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Showing papers by "Wright-Patterson Air Force Base published in 1985"


Journal ArticleDOI
TL;DR: The predictive power of a physiologically based pharmacokinetic model makes it a useful tool for more reliable dose selection before beginning large-scale studies, as well as for the retrospective analysis of experimental results.
Abstract: The process of assessing the risk associated with human exposure to environmental chemicals inevitably relies on a number of assumptions, estimates and rationalizations. One of the more challenging aspects of risk assessment involves the need to extrapolate beyond the range of conditions used in experimental animal studies to predict anticipated human risks. The most obvious extrapolation required is that from the tested animal species to humans; but others are also generally required, including extrapolating from high dose to low dose, from one route of exposure to another and from one exposure timeframe to another. Several avenues are available for attempting these extrapolations, ranging from the assumption of strict correspondence of dose to the use of statistical correlations. One promising alternative for conducting more scientifically sound extrapolations is that of using physiologically based pharmacokinetic models that contain sufficient biological detail to allow pharmacokinetic behavior to be predicted for widely different exposure scenarios. In recent years, successful physiological models have been developed for a variety of volatile and nonvolatile chemicals, and their ability to perform the extrapolations needed in risk assessment has been demonstrated. Techniques for determining the necessary biochemical parameters are readily available, and the computational requirements are now within the scope of even a personal computer. In addition to providing a sound framework for extrapolation, the predictive power of a physiologically based pharmacokinetic model makes it a useful tool for more reliable dose selection before beginning large-scale studies, as well as for the retrospective analysis of experimental results.

223 citations


Journal ArticleDOI
TL;DR: The binding energies of the ground and excited states of a hydrogenic donor associated with the first subband in a GaAs quantum well in the presence of an arbitrary magnetic field are calculated.
Abstract: We have calculated the binding energies of the ground (1s-like) and excited (2${p}_{\ifmmode\pm\else\textpm\fi{}}$-like) states of a hydrogenic donor associated with the first subband in a GaAs quantum well, sandwiched between two semi-infinite layers of ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As. Results have been obtained as a function of the potential-barrier height (or equivalently of Al concentration x) and the size of the quantum well in the presence of an arbitrary magnetic field. We have considered the two cases of donor at the center and at the edge of the well. The applied magnetic field is taken to be parallel to the axis of growth of the quantum-well structure. We have used a variational approach in which the trial wave functions are expanded in terms of appropriate Gaussian basis sets. For a given value of the magnetic field, the binding energies are found to be larger than their values in a zero magnetic field.

198 citations


Journal ArticleDOI
TL;DR: In this paper, the tensile mechanical properties and fracture toughness of a Bisphenol-A type difunctional epoxy resin, cured with different amounts of metaphenylene diamine, using two cure cycles, were determined over a range of temperature.
Abstract: The tensile mechanical properties and fracture toughness of a Bisphenol-A type difunctional epoxy resin, cured with different amounts of metaphenylene diamine, using two cure cycles, were determined over a range of temperature. The tensile modulus in the glassy state was seen to be predominantly related to intermolecular packing, while in the rubbery state crosslink density was the important factor. Yielding appeared to be due to an increase in free volume as a result of dilatation during the tensile test and was related to a critical shear stress. The large strain properties like tensile strength, elongation-to-break, and toughness showed a more complex dependence on chemical structure, molecular architecture, intermolecular packing, and crosslink density. The roles played by the relaxation processes in determining mechanical properties are highlighted.

193 citations


Journal ArticleDOI
TL;DR: It is demonstrated that the nasal respiratory epithelia of rats and hamsters are the most sensitive tissues to the tumorigenic action of hydrazine following inhalation exposures, similar to the reaction of rats to formaldehyde, another highly reactive water-soluble compound.

191 citations


Journal ArticleDOI
TL;DR: In this paper, two dimensional generalizations of Fitts' law were discussed in terms of multidimensional scaling and the obtained pattern of movement times was found to be intermediate to the predictions of Euclidean and City-block models of the movement space.
Abstract: Subjects performed two-dimensional discrete movements either with a helmet-mounted sight or with a joystick. Fitts’ Law was found to be a good predictor of the speed-accuracy tradeoff for both systems. The joystick produced faster movement times than the helmet-mounted sight. For both systems, horizontal and vertical movements were slightly faster than diagonal movements. Two dimensional generalizations of Fitts’ Law were discussed in terms of multidimensional scaling. The obtained pattern of movement times was found to be intermediate to the predictions of Euclidean and City-block models of the movement space. Muscle coordination strategies were considered, and a strictly serial coordination model was rejected. A strictly parallel model was also rejected for the helmet-mounted sight, but not for the joystick.

