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Results for “elastic shear waves”

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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Magnetomechanical damping in cryogenic TbDy

Vibration damping in polycrystalline TbDy alloys was studied at cryogenic temperatures. The material was prepared by cold-rolling to induce crystallographic texture, and was then heat-treated to relieve internal stress. Mechanical hysteretic losses were measured at various strains, frequencies, and loading configurations at 77 K. Some textured TbDy materials demonstrated 22.6% energy dissipation in mechanical measurements at low frequency (0.01 Hz) and a mean logarithmic decrement of 0.23 at a higher frequency (25 kHz). Ultrasonic velocities of longitudinal and shear elastic waves were measured on single and polycrystalline TbDy; little variation in ultrasonic velocities was found evenfor samples with large variation in crystallographic texture and magnetomechanical properties.

magnetomechanical TbDy↗
NASA NTRS1972

Elastic wave velocities and thermal diffusivities of Apollo 14 rocks.

The compressional- and shear-wave velocities of Apollo 14 lunar rocks 14311,50 and 14313,27 as functions of pressure up to 10 kb and the thermal diffusivity of sample 14311,50 over the temperature range 100 to 550 K have been measured. Both samples 14311 and 14313 are polymict fragmental rocks. The overall elastic and anelastic behavior of the Apollo 14 samples are similar to those of Apollo 11 and 12 samples; low velocity and low Q at pressures below 1 kb and rapid increase of velocity and Q with pressure are also typical of the Apollo 14 rocks. The available data of P- and S-wave velocities of lunar rocks show that Birch's law holds for the lunar rocks. The thermal diffusivity of a lunar rock in vacuum is found to be significantly lower than that in air at one atmospheric pressure.

Mizutani, H.↗
NASA NTRS1996

Effect of a hypergravity environment on cortical bone elasticity in rats

There is considerable interest in determining whether hypergravity can be used as a countermeasure for microgravity-induced bone loss. This study was conducted on 20 immature male rats in order to investigate possible elastic adaptations of cortical bone in rapidly growing rats exposed to chronic hypergravity. Ten rats were continuously centrifuged for 14 days at twice gravitational acceleration (2G) on a 12.75 foot radius centrifuge and 10 rats concurrently acted as stationary controls. The effect of hypergravity on the elastic characteristics of cortical bone was quantified via ultrasonic wave propagation. Propagation velocities of longitudinal and shear waves were measured through cubic cortical specimens from the posterior femoral diaphyses. Density was measured with an Archimedes' technique. The orthotropic elastic properties were calculated and used to compare the difference between groups. Results showed an average increase in both the Young's moduli (Eii, + 2.2%) and shear moduli (Gij, + 4.3%) with a statistically significant increase only in G12 (+15.7%, P = 0.046). The ratio of transverse to axial strain (Poisson's ratio, nuij) demonstrated statistically significant changes in nu12, nu21, nu13, and nu31 (P < 0.05). These findings suggest that although slight elastic changes were incurred via a hypergravity environment, the treatment level or duration in this study do not dramatically perturb the normal elastic behavior of cortical bone and that dramatic biomechanical differences noted in previous studies were due more to structural changes than material elasticity changes. Hypergravity applied post facto to a microgravity environment would offer further illucidation of this method as treatment for a degenerative spaceflight experience.

Non-NASA Center↗
NASA NTRS1989

Recent advances in engineering science; Proceedings of the A. Cemal Eringen Symposium, University of California, Berkeley, June 20-22, 1988

Various papers on recent advances in engineering science are presented. Some individual topics addressed include: advances in adaptive methods in computational fluid mechanics, mixtures of two medicomorphic materials, computer tests of rubber elasticity, shear bands in isotropic micropolar elastic materials, nonlinear surface wave and resonator effects in magnetostrictive crystals, simulation of electrically enhanced fibrous filtration, plasticity theory of granular materials, dynamics of viscoelastic media with internal oscillators, postcritical behavior of a cantilever bar, boundary value problems in nonlocal elasticity, stability of flexible structures with random parameters, electromagnetic tornadoes in earth's ionosphere and magnetosphere, helicity fluctuations and the energy cascade in turbulence, mechanics of interfacial zones in bonded materials, propagation of a normal shock in a varying area duct, analytical mechanics of fracture and fatigue.

