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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Computerized Ultrasonic Testing System (CUTS) for in-process thickness determination

A Computerized Ultrasonic Testing System (CUTS) was developed to measure, in real-time, the rate of deposition and thickness of chromium plated on the inside of thick steel tubes. The measurements are made from the outside of the tubes with the ultrasonic pulse-echo technique. The resolution of the system is 2.5 micron. (0.0001 in.) and the accuracy is better than 10 micron (0.0004 in.). The thickness is measured using six transducers mounted at different locations on the tube. In addition, two transducers are mounted on two reference standards, thereby allowing the system to be continuously calibrated. The tube temperature varies during the process, thus the input from eight thermocouples, located at the measurement sites, is used to calculate and compensate for the change in return time of the ultrasonic echo due to the temperature dependence of the sound velocity. CUTS is applicable to any commercial process where real-time change of thickness of a sample has to be known, with the advantage of facilitating increased efficiency and of improving process control.

Frankel, J.↗
NASA NTRS1994

Real-time thickness measurement of MCC ablator material

One of the most favorable characteristics of the Space Shuttle Program is the reusability of two of its primary components: the orbiter itself and the Solid Rocket Boosters (SRB). The SRB's provide the primary source of propulsion for the Space Shuttle during take-off after which they are recovered for refurbishment and reuse. During refurbishment, the SRB's are stripped of all remaining ablative (heat resistant) coating. A new layer is applied to the appropriate sections (nose cone, frustum, forward skirt, and aft skirt). It is the process of applying the ablative coating which provided the impetus for this project. The thickness of this protective layer is considered to be of primary importance to the level of thermal protection provided. The objectives of this effort are to investigate possible techniques for measuring the thickness of MCC, and if possible to test the specific capabilities of those considered good candidates for implementation. The system would be able to take measurements in real-time as close to the spray gun as possible. This will allow the information to be used in the control of the process without an inordinate time delay between a measurement and its appropriate response. The thickness of the deposited material is to be measured with less than 0.100 in if uncertainty. This is the defined tolerance window for the ablator thickness. Finally, it must operate within the confines of the chamber which encloses the turntable, robot, and spray system, and therefore is required to be insensitive to, or at least maintainable in, that environment.

Greenway, R. Bryan, Jr.↗
NASA NTRS1994

Integrated damping mechanics for thick composite laminates and plates

A method for predicting the damped dynamic characteristics of thick composite laminates and plates is presented. Unified damping mechanics relate the damping of composite plates to constituent properties, fiber volume ratio, fiber orientation, laminate configuration, plate geometry, temperature, and moisture. Discrete layer damping mechanics for thick laminates, entailing piecewise continuous displacement fields and including the effects of interlaminar shear damping, are described. A semi-analytical method for predicting the modal damping and natural frequencies of thick simply-supported specialty composite plates is included. Applications demonstrate the validity, merit, and ranges of applicability of the new theory. The applications further illustrate the significance of interlaminar shear damping, and investigate the effects of lamination, thickness aspect ratio, fiber content, and temperature.

Saravanos, D. A.↗
NASA NTRS1995

Percolation effect in thick film superconductors: Using a Bi(Pb)SrCaCuO based paste to prepare a superconducting planar transformer

A thick film superconductor paste has been developed to study the properties of granulated superconductor materials, to observe the percolation effect and to confirm the theory of the conducting mechanism in the superconducting thick films. This paste was also applied to make a superconducting planar transformer. Due to the T(sub c) and advantageous current density properties the base of the past was chosen to be of Bi(Pb)SrCaCu) system. For contacts a conventional Ag/Pt paste was used. The critical temperature of the samples were between 110 K and 115 K depending on the printed layer thickness. The critical current density -at the boiling temperature of the liquid He- was between 200 - 300 A/sq cm. The R(T) and V(I) functions were measured with different parameters. The results of the measurements have confirmed the theory of conducting mechanism in the material. The percolation structure model has been built and described. As an application, a superconducting planar thick film transformer was planned and produced. Ten windings of the transformer were printed on one side of the alumina substrate and one winding was printed on the other side. The coupling between the two sides was possible through the substrate. The samples did not need special drying and firing parameters. After the preparation, the properties of the transformer were measured. The efficiency ans the losses were determined. Finally, some fundamental advantages and problems of the process were discussed.