186 citations


Journal ArticleDOI
TL;DR: In this paper, the authors examined three-point and four-point flexure and ductile-matrix-resin composites in the presence of extremely high combined stress gradients and cast serious doubts on the usefulness of interlaminar-beam experiments for characterizing the delamination resistance of composite materials.
Abstract: Interlaminar beam tests in the form of three-point and four-point flexure are examined both experimentally and analytically. Experimental data are obtained on unidirectional composites. Photomicrographs of actual failure modes and results of a stress analysis based on classical theory of elasticity are utilized to supplement the experimental data. Complex failure modes in the presence of extremely high combined stress gradients are observed and cast serious doubts on the usefulness of interlaminar-beam experiments for characterizing the delamination resistance of composite materials. Further difficulties are encountered with ductile-matrix-resin composites.

139 citations


Journal ArticleDOI
TL;DR: In this article, the moisture transport characteristics of a difunctional epoxy resin cured with different amounts of metaphenylene diamine, using two cure cycles, were reported.
Abstract: The moisture transport characteristics of a difunctional epoxy resin cured with different amounts of metaphenylene diamine, using two cure cycles, are reported. Besides studying the kinetics of moisture sorption at 20, 50, 75, and 100°C, the investigations also included measurement of thermal expansion coefficients and dynamic mechanical transition of the dry and wet samples. The moisture sorption of the sample is shown to be related to its specific volume and hence to its fractional free volume. In the glassy state, the free volume is apparently in the form of frozen voids, and moisture sorption/desorption at this temperature is of the Langmuir type with little or no bond formation. At higher temperatures the free volume is generated predominantly through segmental motion of the α transition. The Henry's Law mode becomes operative, and the moisture can now form bonds. The possible effect of nonuniform crosslink density on moisture sorption is also considered.

112 citations


Journal ArticleDOI
TL;DR: Correlations between measures showed that highway-sign discrimination distance was significantly related to contrast sensitivity at two spatial frequencies, 1.5 and 12 cycles/deg, but discrimination Distance was not related to visual acuity, and implications for highway- sign design and driver vision standards are discussed.
Abstract: This study was conducted to determine if contrast sensitivity could predict age-related differences in the ability to discriminate simple road signs, as these differences have not been predicted by Snellen visual acuity. Contrast sensitivity, Snellen visual acuity, and discrimination distances for projected images of highway signs were measured for 7 older observers, ages 55 to 79, and 13 younger observers, ages 19 to 30. All subjects had 20/20 visual acuity or better, but the older group had significantly lower contrast sensitivity than did the younger group at three spatial frequencies: 3, 6, and 12 cycles/deg of visual angle. The older group required a significantly larger sign symbol in order to determine if it denoted a + or T intersection. Correlations between measures showed that highway-sign discrimination distance was significantly related to contrast sensitivity at two spatial frequencies, 1.5 and 12 cycles/deg, but discrimination distance was not related to visual acuity. Implication for highway-sign design and driver vision standards are discussed. (Author abstract)

109 citations


Journal ArticleDOI
TL;DR: In this article, subjects estimated the perceived contrast of 2°diameter sine-wave grating patches for spatial frequencies of 2, 4, 8, and 16 cycles/deg, at eccentricities from 0° to 40° and contrasts up to 0.8.
Abstract: Subjects estimated the perceived contrast of 2°-diameter sine-wave grating patches for spatial frequencies of 2, 4, 8, and 16 cycles/deg, at eccentricities from 0° to 40° and contrasts up to 0.8. The data were well fitted in all cases by power functions of contrast minus threshold, with exponents of the order of 0.5 implying similar mechanisms in both fovea and periphery. The data also demonstrate that, at high physical contrast, the visual system is generally driven toward an operating state in which two stimuli of equal physical contrast have equal perceived contrast even if the thresholds are quite different. As a consequence, peripheral perceived contrasts produced by high physical contrasts show almost no change with eccentricity, whereas thresholds increase by at least an order of magnitude. This implies that mechanisms mediating threshold detection and suprathreshold perception may be different.

