Showing papers by "Wright-Patterson Air Force Base published in 1992"

Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy

Abstract: A two-phase gamma titanium aluminide alloy, Ti-47Al-1Cr-1V-2.5Nb (in at.%), was studied under forged and various subsequent heat treatment conditions, to investigate the microstructural evolution and the effect of microstructure on room temperature (RT) tensile properties and fracture toughness behavior. Four classes of microstructure and three types of lamellar formation were identified, and their formation mechanisms were analyzed using various analytical techniques including metallography, electron optics, differential thermal analysis (DTA), and crystallography. It was found that both tensile and toughness behavior were profoundly affected by the microstructural variations.

Surface Roughness and Its Effects on the Heat Transfer Mechanism in Spray Cooling

Abstract: In the spray cooling of a heated surface, variations in the surface texture influence the flow field, altering the maximum liquid film thickness, the bubble diameter, vapor entrapment, bubble departure characteristics, and the ability to transfer heat. A new method for determining and designating the surface texture is proposed, and the effects of surface roughness on evaporation/nucleation in the spray cooling flow field studied. A one-dimensional Fourier analysis is applied to determine experimentally the surface profile of a surface polished with emery paper covering a spectrum of grit sizes between 0.3 to 22 {mu}m. Heat transfer measurements of liquid flow rates between 1 to 5 l/h and air flow rates between 0.1 to 0.4 l/s are presented. Maximum heat fluxes of 1,200 W/cm{sup 2} for the 0.3 {mu}m surface at very low superheats were obtained.

Viscoelastic Effects in MIL-L-7808-Type Lubricant, Part I: Analytical Formulation

Abstract: Analytical formulations for the computation of lubricant film thickness and traction in a high-speed rolling-sliding contact are presented with the objective of investigating the viscoelastic response of the MIL-L-7808-type lubricant Two types of relations are used to model the viscous shear strain rate. In the Type I model, a hyperbolic sine relation is used to model the viscous effect which becomes significant when the shear stress reaches a critical value. The Type II model employs a limiting shear stress, which the lubricant can withstand, and an inverse hyperbolic tangent function is considered to model the viscous behavior. Both models are based on three fundamental properties: lubricant viscosity, shear modulus and a critical shear stress. While the viscosity relations may be obtained by direct measurements, estimates of shear modulus and critical shear stress may be derived by curve-fitting the model predictions to experimental traction data. Presented at the 45th Annual Meeting in Denver, Colorad...

The Intentional Spring: A Strategy for Modeling Systems That Learn to Perform Intentional Acts

Abstract: In motor task learning by instruction, the instructor's skill and intention, which, initially, are extrinsic constraints on the learner's perceiving and acting, eventually become internalized as intrinsic constraints by the learner. How is this process to be described formally? This process takes place via a forcing function that acts both as an anticipatory (informing) influence and a hereditary (controlling) influence. A mathematical strategy is suggested by which such intentions and skills might be dynamically learned. A hypothetical task is discussed in which a blindfolded learner is motorically instructed to pull a spring to a specific target in a specific manner. The modeling strategy involves generalizing Hooke's law to the coupled instructor-spring-learner system. Specifically, dual Volterra functions express the anticipatory and hereditary influences passed via an instructor-controlled forcing function on the shared spring. Boundary conditions (task goals) on the instructor-spring system...

Using material behavior models to develop process control strategies

Abstract: Advances in material behavior modeling have enabled the explicit incorporation of material phenomena into process design and control methodologies. A new approach to material behavior modeling, in which stability analysis has been combined with the analysis of activation energy, can be used to identify processing regimes within which the material behavior is essentially deterministic and is amenable to process control. This articles illustrates the application of this dynamic material modeling approach through the example of deformation processing of a δ-TiAl alloy. Additionally, a new control-system design strategy explicitly based on the knowledge of the state of the material under hot-deformation conditions is explored.

Cholecystectomy during pregnancy without fetal loss.

