expressions of the CNF are derived and are in good agreement with finite element results for a wide range of crack lengths. Mode II critical energy release rates for both test methods were found to be in quite good agreement.
TL;DR: In situ surface analysis of the as-deposited films by Auger electron spectroscopy and X-ray photoelectron Spectroscopy showed the films to be stoichiometric MoS 2 as discussed by the authors.
01 Jan 1989
TL;DR: Experimental methods for characterizing interlaminar fracture behavior of composite materials are discussed in this article, where the classic short-beam shear test is presented along with a number of methods based on linear elastic fracture mechanics.
Abstract: Experimental methods for characterizing interlaminar fracture behavior of composite materials are discussed The classic short-beam shear test is presented along with a number of methods based on linear elastic fracture mechanics These methods include the double cantilever beam test, the edge-delamination test, the end-notch flexure test, both the single and double cracked-lap shear test, and the off-axis center notch tensile test Each test method is discussed from an applied mechanics point-of-view Such an approach is appropriate in light of the complex nature of composite materials In addition, relationships between fundamental experimental data and structural performance are considered This includes consideration of mixed-mode failure criteria and a parametric study involving a one-dimensional delamination buckling model for evaluating the materials characterization/structural performance interface Consideration is also given to relationships between basic resin properties and in situ performance as a matrix material in the composite
TL;DR: No correlation was found between subjective symptoms and prolongation of reaction times by hydroxyzine, suggesting that side effect symptoms of traditional antihistamines are unreliable predictors of objective performance.
Abstract: Newer, nonsedating antihistamines provide a therapeutic alternative for the patient with allergy whose work is impaired by the side effects of traditional H 1 antihistamines. To assess the differential effect of these antihistamines on reaction times and subjective symptoms, we compared terfenadine, 60 mg twice daily, to hydroxyzine, 25 mg twice daily, in a double-blind, placebo-controlled, crossover study of 16 healthy, asymptomatic adults. Simple reaction time and choice reaction time were measured with a computer-based, eye-hand, reaction-time testing apparatus. Reaction times and symptom scores were assessed 90 minutes after the fourth and tenth doses of each drug. Hydroxyzine, but not terfenadine, significantly prolonged both simple and choice reaction time ( p ≤ 0.0001). However, decision time, the time to process one bit of spatial information, was not prolonged by either antihistamine. Therefore, hydroxyzine prolonged the interpretation and response to stimuli of the central nervous system without increasing single-bit processing time. Although terfenadine was not different from placebo for any symptom assessed, hydroxyzine produced significant drowsiness ( p = 0.001), dry mouth ( p = 0.022), and irritability ( p = 0.021). During the 5 days of hydroxyzine administration, neither objective nor subjective symptoms demonstrated the development of tolerance. No correlation was found between subjective symptoms and prolongation of reaction times by hydroxyzine, suggesting that side effect symptoms of traditional antihistamines are unreliable predictors of objective performance. Terfenadine provides a promising therapeutic alternative to traditional antihistamines for individuals performing critical tasks.
TL;DR: In this paper, the authors propose a modele stochasticique, which considere la fissuration comm as a processus de Markov vectorise 2D.
Abstract: Ce modele considere la fissuration comme un processus de Markov vectorise 2D. Deux lois de propagation des fissures de fatigue sont utilisees pour l'analyse, et une methode des integrales finies modifiee est proposee pour determiner les parametres du modele stochastique. Ce modele utilise les donnees de Virkler. Excellent accord entre les resultats theoriques et les donnees experimentales
TL;DR: The kinetic constants of chemical metabolism are used to develop physiologically based pharmacokinetic (PB-PK) models which predict the time course distribution of volatile chemicals in mammalian systems and identified fur adsorption of chemical as significantly contributing to the exhalation chamber concentration time course after whole body exposure.
TL;DR: In this article, an implicit algorithm was used to compute the Navier-Stokes equations for the flow over delta wings with emphasis on the separated vortical flows developing on the lee side at high angles of attack.
