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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 1,405 records · Page 78
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Direct numerical simulation of a three-dimensional turbulent boundary layer

The effects of transverse strain on an initially two-dimensional turbulent boundary layer are studied in a direct numerical simulation of a planar channel flow with impulsively started transverse pressure gradient. Consistent with experiments in three-dimensional boundary layers, the simulation shows a decrease in the Reynolds shear stress with increasing transverse strain. Also, the directions of the Reynolds shear stress vector and the mean velocity gradient vector were found to differ. In addition, the simulation shows a drop in the turbulent kinetic energy. Terms in the Reynolds stress transport equations were computed. The balances indicate that the decrease in turbulent kinetic energy is a result of a decrease in turbulence production, along with an increase in turbulent dissipation. Intuitive reasoning and current turbulence models would predict an increase in kinetic energy along with increases in production and dissipation rates as a result of increased mean-flow strain rate. Later in the evolution of the flow, both turbulence production and dissipation increase.

Moin, P.↗
NASA NTRS1991

Estimation of the engineering elastic constants of a directionally solidified superalloy for finite element structural analysis

The temperature-dependent engineering elastic constants of a directionally solidified nickel-base superalloy were estimated from the single-crystal elastic constants of nickel and MAR-MOO2 superalloy by using Wells' method. In this method, the directionally solidified (columnar-grained) nickel-base superalloy was modeled as a transversely isotropic material, and the five independent elastic constants of the transversely isotropic material were determined from the three independent elastic constants of a cubic single crystal. Solidification for both the single crystals and the directionally solidified superalloy was assumed to be along the (001) direction. Temperature-dependent Young's moduli in longitudinal and transverse directions, shear moduli, and Poisson's ratios were tabulated for the directionally solidified nickel-base superalloy. These engineering elastic constants could be used as input for performing finite element structural analysis of directionally solidified turbine engine components.

Abdul-Aziz, Ali↗
NASA NTRS1991

Mechanical characterization and modeling of non-linear deformation and fracture of a fiber reinforced metal matrix composite

The nonlinear anisotropic mechanical behavior of an aluminum alloy metal matrix composite reinforced with continuous alumina fibers was determined experimentally. The mechanical behavior of the composite were modeled by assuming that the composite has a periodical microstructure. The resulting unit cell problem was solved with the finite element method. Excellent agreement was found between theoretically predicted and measured stress-strain responses for various tensile and shear loadings. The stress-strain responses for transverse and inplane shear were found to be identical and this will provide a simplification of the constitutive equations for the composite. The composite has a very low ductility in transverse tension and a limited ductility in transverse shear that was correlated to high hydrostatic stresses that develop in the matrix. The shape of the initial yield surface was calculated and good agreement was found between the calculated shape and the experimentally determined shape.

Jansson, S.↗
NASA NTRS1991

Local delamination in laminates with angle ply matrix cracks. Part 1: Tension tests and stress analysis

Quasi-static tension tests were conducted on AS4/3501-6 graphite epoxy laminates. Dye penetrant enhanced x-radiography was used to document the onset of matrix cracking and the onset of local delaminations at the intersection of the matrix cracks and the free edge. Edge micrographs taken after the onset of damage were used to verify the location of the matrix cracks and local delamination through the laminate thickness. A quasi-3D finite element analysis was conducted to calculate the stresses responsible for matrix cracking in the off-axis plies. Laminated plate theory indicated that the transverse normal stresses were compressive. However, the finite element analysis yielded tensile transverse normal stresses near the free edge. Matrix cracks formed in the off-axis plies near the free edge where in-plane transverse stresses were tensile and had their greatest magnitude. The influence of the matrix crack on interlaminar stresses is also discussed.

Obrien, T. Kevin↗
NASA NTRS1989

A posteriori estimates for shear correction factors in multilayered composite cylinders

A predictor-corrector approach is presented for calculating the composite shear correction factors and analyzing multilayered composite cylinders. In the predictor phase a two-dimensional first-order shear deformation theory is used to predict the gross response characteristics of the cylinder (vibration frequencies, average through-the-thickness displacements, rotations, and transverse shear strain energy per unit area) as well as the in-plane strains and stresses in the thickness direction. The three-dimensional equilibrium equations and constitutive relations are then used to compute the transverse stresses and strains as well as the transverse shear factors, and they are also used to correct the predicted response quantities of the cylinder. For simply supported multilayered cylinders the response quantities obtained by using the proposed approach are shown to be in close agreement with three-dimensional elasticity solutions for a wide range of lamination and geometric parameters. Also, the potential of the proposed approach for use in conjunction with large-scale finite element models of composite cylinders is outlined.