Abstract: Cholecystectomy in the pregnant patient has been generally avoided because of the reported high incidence of associated fetal loss that has been linked to spontaneous and elective abortion during the first trimester and premature labor during the third trimester. Recent developments relating to diagnostic and anesthetic management and the use of tocolytic agents have altered the over-all approach to patients. We have, therefore, retrospectively reviewed the medical records of all women discharged from four area hospitals during 1982 to 1987 with a concurrent diagnosis of cholelithiasis and pregnancy. Twenty-two patients met the review criteria. The incidence of biliary stone disease among gravid patients during the time interval was 0.05 per cent. Of 22 patients, none underwent radiation for diagnosis. Nine patients underwent cholecystectomy while pregnant; two were operated upon during the first trimester, four during the second and three during the third. Three required common bile duct exploration and three had intraoperative cholangiograms. Elective abortion was not recommended to the six patients because of radiation exposure. Two of nine had premature contractions develop that were controlled with tocolytic agents. There were no spontaneous abortions. The mean Apgar scores for neonates born subsequent to cholecystectomy was virtually identical to neonates born to patients in whom cholecystectomy was deferred. It is concluded that the diagnosis and surgical treatment of cholelithiasis can be safely undertaken in the pregnant patient without fetal loss. Delaying appropriate surgical therapy no longer seems warranted.

Friction and wear of diamond and diamondlike carbon coatings

Abstract: Reciprocating sliding friction experiments were conducted with a natural diamond flat, diamond films, and low- and high-density diamondlike carbon (DLC) films in contact with pin specimens of natural diamond and silicon nitride (Si 3 N 4 ) both in humid air and in dry nitrogen. The results indicated that for natural diamond pin contacts the diamond films and the natural diamond flat were not susceptible to moisture but that moisture could increase both the coefficients of friction and wear factors of the DLC films. The coefficients of friction and wear factors of the diamond films were generally similar to those of the natural diamond flat both in humid air and in dry nitrogen. In dry nitrogen the coefficients of friction of the high-density DLC films in contact with pin specimens of both diamond and Si 3 N 4 were generally low (about 0.02) and similar to those of the natural diamond flat and the diamond films. The wear factors of the materials in contact with both natural diamond and Si 3 N 4 were generally in the ascending order of natural diamond flat, diamond film, high-density DLC film, and low-density DLC film. Moisture in the environment increased the coefficients of friction for Si 3 N 4 pins in contact with all the materials (natural diamond flat, diamond films, and DLC films). This increase in friction is due to the silicon oxides film produced on the surface of Si 3 N 4 pins in humid air.

An analysis of the isothermal hot compression test

Abstract: The nominally isothermal, uniaxial hot compression test has been analyzed with special reference to the effects of temperature nonuniformities and friction on sample deformation and flow stress estimates. A simple one-dimensional analysis was used to establish the influence of initial temperature nonuniformities, strain rate, and the temperature dependence of the flow stress on flow localization tendencies. Noticeable strain concentrations were predicted to occur only at high strain rates (∼10 s−1) in materials such as titanium alloys, but not in steels, for typical values of the initial temperature nonuniformity. More extensive numerical (finite element method) simulations of the compression test with various values of the friction shear factor corroborated the conclusions of the flow localization analysis. In addition, it was established that initial temperature nonuniformities, as well as friction, have an almost negligible effect on flow stress data deduced from measurements of average pressurevs true height strain, at least for reductions of the order of 50 pct. The analysis results were supported by observations of the deformation behavior of a near-gamma titanium aluminide and a low-alloy steel.

Electronic structure of planar faults in TiAl

Abstract: The mechanical behavior of intermetallic alloys is related to the mobility of the dislocations found in these compounds. Currently the effect of bonding on dislocation core structure and its influence on deformation behavior is not well understood. However, the unusual properties of these materials, such as the anomalous temperature dependence of flow stress observed in TiAl, are derived in part from the aspects of bonding that determine dislocation mobility. Several recent studies have suggested a particular relationship between directional bonding in TiAl and dislocation mobility. To understand better the flow behavior of high temperature intermetallics, and as a step toward bridging the gap between electronic structure and flow behavior, we have calculated the electronic structure of various planar faults in TiAl. The self consistent electronic structure has been determined using a layered Korringa Kohn Rostoker (LKKR) method which embeds the fault region between two semi-infinite perfect crystals. Calculated defect energies in stoichiometric TiAl agree reasonably well with other theoretical estimates, though overestimating the experimental (111) anti-phase boundary (APB) energy, found for Ti46Al54. We approximate the energy of the (111) APB for the Al-rich stoichiometry by calculating the energy of Al antisites near that defect plane. The calculated (111)APB energy decreases by 6% in going from stoichiometric TiAl to Ti46Al54. The overall hierarchy of fault energies is found to be associated with the number of crystal bond states that are disrupted by the introduction of the fault plane. However, the hierarchy of fault energies is inconsistent with the traditionally accepted ordering. Changes in bonding taking place in the vicinity of the planar defects are illustrated through the density of states and charge density plots. A three body atomistic model is introduced to parameterize the bonding observed in TiAl. The L10 lattice (c/a = 1.00), within a second nearest neighbor three body model, yields a (111)APB energy which is the sum of the complex and superlattice-intrinsic stacking fault energies.

