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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Dynamic response of lined tunnels by boundary elements

The dynamic stress concentration manifested around a lined cylindrical tunnel buried in an infinitely extending linear elastic or viscoelastic medium due to the passage of transient disturbances was investigated. Plane strain is assumed to hold and the transient disturbances can be of any arbitrary time variation. The boundary element method formulated in the Laplace transform domain is employed. Isoparametric boundary elements are used in the discretization of the liner and tunnel surfaces. Viscoelastic material behavior can be readily obtained from the linear elastic case in the Laplace transform domain through the use of the correspondence principle. The transient solution is recovered by numerical inversion of the solution obtained in the transformed domain.

Manolis, G. D.↗
NASA NTRS1988

Cracking of a layered medium on an elastic foundation under thermal shock

The cladded pressure vessel under thermal shock conditions which is simulated by using two simpler models was studied. The first model (Model 1) assumes that, if the crack size is very small compared to the vessel thickness, the problem can be treated as a semi-infinite elastic medium bonded to a very thin layer of different material. However, if the crack size is of the same order as the vessel thickness, the curvature effects may not be negligible. In this case it is assumed that the relatively thin walled hollow cylinder with cladding can be treated as a composite beam on an elastic foundation (Model 2). In both models, the effect of surface cooling rate is studied by assuming the temperature boundary condition to be a ramp function. The calculated results include the transient temperature, thermal stresses in the uncracked medium and stress intensity factors which are presented as a function of time, and the duration of cooling ramp. The stress intensity factors are also presented as a function of the size and the location of the crack. The problem is solved for two bonded materials of different thermal and mechanical properties. The mathematical formulation results in two singular integral equations which are solved numerically. The results are given for two material pairs, namely an austenitic steel layer welded on a ferritic steel substrate, and a ceramic coating on ferritic steel. In the case of the yielded clad, the stress intensity factors for a crack under the clad are determined by using a plastic strip model and are compared with elastic clad results.

Rizk, Abd El-Fattah A.↗
NASA NTRS1984

The role of an intracrustal asthenosphere on the behavior of major strike-slip faults

Strain accumulation measurements adjacent to the San Andreas fault have indicated that the strain accumulation zone extends only a few tens of kms away from the fault. While the restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike-slip faults has been attributed to a viscoelastic asthenosphere's damping effect, the narrowness of the San Andreas zone implies a thickness of the lithosphere that, at 10-20 km, may not be consistent with the relatively low surface heat flow measurements obtained. It is presently proposed that an upper elastic plate extends to a depth of 15 km, and that beneath this upper elastic plate is a soft, intracrustal asthenosphere exhibiting a viscoelastic behavior. A second elastic layer lies under this, followed by the asthenosphere. It is shown that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release.