Sali, Robert↗
NASA NTRS1996

Effects of Core Thickness and Fiber Orientation on Composite Beam Stiffness

This laboratory experiment is intended for students in an introductory polymer materials and processes course or engineering materials course. It can be conducted as an introduction to the hand lay-up process, with additional observations regarding the stiffness of the completed composite beams based on core thickness and fiber orientation. Students gain hands-on experience with the hand lay-up process by constructing glass/epoxy composite panels. Each lab group produces a panel with different core thickness or fiber orientation. The panels are then cut into strips and tested for flexural stiffness in a three-point bending fixture. Students plot deflection versus load data for composite beams with two different fiber orientations, two core thicknesses and one beam with laminate plies only (no core). The deflection plots highlight the effects of core thickness and fiber orientation on composite beam stiffness.

Hillsman, Vernon S.↗
NASA NTRS1999

Correction Factor for Gaussian Deconvolution of Optically Thick Linewidths in Homogeneous Sources

Profiles of optically thick, non-Gaussian emission line profiles convoluted with Gaussian instrumental profiles are constructed, and are deconvoluted on the usual Gaussian basis to examine the departure from accuracy thereby caused in "measured" linewidths. It is found that "measured" linewidths underestimate the true linewidths of optically thick lines, by a factor which depends on the resolution factor r congruent to Doppler width/instrumental width and on the optical thickness tau(sub 0). An approximating expression is obtained for this factor, applicable in the range of at least 0 <= tau(sub 0) <= 10, which can provide estimates of the true linewidth and optical thickness.

Kastner, S. O.↗
NASA NTRS1999

The Effect of Interface Roughness and Oxide Film Thickness on the Inelastic Response of Thermal Barrier Coatings to Thermal Cycling

The effects of interfacial roughness and oxide film thickness on thermally-induced stresses in plasma-sprayed thermal barrier coatings subjected to thermal cycling are investigated using the recently developed higher-order theory for functionally graded materials. The higher-order theory is shown to be a viable alternative to the finite-element approach, capable of modeling different interfacial roughness architectures in the presence of an aluminum oxide layer and capturing the high stress gradients that occur at the top coat/bond coat interface. The oxide layer thickness is demonstrated to have a substantially greater effect on the evolution of residual stresses than local variations in interfacial roughness. Further, the location of delamination initiation in the top coat is predicted to change with increasing oxide layer thickness. This result can be used to optimize the thickness of a pre-oxidized layer introduced at the top coat/bond coat interface in order to enhance TBC durability as suggested by some researchers. The results of our investigation also support a recently proposed hypothesis regarding delamination initiation and propagation in the presence of an evolving bond coat oxidation, while pointing to the importance of interfacial roughness details and specimen geometry in modeling this phenomenon.

Pindera, Marek-Jerzy↗
NASA NTRS2000

Development of Thick-Film Thermoelectric Microcoolers Using Electrochemical Deposition

Advanced thermoelectric microdevices integrated into thermal management packages and low power, electrical source systems are of interest for a variety of space and terrestrial applications. By shrinking the size of the thermoelements, or legs, of these devices, it becomes possible to handle much higher heat fluxes, as well as operate at much lower currents and higher voltages that are more compatible with electronic components. The miniaturization of state-of-the-art thermoelectric module technology based on Bi2Te3 alloys is limited due to mechanical and manufacturing constraints for both leg dimensions (100-200 gm thick minimum) and the number of legs (100-200 legs maximum). We are investigating the development of novel microdevices combining high thermal conductivity substrate materials such as diamond, thin film metallization and patterning technology, and electrochemical deposition of thick thermoelectric films. It is anticipated that thermoelectric microcoolers with thousands of thermocouples and capable of pumping more than 200 W/sq cm over a 30 to 60 K temperature difference can be fabricated. In this paper, we report on our progress in developing an electrochemical deposition process for obtaining 10-50 microns thick films of Bi2Te3 and its solid solutions. Results presented here indicate that good quality n-type Bi2Te3, n-type Bi2Te(2.95)Se(0.05) and p-type Bi(0.5)Sb(1.5)Te3 thick films can be deposited by this technique. Some details about the fabrication of the miniature thermoelements are also described.