Noor, Ahmed K.↗
NASA NTRS1990

Dynamic response of anisotropic composite panels to time-dependent external excitations

This paper deals with the dynamic response of anisotropic laminated composite flat panels exposed to sonic boom and explosive blast-type loadings. The pertinent governing equations incorporating transverse shear deformation, transverse normal stress, the higher order effects as well as the viscous structural damping are solved by using the integral-transform technique. The obtained results are compared with their counterparts obtained within the framework of the first order transverse shear deformation and the classical plate theories and some conclusions concerning their range of applicability are outlined. The paper also contains a detailed analysis of the influence played by the various parameters characterizing the considered pressure pulses as well as the material and geometry of the plate.

Librescu, L.↗
NASA NTRS1991

Formulation of an elastodynamic theory of laminated shear-deformable flat panels

A higher order shear deformation theory (HSDT) is developed, designed to preserve all the characteristics of the first-order shear deformation theory (FSDT) with regard to the number of the unknowns and to the order of the associated governing equations. In contrast to the FSDT theory, the HSDT theory is characterized by a parabolic distribution of transverse shear stress and strain components throughout the laminate thickness thus eliminating the need of a shear correction factor, and by the elimination of the contradictory assumption of the FSDT concerning the simultaneous consideration of zero transverse normal stress and zero transverse normal strain. The governing equations of the HSDT are presented, and several special applications are discussed.

Librescu, L.↗
NASA NTRS1991

Apparatus for precision focussing and positioning of a beam waist on a target

The invention relates to optical focussing apparatus and, more particularly, to optical apparatus for focussing a highly collimated Gaussian beam which provides independent and fine control over the focus waist diameter, the focus position both along the beam axis and transverse to the beam, and the focus angle. A beam focussing and positioning apparatus provides focussing and positioning for the waist of a waisted beam at a desired location on a target such as an optical fiber. The apparatus includes a first lens, having a focal plane f sub 1, disposed in the path of an incoming beam and a second lens, having a focal plane f sub 2 and being spaced downstream from the first lens by a distance at least equal to f sub 1 + 10 f sub 2, which cooperates with the first lens to focus the waist of the beam on the target. A rotatable optical device, disposed upstream of the first lens, adjusts the angular orientation of the beam waist. The transverse position of the first lens relative to the axis of the beam is varied to control the transverse position of the beam waist relative to the target (a fiber optic as shown) while the relative axial positions of the lenses are varied to control the diameter of the beam waist and to control the axial position of the beam waist. Mechanical controllers C sub 1, C sub 2, C sub 3, C sub 4, and C sub 5 control the elements of the optical system. How seven adjustments can be made to correctly couple a laser beam into an optical fiber is illustrated. Prior art systems employing optical techniques to couple a laser beam into an optical fiber or other target simply do not provide the seven necessary adjustments. The closest known prior art, a Newport coupler, provides only two of the seven required adjustments.

Lynch, Dana H.↗
NASA NTRS1991

A constitutive relation for the viscous flow of an oriented fiber assembly

A constitutive relation for an equivalent, homogeneous fluid is developed for the anisotropic viscous flow of an oriented assembly of discontinuous fibers suspended in a viscous fluid. The anisotropic viscous compliance matrix can be expressed in terms of three constants by assuming the equivalent fluid to be incompressible and the microstructure to have axial symmetry (transversely isotropic). By means of a micromechanics analysis, the three terms of the constitutive relation are expressed in terms of the viscosity of the matrix fluid, the fiber aspect ratio, and the fiber volume fraction. A comparison of the viscosity terms reveals that the elongational viscosity in the fiber direction varies as the square of the fiber aspect ratio and a complex function of the fiber volume fraction. Furthermore, the ratio of the axial elongational viscosity to the transverse elongational viscosity and both axial and transverse shear viscosities was shown to be 10 exp 4 - 10 exp 6 for fiber aspect ratio of 100-1000, except at extreme values of the fiber volume fraction.

Pipes, R. B.↗
NASA NTRS1992

Micromechanical combined stress analysis: MICSTRAN, a user manual

Composite materials are currently being used in aerospace and other applications. The ability to tailor the composite properties by the appropriate selection of its constituents, the fiber and matrix, is a major advantage of composite materials. The Micromechanical Combined Stress Analysis (MICSTRAN) code provides the materials engineer with a user-friendly personal computer (PC) based tool to calculate overall composite properties given the constituent fiber and matrix properties. To assess the ability of the composite to carry structural loads, the materials engineer also needs to calculate the internal stresses in the composite material. MICSTRAN is a simple tool to calculate such internal stresses with a composite ply under combined thermomechanical loading. It assumes that the fibers have a circular cross-section and are arranged either in a repeating square or diamond array pattern within a ply. It uses a classical elasticity solution technique that has been demonstrated to calculate accurate stress results. Input to the program consists of transversely isotropic fiber properties and isotropic matrix properties such as moduli, Poisson's ratios, coefficients of thermal expansion, and volume fraction. Output consists of overall thermoelastic constants and stresses. Stresses can be computed under the combined action of thermal, transverse, longitudinal, transverse shear, and longitudinal shear loadings. Stress output can be requested along the fiber-matrix interface, the model boundaries, circular arcs, or at user-specified points located anywhere in the model. The MICSTRAN program is Windows compatible and takes advantage of the Microsoft Windows graphical user interface which facilitates multitasking and extends memory access far beyond the limits imposed by the DOS operating system.

