Showing papers by "Wright-Patterson Air Force Base published in 1988"

The Subjective Workload Assessment Technique: A Scaling Procedure for Measuring Mental Workload

Abstract: Mental workload is proposed to be a multidimensional construct that can be largely explained by three component factors: Time Load, Mental Effort Load, and Psychological Stress Load. In this paper, we describe a subjective scaling approach, the Subjective Workload Assessment Technique (SWAT), that captures this multidimensional nature of mental workload. We describe the SWAT procedure as a two-phased method that includes (a) a scale development phase based on conjoint measurement and nonmetric scaling, and (b) an event scoring phase. The development of SWAT and its measurement foundations are discussed. Recent research illustrating SWAT's widespread utility and its sensitivity as a measure of perceived mental workload is summarized.

Phase Relationships in the Silicon Carbide-Aluminum Nitride System

Abstract: Phase relationships in the SiC-AlN system were determined by analytical electron microscopy of local equilibria among adjacent phases in hot-pressed samples and in diffusion couples. At 2100° to 2300°C, a 4H–2H equilibrium exists, the 4H field extending from ∼2 mol% AlN to an upper limit of 11 to 14 mol% AlN. The wurtzite-type 2H (δ) solid solution extends from an impurity-sensitive lower limit of 17 to 24 mol% AlN up to 100 mol% AlN. Semiquantitative Auger electron spectroscopy and energy dispersive X-ray spectroscopy ultrathin-window detector measurements confirmed the assumption that the solid solutions have the general formula (SiC)1-x(AlN)x and belong to the SiC-AlN pseudobinary system. The existence of a miscibility gap below 1950°C was confirmed, but its limits were not determined accurately. A faulted metastable cubic phase, β′, exists below ∼2000°C and contains up to ∼4 mol% AlN. The transformations into the stable a structures occur through diffusion-controlled stacking rearrangements.

Structural optimization with frequency constraints

Abstract: This paper presents a design optimization algorithm for structural weight minimization with multiple frequency constraints. An optimality criterion method based on uniform Lagrangian density for resizing and a scaling procedure to locate the constraint boundary were used in optimization. Multiple frequency constraints of equality and inequality types were addressed. The effectiveness of the algorithm was demonstrated by designing a number of truss structures with as many as 489 design variables. No attempt was made to reduce the number of design variables by such procedures as linking and/or invoking symmetry conditions. The design examples include a 10-bar truss, 200-bar truss, a modified ACOSS-II, and COFS (Control of Flexible Structures) mast truss. All the structures contain nonstructural mass besides their own mass. The algorithm is extremely stable and, in all cases, the optimum designs were obtained in less than 20 iterations regardless of the size of the structure and the number of design variables.

Game theory approach for the integrated design of structures and controls

Abstract: The problem of design of actively controlled structures subject to constraints on the damping parameters of the closed-loop system is formulated as a multiobjeetive optimization problem. The structural weight and the controlled system energy are considered as objective functions for minimization with cross-sectional areas of members as design variables. A computational procedure is developed for solving the multiobjeetive optimization problem using cooperative game theory. The feasibility of the procedure is demonstrated through the design of two truss structures.

Evaluating project scheduling and due date assignment procedures: an experimental analysis

Abstract: Managers of construction projects, maintenance activities, auditing contracts, software shops, etc. are frequently faced with the task of establishing a new project's due date, which must compete with other projects already in progress or expected (forecasted) to start in the future. The study reported here addresses the problem of establishing due dates for projects which require limited resources, in an environment where new projects arrive continuously and randomly over time. A set of procedures is developed which set each project's due date when it arrives using information about the new project, current projects, and available resources. The due date setting procedures are tested via simulation with four activity scheduling heuristics that control the assignment of resources to specific activities of available projects. A second test demonstrates the performance of the due date procedures, where a portion of arriving projects have their due dates established by external forces beyond management's con...

A Theory of Situation Assessment : Implications for Measuring Situation Awareness

Abstract: Measures of pilot situation awareness (SA) are needed in order to know whether new concepts in display design help pilots keep track of rapidly changing tactical situations. In order to measure SA,...

Excitonic transitions in GaAs/GaxAl1-xAs quantum wells observed by photoreflectance spectroscopy: Comparison with a first-principles theory.

Abstract: Excitonic transitions in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As multiple quantum wells are observed using photoreflectance spectra obtained with a novel double-monochrometer spectrometer. The first pass of the monochrometer produces the infrared probe beam while the second pass is used as a synchronous band-pass filter, resulting in a better signal-to-noise ratio than the low-pass filter conventionally used, and allowing simultaneous collection of the photoluminescence spectrum. The photoreflectance data are analyzed using a multiband effective-mass theory which includes valence-band mixing and assumes a first-derivative modulation of the built-in field as the dominant modulation mechanism. The entire photoreflectance spectrum is simultaneously fit using a first-principles theory in which only the exciton linewidth and value of the built-in electric field are not determined by the theory. Full advantage is thus taken of the signal intensity information, in contrast to comparisons currently encountered in the literature where positions of excitons are usually compared to values predicted by a finite-square-well calculation, without valence-band mixing effects. The theory is shown to have much success in explaining the observed photoreflectance signal from a 200-A\r{} multiple quantum well at 77 K, including the qualitative line shape for any given exciton.

Transsexuals in the military: flight into hypermasculinity.