Fleurial, J.-P.↗
NASA NTRS2000

A Mathematical Model of Diffusion-Limited Gas Bubble Dynamics in Tissue with Varying Diffusion Region Thickness

A three-region mathematical model of gas bubble dynamics has been shown suitable for describing diffusion-limited dynamics of more than one bubble in a given volume of extravascular tissue. The model is based on the dynamics of gas exchange between a bubble and a well-stirred tissue region through an intervening unperfused diffusion region previously assumed to have constant thickness and uniform gas diffusivity. As a result, the gas content of the diffusion region remains constant as the volume of the region increases with bubble growth, causing dissolved gas in the region to violate Henry's law. Earlier work also neglected the relationship between the varying diffusion region volume and the fixed total tissue volume, because only cases in which the diffusion region volume is a small fraction of the overall tissue volume were considered. We herein extend the three-region model to correct these theoretical inconsistencies by allowing both the thickness and gas content of the diffusion region to vary during bubble evolution. A postulated difference in gas diffusivity between an infinitesimally thin layer at the bubble surface and the remainder of the diffusion region leads to variation in diffusion region gas content and thickness during bubble growth and resolution. This variable thickness, differential diffusivity (VTDD) model can yield bubble lifetimes considerably longer than those yielded by earlier three-region models for given model and decompression parameters, and meets a need for theoretically consistent but relatively simple bubble dynamics models for use in studies of decompression sickness (DCS) in human subjects, Keywords: decompression sickness, gas diffusion in tissue, diffusivity

Srinivasan, R. Srini↗
NASA NTRS2001

Constraining the Mean Crustal Thickness on Mercury

The topography of Mercury is poorly known, with only limited radar and stereo coverage available. However, radar profiles reveal topographic contrasts of several kilometers over wavelengths of approximately 1000 km. The bulk of Mercury's geologic activity took place within the first 1 Ga of the planet's history), and it is therefore likely that these topographic features derive from this period. On Earth, long wavelength topographic features are supported either convectively, or through some combination of isostasy and flexure. Photographic images show no evidence for plume-like features, nor for plate tectonics; I therefore assume that neither convective support nor Pratt isostasy are operating. The composition and structure of the crust of Mercury are almost unknown. The reflectance spectrum of the surface of Mercury is similar to that of the lunar highlands, which are predominantly plagioclase. Anderson et al. used the observed center-of-mass center-of-figure offset together with an assumption of Airy isostasy to infer a crustal thickness of 100-300 km. Based on tidal despinning arguments, the early elastic thickness (T(sub e)) of the (unfractured) lithosphere was approximately equal to or less than 100 km. Thrust faults with lengths of up to 500 km and ages of about 4 Ga B.P. are known to exist on Mercury. Assuming a semicircular slip distribution and a typical thrust fault angle of 10 degrees, the likely vertical depth to the base of these faults is about 45 km. More sophisticated modelling gives similar or slightly smaller answers. The depth to the base of faulting and the elastic layer are usually similar on Earth, and both are thought to be thermally controlled. Assuming that the characteristic temperature is about 750 K, the observed fault depth implies that the heat flux at 4 Ga B.P. is unlikely to be less than 20 mW m(exp -2) for a linear temperature gradient. For an elastic thickness of 45 km, topography at 1000 km wavelength is likely to be about 60% compensated. There are thus likely to be considerable lateral variations in crustal thickness. Additional information is contained in the original extended abstract.

Nimmo, F.↗
NASA NTRS2001

Radiation-Driven Flame Spread Over Thermally-Thick Fuels in Quiescent Microgravity Environments

Microgravity experiments on flame spread over thermally thick fuels were conducted using foam fuels to obtain low density and thermal conductivity, and thus large spread rate (Sf) compared to dense fuels such as PMMA. This scheme enabled meaningful results to lie obtained even in 2.2 second drop tower experiments. It was found that, in contrast conventional understanding; steady spread can occur over thick fuels in quiescent microgravity environments, especially when a radiatively active diluent gas such as CO2 is employed. This is proposed to be due to radiative transfer from the flame to the fuel surface. Additionally, the transition from thermally thick to thermally thin behavior with decreasing bed thickness is demonstrated.