109 citations


Journal ArticleDOI
TL;DR: In this article, a phase transformation of the Ti-6Al-4V alloy was performed and the microstructures and tensile properties of sheet specimens were determined after such transformation followed by dehydrogenation at temperatures between 650°C and 760°C.
Abstract: Ti-6Al-4V alloy, to which 0.6 wt pct to 1.0 wt pct (22 to 33 at. pct) hydrogen has been added, can undergo a phase transformation which produces unique, fine microstructures. Specimens of the alloy were heated to 870°C, transformed at temperatures between 540°C and 700°C, and the microstructures were determined as a function of hydrogen content and transformation temperature. Microstructures and tensile properties of sheet specimens were determined after such transformation followed by dehydrogenation at temperatures between 650°C and 760°C. The highest yield strength (1130 MPa) and good ductility (9 pct El) were associated with a fine equiaxed microstructure obtained in material charged with approximately 1.0 wt pct hydrogen, transformed at 565°C and dehydrogenated at 675°C. Lower strengths and ductilities were associated with acicular microstructures produced by transformation at higher temperatures or coarser structures producted at higher dehydrogenation temperatures.

89 citations


Journal ArticleDOI
TL;DR: In this article, a tensile test for the combined Mode I (crack opening) and Mode II (forward shearing) fracture behavior of unidirectional T300/1034C graphite/epoxy and graphite-reinforced APC-1 polyetheretherketone is described.

Book ChapterDOI
TL;DR: This chapter focuses on the reason that determines infants' visual preferences at different ages and proposes a quantitative model of preferences based on linear systems techniques and test it against data from several well-known preference experiments, finding that the model's predictions agree quite well with observed preferences for a variety of stimuli.
Abstract: Publisher Summary This chapter focuses on the reason that determines infants' visual preferences at different ages and proposes a quantitative model of preferences based on linear systems techniques and test it against data from several well-known preference experiments The model's predictions agree quite well with observed preferences for a variety of stimuli The success of this model implies that infants' visual preferences are governed simply by a tendency to look at highly visible patterns This account of early preferential looking is thus consonant with the understanding of how the growth of basic sensory mechanisms affects visual perception during the first months of life Linear systems analysis is based on Fourier's theorem This powerful theorem implies that any two-dimensional, time-invariant visual stimulus can be exactly described by combining a set of more basic stimuli These basic stimuli are sine wave gratings A sine wave grating is a pattern of light and dark stripes whose intensity varies sinusoidally with position Sine wave gratings are specified by four parameters—spatial frequency, orientation, phase, and contrast Fourier's theorem implies then that even a complex, two-dimensional visual stimulus, such as the picture of a face, can be described exactly by the combination of a set of gratings of various frequencies, orientations, phases, and contrasts

Journal ArticleDOI
TL;DR: In this article, the phenomenon of self-excited shock oscillations on a spike-tipped body at Mach 3 was investigated and various modes of shock oscillation were observed.
Abstract: The phenomenon of self-excited shock oscillations on a spike-tipped body at Mach 3 is investigated Various modes of shock oscillations are observed For some spike lengths, the shock oscillations are symmetric For other spike lengths, the shock structure oscillates asymmetrically with respect to the spike's axis In this case, experimental evidence confirms the presence of standing rotational waves on the body face and spike Spike configurations resulting in stable shock systems are also observed Holography and interferometry are used to obtain a detailed flow visualization

Journal ArticleDOI
TL;DR: In this article, the effect of structural optimization on optimal control design is studied in a simple truss structure, in which vibration suppression with only initial disturbances was considered. But, the conclusion was that modification of the structural parameters (stiffness and structural mass) did not significantly alter the control design in this study.
Abstract: The effect of structural optimization on optimal control design is studied in this paper. Structural optimiza- tion was treated as a problem of mass minimization with constraint on the open-loop frequency. The quadratic performance index, involving the state and control variables, was used in the design of the control system. A control system with only full-state feedback was considered. A procedure for generating the state and control weighting matrices by structural dynamics programs was outlined. By introducing simple scaling parameters, the weighting matrices were used effectively to achieve the desired control objectives. A number of case studies using a simple truss structure were made, in which vibration suppression with only initial disturbances was considered. The conclusion was that modification of the structural parameters (stiffness and structural mass) did not significantly alter the control design in this study. IBRATION control is an important consideration in the design of dynamic systems on the ground, in the air, and in space. The disturbances in ground and air vehicles are primarily caused by rough road (runway) profiles and airflow, such as gusts and powerplants. Similarly, in large space structures the disturbances are the result of slew- ing/pointing maneuvers, thermal transients, and mechanical machinery such as coolers, generators, etc. Control of the dynamic response is essential for maintaining the ride quality and performance requirements, as well as for the safety of the structure. The response of a structure is basically governed by three sets of parameters. The mass, damping, and stiffness repre- sent the structural parameters. The second set of parameters is due to the sources of external disturbances. These are generally external to the system and are considered as fixed inputs; thus their alteration is not within the realm of the structures/controls designer. The third set represents the control system, assuming that the structure is actively con- trolled. Control of the dynamic response by modification of the structural parameters alone is considered to be passive. Passive control is most appealing from both the reliability and maintainability points of view, if it can be achieved at all economically. Basically, the stiffness and mass modifica- tions result in frequency and mode changes, while the damp- ing affects the dissipation energy of the system. The damping can be significantly altered by either viscoelastic coatings (or constrained layer damping) or the provision of discrete dashpot mechanisms. The objective of vibration control is to design the structure and its controls either to eliminate vibration completely or to reduce the mean square response of the system to a desired level within a reasonable span of time. In addition, it is im- portant that this objective be achieved in some optimal way. For a structural designer, the optimal design represents an