Abstract: A sample of 11 male gender-dysphoric patients meeting DSM-III criteria for transsexualism was seen over a 3-year period by a military psychiatrist. Eight patients had extensive military experience, including combat duty in some cases. At the time of evaluation three were on active duty, one was a Department of Defense employee, and four were veterans. Evidence is presented for a hypermasculine phase of development that coincides with the age of enlistment in nearly all cases. The psychodynamic underpinnings of the choice to enlist in transsexual males are discussed. Outcome of military service was premature discharge in over 60%. The military's management of genderdysphoric servicemen is described. Current military policies, in association with the proposed hypermasculine phase of transsexual development, may actually result in a higher prevalence of transsexualism in the military than in the civilian population.

Mechanical and Microstructural Characterization of Mullite and Mullite‐SiC‐Whisker and ZrO2‐Toughened‐Mullite—SiC‐Whisker Composites

Abstract: High-purity mullite-SiC-whisker composites and mullite-ZrO2-SiC-whisker composites were fabricated in situ by hot-pressing using a matrix prepared by the alkoxide process. Varying degrees of ZrO2 stabilization were achieved by varying amounts of Y2O3 or MgO addition. Microstructural characterization was accomplished using SEM and energy dispersive analysis. Room-temperature flexural strength and fracture toughness were determined as a function of SiC-whisker content (0% to 30%) and ZrO2-stabilizer content. The flexural strength of mullite varied with composition and was increased ∼50% by the addition of ∼30% ZrO2 phase. The flexural strength of mullite and mullite + 30% ZrO2 was increased ∼50% for 30% SiC-whisker additions. The fracture toughness of mullite + 30% ZrO2 was nearly twice that of mullite. For a 30% SiC-whisker addition, the fracture toughness of mullite was doubled, and the fracture toughness of mullite + 30% ZrO2 was increased 25% to 50%.

Cognitive, Affective, and Behavioral Outcomes of Participation in Quality Circles: Conceptual and Empirical Findings

Abstract: Little reliable longitudinal research has investigated the outcomes of participating in quality circles (QCs). This article reports the results of such a study whose evaluation criteria were developed according to a heuristic model of key cognitive, affective, and behavioral results of the QC process. A sample of 225 military and civilian employees of a U.S. Air Force base was evaluated using survey and performance appraisal measures administered within the framework of a non-equivalent control design. Analyses of covariance yielded evidence that QC participation significantly affected the study's criterion variables. The authors discuss the limitations and implications of their study and offer suggestions for future research.

Wind-tunnel investigation of wing-in-ground effects

Abstract: Wing-in-Ground effects from a wind-tunnel study of a NACA 4415-profile wing model with an aspect ratio of 2.33 are described. The wing model contains a 20%-chord, full-span, adjustable flap and removable end and enter plates. Ground boards are used in the wind tunnel to simulate the ground. In this study, the ground effects are expressed as variations to the aerodynamic coefficients (lift and drag) and lift-to-drag ratio. The ground effects are described in terms of angle of attack, flap angle, wing height above ground and use and size of end and center plates.

Uniaxial Compressive Strength of High Modulus Fibers for Composites

Abstract: Axial compressive behavior of carbon and high modulus organic fibers (such as Kevlar and PBT) has been discussed. The compressive strength of carbon fibers is found to in crease with a decrease in ...

Quantification of reaction time and time perception during Space Shuttle operations.

Abstract: A microprocessor-based test battery containing simple reaction time, choice reaction time, and time perception tasks was flown aboard a 1985 Space Shuttle flight. Data were obtained from four crewmembers. Individual subject means indicate a correlation between change in reaction time during the flight and the presence of space motion sickness symptoms. The time perception task results indicate that the shortest duration task time (2 s) is progressively overestimated as the mission proceeds and is statistically significant (p less than 0.01) when comparing preflight and postflight baselines. The tasks that required longer periods of time to estimate (8, 12, and 16 s) are less affected.

Compressive Strength of high Performance Fibers

Abstract: Significant research efforts have been carried out to improve the tensile modulus and tensile strength of high performance carbon and polymeric fibers Experimental polymeric fibers (ordered polymer fibers) have been prepared with moduli >50 MPSI and tensile strength approaching one MPSI However, the benefits of the above improvements in tensile properties for aerospace applications are limited because composites of these fibers have low axial compressive strength, which is a direct result of the poor axial fiber compressive strength The poor axial fiber compressive strength has usually been attributed to the microfibrillar/fibrillar buckling However, questions concerning the intrinsic limitations at the molecular level and the effects of intermolecular interactions are also considered important Better understanding of these aspects will help in determining the theoretically achievable compressive strength and may aid in the development of higher compressive strength high performance fibers These and other issues related to the compressive strength of high performance polymeric and carbon fibers are discussed in this paper

Microstructures and Mechanical Behavior of Two-Phase Niobium Silicide-Niobium Alloys

Abstract: In the Nb-Si system, it is possible to produce in-situ composites consisting of a brittle Nb5Si3 intermetallic matrix and ductile Nb particles. The two phases are thermochemically stable up to ∼ 1500∼C and are amenable to wide microstructural variations including morphology, volume fraction, and the size of the individual microconstituents. This paper presents microstructures and phase transformations in these composites as a function of composition and heat treatments and bend properties from room-temperature to 1400°C.

Crystallization kinetics of As−Sb−S glass in bulk and thin film form

Abstract: The kinetics of crystallization in As2−xSbxS3 glasses on a melt quenched bulk form and a thin film form was studied with a differential scanning calorimeter used in dynamic heating and isothermal mode. Our understanding of the crystallization mechanism was aided by SEM and X-ray diffraction analysis. The crystallization initiated at the surface of the samples, and for this reason, the films crystallized much quicker even though the apparent activation energy was higher in films. Data indicated the difference in the initial glassy state between bulk glasses and films.