Honda, Linton K.↗
NASA NTRS2002

Method and Apparatus for Obtaining a Precision Thickness in Semiconductor and Other Wafers

A method and apparatus for processing a wafer comprising a material selected from an electrical semiconducting material and an electrical insulating material is presented. The wafer has opposed generally planar front and rear sides and a peripheral edge, wherein said wafer is pressed against a pad in the presence of a slurry to reduce its thickness. The thickness of the wafer is controlled by first forming a recess such as a dimple on the rear side of the wafer. A first electrical conducting strip extends from a first electrical connection means to the base surface of the recess to the second electrical connector. The first electrical conducting strip overlies the base surface of the recess. There is also a second electrical conductor with an electrical potential source between the first electrical connector and the second electrical connector to form. In combination with the first electrical conducting strip, the second electrical conductor forms a closed electrical circuit, and an electrical current flows through the closed electrical circuit. From the front side of the wafer the initial thickness of the wafer is reduced by lapping until the base surface of the recess is reached. The conductive strip is at least partially removed from the base surface to automatically stop the lapping procedure and thereby achieve the desired thickness.

Okojie, Robert S.↗
NASA NTRS2003

Ice-Accretion Scaling Using Water-Film Thickness Parameters

Studies were performed at INTA in Spain to determine water-film thickness on a stagnation-point probe inserted in a simulated cloud. The measurements were correlated with non-dimensional parameters describing the flow and the cloud conditions. Icing scaling tests in the NASA Glenn Icing Research Tunnel were then conducted using the Ruff scaling method with the scale velocity found by matching scale and reference values of either the INTA non-dimensional water-film thickness or a Weber number based on that film thickness. For comparison, tests were also performed using the constant drop-size Weber number and the average-velocity methods. The reference and scale models were both aluminum, 61-cm-span, NACA 0012 airfoil sections at 0 deg. AOA. The reference had a 53-cm-chord and the scale, 27 cm (1/2 size). Both models were mounted vertically in the center of the IRT test section. Tests covered a freezing fraction range of 0.28 to 1.0. Rime ice (n = 1.0) tests showed the consistency of the IRT calibration over a range of velocities. At a freezing fraction of 0.76, there was no significant difference in the scale ice shapes produced by the different methods. For freezing fractions of 0.40, 0.52 and 0.61, somewhat better agreement with the reference horn angles was typically achieved with the average-velocity and constant-film thickness methods than when either of the two Weber numbers was matched to the reference value. At a freezing fraction of 0.28, the four methods were judged equal in providing simulations of the reference shape.

Anderson, David N.↗
NASA NTRS2004

Improvements to the Two-Thickness Method for Deriving Acoustic Properties of Materials

The characteristic impedance and other derivative acoustic properties of a material can be derived from impedance tube data using the specific impedance measured from samples with two different thicknesses. In practice, samples are chosen so that their respective thicknesses differ by a factor of 2. This simplifies the solution of the equations relating the properties of the two samples so that the computation of the characteristic impedance is straightforward. This approach has at least two drawbacks. One is that it is often difficult to acquire or produce samples with precisely a factor of 2 difference in thickness. A second drawback is that the phase information contained in the imaginary part of the propagation constant must be unwrapped before subsequent computations are performed. For well-behaved samples, this is not a problem. For ill behaved samples of unknown properties, the phase unwrapping process can be tedious and difficult to automate. Two alternative approaches have been evaluated which remove the factor-of-2 sample thickness requirement and directly compute unwrapped phase angles. One uses a Newton-Raphson approach to solve for the roots of the samples' simultaneous equations. The other produces a wave number space diagram in which the roots are clearly discernable and easily extracted. Results are presented which illustrate the flexibility of analysis provided by the new approaches and how this can be used to better understand the limitations of the impedance tube data.

Palumbo, Daniel L.↗
NASA NTRS1996

Permeation of protons, potassium ions, and small polar molecules through phospholipid bilayers as a function of membrane thickness

Two mechanisms have been proposed to account for solute permeation of lipid bilayers. Partitioning into the hydrophobic phase of the bilayer, followed by diffusion, is accepted by many for the permeation of water and other small neutral solutes, but transient pores have also been proposed to account for both water and ionic solute permeation. These two mechanisms make distinctively different predictions about the permeability coefficient as a function of bilayer thickness. Whereas the solubility-diffusion mechanism predicts only a modest variation related to bilayer thickness, the pore model predicts an exponential relationship. To test these models, we measured the permeability of phospholipid bilayers to protons, potassium ions, water, urea, and glycerol. Bilayers were prepared as liposomes, and thickness was varied systematically by using unsaturated lipids with chain lengths ranging from 14 to 24 carbon atoms. The permeability coefficient of water and neutral polar solutes displayed a modest dependence on bilayer thickness, with an approximately linear fivefold decrease as the carbon number varied from 14 to 24 atoms. In contrast, the permeability to protons and potassium ions decreased sharply by two orders of magnitude between 14 and 18 carbon atoms, and leveled off, when the chain length was further extended to 24 carbon atoms. The results for water and the neutral permeating solutes are best explained by the solubility-diffusion mechanism. The results for protons and potassium ions in shorter-chain lipids are consistent with the transient pore model, but better fit the theoretical line predicted by the solubility-diffusion model at longer chain lengths.