Journal ArticleDOI
TL;DR: In this article, single fibers were encapsulated in an epoxy-mPDA matrix so that interfacial shear strength and matrix response to shear loading could be quantified and observed microscopically with polarized light as a function of moisture exposure.
Abstract: Surface treatments have been used to promote shear strength between graphite fibers and epoxy matrices by adding polar surface groups to increase the attraction between the fiber surface and the polar epoxy matrix. Because of the polar interactions between fiber and matrix, interfacial bonding may be susceptable to moisture attack resulting in permanent loss of fiber matrix bonding with a resultant loss in composite shear strength. Fiber “finishes” are also used to improve the processability of surface treated graphite fibers. These “finish” layers may not respond to hygrothermal exposure in the same manner as the matrix. Type AS1 surface treated graphite fibers and surface treated and “finished” (ASIC) for improved adhesion to epoxy matrices were used in this study. Single fibers were encapsulated in an epoxy-mPDA matrix so that interfacial shear strength and matrix response to shear loading could be quantified and observed microscopically with polarized light as a function of moisture exposure ...

Journal ArticleDOI
TL;DR: In this article, a stress analysis of orthotropic beams subjected to concentrated loads is performed, within the framework of the classical theory of elasticity, on both three-point and four-point bending.

Journal ArticleDOI
TL;DR: In this paper, the problem of designing a structure with multiple frequency constraints by using an optimality criterion method was investigated, where the frequency constraints include the condition that the frequencies are equal or separated by a specified interval with a preselected vibration mode associated with the fundamental frequency.

Journal ArticleDOI
TL;DR: In this article, a higher-order plate theory was used to analyze a complete double cantilever beam specimen and the effect of specimen geometry on energy release rate was investigated numerically.

Journal ArticleDOI
TL;DR: In this article, a bound on the perturbation of an asymptotically stable linear system is obtained to maintain stability using Liapunov matrix equation solution, which is shown to be an improved upper bound over the ones recently reported in the literature.
Abstract: The stability robustness aspect of linear systems is analyzed in the time domain. A bound on the perturbation of an asymptotically stable linear system is obtained to maintain stability using Liapunov matrix equation solution. The resulting bound is shown to be an improved upper bound over the ones recently reported in the literature. The proposed methodology is then extended to Linear Quadratic (LQ) and Linear Quadratic Gaussian (LQG) regulators. Examples given include comparison with an aircraft control problem previously analyzed.

Journal ArticleDOI
TL;DR: In this paper, the binding energies of the ground state of both the heavy-hole and light-hole excitons in a GaAs quantum well sandwiched between two semi-infinite layers were calculated as a function of the size of the well in the presence of an arbitrary magnetic field.
Abstract: Binding energies of the ground state of both the heavy-hole and light-hole excitons in a GaAs quantum well sandwiched between two semi-infinite layers of ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As, are calculated as a function of the size of the well in the presence of an arbitrary magnetic field. A variational approach is followed where the trial wave functions are expressed in terms of Gaussian basis sets. The applied magnetic field is assumed to be parallel to the axis of growth and the binding energies are calculated for a finite value of the height of the potential barrier. As expected, for a given value of the magnetic field, the binding energies are found to be larger than their values in a zero magnetic field. The contribution to the binding energy due to the magnetic field, at a given field, increases slowly as the well size is reduced. A comparison with the available experimental data is made.

