Showing papers by "Wright-Patterson Air Force Base published in 1984"

Modeling of dynamic material behavior in hot deformation: Forging of Ti-6242

Abstract: A new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented. The approach in this method is to consider the workpiece as a dissipator of power in the total processing system and to evaluate the dissipated power co-contentJ = ∫o σ e ⋅dσ from the constitutive equation relating the strain rate (e) to the flow stress (σ). The optimum processing conditions of temperature and strain rate are those corresponding to the maximum or peak inJ. It is shown thatJ is related to the strain-rate sensitivity (m) of the material and reaches a maximum value(J max) whenm = 1. The efficiency of the power dissipation(J/J max) through metallurgical processes is shown to be an index of the dynamic behavior of the material and is useful in obtaining a unique combination of temperature and strain rate for processing and also in delineating the regions of internal fracture. In this method of modeling, noa priori knowledge or evaluation of the atomistic mechanisms is required, and the method is effective even when more than one dissipation process occurs, which is particularly advantageous in the hot processing of commercial alloys having complex microstructures. This method has been applied to modeling of the behavior of Ti-6242 during hot forging. The behavior of α+ β andβ preform microstructures has been exam-ined, and the results show that the optimum condition for hot forging of these preforms is obtained at 927 °C (1200 K) and a strain rate of 1CT•3 s•1. Variations in the efficiency of dissipation with temperature and strain rate are correlated with the dynamic microstructural changes occurring in the material.

Energy levels of Wannier excitons in G a A s − Ga 1 − x Al x As quantum-well structures

Abstract: Energy levels of Wannier excitons in a quantum-well structure consisting of a single slab of GaAs sandwiched between two semi-infinite layers of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ are calculated with the use of a variational approach. Owing to lowering of symmetry along the axis of growth of this quantum-well structure and the presence of energy-band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed, leading to two exciton systems, namely, the heavy-hole exciton and the light-hole exciton. The values of the binding energies of the ground state and of a few lowlying excited states of these two exciton systems are calculated as a function of the size of the GaAs quantum well for several values of the heights of the potential barriers and their behavior is discussed. The results thus obtained are also compared with the available experimental data. The reliability of the various approximations made in this calculation is discussed.

Thermal stability of SiC fibres (Nicalon

Abstract: The degradation behaviour of Nippon Carbon Co. SiC fibres (Nicalon®) after heat treatment in various environments was studied. Regardless of the heat-treatment conditions, the Nicalon® fibre strength degraded when the fibres were subjected to temperatures higher than 1200° C (temperatures below 1200° C were not investigated). This degradation is associated with the evaporation of CO from the fibres as well as withβ-SiC grain growth in the fibres.

Acid–base complexes of polymers

Abstract: Les polymeres acides (chlorure polyvinylique surchlore, fluorure de polyvinyle, fluorure de polyvinylidene, polybutyral vinylique) absorbent fortement les vapeurs de solvants basiques et les polymeres basiques (PMMA) absorbent fortement les vapeurs de solvants acides. Importance des proprietes acidobasiques des polymeres dans l'adherence au verre

Aeroelastic flutter and divergence of stiffness coupled, graphite/epoxy cantilevered plates

Abstract: An analytical and experimental investigation was conducted to determine the aeroelastic flutter and divergence behavior of unswept, rectangular wings simulated by graphite/epox y, cantilevered plates with various amounts of bending-torsi on stiffness coupling. The analytical approach incorporated a Rayleigh-Ritz energy formulation and unsteady, incompressible two-dimensional aerodynamic theory. Flutter and divergence velocities were obtained using the \ -g method and compared to results of low-speed wind tunnel tests. Stall flutter behavior was also examined experimentally. There was good agreement between analytical and experimental results. Wings with negative stiffness coupling exhibited divergence, while positive coupling delayed the onset of stall flutter.

A survey of macroscopic failure criteria for composite materials

Abstract: : Popular failure criteria of fiber-reinforced composite materials are described and compared. These criteria are empirical and should only be judged from the standpoint of the fitness to data and the ease of application. The criteria for orthotropic plies of unidirectional composites are extensions of those for isotropic materials. The quadratic criteria are considered to be the most suitable for both isotropic and composite materials. Macroscopic criteria are essential for design and for providing guidelines for materials improvements. Strictly speaking, failure criteria for multidirectional laminants are valid up to the first-ply failure envelope; i.e., before transverse cracking and delamination occur. Finally, conditions for fully optimized ply properties are easily derived from the quadratic failure criterion. (Author)

Effect of nonparabolicity on the energy levels of hydrogenic donors in G a A s − Ga 1 − x Al x As quantum-well structures

Abstract: Binding energies of the ground state and of a few low-lying excited states of a hydrogenic donor in a quantum-well structure consisting of a single layer of GaAs sandwiched between two-semi-infinite layers of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ are calculated, including the effect of nonparabolicity of the conduction band and following a variational approach. The effect of nonparabolicity of the conduction band is included by using an expression for the energy-dependent effective mass based on the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{p}}$ approximation. The variations of the binding energies of these states as a function of the size of the GaAs quantum well for different values of the potential barrier (or equivalently for different values of Al concentration $x$) are calculated. These results are compared with those obtained with the use of a parabolic conduction band.