Naik, R. A.↗
NASA NTRS1992

Palinspastic reconstruction of southeastern California and southwestern Arizona for the middle Miocene

A paleogeographic reconstruction of southeastern California and southwestern Arizona at 10 Ma was made based on available geologic and geophysical data. Clockwise rotation of 39 deg was reconstructed in the eastern Transverse Ranges, consistent with paleomagnetic data from late Miocene volcanic rocks, and with slip estimates for left-lateral faults within the eastern Transverse Ranges and NW-trending right lateral faults in the Mojave Desert. This domain of rotated rocks is bounded by the Pinto Mountain fault on the north. In the absence of evidence for rotation of the San Bernardino Mountains or for significant right slip faults within the San Bernardino Mountains, the model requires that the late Miocene Pinto Mountain fault become a thrust fault gaining displacement to the west. The Squaw Peak thrust system of Meisling and Weldon may be a western continuation of this fault system. The Sheep Hole fault bounds the rotating domain on the east. East of this fault an array of NW-trending right slip faults and south-trending extensional transfer zones has produced a basin and range physiography while accumulating up to 14 km of right slip. This maximum is significantly less than the 37.5 km of right slip required in this region by a recent reconstruction of the central Mojave Desert. Geologic relations along the southern boundary of the rotating domain are poorly known, but this boundary is interpreted to involve a series of curved strike slip faults and non-coaxial extension, bounded on the southeast by the Mammoth Wash and related faults in the eastern Chocolate Mountains. Available constraints on timing suggest that Quaternary movement on the Pinto Mountain and nearby faults is unrelated to the rotation of the eastern Transverse Ranges, and was preceded by a hiatus during part of Pliocene time which followed the deformation producing the rotation. The reconstructed Clemens Well fault in the Orocopia Mountains, proposed as a major early Miocene strand of the San Andreas fault, projects eastward towards Arizona, where early Miocene rocks and structures are continuous across its trace. The model predicts a 14 deg clockwise rotation and 55 km extension along the present trace of the San Andreas fault during late Miocene and early Pliocene time. Palinspastic reconstructions of the San Andreas system based on this proposed reconstruction may be significantly modified from current models.

Richard, Stephen M.↗
NASA NTRS1992

Vibration of compressively loaded shear deformable flat panels exhibiting initial geometric imperfections

Numerical study results are presented which indicate that, in the prebuckling range, the natural frequency predicted by the shear-deformable plate theory is smaller than that associated with its transverse-shear rigid counterpart. The opposite behavior occurs in the postbuckling/postcritical range. With increasing transverse shear flexibility, greater differences between frequencies predicted by shear deformation and infinitely rigid transverse shear theories emerge. The character of in-plane boundary conditions is a determinant of both increasing/decreasing buckling loads and increasing/decreasing vibratory frequencies.

Librescu, L.↗
NASA NTRS1993

Gravitational microlensing - The effect of random motion of individual stars in the lensing galaxy

We investigate the influence of random motion of individual stars in the lensing galaxy on the light curve of a gravitationally lensed background quasar. We compare this with the effects of the transverse motion of the galaxy. We find that three-dimensional random motion of stars with a velocity dispersion sigma in each dimension is more effective in producing 'peaks' in a microlensed light curve by a factor a about 1.3 than motion of the galaxy with a transverse velocity v(t) = sigma. This effectiveness parameter a seems to depend only weakly on the surface mass density. With an assumed transverse velocity of v(t) = 600 km/s of the galaxy lensing the QSO 2237+0305 and a measured velocity dispersion of sigma = 215 km/s, the expected rate of maxima in the light curves calculated for bulk motion alone has to be increased by about 10 percent due to the random motion of stars. As a consequence, the average time interval Delta t between two high-magnification events is smaller than the time interval Delta(t) bulk, calculated for bulk motion alone, Delta t about 0.9 Delta(t) bulk.