Journal ArticleDOI
TL;DR: In this article, the authors defined the Jacobian of coordinate transformation as a Jacobian-of-coordinate transformation (JOTC) of a coordinate transformation with respect to specific heat conductivities.
Abstract: Nomenclature e = internal energy e = e{ + ¥211-u e^ = specific internal energy / =force F, G, H = vector fluxes J = Jacobian of coordinate transformation k = molecular heat conductivity n — outward normal P = pressure Pr = Prandtl number Prt = turbulent Prandtl number q =heat exchange Q = heat release t = time T = temperature u, v, w = velocity components in Cartesian frame x, y, z = Cartesian coordinates 7 = ratio of specific heats e = eddy viscosity coefficient X = bulk viscosity coefficient fji = molecular viscosity coefficient £, 17, f = transformed coordinates p = density T = stress tensor

Journal ArticleDOI
TL;DR: In this article, composites reinforced with SiC fibers were tested at room temperature both in, flexural and tensile configurations and the stress-strain behavior for composites tested in tension was correlated with progressive microcracking and failure processes.
Abstract: Ceramic-matrix composites reinforced with SiC fibers were tested at room temperature both in, flexural and tensile configurations. The stress-strain behavior for composites tested in tension was correlated with progressive microcracking and failure processes. Significant differences between failure modes in tension and flexure were observed.


Journal ArticleDOI
TL;DR: In this article, LiMAluminosilicate glass-ceramic-matrix composites reinforced with Sic fibers were tested at 900° and 1000°C in flexural and tensile configurations and showed severe thermomechanical degradation when tested in oxidizing atmospheres.
Abstract: Lithium aluminosilicate glass-ceramic-matrix composites reinforced with Sic fibers were tested at 900° and 1000°C in flexural and tensile configurations. The composites showed severe thermomechanical degradation when tested in oxidizing atmospheres. This degradation was found to depend on oxygen partial pressure. For Po2≳103 Pa the composites failed in a brittle fashion; a single crack initiated and propagated through the matrix and fibers. At lower Po2, multiple matrix cracking was observed, and the fibers bridged the matrix cracks.

Journal ArticleDOI
TL;DR: In this paper, the characteristics of a dual-inlet side-dump combustor configuration suitable for ducted rocket missiles were investigated by injecting a liquid fuel (JP-4) into the two rectangular inlet ducts upstream of the dump plane.
Abstract: The characteristics of a dual-inlet side-dump combustor configuration suitable for ducted rocket missiles was investigated by injecting a liquid fuel (JP-4) into the two rectangular inlet ducts upstream of the dump plane. Cold flow water tunnel simulations were obtained by injecting small air bubbles into the inlet arms of a plexiglas model and illuminating these by a high intensity slit light source. Combustor performance was obtained by testing a similar stainless steel model on a combustor thrust rig. Combustion efficiency, combustor total pressure loss, lean blow-out limits, combustor pressure oscillations, and surface heating patterns were obtained. A threedimensional computer code using the k-e turbulence model was used to predict the corresponding cold flowfields with good agreement.

Journal ArticleDOI
TL;DR: In this article, the adequacy of the U.S. Air Force damage tolerance inspection criterion for protecting the safety of the flight of an aging military trainer aircraft is examined through a risk assessment based on cracks found in teardown inspections of retired wings.
Abstract: The paper examines the adequacy of the U.S. Air Force damage tolerance inspection criterion for protecting the safety of the flight of an aging military trainer aircraft. This is done through a risk assessment based on cracks found in teardown inspections of retired wings. The crack population is combined with stress probabilities representing service experience to determine single flight probability of failure and the single aircraft probability of failure after a given time. These quantities are then used as a basis for judging the required inspection interval. For the case studied, the 0.9 probability of detection inspection criterion in the Air Force damage tolerance requirements may be nnconsewative.

Journal ArticleDOI
TL;DR: The effects of AlN and BN additives on hot-pressed SiC were investigated in this article, where an analytical scanning transmission electron microscopy was used to determine the phases in contact, the local compositions and the composition gradients.

Journal ArticleDOI
TL;DR: In this article, a review of the theoretical and experimental work on shallow impurity centers in superlattices and quantum well structures is presented, and the behavior of these levels as a function of the quantum well size is discussed.

Journal ArticleDOI
TL;DR: A review of current modeling concepts is presented in this paper, where the earliest modeling involved interaction schemes and, primarily, time-independent material behavior, and more recent work has focused on time-dependence and creep-fatigue interaction effects.

Journal ArticleDOI
TL;DR: The data support a suggestion that the potentiation of hexacarbon neurotoxicity by MEK is the result of the persistence of the neurotoxic metabolite in the blood and not the enhanced metabolism of parent Hexacarbon to 2,5-HD.