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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Variational theorems for superimposed motions in elasticity, with application to beams

Variational theorems are presented for a theory of small motions superimposed on large static deformations and governing equations for prestressed beams on the basis of 3-D theory of elastodynamics. First, the principle of virtual work is modified through Friedrichs's transformation so as to describe the initial stress problem of elastodynamics. Next, the modified principle together with a chosen displacement field is used to derive a set of 1-D macroscopic governing equations of prestressed beams. The resulting equations describe all the types of superimposed motions in elastic beams, and they include all the effects of transverse shear and normal strains, and the rotatory inertia. The instability of the governing equations is discussed briefly.

Doekmeci, M. C.↗
NASA NTRS1976

Response of long, flexible cantilever beams applied root motions

Results are presented for an analysis of the response of long, flexible cantilever beams to applied root rotational accelerations. Maximum values of deformation, slope, bending moment, and shear are found as a function of magnitude and duration of acceleration input. Effects of tip mass and its eccentricity and rotatory inertia on the response are also investigated. It is shown that flexible beams can withstand large root accelerations provided the period of applied acceleration can be kept small relative to the beam fundamental period.

Fralich, R. W.↗
NASA NTRS1976

A study on the forced vibration of a Timoshenko beam

By using Galerkin's variational method an approximate solution is developed for a system of two differential equations with linear partial derivatives of the second order. This system of differential equations corresponds to the physical model, known in the literature as the Timoshenko Beam. The results obtained are applied to two particular cases representing respectively: the case of a beam with a rectangular section, with a constant height and a basis with a linear variation; and the case of a beam with a constant basis and a height with cubic variation.

Zainea, B.↗
NASA NTRS1976

Considerations for the application of finite element beam modeling to vibration analysis of flight vehicle structures

The manner of representing a flight vehicle structure as an assembly of beam, spring, and rigid-body components for vibration analysis is described. The development is couched in terms of a substructures methodology which is based on the finite-element stiffness method. The particular manner of employing beam, spring, and rigid-body components to model such items as wing structures, external stores, pylons supporting engines or external stores, and sprung masses associated with launch vehicle fuel slosh is described by means of several simple qualitative examples. A detailed numerical example consisting of a tilt-rotor VTOL aircraft is included to provide a unified illustration of the procedure for representing a structure as an equivalent system of beams, springs, and rigid bodies, the manner of forming the substructure mass and stiffness matrices, and the mechanics of writing the equations of constraint which enforce deflection compatibility at the junctions of the substructures. Since many structures, or selected components of structures, can be represented in this manner for vibration analysis, the modeling concepts described and their application in the numerical example shown should prove generally useful to the dynamicist.

Kvaternik, R. G.↗
NASA NTRS1976

Six-beam models in radiative transfer theory

A critical analysis is given of the applicability of six-beam models to radiative transfer in particulate materials. The method of introducing transverse scattering in these models is shown to cause fundamental difficulties in the case of physically plausible phase functions; in particular, the effective absorptivity is abnormally large and thus results in incorrect reflectances and transmittances. Six-beam calculations for several media are compared with accurate solutions, with Chu-Churchill two-beam results, and with a simple modification to the Eddington approximation, the last being generally superior over a wide range of conditions.

Meador, W. E.↗
NASA NTRS1976

Applications of beam-foil spectroscopy to atomic collisions in solids

Some selected papers presented at the Fourth International Conference on Beam-Foil Spectroscopy, whose results are of particular pertinence to ionic collision phenomena in solids, are reviewed. The topics discussed include solid target effects and means of surmounting them in the measurement of excited projectile ion lifetimes for low-energy heavy element ions; the electron emission accompanying the passage of heavy particles through solid targets; the collision broadening of X rays emitted from 100 keV ions moving in solids; residual K-shell excitation in chlorine ions penetrating carbon; comparison between 40 MeV Si on gaseous SiH4 targets at 300 mtorr and 40 MeV Si on Al; and the emergent surface interaction in beam-foil spectroscopy. A distinct overlap of interests between the sciences of beam-foil spectroscopy and atomic collisions in solids is pointed out.

