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Showing papers by "Wright-Patterson Air Force Base published in 1991"


Journal ArticleDOI
TL;DR: In this article, the effects of residual axial strain in the fiber and fiber surface topography were analyzed to predict the load-displacement behavior in terms of fiber/matrix interface parameters.
Abstract: The fiber pullout and pushout tests have been analyzed to predict the load-displacement behavior in terms of fiber/matrix interface parameters. The effects of residual axial strain in the fiber and fiber surface topography were included. The residual axial strain was found to be a significant parameter. It is shown that the interface failure can be progressive or catastrophic. In the case of a progressive failure of the interface, the load-displacement curve is nonlinear. The portion of the curve from above the first nonlinearity to near the peak load can be predicted in terms of parameters of the interface, viz., the friction coefficient, the radial stress at the interface, the fracture toughness of the interface, and the residual axial strain in the fiber. Values for these parameters can be obtained from a single loaddeflection curve. The peak load and load drop, which are usually reported, are found not to be directly relatable to any interface property, since the length of the last portion of the fiber to debond is influenced by end effects and hence not easily predicted. However, for data which describe the peak load as a function of initial embedded length, that factor can be eliminated and the data reduced to yield the relevant interface parameters. In pullout, the peak and friction loads saturate with large specimen thickness. Catastrophic failure is favored when the debond initiation load is high or when residual stress is low. Finally, a methodology to extract interface parameters from experimental data is suggested.

407 citations


Journal ArticleDOI
TL;DR: The data are consistent with the existence of a complex network of inhibitory interconnections among mechanisms that mediate contrast perception and must extend over spatial distances equivalent to more than 12 cycles of the central grating patch.

245 citations


Journal ArticleDOI
TL;DR: The actuator location selection problem is cast in the framework of a zero-one optimization problem and a genetic algorithmic approach is developed that involves three basic operations: reproduction, crossover, and mutation.
Abstract: The actuator location selection problem is cast in the framework of a zero-one optimization problem. A genetic algorithmic approach is developed. To obtain successive generations that yield the solution corresponding to the maximum fitness value, this approach involves three basic operations: reproduction, crossover, and mutation.

192 citations


Journal ArticleDOI
TL;DR: In this article, it is suggested that interfacial roughness causes this phenomenon and that the decrease in load (friction) is associated with the fiber seating back into its original position.
Abstract: A fiber which is partially pushed out of a surrounding matrix and subsequently pushed in the opposite direction exhibits a substantial decrease in sliding friction as it passes through its original position (its “origin”). This is manifest by a decrease in the load required to push the fiber. It is suggested that interfacial roughness causes this phenomenon and that the decrease in load (friction) is associated with the fiber seating back into its original position. The period of the drop has been correlated with the spatial extent of the interfacial surface roughness, and the magnitude of the drop (referred to hereafter as the seating drop) has been correlated with the amplitude of the interfacial roughness. Observation of the seating drop allows separation of the friction associated with interfacial irregularities from that resulting primarily from residual stresses at the interface. Implications for composite design and use are discussed. The effect of abrasion at the sliding interface is also addressed.

157 citations


Journal ArticleDOI
TL;DR: In this paper, the authors describe the initiation of matrix cracking for glass and glass-ceramic matrix composites reinforced with small-diameter silicon carbide and carbon fibers under uniaxial tensile loading.
Abstract: This paper describes the initiation of matrix cracking for glass and glass-ceramic matrix composites reinforced with small-diameter silicon carbide and carbon fibers under uniaxial tensile loading. Acoustic emission, replication, and optical microscopy in conjunction with stress-strain curves are employed to detect the initiation of matrix cracking. The proportional limit of the stress-strain curve is found to overestimate the initiation of matrix cracking in the material systems studied. The matrix cracking iniates at axial strains from 0.07% to 0.15%. The ACK model overestimates the initiation of the matrix cracking for the material systems studied in this paper.