Abstract: Solutions to the Navier-Stokes equations for the flow over delta wings are computed with emphasis on the separated vortical flows developing on the lee side at high angles of attack. A recently developed implicit algorithm is used which employs upwind differencing for the pressure and convection terms and central differencing for the shear stress and heat transfer terms. Solutions to both the three-dimensional equations and the approximate conical flow equations are compared parametrically with an extensive experimental data base at supersonic speeds. The computations indicate that the conical flow approximation provides results in close agreement with the three-dimensional equations, even to angles of attack as high as 20 degrees. Good agreement with experimentally measured pressures and vapor screen photographs is obtained for the conditions investigated. The method predicts the classical pattern of vortical flow over a delta wing and transition to other flow patterns as the leading edge sweep angle and leading edge normal Mach number are varied.
TL;DR: Dans l'energie de liaison des excitons aux donneurs neutres dans les puits quantiques de GaAs-Al x Ga 1-x As, est determinee par mesure de la photoluminescence a excitation resonante a haute resolution and de the dependance of the photolominescence vis-a-vis de the temperature.
Abstract: The binding energy of excitons to neutral donors (${D}^{0}$,X) in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells is determined by high-resolution resonant-excitation photoluminescence and temperature-dependent photoluminescence measurements. Changes in the binding energy of excitons are observed when donors are located in the center of the well, at the edge of the well in the interface region, or in the center of the barrier. The variations in these binding energies are investigated as a function of well size from 75 to 350 A\r{}. The binding energies are found to increase as well size was reduced until about 100 A\r{}, after which they decreased. An additional transition is observed which is tentatively assigned to excitons bound to ionized donors located at the center or edges of the well.
01 Jan 1989
TL;DR: In this paper, a review of the development of analytical models of the free-edge delamination phenomenon is given, with a focus on the role and importance of free edge problem in laminate elasticity in fostering an understanding of interlaminar stresses and their influence on composite response.
Abstract: A historical/technical review is given, depicting the development of analytical models of the free-edge delamination phenomenon. Emphasis is placed on the role and importance of the free-edge problem in laminate elasticity in fostering an understanding of interlaminar stresses and their influence on composite response. From the early modeling work of Hayashi in 1967, and the definitive experiments of Foye and Baker in 1970, we trace analytical developments over the past two decades. The concept of ply elasticity or effective-modulus representation is discussed, as well as its consequences in laminate modeling. The first elasticity solution of the free-edge problem by Pipes and Pagano using finite differences is then described. This solution has been very instrumental in defining the general character of the interlaminar stress field in the neighborhood of a free edge. We then provide results for the elementary modeling of the effect of stacking sequence on laminate response and derivation of simplified equations to either optimize or minimize this effect in test specimens. Thence, a description of a model based upon the concept of a plate on a smooth foundation is given to capture the essence of the distribution of interlaminar normal stress, its boundary layer zone, and its implication on initiation of delamination. This model leads naturally to the formulation of a variational theorem for laminates that provides an accurate way to compute composite stresses. The chapter culminates with a derivation of the global-local model, which provides a practical way to describe the stress field in a multi-layered composite laminate, and a review of some recent modeling activities that have their basis in the concepts given earlier here. Numerous results are shown for stress fields within laminates which have both practical and theoretical importance.
TL;DR: In this paper, the authors considered the problem of time-fixed impulsive intercept trajectories from a point at rest on the surface of a planet or moon to a target in circular orbit.
Abstract: PTIMAL time-fixed impulsive intercept trajectories from a point at rest on the surface of a planet or moon to a target in circular orbit are obtained. Some of the results in this study are in Heckathorn1 and form a companion analysis to the related problem of optimal direct-ascent rendezvous treated previously by Gross and Prussing.2 Previous optimal intercept studies in the literature deal primarily with time-open disorbit problems.3 Applications of the results obtained include interception or reconnaissance of orbiting satellites and deployment of satellites at specified locations that are intercepted by the deploying vehicle. The idealized results obtained assuming impulsive thrust and absence of atmospheric effects can be applied to more realistic cases by adding gravity and drag losses to the velocity requirements obtained to account for finite burn times and atmospheric drag. In addition, the case of airborne launch rather than launch from the surface can be obtained by interpreting the launch conditions differently, e.g., the contact force of the planet prior to launch becomes the lift on the aircraft. The optimal time-fixed results obtained can be used to perform time vs fuel trade-offs for missions that have operational time constraints.
TL;DR: An overview of the biologically motivated modeling approach is provided and examples of models for styrene and methylene chloride are discussed in relation to their ability to predict human kinetics for these chemicals and their use in estimating the risk of chemicals to exposed humans.