Helmet mounted area of interest (HMAoI) for the display for advanced research and training (DART)

Abstract: The Display for Advanced Research and Training (DART), a wrap-around display system for presenting out-the-window visual imagery in a flight simulator, is substantially improved by adding a helmet mounted display of modest field-of-view that displays only an area-of-interest view (HMAoI) The HMAoI display displays a modest field-of-view, high resolution image, according to the head position of a simulator pilot as determined by a helmet mounted position indicator, that replaces the lower resolution view provided by the DART for the same image. The corresponding DART image is not displayed to prevent displaying a confusing view. The remaining DART display provides a peripheral view for a simulator pilot. The imagery provided through the HMAoI may be a two part image comprising a very high resolution central image surrounded by a lower, but still higher than the DART, resolution image.

Clinical parameters of evaluation during implant maintenance.

Abstract: This paper reviews investigations concerning the clinical parameters of evaluating dental implants during the maintenance phase of therapy. Clinical parameters that are discussed include mobility, gingival alterations, tissue movement, probing and attachment level measurements, bleeding upon probing, occlusion, and microbial monitoring. The role of implant radiology is discussed and encompasses radiographic interpretation, interval, and technique.

Thermal Conductivity of Boron Carbide–Boron Nitride Composites

Abstract: Because of their preferred orientation, the addition of boron nitride dispersions to hot-pressed boron carbide was found to result in a considerable degree of anisotropy in thermal conductivity of the resulting composite, indicated by an increase in the thermal conductivity perpendicular to the hot-pressing direction by as much as a factor of 3 at the highest boron nitride volume fractions of this study, and a decrease in the thermal conductivity parallel to the hot-pressing direction by as much as a factor of 2. The composite data were found to be below the values expected from composite theory, which may represent indirect evidence for the existence of an interfacial thermal barrier.

Phase and Property Studies of Boron Carbide–Boron Nitride Composites

Abstract: This paper reports on boron carbide-boron nitride particulate composites that were fabricated by vacuum hot-pressing. Near-theoretical densities of B{sub 4}C were obtained, but percent theoretical densities decreased with increasing amounts of BN. The grain size of B{sub 4}C and BN was not affected by composition, but the amount of twinning in B{sub 4}C decreased with increasing BN content. No third phase was found at the B{sub 4}C-BN interface by analytical STEM analysis. Lattice parameter measurements indicated slight solubility of B{sub 4}C in BN, but no solubility of BN in B{sub 4}C for samples hot-pressed at 2250{degrees} C. Room-temperature flexural strength measurements revealed a sharply decreasing strength with increasing BN content up to 40% BN, and then relatively constant values with greater amounts of BN.

Optical modulation and switching with enhanced third order nonlinearity multiple quantum well effects

Abstract: A multiple quantum well arrangement which achieves significantly improved third order optical nonlinearity in a semiconductor device by way of spatially periodic electrodes applied to the semiconductor device. The spatial period of the applied electrodes and the resulting exciton confinement dimension is improved over that of previous multiple quantum well structures and to the Bohr radius range of dimensions for the semicondcutor material by way of av RIGHTS OF THE GOVERNMENT The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.

Plaque-induced inflammation around implants.

Abstract: This article is a review of plaque-induced inflammation around dental implants. The microflora around successful implants is similar to healthy sulci, while that associated with failing implants is similar to periodontally diseased sites. Implant microflora is similar to the tooth microflora in the partially edentulous mouth. The microflora of implants in partially edentulous mouths differs from that in edentulous mouths. This seems to indicate a bacterial reservoir around the teeth and the possibility of reinfection of the implant sulcus by periodontal pathogens. The maintenance of a tooth microflora consistent with periodontal health in partially edentulous mouths may lead to maintaining an implant microflora consistent with peri-implant health. Thus, periodontal and implant maintenance are linked and neither can be overlooked.