136 citations


Journal ArticleDOI
TL;DR: In this paper, a computational study of both steady and periodic laminar horseshoe vortex flows generated upstream of a cylinder/flat plate juncture is presented, and the flowfields are simulated using the full three-dimensional unsteady Navier-Stokes equations and a time-accurate implicit algorithm.
Abstract: A computational study of both steady and periodic laminar horseshoe vortex flows generated upstream of a cylinder/flat plate juncture is presented. The flowfields are simulated using the full three-dimensional unsteady Navier-Stokes equations and a time-accurate implicit algorithm. A new type of laminar horseshoe vortex topology is identified. For the case of a single primary vortex, this new topology is found to be independent of the computational grid and is also supported by recent experimental flow visualizations. The flat plate skin-friction portraits corresponding to the new and to the standard horseshoe vortex topologies are equivalent, pointing out the nonunique relation between the wall limiting streamline pattern and the three-dimensional flow above the plate. For the new topology, the foremost line of coalescense is an attachment rather than a separation line. This unusual feature illustrates the fact that convergence of skin-friction lines is a necessary but not sufficient condition for separation. As the Reynolds number increases, the flow topology evolves from a single to multiple primary horseshoe vortices, in agreement with experimental observations. At least two different types of triple horseshoe vortex systems are shown to be possible. Above a certain value of the Reynolds number, the juncture flow becomes unsteady and periodic at a frequency that increases with Reynolds number. The unsteady horseshoe vortex process upstream of the cylinder is found in qualitative agreement with experiment. Horseshoe vortices are periodically generated and convected toward the juncture. Vorticity intensification by vortex stretching, and the eruption of vorticity from the plate surface are observed.

132 citations


Book ChapterDOI
01 Jan 1991
TL;DR: In this article, various methods for predicting the fatigue life of resin-matrix composite materials are reviewed, including empirical, residual strength degradation, stiffness degradation, and actual damage state based theories.
Abstract: Available methods for predicting the fatigue life of resin–matrix composite materials are reviewed. These include empirical, residual strength degradation, stiffness degradation, and actual damage state based theories. The residual strength degradation fatigue theories are discussed in detail. They are shown to possess a damage metric that can be summed cycle-by-cycle to give cumulative damage theories. Moreover, they exhibit the sudden death behavior observed in some experimental data. The stiffness degradation theories are shown to be logical generalizations of the residual strength degradation fatigue theories. Statistically rigorous procedures for determining how well particular fatigue theories fit experimental data are presented. Their use for checking for panel-to-panel fabrication variability and different fatigue theory fits is illustrated. Finally, a procedure for determining whether a particular theory is statistically valid is discussed and used to show that the residual strength degradation based fatigue models may not be valid for composite materials.

131 citations


Journal ArticleDOI
TL;DR: A physiologically based pharmacokinetic (PB-PK) model which describes the kinetics of CO, carboxyhemoglobin (HbCO), and parent dihalomethane, and has applied this model to examine the inhalation kinetic patterns of CO and of DCM in rats and humans is developed.

124 citations


Journal ArticleDOI
TL;DR: In this paper, the onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding and tension tests were used to determine the stress and strain levels to first initiate micro-cracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass.
Abstract: The onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross-ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.

110 citations


Journal ArticleDOI
01 Sep 1991
TL;DR: The present research tests a subjective situation awareness (SA) metric, called the SA-SWORD technique, and suggests that it can be a useful tool for assessing subjective SA.
Abstract: The present research tested a subjective situation awareness (SA) metric, called the SA-SWORD technique. The SA-SWORD technique was an SA adaptation of a subjective workload assessment tool (called the Subjective WORkload Dominance (SWORD) technique). The utility of the SA-SWORD technique was tested in a simulator evaluation of cockpit displays, using Air Force pilots as subjects. Two displays were compared: the Fire Control Radar (FCR) display, and the Horizontal Situation Format (HSF) display. The FCR display provided a functional simulation of current air-to-air radar displays. The HSF was a map-like display that combined data-linked information from outside sources (e.g., an AWACS) with the aircraft's own data to provide a bird's-eye, symbolic view of the entire area. Following collection of the performance data, SA-SWORD scores were collected. Pilots rated their SA higher when using the HSF display, than when using the FCR display. An implication of the current results is that subjective SA metrics c...

100 citations


Journal ArticleDOI
TL;DR: The PB-PK model for TCE adequately described the uptake and clearance of TCE in male and female rats exposed to a single, constant concentration of T CE vapor, but failed to describe the uptake or clearance inmale and female mice exposure to a wide range TCE vapor concentrations.

Journal ArticleDOI
TL;DR: In this paper, the utility of the Subjective Workload Dominance (SWORD) technique as a projective workload tool was examined and two groups predicted the workload associated with using six possible hea...
Abstract: The present study examined the utility of the Subjective Workload Dominance (SWORD) technique as a projective workload tool. Two groups predicted the workload associated with using six possible hea...