Sellin, I. A.↗
NASA NTRS1978

Laser beam color separator

Multiwavelength laser beam is separated into series of parallel color beams using prism and retroreflector. Setup is inexpensive and needs no critical adjustments. It can incorporate several prisms to increase dispersion and reduce overall size. Transmission grating can be used instead of prism with sacrifice in efficiency. Spatial filter can remove unwanted beams.

Franke, J. M.↗
NASA NTRS1978

RF beam center location method and apparatus for power transmission system

The receiving element in wireless power transmission systems intercepts the greatest possible portion of the transmitted energy beam. Summing the output energy of all receivers in a planar array makes it possible to determine the location of the center of energy of the incident beam on a receiving array of antenna elements so that the incident beam is in the microwave region.

Dickinson, R. M.↗
NASA NTRS1978

A simple element for multilayer beams in NASTRAN thermal stress analysis

In the application of NASTRAN, structural members are usually represented by bar elements with multipoint constraint cards to enforce the interface conditions. While this is a very powerful method in principle, it was found that in practice the process for specification of constraints became tedious and error prone, unless the geometry was simple and the number of grid points low. An alternative approach was found within the framework of the NASTRAN program. This approach made use of the idea that a thermal distortion in a multilayer beam may be similar to a homogeneous beam with a thermal gradient across the cross section. The exact mathematical derivation for the equivalent beam, and all the necessary formulae for the equivalent parameters in NASTRAN analysis are presented. Some numerical examples illustrate the simplicity and ease of this approach for finite element analysis.

Chen, W. T.↗
NASA NTRS1977

Observation of beam-generated VLF hiss in a large laboratory plasma

The paper describes a lab experiment on a beam-plasma instability under conditions for which the theory of VLF hiss is applicable. An energetic electron beam is injected into a cold, dense, almost collisionless magnetoplasma. Beam and plasma dimensions are so large that oblique whistler wave propagation and growth are readily observable. The observations of frequency and wave number spectra, growth rate, and instability level are consistent with theory and satellite observation in regions of electron precipitation.

Stenzel, R. L.↗
NASA NTRS1978

Experiments on current-driven three-dimensional ion sound turbulence. I - Return-current limited electron beam injection. II - Wave dynamics

Pulsed electron beam injection into a weakly collisional magnetized background plasma is investigated experimentally; properties of the electron beam and background plasma, as well as the low-frequency instabilities and wave dynamics, are discussed. The current of the injected beam closes via a field-aligned return current of background electrons. Through study of the frequency and wavenumber distribution, together with the electron distribution function, the low-frequency instabilities associated with the pulsed injection are identified as ion acoustic waves driven unstable by the return current. The frequency cut-off of the instabilities predicted from renormalized plasma turbulence theory, has been verified experimentally.

Stenzel, R. L.↗
NASA NTRS1978

Nonlinear flap-lag-axial equations of a rotating beam with arbitrary precone angle

In an attempt both to unify and extend the analytical basis of several aspects of the dynamic behavior of flexible rotating beams, the second-degree nonlinear equations of motion for the coupled flapwise bending, lagwise bending, and axial extension of an untwisted, torsionally rigid, nonuniform, rotating beam having an arbitrary angle of precone with the plane perpendicular to the axis of rotation are derived using Hamilton's principle. The derivation of the equations is based on the geometric nonlinear theory of elasticity and the resulting equations are consistent with the assumption that the strains are negligible compared to unity. No restrictions are imposed on the relative displacements or angular rotations of the cross sections of the beam other than those implied by the assumption of small strains. Illustrative numerical results, obtained by using an integrating matrix as the basis for the method of solution, are presented both for the purpose of validating the present method of solution and indicating the range of applicability of the equations of motion and the method of solution.