Turcotte, D. L.↗
NASA NTRS2004

Gear Performance Improved by Coating

Gears, bearings, and other mechanical elements transmit loads through contacting surfaces. Even if properly designed, manufactured, installed, and maintained, gears and bearings will eventually fail because of the fatigue of the working surfaces. Economical means for extending the fatigue lives of gears and bearings are highly desired, and coatings offer the opportunity to engineer surfaces to extend the fatigue lives of mechanical components. A tungsten-containing diamondlike-carbon coating exhibiting high hardness, low friction, and good toughness was evaluated for application to spur gears. Fatigue testing was done at the NASA Glenn Research Center on both uncoated and coated spur gears. The results showed that the coating extended the surface fatigue lives of the gears by a factor of about 5 relative to the uncoated gears. For the experiments, a lot of spur test gears made from AISI 9310 gear steel were case-carburized and ground to aerospace specifications. The geometries of the 28-tooth, 8-pitch gears were verified as meeting American Gear Manufacturing Association (AGMA) quality class 12. One-half of the gears were randomly selected for coating. The method of coating was selected to achieve desired adherence, toughness, hardness, and low-friction characteristics. First the gears to be coated were prepared by blasting (vapor honing) with Al2O3 particles and cleaning. Then, the gears were provided with a thin adhesion layer of elemental chromium followed by magnetron sputtering of the outer coating consisting of carbon (70 at.%), hydrogen (15 at.%), tungsten (12 at.%), and nickel (3 at.%) (atomic percent at the surface). In total, the coating thickness was about 2.5 to 3 microns. As compared with the steel substrate, the coated surface was harder by a factor of about 2 and had a smaller elastic modulus. All gears were tested using a 5-centistoke synthetic oil, a 10,000-rpm rotation speed, and a hertzian contact stress of at least 1.7 GPa (250 ksi). Tests were run until either surface fatigue occurred or 300 million stress cycles were completed. Tests were run using either a pair of uncoated gears or a pair of coated gears (coated gears mated with uncoated gears were not evaluated). The fatigue test results, shown on Weibull coordinates in the graph, demonstrate that the coating provided substantially longer fatigue lives even though some of the coated gears endured larger stresses. The increase in fatigue life was a factor of about 5 and the statistical confidence for the improvement is high (greater than 99 percent). Examination of the tested gears revealed substantial reductions of total wear for coated gears in comparison to uncoated gears. The coated gear surface topography changed with running, with localized areas of the tooth surface becoming smoother with running. Theories explaining how coatings can extend gear fatigue lives are research topics for coating, tribology, and fatigue specialists. This work was done as a partnership between NASA, the U.S. Army Research Laboratory, United Technologies Research Corporation, and Sikorsky Aircraft.

Krantz, Timothy L.↗
NASA NTRS1992

Evaluation of microcrack thermal shock damage in ceramics: Modeling and experiment

In this paper we present an experimental and theoretical study of the effect of microcrack damage on ceramic properties. For the experimental investigation, ceramic samples of aluminum oxide and reaction bonded silicon nitride (RBSN) are used. Thermal shock treatment from different temperatures up to 1000 C is applied to produce the microcracks. Both surface and bulk ultrasonic wave methods are used to correlate the change of elastic constants to microstructural degradation and to determine the change in elastic anisotropy induced by microcrack damage. For the theoretical investigation, damage mechanics, which relates microstructural damage to material service life and mechanical failure, is used. The change in elastic properties due to microcrack damage calculated from the theoretical model is compared with the experimental results for determination of the applicability of damage theory. It is shown that two independent experimental methods (bulk wave and surface wave) give the same results for shear moduli of damaged ceramics. The experimental results aagree reasonably well with the moduli predicted from the cracked solid model.

Chu, Y. C.↗
NASA NTRS1985

Numerical Techniques for Scattering from Submerged Objects

To represent the final results in terms of matrices, one expands all appropriate physical quantities in terms of partial wave basis states. This includes expansions for the incident and scattered fields and the surface quantities. The method then utilizes the Huygen-Poincare integral representation for both the exterior and interior solutions, leading to the required matrix equations. One thus deals with matrix equations, the complexity of which depends on the nature of the problem. It is shown that in general a transition matrix T can be obtained relating the incident field A with the scattered field f having the form T = PQ(-1), where f = TA. The structure of Q can be quite complicated and can itself be composed of other matrix inversions such as arise from layered objects. Recent improvements in this method appropriate for a variety of physical problems are focused on, and on their implementation. Results are outlined from scattering simulations for very elongated submerged objects and resonance scattering from elastic solids and shells. The final improvement concerns eigenfunction expansions of surface terms, arising from solution of the interior problem, obtained via a preconditioning technique. This effectively reduces the problem to that of obtaining eigenvalues of a Hermitian operator. This formalism is reviewed for scattering from targets that are rigid, sound-soft, acoustic, elastic solids, elastic shells, and elastic layered objects. Two sets of the more interesting results are presented. The first concerns scattering from elongated objects, and the second to thin elastic spheroids.