Book ChapterDOI
01 Jan 1991
TL;DR: In this paper, the authors applied fracture mechanics principles in a deterministic manner to evaluate the durability and damage tolerance requirements of MIL-STD-1530A in three areas: (1) designing new aircraft, (2) evaluating the durability, and (3) evaluating structural repairs and modifications.
Abstract: To date, the Air Force has applied the durability and damage tolerance requirements of MIL-STD-1530A in three areas: a) designing new aircraft; b) evaluating the durability and damage tolerance characteristics of aircraft which were designed prior to the current requirements of MIL-STD-1530A; and, c) evaluating structural repairs and modifications. These applications have used fracture mechanics principles in a deterministic manner. That is, flaw growth was predicted using a fixed potential flaw size, a fixed da/dn vs AK relationship, and a stress spectrum derived from a predicted average usage. While it was realized that there are many stochastic elements in the initiation and growth of cracks, the applied process was considered conservative. Initial flaw size assumptions were generally severe, tracking programs accounted for variations in usage severity, and inspections (if necessary) were scheduled at half the time required for the specified initial flaws to grow to a critical size. At the time of the assessments, there was a high (but unquantified) degree of assurance that fatigue failures and widespread cracking would not occur within the design operational lifetime.

Journal ArticleDOI
TL;DR: In this paper, the authors review and assess the data base for shock boundary-layer interaction that is pertinent to the flow prediction in supersonic inlets and identify specific areas related to shock wave/boundary layer interaction bleed, for flow separation control.
Abstract: The performance of supersonic inlets is strongly affected by the boundary-layer development over its internal surfaces. Boundary-layer bleed is used to suppress separation and to provide the desired inlet performance. The gain in pressure recovery and stability is accompanied, however, with a loss in mass flow and an increase in drag that must be minimized by optimizing the amount of bleed and bleed configuration. The purpose of this work is to review and assess the data base for shock boundary-layer interaction that is pertinent to the flow prediction in supersonic inlets. The first part of the review concerns mixed compression supersonic inlets and their bleed system performance at design and off-design conditions. Based on the assessment of this data, specific areas related to shock wave/boundary-layer interaction bleed, for flow-separation control, are identified, the last part of the review addresses this phenomena in various twoand three-dimensional flow configurations. The effect of bleed in the interaction zone is especially emphasized.

Journal ArticleDOI
TL;DR: In this article, the effect of colony size on ΔK th, ΔK eff,th and K cl,th has been investigated as a function of Widmanstatten microstructures and it has been found that crack growth rates are strongly affected by the size of microstructural features such as colonies and α laths.
Abstract: Near threshold fatigue crack growth behavior of Ti-6A1-4V alloy was investigated as function of Widmanstatten microstructures. In particular, the effect of colony size on ΔK th , ΔK eff,th and K cl,th has been studied. It has been found that crack growth rates are strongly affected by the size of microstructural features such as colonies and α laths. However, the microstructural units controlling crack growth are colonies in fast cooled microstructures consisting of fine Widmanstatten colonies while they are α laths in relatively slow cooled ones with coarse colonies. As a result, the projection in literature of increased fatigue crack growth resistance at large colony sizes in titanium alloys can not be generalized. This distinction appears to be brought about by the thick continuous interplatelet β phase present in slow cooled structures. In fast cooled structures, thin discontinuous β phase is seen to be ineffective in arresting slip or crack. However, in slow cooled ones thick β phase appears to effectively retard slip/crack in fatigue. The thickness, composition, intrinsic properties such as modulus, ductility of β phase and an added environmental effect have been suggested to be important in this respect. The crack growth rates and the magnitudes of ΔK th and ΔK eff,th can be uniquely ordered when compared in terms of the controlling microstructural units in respective microstructures. However, crack closure levels at threshold appear to be dependent on colony size. In addition, an increase in the intrinsic crack growth resistance ΔK eff,th appears to exist when the cyclic plastic zone size approaches the thickness of α laths in the microstructure.

Journal ArticleDOI
TL;DR: The limits of maximal translational motion of the distal radioulnar joints can be determined by computed tomography stress analysis, and bilateral analysis allows the determination of frank instability, subluxation, and dynamic instability.
Abstract: Sixteen distal radioulnar joints in eight normal volunteers (five females, three males) were evaluated by use of a special stress computed tomography technique. The mean palmar and dorsal translational motion was 2.2 millimeters ± 1.6 and 1.0 millimeters ± 0.9, respectively, for the combined group. The mean stress range was 3.3 millimeters ± 1.4 for the entire group. There was no statistical difference for the amount of motion noted on palmar stress, dorsal stress, and the stress-range between males and females. There was also no statistical difference between contralateral sides and hand dominance. The average contralateral difference was 0.1 millimeters, 0.1 millimeters, and 0.3 millimeters for palmar, no stress, dorsal studies, respectively. We conclude the limits of maximal translational motion of the distal radioulnar joints can be determined by computed tomography stress analysis. Unilateral analysis allows the determination of frank instability. However, bilateral analysis allows the determination of frank instability, subluxation, and dynamic instability. This technique is useful in evaluating distal radioulnar joint stability.