Koh, Severino L.↗
NASA NTRS1994

Numerical simulations of upstream blocking, columnar disturbances, and bores in stably stratified shear flows over an obstacle

A two-dimensional, nonhydrostatic, elastic numerical model has been used to study the generation of gravity waves for a stably stratified shear flow over an obstacle. When a low-level wind shear is included in the simulation, we find that the predictions for noticeable upstream effects based on Froude number for a uniform flow are no longer accurate. Upstream effects are encountered in the form of upstream propagating columnar disturbances and internal bores away from the obstacle. The limited parameter space studies conducted in this study suggest that the ratio of the shear depth to the obstacle heigh (d/H), the obstacle aspect ratio (H/L), and the Froude number (U/NH) are instrumental in determining the strength and the existence of these upstream disturbances. Thus, the present theoretical and empirical understanding of the importance of the Froude number for determining the nature of upstream effects should be modified substantially to include additional nondimensional parameters when shear is present.

Chen, Chaing↗
NASA NTRS1973

Elasticity of water-saturated rocks as a function of temperature and pressure.

Compressional and shear wave velocities of water-saturated rocks were measured as a function of both pressure and temperature near the melting point of ice to confining pressure of 2 kb. The pore pressure was kept at about 1 bar before the water froze. The presence of a liquid phase (rather than ice) in microcracks of about 0.3% porosity affected the compressional wave velocity by about 5% and the shear wave velocity by about 10%. The calculated effective bulk modulus of the rocks changes rapidly over a narrow range of temperature near the melting point of ice, but the effective shear modulus changes gradually over a wider range of temperature. This phenomenon, termed elastic anomaly, is attributed to the existence of liquid on the boundary between rock and ice due to local stresses and anomalous melting of ice under pressure.

Takeuchi, S.↗
NASA NTRS1999

Elastic Moduli of Pyrolytic Boron Nitride Measured Using 3-Point Bending and Ultrasonic Testing

Three-point bending and ultrasonic testing were performed on a flat plate of PBN. In the bending experiment, the deformation mechanism was believed to be shear between the pyrolytic layers, which yielded a shear modulus, c (sub 44), of 2.60 plus or minus .31 GPa. Calculations based on the longitudinal and shear wave velocity measurements yielded values of 0.341 plus or minus 0.006 for Poisson's ratio, 10.34 plus or minus .30 GPa for the elastic modulus (c (sub 33)), and 3.85 plus or minus 0.02 GPa for the shear modulus (c (sub 44)). Since free basal dislocations have been reported to affect the value of c (sub 44) found using ultrasonic methods, the value from the bending experiment was assumed to be the more accurate value.

Kaforey, M. L.↗
NASA NTRS1985

The speed of wave-wave interactions in the atmosphere

Resonant wave-wave interactions are considered. Studies of the interaction coefficient show that rapid transfer of wave action can take place in the disjoined parts of the spectrum for three processes, namely: elastic scattering, parametric subharmonic instability and induced diffusion. Of the three processes, the vertical shear plays a role in two. The vertical shear of a moderate scale interacts through elastic scattering to make the spectrum vertically symmetric. On the other hand, the vertical shear of a large scale interacts through induced diffusion and is responsible for diffusion in k sub z space. When interacting with a vertical shear, it is known that the vertical shear acts as a catalyst and is not involved in energy transfer. Consequently, in both elastic scattering and induced diffusion, the vertical shear does not gain or lose energy. Through parametric subharmonic instability the more energetic large-scale waves are feeding energy into moderate and small scale waves of an elevation angle of 60 deg or larger.

Yeh, K. C.↗
NASA NTRS1971

Theoretical calculation of plane wave speeds for alkali metals under pressure.

Theoretical calculations of the variation with pressure of small amplitude plane wave speeds are performed for sodium and potassium at zero temperature. The results obtained for wave speeds associated with volume dependent second-order elastic coefficients show better agreement with experimental data than for wave speeds associated with shear dependent coefficients. This result is believed to be due to omission of the band structure correction to the strain energy density.

Eftis, J.↗
NASA NTRS1986

Ultrasonic determination of the elastic constants of the stiffness matrix for unidirectional fiberglass epoxy composites

The elastic constants of a fiberglass epoxy unidirectional composite are determined by measuring the phase velocities of longitudinal and shear stress waves via the through transmission ultrasonic technique. The waves introduced into the composite specimens were generated by piezoceramic transducers. Geometric lengths and the times required to travel those lengths were used to calculate the phase velocities. The model of the transversely isotropic medium was adopted to relate the velocities and elastic constants.