Numerical Simulation of Near-Critical and Unsteady, Subcritical Inlet Flow

Abstract: The unsteady, Reynolds-averaged Navier-Stokes equations were solved for the flow about an experimentally tested external compression axisymmetric inlet operating in the near-critical and unsteady subcritical flow regimes. The near-critical solution reached a stable steady state, while the subcritical solution attained an unstable, bounded oscillatory state characterized by large-amplitude pressure oscillations and traveling shock waves. The self-sustained oscillations, known as inlet buzz, result from a shear layer instability combined with a closed-loop feedback of reflected disturbances. Solutions are compared with experimental results.

Developments in Titanium Metal Matrix Composites

Abstract: A review is given of developments in the titanium metal matrix composites (MMC) area over the past two decades. Particular topics of discussion include: fibers, matrices, fabrication techniques, mechanical properties of both the fiber and composite, and fractography. It is demonstrated that the increased metallurgical understanding of the composite has led to a product with significantly improved mechanical properties. Conclusions regarding the status of the titanium MMC technology are presented along with suggestions for further metallurgical work necessary to fully realize the potential of these materials.

Sharp-line photoluminescence spectra from GaAs-GaAlAs multiple-quantum-well structures

Abstract: The photoluminescence spectra from four ${\mathrm{Al}}_{0.25}$${\mathrm{Ga}}_{0.75}$As-GaAs multiple-quantum-well (MQW) structures with well thicknesses of 100, 200, 300, and 400 \AA{} and barrier thicknessess of 100 \AA{} have been interpreted. The extrinsic emission is predominantly in the donor region. A number of very sharp transitions have been observed for the first time. Several of the transitions are no more than 0.15 meV in width. The light- and the heavy-hole free excitons were observed in the 400 \AA{} well. The heavy-hole free excitons were observed in the other wells. The radiative transitions associated with the neutral-donor bound excitons as well as the free-to-bound and neutral-acceptor bound excitons are discussed. Magnetic field splittings were observed for several of the optical transitions. These data together with the diamagnetic shifts and excitation-dependent measurements are used in the identification of the multiple-quantum-well transitions.

Acute tumor lysis syndrome with metastatic medulloblastoma. A case report.

Abstract: The acute tumor lysis syndrome occurs rarely in nonhematologic malignancies. This patient, a 34-year-old woman with metastatic medulloblastoma, was receiving palliative radiotherapy for a rapidly expanding abdominopelvic mass. After a total of 300 rad, the patient developed the biochemical hallmarks of the acute tumor lysis syndrome, hyperuricemia, hyperkalemia, hyperphosphatemia, and hypocalcemia. This was complicated by oliguria from hyperuricemic acute renal failure. The patient responded well to hydration, alkaline diuresis, phosphate and potassium binders, and allopurinol. The potential for acute tumor lysis syndrome should be anticipated when treating metastatic medulloblastoma.

Nickel-aluminum-molybdenum phase equilibria

Abstract: Recent work on alloys based on the Ni-Al-Mo system has brought to light several inconsistencies with published equilibrium phase diagrams for this system. Published diagrams have been based on theoretical computer models and on data gathered ostensibly before equilibrium was achieved, especially at temperatures below 1100 °C. The intent of this effort was to produce an experimentally validated ternary equilibrium phase diagram for the Ni-Al-Mo system. Specimens for this task were produced by both conventional casting and powder metallurgy techniques. The temperatures studied included 1260, 1171, 1093, 1038, and 927 °C (2300, 2140, 2000, 1900, and 1700 °F) for times up to 2500 hours. Phases were identified using an electron probe microanalyzer and X-ray diffraction. The results show significant deviations from the proposed phase diagrams published in the literature in the temperature range investigated. In particular, a class II four-phase equilibrium reaction γ + α cooling // heating γ′+ δ has been shown to occur at 1127 ± 2 °C (2060 ± 4 °F).

Phase Relations in the System ZrO2‐Y2O3 at Low Y2O3 Contents

Abstract: Subsolidus phase relations in the low-Y2O3 portion of the system ZrO2-Y2O3 were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y2O3 is soluble in monoclinic ZrO2, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y2O3 below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y2O3 from ∼500° to ∼1600°C. At higher Y2O3 compositions, cubic ZrO2 solid solution occurs.

Powder Metallurgy of Light Metal Alloys for Demanding Applications

Abstract: Powder metallurgy (P/M) processing offers advantages over conventional processing for both the aluminum and titanium systems. For aluminum, rapid solidification and mechanical attrition processes produce P/M alloys having improved mechanical and corrosion properties. The processing of these materials and the alloy classes being exploited—high strength/corrosion resistant, reduced density/increased modulus, and high-temperature alloys—are discussed. For titanium, the P/M approach has generally been used to reduce costs. However, recent improvements in properties, due to rapid solidification, in both conventional (Ti-6Al-4V) and new alloys have been obtained. The state of the art in these two alloy systems is presented and future developments projected.