Kvaternik, R. G.↗
NASA NTRS1978

Continuum models for beam-like and plate-like lattice structures

A simple, rational approach is presented for developing continuum models for large repetitive beam-like and plate-like lattices with arbitrary configurations subjected to static, thermal and dynamic loadings. The continuum models for these structures are shear flexible beams and plates. They account for local effects in the repeating element of the actual structure and are characterized by their thermoelastic strain and kinetic energies from which the equations of motion and constitutive relations can be derived. The procedure for developing the expressions for thermoelastic strain and kinetic energies of the continuum involves introducing basic assumptions regarding the variation of the temperature, displacement and strain components in one or two directions (for plate-like and beam-like lattices) and obtaining effective thermoelastic and dynamic coefficients of the continuum in terms of material properties and geometry of the original lattice structure. The high accuracy of the continuum models developed is demonstrated by means of numerical examples.

Noor, A. K.↗
NASA NTRS1979

Development of a beam builder for automatic fabrication of large composite space structures

The composite material beam builder which will produce triangular beams from pre-consolidated graphite/glass/thermoplastic composite material through automated mechanical processes is presented, side member storage, feed and positioning, ultrasonic welding, and beam cutoff are formed. Each process lends itself to modular subsystem development. Initial development is concentrated on the key processes for roll forming and ultrasonic welding composite thermoplastic materials. The construction and test of an experimental roll forming machine and ultrasonic welding process control techniques are described.

Bodle, J. G.↗
NASA NTRS1979

Latitudinal beaming of Jupiter's low frequency radio emissions

By comparing RAE-1 and IMP-6 satellite measurements of Jupiter's radio emission near 1MHz with recent Voyager-1 and 2 observations in the same frequency range, the properties of the low frequency radiation pattern over a 10 deg range of latitudes with respect to the Jovian rotation equator can be studied. These observations, which cover a wider latitudinal range than is possible from the earth, are consistent with many aspects of earlier ground-based measurements used to infer a sharp beaming pattern for the decameter wavelength emissions. Marked, systematic changes are found in the statistical occurrence probability distributions with system 3 central meridian longitude as the jovigraphic latitude of the observer changes over this range. Simultaneous observations by the two Voyager spacecraft suggest that the instantaneous beam width may be no more than a few degrees at times. The new hectometer-wave results can be interpreted in terms of a narrow, curved sheet at a fixed magnetic latitude into which the emission is beamed to escape the planet.

Alexander, J. K.↗
NASA NTRS1979

Mapping of electrical potential distribution with charged particle beams

Potentials were measured using a beam of soft X-rays in air at 2 x 10 to the -5 power Torr. Ions were detected by a continuous-dynode electron multiplier after they passed through a retarding field. Ultimate resolution depends upon the diameter of the X-ray beam which was 3 mm. When the fields in the region of interest were such to disperse the ions, only a small fraction were detected and the method of measurement was not very reliable. Yet reasonable data could be collected if the ions traveled in parallel paths toward the detector. Development should concentrate on increasing the aperture of the detector from the pinhole which was used to something measured in centimeters. Also increasing the strength of the source would provide a stronger signal and more reliable data. Measurements were made at an estimated ion current to 10 to the -15 power A from a 10 cm length of the X-ray beam, this current being several orders of magnitude below what would have a perturbing effect on the region to be measured. Consequently, the source strength can be increased and prospects for this method of measurement are good.

Robinson, J. W.↗
NASA NTRS1979

Possibilities and limitations of rod-beam theories

Rod-beam theories are founded on hypotheses such as Bernouilli's suggesting flat cross-sections under deformation. These assumptions, which make rod-beam theories possible, also limit the accuracy of their analysis. It is shown that from a certain order upward terms of geometrically nonlinear deformations contradict the rod-beam hypotheses. Consistent application of differential geometry calculus also reveals differences from existing rod theories of higher order. These differences are explained by simple examples.

Peterson, D.↗