Werby, M. F.↗
NASA NTRS1980

Interpretation of postseismic deformation with a viscoelastic relaxation model

A viscoelastic relaxation model is used to interpret postseismic surface deformation for the large-magnitude Aleutian earthquakes of 1957 and 1965. The lithosphere and asthenosphere are modeled as elastic solids with an anomalous viscoelastic inclusion below the island arc volcanoes. It is found that the observed postseismic surface deformation is a corollary of the known island arc structure. A satisfactory fit to the uplift following the 1957 earthquake is found for a viscoelastic volume with 80-km width extending from a depth of 50 to 200 km.

Wahr, J.↗
NASA NTRS1982

Tribological characteristics of a composite total-surface hip replacement

Continuous fiber, woven E glass composite femoral shells having the same elastic properties as bone were fabricated. The shells were then encrusted with filled epoxy wear resistant coatings and run dry against ultrahigh molecular weight polyethylene acetabular cups in 42,000 and 250,000 cycle wear tests on a total hip simulator. The tribological characteristics of these continuous fiber particulate composite femoral shells articulating with ultrahigh molecular weight polyethylene acetabular cups were comparable to those of a vitallium ball articulating with an ultrahigh molecular weight polyethylene acetabular cup.

Jones, W. R., Jr.↗
NASA NTRS1975

Seismically induced modification of lunar surface features

The formation of large impact craters and basins produced not only large volumes of ejecta but also catastrophic seismic waves. Theoretical models, extrapolation of terrestrial explosion data, and extrapolation of lunar impact data suggest surface displacements of 1-10 m at four crater radii from a Copernicus-size impact (4 x 10 to the 30th power ergs). Independent estimates of impact-generated stresses indicate that regions within 4 crater radii of an impact will receive shock waves with pressures exceeding 1 kbar. Greater areas also may receive shock effects as elastic waves following deep ray paths transform into shock waves near the surface. Additional seismic energy will be generated by impacts from secondary ejecta. The resulting large and long-lasting surface movement may contribute to the subdued appearance of the continuous ejecta blanket and secondary craters around large impact craters and to the degradation of small craters outside these zones.

Schultz, P. H.↗
NASA NTRS1972

On the tidal effects in the motion of earth satellites and the love parameters of the earth

The tidal effects in the motion of artificial satellites are studied to determine the elastic properties of the earth as they are observed from extraterrestrial space. Considering Love numbers, the disturbing potential is obtained as the analytical continuation of the tidal potential from the surface of the earth into-outer space, with parameters which characterize the earth's elastic response to tidal attraction by the moon and the sun. It is concluded that the tidal effects represent a superposition of a large number of periodic terms, and the rotation of the lunar orbital plane produces a term of 18 years period in tidal perturbations of the ascending node of the satellite's orbit.

Musen, P.↗
NASA NTRS1973

Unsteady aerodynamic analysis of space shuttle vehicles. Part 2: Steady and unsteady aerodynamics of sharp-edged delta wings

An analysis of the steady and unsteady aerodynamics of sharp-edged slender wings has been performed. The results show that slender wing theory can be modified to give the potential flow static and dynamic characteristics in incompressible flow. A semiempirical approximation is developed for the vortex-induced loads, and it is shown that the analytic approximation for sharp-edged slender wings gives good prediction of experimentally determined steady and unsteady aerodynamics at M = 0 and M = 1. The predictions are good not only for delta wings but also for so-called arrow and diamond wings. The results indicate that the effects of delta planform lifting surfaces can be included in a simple manner when determining elastic launch vehicle dynamic characteristics. For Part 1 see (N73-32763).

Ericsson, L. E.↗
NASA NTRS1979

Analysis of the vibratory excitation of gear systems. II - Tooth error representations, approximations, and application

The second part of a theory for predicting the vibratory excitation of gear systems from fundamental descriptions of gear tooth elastic properties and deviations of tooth faces from perfect involute surfaces is presented. The first part of the theory provides expressions for the Fourier-series coefficients of the vibratory excitation, and this paper gives expressions for these Fourier-series coefficients in terms of easily interpreted gear tooth metrics that are readily evaluated from tooth-face measurements. Results are given for rectangular tooth-face contact regions using two-dimensional Legendre polynomial expansions of local tooth-pair stiffnesses and stiffness-weighted deviations of tooth faces from perfect involute surfaces. A rigorous transfer function approach is developed that permits separation of the effects of gear tooth errors and gear design parameters; the theory is applicable to helical and spur gears and is illustrated with measurements of tooth-spacing errors and tooth profiles obtained from a pair of spur gears.