Optimization of Shallow Trusses Against Limit Point Instability

Abstract: The problem of the maximization of the critical load (limit point instability) of shallow space trusses of constant volume or weight is considered Exact solutions being possible in simple cases, a detailed investigation of the question of the uniqueness of the optimum designs for two- and four-bar shallow truss problems is carried out The optimized trusses satisfy the constant-strain energy density criteria and the load maximization problem is verified to be the dual of the minimization of the total potential energy

Structure and properties of rapidly solidified dispersion strengthened titanium alloys: Part II. tensile and creep properties

Abstract: The effects of incoherent dispersoids on tensile and creep properties were determined in rapidly solidified Ti-Er and Ti-Nd alloys. Uniform distributions of. fine incoherent dispersoids in Ti matrix were produced by rapid solidification at cooling rates > 103 °C per second and subsequent annealing at 700 to 800°C of Ti-1.0Er, Ti-2.0Er, Ti-1.5Nd, and Ti-3.0Nd alloys. The rapidly solidified particulates consolidated by vacuum hot pressing were isothermally forged, rolled, and annealed to produce fully recrystallized microstructures. The incoherent dispersoids in Ti-Er and Ti-Nd alloys increase by 40 to 110 pct the yield strength and ultimate tensile strength of Ti with no significant loss in ductility. The strength increments were analyzed in terms of the superposition of dispersion-, solid solution-, and fine grain-strengthening. Dispersion strengthening is offset to some extent by the reduction in interstitial oxygen solid solution strengthening caused by the scavenging of oxygen by Er and Nd. The dispersoids decrease the creep rates and increase the stress rupture lifetimes of Ti at 482 to 700 °C.

Investigation into the mass distribution properties of the human body and its segments

Abstract: A series of studies was conducted to develop information about mass distribution characteristics of the living human body and its segments, and to establish a reliable means for estimating these properties from easily measured body dimensions. While investigators over the years have established a number of laborious methods for determining body mass and moments of inertia of individuals, comparable data for segments of the body have till recently been available only through the study of cadavers. The use of stereophotography now makes possible the segmentation of living subjects and provides a means for measuring mass distribution properties on body segments as well as on the total body. In the studies described here investigators combined stereophotometric and anthropometric techniques to measure 31 male subjects and 46 female subjects. Bodies were photographically segmented and their volumes, centres of volume and principal moments of inertia established stereophotometrically. The principal axes of iner...

Computer sizing of composite laminates for strength

Abstract: This report presents a procedure to design laminated composite panels subjected to bending loads and in plane loads. The procedure takes advantage of the directionality of the mechanical properties of the composite material using the laminated plates theory.

The effects of stoichiometry and structure on the dynamic torsional properties of a cured epoxy resin system

Abstract: The dynamic torsional characteristics of a difunctional, bisphenol-A-type epoxy resin cured with different amounts of metaphenylene diamine, using two cure cycles, were determined in torsion at fou...

Numerical Simulation of Wing-Fuselage Aerodynamic Interaction

Abstract: Numerical solutions of the mass-averaged Navier-Stokes equations were accomplished for a wing-body configuration at a nominal Mach number of 6 and a Reynolds number of 15 x 10. The computations were performed on a CRAY-1 computer utilizing a grid consisting of 56,730 points. The wing-fuselage configuration is comprised of a tangent-ogive forebody and highly swept wedge delta wing. The present results, using a branchcut mesh system, were verified by comparing with experimental measurements. An improvement in numerical resolution over the previous solution, utilizing a wraparound grid distribution, is clearly demonstrated. The flowfield structure is delineated by identifying the shock wave system and the cross-flow velocity field.

Kinematic and force data collection in biomechanics by means of sonic emitters--I: Kinematic data collection methodology.

Abstract: In this paper, first, the principles of sonic digitizing are presented. Next, a description of quantitative determination of the relative motion between two body segments by utilization of sonic emitters is provided. A new kinematic data collection methodology and data analysis is proposed to check continuously the accuracy of the data collected by means of the sonic emitters. The first part of the paper is terminated by establishment of an accuracy criteria and selection of the most accurate data set and associated error analysis. Quantitative results based upon the kinematic data collection methodology of Part I were obtained for the forced kinematic motion of the human shoulder complex and are presented in Part II.

Eye-Controlled Switching for Crew Station Design

Abstract: Integration of eye and head position monitoring devices may enable practical control of systems using the operator's eye line-of-sight (LOS) under conditions of free head and eye movement This paper describes the components of an eye-control system developed to examine the use of eye LOS as an alternate control interface for crew station design The implications of the performance of this system to the implementation of eye-controlled switching are discussed