Abstract: Assessing the risk associated with human exposure to environmental chemicals depends to a large extent on the ability to extrapolate from a particular range of exposure conditions in the test animal species to a very different range of exposure conditions in the human. One of the more promising tools for accomplishing this extrapolation is the biologically motivated pharmacokinetic/pharmacodynamic model. In a biologically motivated model, the structure is based on the physiological and biochemical structure of the animal system being described. This paper provides an overview of the biologically motivated modeling approach. Examples of models for styrene and methylene chloride are discussed in relation to their ability to predict human kinetics for these chemicals and their use in estimating the risk of chemicals to exposed humans. Finally, the use of a biologically motivated model to analyze the mechanistic basis of chemical carcinogenesis is discussed.
TL;DR: In this article, a cross coupling reaction between perfluoroalkyl iodides, copper and mono and dibromobenzenes, as well as other functional groups containing functional groups e.g., CO 2 H, CO 2 R, NO 2, NH 2 OCH 3 and C(O)CH 3 has been investigated.
TL;DR: Results indicate that a model of cognitive complexity based on a semantic network formalism may provide a useful technique for quantitatively evaluating the quality of competing display format concepts.
Abstract: An approach for measuring the cognitive complexity of visual displays is discussed and applied to a dynamic display of avionic information. A semantic network formalism is used to model two interrelated knowledge systems, world knowledge and display knowledge. The information the operator receives during training about the general display format characteristics and the task requirements, along with other previously stored information, constitutes world knowledge. The semantic content of a particular configuration of information encountered during task performance constitutes display knowledge. Four orthogonal predictor measures of cognitive complexity were derived from the networks. In an experiment three of the orthogonal predictors were significantly correlated with task performance. After averaging across operators, the three significant predictors accounted for 99% of the variation of display effectiveness. Results indicate that a model of cognitive complexity based on a semantic network formalism may provide a useful technique for quantitatively evaluating the quality of competing display format concepts. Language: en
TL;DR: In this article, the thermally induced phase separation of poly(p- phenylene benzobisthiazole ) Nylon 66 molecular composites was followed by small-angle light scattering which showed the development of a scattering ring.
Journal Article•
TL;DR: In this paper, a self-contained impedance device is described which may be used to measure the electrical transmission characteristics produced by blood flow and volume changes in six segments of the human body during head-up tilt, bed rest, and lower body negative pressure.
Abstract: Definition of multisegment hemodynamic changes that take place in the body would provide a more complete understanding of the physiologic responses to various orthostatic stress techniques A self-contained impedance device is described which may be used to measure the electrical transmission characteristics produced by blood flow and volume changes in six segments of the human body during head-up tilt, bed rest, and lower body negative pressure The device consists of a module that contains the electronics for the impedance system, a separate controller/multiplexer, a personal computer interface/analog to digital conversion/power supply system, and the associated computer control software The instrument is linear over a range of 0 to 200 ohms; provides analog outputs of base impedance, phase angle, pulsatile impedance change, and the first derivative of the pulsatile impedance changes; and can be used to automatically record basal impedance values into spread-sheet format with cycle times between 12 s and 1 h Typical results are presented to illustrate its application in aerospace research
TL;DR: The importance of materials in developing new and improved aerospace systems for the twenty-first century is discussed in this article, with examples drawn from both monolithic and composite metals, polymers and ceramics.
TL;DR: In this article, a finite element analysis with inverse formulation is carried out to study the stress distributions, and the results are concisely presented by defining suitable parameters The results show some interesting features The normalised maximum bearing stress on the hole boundary not only varies nonlinearly with the magnitude of the load parameter but the nonlinear law depends on the type of load (push or pull) on the pin.
TL;DR: In this paper, an interactive viscous/inviscid procedure combining a three-dimensional panel method with an inverse finite-difference boundary-layer method was developed to compute flows with significant regions of boundary layer separation.
Abstract: An interactive viscous/inviscid procedure has been developed combining a three-dimensional panel method with an inverse finite-difference boundary-layer method. The scheme incorporates both a two-dimensional and a quasi-three-dimensional boundary-layer scheme. The resulting method has been applied to the calculation of the flow over three-dimensional wings and wing/body configurations and it has been shown that the procedure can compute flows with significant regions of boundary-layer separation.