Journal Article
TL;DR: Combined cardiac and eye blink data produced better classifications than when each was used alone, and the advantage of collecting multiple physiological signals was demonstrated.
Abstract: Discriminant analysis techniques were used to classify 8 flight segments for 19 F4 crewmembers and to classify pilots from Weapons Systems Officers (WSOs). Heart rate and eye blink data were used as variables. Pilots and WSOs were correctly classified 84% of the time, while 92% of the flight segments were correctly classified for pilots and 89% for WSOs. The percent correct classifications of flight segments using the jackknife procedure were 69% and 68%, respectively. The advantage of collecting multiple physiological signals was demonstrated. Combined cardiac and eye blink data produced better classifications than when each was used alone. Application of this technique to the flight environment is discussed.

Journal ArticleDOI
TL;DR: The case of a 39-year-old patient with a large LMA aneurysm associated with atherosclerosis in patients more than 56 years old is detailed.
Abstract: Aneurysms of the coronary arteries occur in from 0.3% to 4.9% of angiograms. Only 12 cases of left main artery (LMA) aneurysms have been reported. Of these, seven were associated with atherosclerosis in patients more than 56 years old. This report details the case of a 39-year-old patient with a large LMA aneurysm associated with atherosclerosis.

Journal ArticleDOI
TL;DR: A model that allows us to simulate contrast detection, contrast perception and the gradual disappearance of spatial response pooling with contrast is proposed and successfully predicts thresholds and perceived contrast functions for several multiple component stimuli.

Journal ArticleDOI
TL;DR: The role of silicon morphology in the formation of tribo-induced graphite films is explained in this paper, where the performance of heat-treated composites was found to be superior to the rest under both dry and partially liquid lubricated conditions.
Abstract: AlSi (LM6) alloy and 3 wt.% graphite particle composites were synthesised using a solidification processing technique. The morphology of silicon was altered by sodium modification and heat treatment. Wear tests were conducted under dry and partially liquid lubricated conditions using a pin-on-disc wear test apparatus. Wear surfaces and debris were examined by scanning electron microscopy. Under both dry and partially liquid lubricated conditions, the performance of heat-treated composites was found to be superior to the rest. Wear surfaces of heat-treated composites showed the presence of patchy layers of graphite. The role of silicon morphology in the formation of triboinduced graphite films is explained.

Journal ArticleDOI
01 Jan 1991-Polymer
TL;DR: In this paper, the authors used a synchrotron source of X-rays to obtain high-resoultion small-angle X-ray scattering (SAXS) patterns from commercial poly( p -phenylene terephthalamide) (Kevlar) fibres.

Journal ArticleDOI
TL;DR: Blood levels of 1,2-dichloroethane and benzene continued to increase during the 24-hr exposure to neat chemical, while blood levels of the other neat VOCs peaked within 4 hr and then either decreased or remained about the same for the duration of the exposure.

Journal ArticleDOI
TL;DR: It is found that it is necessary to incorporate many-body corrections (in particular electron-electron intrasubband exchange and direct Coulomb interchange energies), depolarization, and excitonlike shifts in the EFA calculations in order to account for the experimental peak-position energy and blueshift as \ensuremath{\sigma} is increased.
Abstract: The intersubband transition (IT) in GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well samples measured by the infrared-absorption technique at 5 K is studied as a function of the two-dimensional electron-gas density (\ensuremath{\sigma}). A blueshift in the peak-position energy of the IT is observed as \ensuremath{\sigma} increases. Single-particle calculations of the optical-absorption spectra, which are obtained by using the nonparabolic-anisotropic envelope-function approximation (EFA), indicate that the peak-position energy should show a redshift as \ensuremath{\sigma} is increased. We found that it is necessary to incorporate many-body corrections (in particular electron-electron intrasubband exchange and direct Coulomb interchange energies), depolarization, and excitonlike shifts in the EFA calculations in order to account for the experimental peak-position energy and blueshift as \ensuremath{\sigma} is increased.