Mark, W. D.↗
NASA NTRS1981

Test progress on the electrostatic membrane reflector

NASA is currently developing a low mass antenna which derives its reflector surface quality from the application of electrostatic forces to form a thin membrane into the desired concave reflector surface. The shuttle-deployed antenna would have a diameter of 100 m and an RMS surface smoothness of 10 to 1 mm for operation at 1 to 10 GHz. Surface quality measurements have been made on a highly deformable elastic membrane, pressurized by electrostatic forces. Included are the effects of the perimeter boundary, splicing of the membrane, the long-scale smoothness of commercial membranes, and the spatial controllability of the membrane using voltage adjustments to alter the electrostatic forces. The electrostatic membrane was found to operate well in an open-loop sense, showing a high degree of position stability and negligible power consumption in dry air. Visco-electric creep was not evident, but the polymer membrane did expand and contract considerably due to its hygroscopic expansion coefficient. A residual roughness of about 0.75 mm existed with the polymer used in these tests; this error is attributed to seams and membrane anisotropy where the material is stiffer in one direction.

Goslee, J. W.↗
NASA NTRS1983

Wrinkling phenomenon in structures. I - Model formulations

Those structural element models which can suffer the wrinkling phenomenon are analyzed. A laminated elastic material, defined as a one-parametrical family of surfaces whose in-plane strains are negligible in comparison to the strains in the transverse direction, is analyzed in terms of its geometry, geometrical compatibility equations, layer geometry, kinematics, kinematical compatibility equations, and governing dynamical equations. These aspects are also examined for the case of a soft shell, defined as a shell for which the in-plane strains of the middle surface are negligible compared to the strains in the transverse direction, and for the case of a soft rod, defined as a rod for which the strains along the axis are negligible compared to the strains in the transverse direction. The influence of internal fluid flow on the wrinkling phenomenon for layers of laminated material and for a flexible pipe containing an inviscid and incompressible fluid is also analyzed.

Zak, M.↗
NASA NTRS1986

Predicting the Cyclic Response of High-Temperature Materials

Simplified analytical procedure for estimating local stress/strain response in high-temperature structural component developed. Simplified procedure relies on assumption local inelastic response in hightemperature structure constrained by surrounding elastic material. Procedure assumes local response composed of elastic, time-independent plastic, and creep components. Conventional yield surface concept used to determine onset of plastic action.

Moreno, V.↗
NASA NTRS1990

Crack problems involving nonhomogeneous interfacial regions in bonded materials

Consideration is given to two classes of fracture-related solid mechanics problems in which the model leads to some physically anomalous results. The first is the interface crack problem associated with the debonding process in which the corresponding elasticity solution predicts severe oscillations of stresses and the crack surface displacements vary near the crack tip. The second deals with crack intersecting the interface. The nature of the solutions around the crack tips arising from these problems is reviewed. The rationale for introducing a new interfacial zone model is discussed, its analytical consequences within the context of the two crack-problem classes are described, and some examples are presented.

Erdogan, F.↗
NASA NTRS1994

Rheology, tectonics, and the structure of the Venus lithosphere

Given the absence of ground truth information on seismic structure, heat flow, and rock strength, or short wavelength gravity or magnetic data for Venus, information on the thermal, mechanical and compositional nature of the shallow interior must be obtained by indirect methods. Using pre-Magellan data, theoretical models constrained by the depths of impact craters and the length scales of tectonic features yielded estimates on the thickness of Venus' brittle-elastic lithosphere and the allowable range of crustal thickness and surface thermal gradient. The purpose of this study is to revisit the question of the shallow structure of Venus based on Magellan observations of the surface and recent experiments that address Venus' crustal rheology.

Zuber, M. T.↗