Mechanical and Electrical Properties of Heat-Treated Ladder Polymer Fiber

Abstract: The ladder polymer poly[(7-oxo-7H, 10H-benz(d, e)imidazo(4', 5':5, 6)-benzimidazo(2, l-a)isoquinoline-3, 4:10, ll-tetrayl)-10-carbonyl] (BBL) was synthesized in polyphosphoric acid (PPA) at a concentration of 15% by weight. The polymer had an intrinsic viscosity of 8.75 dl/g in methanesulfonic acid (MSA) at 30°C. Highly oriented BBL fiber was spun by a dry-jet wet-spin process with a spin-draw-ratio of 8 to 1. After neutralization, the fiber exhibited a phosphoms content below the detectable limit (0.04%) by elemental analysis. This fiber, after a heat treatment at 300°C for 30 seconds under tension, showed a Young's modulus of 120 GPa, a tensile strength of 830 MPa and a compressive strength of 410 MPa. The fiber was also heat-treated at temperatures from 100°C to 600°C at 50°C increments for electrical conductivity measurements. It was found that room temperature DC conductivity of the BBL fiber increased dramatically from 2×10-8 S/cm with 100°C heat treatment to 3×10-4 S/cm with 350°C heat treatment and then decreased to 6×10-7 S/cm with 600°C heat treatment. The enhanced conductivity of heat-treated BBL fiber is believed due to the improved structural order in the BBL fiber and the thermally excited charge carriers.

A survey of toughness in ductile phase composites

Abstract: The available literature data on toughness in ductile phase composites are overviewed, and the known theoretical toughness models are assessed for their usefulness for predicting fracture toughness in ductile phase composites. It is shown that the predictions of simple equations incorporating ductile phase properties agree with the measured fracture toughness of ductile phase composites. It was found that the fracture toughness of ductile phase composites is governed by the modulus of the composite, the flow stress, and the plastic rupture displacement of the ductile phase. It is suggested that these equations should be useful for the alloy design of high-temperature materials consisting of ductile phases. 32 refs.

Optical properties of CdTe/Cd 1-x Zn x Te strained-layer single quantum wells

Abstract: The electronic states in a series of molecular-beam-epitaxy-grown CdTe/${\mathrm{Cd}}_{0.79}$${\mathrm{Zn}}_{0.21}$Te strained-layer single quantum wells have been studied both theoretically and experimentally. Calculations based on model solid theory and deformation-potential theory have shown that, at Zn concentration of 0.21 in the barriers, the band lineup is type I for heavy holes and type II for light holes. The confined states were calculated using the three-band envelope function model. Photoluminescence measurement at temperatures between 9 and 100 K indicates that the broadening mechanism is dominated by the polar optical phonon (longitudinal optical) interactions. The circularly polarized excitation spectra have shown predominant heavy-hole feature confirming that the band lineup is type I for heavy holes and type II for light holes. With well width between 25 and 75 \AA{}, our study has shown that exciton binding energy decreases with decreasing well width.

Plastic-flow behavior and microstructural development in a cast alpha-two titanium aluminide

Abstract: Plastic-flow behavior and microstructural development were investigated for a cast α2 titanium aluminide, Ti-24Al-11Nb (atomic percent), using the isothermal hot-compression test. Regimes of warm- and hot-working behavior were inferred from flow curves adjusted for deformation heating effects. Plots of flow stress as a function of inverse temperature and estimates of the strain-rate-sensitivity index confirmed the transition from warm to hot-working conditions over a rather narrow temperature range. Hot working in theα 2 +β phase field was also marked by a rather high activation energy (viz., ∼1080 kJ/mole) for the controlling deformation process, which appeared to consist of dynamic globularization of the ordered-α 2 phase. A sharp decrease in the activation energy was noticed when the deformation temperature was increased above the β-transus. Microstructural observations also indicated development of an unrecrystallized structure during warm working, with cavities and wedge cracks being found near the bulged free surfaces of the upset specimens. The plastic-flow phenomenology exhibited a number of similarities to that found in the wrought version of the Ti-24Al-11Nb alloy.

Impact response of graphite/epoxy cylindrical panels

Abstract: Graphite/epoxy curved cylindrical panels were impacted in the center by an impact machine capable of measuring load during the test Load, deflection, and strain as functions of time were measured for six symmetric lay-up configurations for impact energies of 05-45 ft-lb Damage was produced in all panels for certain impact energies The extent and location of damage was determined from C-scans and optical microscopy of panel crow sections An in-house nonlinear finite element code was used to predict the panel deflections and stresses