Kundic, Tomislav↗
NASA NTRS1993

Nonlinear growth of dynamical overstabilities in blast waves

The numerical gasdynamics code ZEUS-2D is used to directly model the dynamical overstabilities in blast waves. The linear analysis is confirmed by perturbing a blast wave with a low-amplitude eigenfunction of the overstability. The amplitude of the perturbations is increased in order to determine the nonlinear behavior of the overstabilities. The overstability is found to saturate due to weak transverse shocks in the shell. Transverse velocities in the dense shell reach the postshock sound speed, and high-density regions with sizes of the order of the shell thickness form. Transverse oscillations continue even after saturation. This confirms and explains the damping of the overstability experimentally discovered by Grun et al. (1991).

Mac Low, Mordecai-Mark↗
NASA NTRS1993

Streamwise vortex meander in a plane mixing layer

The present experimental study was conducted in order to determine the existence of streamwise vortex meander in a mixing layer, and if present, its significance on the measured properties. The dependence of the velocity cross-correlation on the fixed probe location was shown to be a good indicator of the stationarity of the streamwise vortex location. The cross-correlation measurements obtained here indicate that spanwise meander is negligible, although transverse apparent meander (normal to the plane of the mixing layer) was indicated. The transverse meander, exemplified by the elliptical shape of the mean streamwise vorticity contours, was expected, since the streamwise vorticity in the braid region is essentially inclined, with respect to the streamwise direction. These conclusions were supported by results of estimated spanwise profiles of the transverse velocity component. The balance of evidence suggests that the measured mean streamwise vorticity decay is representative of the decay of the vorticity rather than an artifact of meander.

Leboeuf, Richard L.↗
NASA NTRS1993

Ultrasonic assessment of interfacial oxidation damage in ceramic matrix composites

A new approach to characterizing oxidation damage in ceramic matrix composites using ultrasonic techniques is proposed. In this approach, the elastic constants of the composite are determined nondestructively by measuring the angular dependence of both longitudinal and transverse wave velocities. A micromechanical model for composites with anisotropic constituents is used to find the anisotropic properties of an effective fiber, which is a combination of the fiber and the interface. Interfacial properties are extracted from the properties of this effective fiber by analyzing the difference between effective and actual fiber properties. Unidirectional /0/28 SiC/Si3N4 composites with 30 percent fiber volume fraction and 30 percent matrix porosity are used. The samples are exposed in a flowing oxygen environment at elevated temperatures, up to 1400 C, for 100 hours and then measured by ultrasonic methods at room temperature. The Young's modulus in the fiber direction of the sample oxidized at 600 C decreased significantly but it was unchanged for samples oxidized at temperatures above 1200 C. The transverse moduli obtained from ultrasonic measurements decrease continuously up to 1200 C. The shear stiffnesses show behavior similar to the transverse moduli. The effective elastic moduli of the interfacial carbon coating are determined from the experimental data, and their change due to thermal oxidation is discussed.

Chu, Y. C.↗
NASA NTRS1942

Tensile and pack compressive tests of some sheets of aluminum alloy, 1025 carbon steel, and chromium-nickel steel

Tensile and compressive stress-strain curves, stress-deviation curves, and secant modulus-stress curves are given for longitudinal and transverse specimens of 17S-T, 24S-T, and 24S-RT aluminum-alloy sheet in thicknesses from 0.032 to 0.081 inch, 1025 carbon steel sheet in thicknesses of 0.054 and 0.120 inch, and chromium-nickel steel sheet in thicknesses form 0.020 to 0.0275 inch. Significant differences were found between the tensile and the compressive stress-strain curves, and also the corresponding corollary curves; similarly, differences were found between the curves for the longitudinal and transverse directions. These differences are of particular importance in considering the compressive strength of aircraft structures made of thin sheet. They are explored further for the case of compression by giving tangent modulus-stress curves in longitudinal and transverse compression and dimensionless curves of the ratio of tangent modulus to Young's modulus and of the ratio of reduced modulus for a rectangular section to Young's modulus, both plotted against the ratio of stress to secant yield strength.

Atchison, C S↗
NASA NTRS1950

Small bending and stretching of sandwich-type shells

A theory has been developed for small bending and stretching of sandwich-type shells. This theory is an extension of the known theory of homogeneous thin elastic shells. It was found that two effects are important in the present problem, which are not normally of importance in the theory of curved shells: (1) the effect of transverse shear deformation and (2) the effect of transverse normal stress deformation. The first of these two effects has been known to be of importance in the theory of plates and beams. The second effect was found to occur in a manner which is typical for shells and has no counterpart in flat-plate theory. The general results of this report have been applied to the solution of problems concerning flat plates, circular rings, circular cylindrical shells, and spherical shells. In each case numerical examples have been given, illustrating the magnitude of the effects of transverse shear and normal stress deformation.

Reissner, Eric↗