Marques, E. R. C.↗
NASA NTRS1972

Rayleigh wave effects in an elastic half-space.

Consideration of Rayleigh wave effects in a homogeneous isotropic linearly elastic half-space subject to an impulsive uniform disk pressure loading. An approximate formula is obtained for the Rayleigh wave effects. It is shown that the Rayleigh waves near the center of loading arise from the portion of the dilatational and shear waves moving toward the axis, after they originate at the edge of the load disk. A study is made of the vertical displacement due to Rayleigh waves at points on the axis near the surface of the elastic half-space.

Aggarwal, H. R.↗
NASA NTRS1994

Hydrogen and related materials at high density: Physics, chemistry and planetary implications

Recent studies of low-Z molecular materials including hydrogen to multimegabar pressures (less than 300 GPa) have uncovered a range of phenomena relevant to understanding the nature of the interiors of the outer planets and their satellites. Synchrotron x ray diffraction measurements (to 42 GPa) have been used to determine the crystal structure of the solid (hexagonal-close packed) and equation of state. Sound velocities in fluid and solid hydrogen (to 24 GPa) have been inverted to obtain elastic constants and aggregate bulk and shear moduli. In addition, an improved intermolecular potential has been determined which fits both static and shock-wave data. Use of the new potential for the molecular envelope of Jupiter suggests the need for major revisions of existing Jovian models or a reanalysis of reported free oscillations for the planet. Studies at higher pressures (greater than 100 GPa) reveal a sequence of pressure-induced symmetry-breaking transitions in molecular hydrogen, giving rise to three high-pressure phases (1, 2, and 3). Phase 1 is the rotationally disordered hcp phase which persists from low pressure to well above 100 GPa at high temperature (e.g., 300 K). Phase 2 is a low-temperature, high-pressure phase (transition at 100 GPa and 77 K in H2) with spectral features indicative of partial rotational ordering and crystallographic distortion. The transition to Phase 3 at 150 GPa is accompanied by a weakening of the molecular bond, gradual changes in orientational ordering, strong enhancement of the infrared intramolecular vibrational absorption, and strong intermolecular interactions similar to those of ambient-pressure network solids. Studies of the phase diagram reveal a triple point near 130 K and 160 GPa. Higher pressure measurements of vibrational spectra place a lower bound of approximately 250 GPa on the predicted transition pressure for dissociation of molecular hydrogen to form a monatomic metal.

Hemley, R. J.↗
NASA NTRS1973

BUCLAP2: A computer program for instability analysis of laminated long plates subjected to combined inplane loads

This program description document describes the program structure and the design details of a CDC 6600 FORTRAN 4 digital computer program, BUCLAP2, which uses minimum energy principles to do an elastic stability analysis of curved and flat laminated rectangular long plates subjected to combined inplane normal and shear loads. Given the geometry, the material properties, and slected boundary conditions for the plate element, the program calculates the minimum buckling load for various wave lengths. The two parallel ends of the program calculates the minimum buckling load for various wave lengths. The two parallel ends of the long plate must be simply supported and arbitrary elastic boundary conditions may be imposed along either one or both external longitudinal sides. For guide to program use, see

Halstead, D. W.↗
NASA NTRS1986

Measurement of the speed of sound in ice

The data presented demonstrate the gathering of sound speed measurements in refrigerated ice, using both compressional and shear waves, on the basis of ice thickness and sound wave time-of-travel parameters. Clear and sharp echo reflections were obtained for both compressional and shear waves at the aluminum/ice and ice/aluminum interfaces for ice at -26 C; ice formed at various conditions can have different sound speeds, depending on the density and elastic constants.

Smith, A. C.↗
NASA NTRS1991

Neutron starquake model for gamma-ray bursts

A neutron starquake model for gamma ray bursts is presented and critically analyzed. It is suggested that a slowly accreting neutron star may develop density inversions deep in its crust. These unstable layers, may be subject to elastic Rayleigh-Taylor instability which can liberate sufficient gravitational and perhaps also nuclear energy to account for individual bursts. Energy can be transported to the surface by shear waves and slowly transmitted into the magnetosphere as relativistic Alfven waves. Particle acceleration and gamma ray emission from the outer magnetosphere should ensue. Some observational implications are mentioned.

Blandford, R. D.↗