Journal ArticleDOI
TL;DR: In this paper, the effect of short-tube orifice diameter on the response of a heat pump during the reverse-cycle defrost was investigated. But the authors did not consider the effects of different sizes of orifices on defrost performance.
Abstract: Many air source heat pumps use the reverse-cycle defrost to eliminate frost that forms on the outdoor heat exchanger during normal winter operation. During the defrost, the heat pump is switched from the heating to the cooling mode to provide heat to the outdoor heat exchanger to melt the frost. Once the frost is melted and drained from the heat exchanger, the unit is switched back to the heating mode. The objective of this research was to characterize the effect of short-tube orifice diameter on the response of a heat pump during the reverse-cycle defrost. An experimental apparatus was constructed containing a nominal 3 ton (cooling capacity) residential air source heat pump. A manifold was constructed which allowed for the switching of different sized orifices by turning a shut-off valve. Refrigerant temperature and pressure measurements were made throughout the system as well as refrigerant flow-rates, air-side capacity, compressor/outdoor fan power and refrigerant level in the accumulator. A detailed comparison of the effect on defrost performance is provided.

Journal ArticleDOI
TL;DR: In this article, an investigation was conducted in the Thermal Acoustic Fatigue Apparatus at the Langley Research Center to study the acoustically excited random motion of an aluminum plate which is buckled due to thermal stresses.
Abstract: An investigation was conducted in the Thermal Acoustic Fatigue Apparatus at the Langley Research Center to study the acoustically excited random motion of an aluminum plate which is buckled due to thermal stresses. The thermal buckling displacements were measured and compared with theory. The general trends of the changes in resonances frequencies and random responses of the plate agree with previous theoretical prediction and experimental results for a mechanically buckled plate.

Journal ArticleDOI
TL;DR: Late luteal endometrial histology was abnormal in one of four available treatment cycle specimens, but the endocrine characteristics and number and subcellular distribution of estrogen receptor and progesterone receptor in the abnormal cycle were not different from those of normal, in-phase cycles.

Journal ArticleDOI
TL;DR: The studies demonstrate that this series of polyenes consists of efficient broadband nonresonant optical-limiting materials and the predominant nonlinear mechanism was quasi-steady-state self-focusing.
Abstract: The optical-limiting behavior of a series of trans-alpha, omega-diphenyl polyene compounds was observed in solutions of chloroform. The influence of planarity, the substitution of donor and acceptor groups, and the extent of pi-electron delocalization on the nonlinear thresholds of the diphenyl polyenes in an optical-limiting geometry were examined. A saturation effect of optical-limiting nonlinear thresholds, consistent with the theoretical work, was observed at 10-11 pi-electron bonds. The temporal profiles of the transmitted laser pulses and the power dependence of the nonlinear thresholds as a function of spot size and wavelength were examined. These examinations led to the conclusion that the predominant nonlinear mechanism was quasi-steady-state self-focusing. Nonlinear thresholds an order of magnitude lower, and thus effective n(2) and X(3) values an order of magnitude higher, than the well-known self-focusing medium CS(2) were observed. Our studies demonstrate that this series of polyenes consists of efficient broadband nonresonant optical-limiting materials.

Journal ArticleDOI
30 Sep 1991-Wear
TL;DR: In this article, a study was performed to examine the tribological compatibility of commercial aluminum metal-matrix composites (MMCs) with graphite as a solid lubricant.

Journal ArticleDOI
TL;DR: In this paper, the conventional three-dimensional alternating-direction implicit (ADI) method is modified by introducing an f factor (0 < f < 1), which allows the time step limit to be increased by a factor of 1/f with the solutions remaining stable and high accuracy being retained.
Abstract: The conventional three-dimensional alternating-direction implicit (ADI) method is modified by introducing an f factor (0 < f < 1). This modification allows the time step limit to be increased by a factor of 1/f with the solutions remaining stable and high accuracy being retained. This new method is tested for two different boundary conditions: a constant heat flux and a sudden heating of the surface to a constant temperature. In addition, it is compared with the popular Brian and Douglas methods, the results showing that the new ADI method has higher accuracy and requires less computer storage than those methods.

Journal ArticleDOI
TL;DR: In this article, thin films of TiC have been grown on Si (100) substrates by pulsed laser evaporation and analysis by X-ray photoelectron spectroscopy and by Auger sputter depth profiles indicates that the films grown between RT and 500° C are stoichiometric TiC.