Application of Active Flexible Wing technology to the Agile Falcon

Abstract: The Active Flexible Wing is an integrated technology wing concept of special interest to systems requiring high maneuverability at high speed. The following article examines the results from an application of the Active Flexible Wing (AFW) concept to an F-16 Agile Falcon, low-speed aeroelastic model. The article also summarizes, for archival purposes, design and fabrication procedures typical for construction of low-speed aeroelastic models. The Active Flexible Wing concept is applied to a 4 scale model design based upon full scale preliminary wing design studies conducted for the U.S. Air Force F-16 Agile Falcon fighter program. The model is equipped with multiple leadingand trailing-edge control surfaces. Wind tunnel test results are presented showing wing control power increases generated through the use of the leadingand trailing-edge control surfaces in conjunction with increased wing flexibility. The results show that the Active Flexible Wing concept is a viable concept for the Agile Falcon planform and that a leading-edge outboard control surface can be a particularly powerful control power effector.

Segregation and homogenization of a near-gamma titanium aluminide

Abstract: The nature of microsegregation and the kinetics of homogenization for wrought ingot-metallurgy near-gamma titanium aluminides were investigated experimentally and theoretically. This included heat treatments of the alloy Ti-47Al-2.5Nb-0.3Ta (in atomic percent) in the two-phase (alpha + gamma) and single-phase (alpha) regimes and subsequent electron microprobe analysis to determine the composition of the microstructural constituents. The equivalence of the composition of the gamma constituent in both the single-phase bands and two-phase (alpha-two + gamma) regions of the as-received material and in microstructures developed during subtransus heat treatment revealed that the observed microsegregation is principally morphological in character. Measurements of the kinetics of gamma grain dissolution above the transus temperature, coupled with microprobe measurements of concentration profiles, were used to conclude that the rate-controlling mechanism for homogenization of the near-gamma titanium aluminides is a reaction at alpha-gamma interfaces rather than diffusion-limited dissolution of the gamma constituent. This conclusion was supported by estimates of the interdiffusion coefficient using a spherical-infinite model for gamma-phase dissolution and finite difference calculations of the rate at which concentration gradients in both the alpha and gamma phases would be expected to dampenvia diffusional processes.

Potential neurotoxin formation in thermally degraded synthetic ester turbine lubricants.

Abstract: Under extreme conditions of thermal degradation, a potent neurotoxin, trimethylolpropane phosphate (TMPP), may be formed by reaction of tricresyl phosphate (TCP, an anti-wear additive) and trimethylolpropane esters (TMPE) of carboxylic acids that may be contained in turbine engine lubricant formulations. The neurotoxin is only formed at extreme temperatures (350–700° C), which has minimized concern for use of the synthetic lubricants under intended applications; however, since researchers are often not privy to the proprietary composition of such lubricants, extreme caution should be exercised in studies of or applications of synthetic lubricatns degraded under extreme conditions.

Biological aerosol particle detector and method having an electronic pulse detection means

Abstract: System and method for detection and measurement of airborne biological aerosol particles in a gaseous sample are described comprising a source of low energy radiation for irradiating the gaseous sample whereby biological particles in the sample are ionized, a detector for detecting ionized biological particles in the sample including a pair of electrically charged conducting plates disposed in parallel confronting relationship to each other with a preselected space therebetween, a source of electrical power operatively connected to the plates for applying a preselected electrical potential thereacross, and electronics for sensing collisions on the plates of ionized biological particles in the sample.

Kinetic study of the helix to coil dark reaction of poly(spiropyran-L-glutamate).

Abstract: An investigation of kinetics of the helix to coil dark reaction of light adapted poly(spiropyran-L-glutamic acid) (PSLG) dissolved in hexafluoroisopropanol was performed. The reaction was associated with the spiropyran (SP) to merocyanine (MC) ring opening. The ring opening reaction monitored with UV/VIS spectroscopy showed first order kinetics. Chromophore and polypeptide backbone circular dichroism data fit to an expression consistent with a single intermediate series mechanism. By FTIR, we monitored the polypeptide alpha-helix amide I, the MC chromophore--C = C--stretch and the protonated unmodified carboxylate C = O stretch bands. During the first step of the series mechanism, changes in the hydrogen bonding of the unmodified carboxylate groups occurred, suggesting breakup of polypeptide aggregates. The second step of the proposed series mechanism was dominated by the helix to coil transition and the ring opening of SP to MC. The CD spectrum of MC in the dark adapted PSLG was red shifted and had a narrower bandwidth than the UV/VIS spectrum. The kinetic and spectroscopic data suggested that a fraction (population I) of the MC chromophores experienced optical activity induced by the chiral polypeptide environment, while the remainder of the MC chromophores (population II) were solvated and enantiomeric.