NASA Program Exobiology↗
NASA NTRS2005

Effect of Cover Thickness on the Relationship of Surface Relief to Diameter of Northern Lowland QCDs on Mars

Previous work has established that there is a relationship of surface relief to diameter for quasi-circular depressions (QCDs) around the Utopia Basin [1]. This relationship has been used to support the contention that the QCDs represent impact craters buried beneath a differentially compacting cover material. For any given regional cover thickness, total cover thickness is greater over the centers of completely buried craters than over their rims; thus total compaction is greater over the center of craters than their rims and topographic depressions will form. Since large craters are deeper than small craters, differential compaction models also predict that surface relief will be proportional to the diameter of the buried crater [2]. It is highly unlikely, however, that the material covering the QCD impact craters is a consistent thickness throughout the entire northern lowlands of Mars. We explore the effects that changes in cover thickness would have on the surface relief vs. diameter relationship of QCDs.

Buczkowski, D. L.↗
NASA NTRS2005

Sea Ice Kinematics and Thickness from RGPS: Observations and Theory

The RADARSAT Geophysical Processor System (RGPS) has produced a wealth of data on Arctic sea ice motion, deformation, and thickness with broad geographical coverage and good temporal resolution. These data provide unprecedented spatial detail of the structure and evolution of the sea ice cover. The broad purpose of this study was to take advantage of the strengths of the RGPS data set to investigate sea ice kinematics and thickness, which affect the climate through their influence on ice production, ridging, and transport (i.e. mass balance); heat flux to the atmosphere; and structure of the upper ocean mixed layer. The objectives of this study were to: (1) Explain the relationship between the discontinuous motion of the ice cover and the large-scale, smooth wind field that drives the ice; (2) Characterize the sea ice deformation in the Arctic at different temporal and spatial scales, and compare it with deformation predicted by a state-of-theart ice/ocean model; and (3) Compare RGPS-derived sea ice thickness with other data, and investigate the thinning of the Arctic sea ice cover as seen in ULS data obtained by U.S. Navy submarines. We briefly review the results of our work below, separated into the topics of sea ice deformation and sea ice thickness. This is followed by a list of publications, meetings and presentations, and other activities supported under this grant. We are attaching to this report copies of all the listed publications. Finally, we would like to point out our community service to NASA through our involvement with the ASF User Working Group and the RGPS Science Working Group, as evidenced in the list of meetings and presentations below.

Stern, Harry↗
NASA NTRS2005

Dynamic Harris current sheet thickness from Cluster current density and plasma measurements

We use the first accurate measurements of current densities in the plasma sheet to calculate the half-thickness and position of the current sheet as a function of time. Our technique assumes a Harris current sheet model, which is parameterized by lobe magnetic field B(o), current sheet half-thickness h, and current sheet position z(sub o). Cluster measurements of magnetic field, current density, and plasma pressure are used to infer the three parameters as a function of time. We find that most long timescale (6-12 hours) current sheet crossings observed by Cluster cannot be described by a static Harris current sheet with a single set of parameters B(sub o), h, and z(sub o). Noting the presence of high-frequency fluctuations that appear to be superimposed on lower frequency variations, we average over running 6-min intervals and use the smoothed data to infer the parameters h(t) and z(sub o)(t), constrained by the pressure balance lobe magnetic field B(sub o)(t). Whereas this approach has been used in previous studies, the spatial gnuhen& now provided by the Cluster magnetometers were unavailable or not well constrained in earlier studies. We place the calculated hdf&cknessa in a magnetospheric context by examining the change in thickness with substorm phase for three case study events and 21 events in a superposed epoch analysis. We find that the inferred half-thickness in many cases reflects the nominal changes experienced by the plasma sheet during substorms (i.e., thinning during growth phase, thickening following substorm onset). We conclude with an analysis of the relative contribution of (Delta)B(sub z)/(Delta)X to the cross-tail current density during substorms. We find that (Delta)B(sub z)/(Delta)X can contribute a significant portion of the cross-tail c m n t around substorm onset.

Thompson, S. M.↗