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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

A test of alternative Caribbean plate relative motion models

The new NUVEL-1 data set for global relative plate motions is used here to discriminate between the two prevailing models for Caribbean plate motion. One model, by Jordan (1975), assumes that North America-Caribbean motion is reflected by the spreading rate inferred from magnetic anomalies at the Cayman Spreading Center and the azimuths of nearby transforms. The other model, by Sykes et al. (1982), uses rates and azimuths inferred from the geometry of the Lesser Antilles Wadati-Benioff zone. Overall, it is found that the data fit the Jordan geometry better, that the data used in global plate motion models are more suitable than rates and azimuths inferred from the geometry of the Wadati-Benioff zone for determining relative motions, and that incorporation of all relevant plate boundaries is essential.

Stein, Seth↗
NASA NTRS1987

Rapidly solidified metal coatings by peen plating

Specimens of 7075-T6 aluminum alloy were peen plated with rapidly solidified tin-lead and aluminum powders, and the cross-sections of the coated specimens were examined by light and electron microscopy. The properties of the peen plated specimens were also compared with those of shot peened specimens without any coating. It is found that peen plating with rapidly solidified metals improves the fatigue properties of the coated samples to a greater extent than shot peening alone. Specimens of 7075-T6 alloy peen plated with rapidly solidified tin-lead and aluminum exhibited better fatigue resistance than shot peened specimens in both air and salt water.

Chu, H. P.↗
NASA NTRS1988

Analysis of symmetric cross-ply laminated elastic plates using a higher-order theory. II - Buckling and free vibration

A previously developed higher-order plate theory and a technique based on the state space concept are used to investigate free vibration and buckling problems of rectangular cross-ply laminated plates. Unified results are presented for the case of arbitrary boundary conditions on two opposite edges. Good agreement is obtained with previous data for simply supported edge conditions. It is pointed out that classical laminated plate theory tends to overpredict both eigenfrequencies and buckling loads, leading to an increase of the degree of orthotropicity of individual layers and of the thickness ratio of the plate.

Khdeir, A. A.↗
NASA NTRS1988

Measurements of matrix cracking and delamination caused by impact on composite plates

Tests were performed measuring the locations and extents of delaminations and matrix crackings in 3-in. by 3-in. Fiberite T300/934 graphite/epoxy plates impacted by 1/2-in. diameter aluminum spheres. Plates with six ply orientations were tested. The impactor speeds ranged from 700 to 1300 in/sec. The plates were inspected by C-scan both before and after impact. The plates were also dissected after impact and examined visually for damage. In addition, the longitudinal and transverse tensile, compressive, and shear properties of the material were measured.

Wu, Hsi-Yung T.↗
NASA NTRS1988

Plate-driving forces over the Cenozoic era

Under the assumptions of a dynamical balance between active torques and plate drag as the passive torque, plate reconstructions have been used to determine plate torques for the Cenozoic era. A torque balance equation is derived in which slab-pull and ridge-push torques are proportional to boundary chord vectors, with the weights depending on powers of the subduction velocity at the middle of the chords. The unique angular velocity satisfying the torque balance requirements is obtained for each plate. Torques are found to be fairly stable throughout the Cenozoic, with the misfit between the balanced torque and drag torque increasing systematically for earlier reconstructions.

Jurdy, Donna M.↗
NASA NTRS1988

The stress field in an edge cracked plate in antiplane deformation

The antiplane strain problem of an edge cracked elastic plate subjected to a surface displacement load is solved, and the surface line loads are obtained using conformal mapping techniques. Results are presented for the yield, stress, strain and displacement distributions, and stress intensity factors in the plate. A superposition technique and Green's functions are used to determine the strain field on the upper plate surface due to arbitrarily applied tensile stress on the lower plate surface.

Li, V. C.↗
NASA NTRS1989

Transient dynamic analysis of laminated composite plates subjected to transverse impact

The dynamic response of a composite laminate plate to transverse impact loading is simulated numerically. An FEM scheme based on an eight-point brick element and three-dimensional linear elasticity theory is employed, assuming that the laminate layers are homogeneous and orthotropic. The derivation of the model is given in detail; the numerical implementation is briefly outlined; and results for the impact of 0.5-inch-diameter Al spheres at 500, 1000, and 1500 inch/sec on clamped or simply supported 3 x 3-inch plates made of T300/934 graphite/epoxy prepreg with ply orientation (0/-45/45/90)2S are presented in extensive graphs. The model is shown to provide accurate information on the histories of impactor-plate contact force; impactor displacement and velocity; and displacement, velocity, strain, and stress throughout the plate.

Wu, Hsi-Yung T.↗
NASA NTRS1989

Closure of the Africa-Eurasia-North America plate motion circuit and tectonics of the Gloria fault

The current motions of the African, Eurasian, and North American plates are examined. The problems addressed include whether there is resolvable motion of a Spitsbergen microplate, the direction of motion between the African and North American plates, whether the Gloria fault is an active transform fault, and the implications of plate circuit closures for rates of intraplate deformation. Marine geophysical data and magnetic profiles are used to construct a model which predicts about 4 mm/yr slip across the Azores-Gibraltar Ridge, and west-northwest convergence near Gibraltar. The analyzed data are consistent with a rigid plate model with the Gloria fault being a transform fault.

Argus, Donald F.↗
NASA NTRS1989

Random vibrations of laminated plates modeled within the first order shear deformation theory

This paper deals with the dynamic response of anisotropic laminated composite plates to stationary random excitation. The theory of laminated plates used here includes shear deformation and rotary inertia effects in the same manner as Mindlin's theory for isotropic homogeneous plates. Two cases of random pressure fields are considered in this analysis. In the first case, it is modeled as a uniformly distributed load, random in time with an exponential decaying correlation function, while in the second one as a jet noise, fully correlated in one direction and homogeneous in the perpendicular one. The analysis presented herein, as well as the obtained response characteristics, expressed in terms of mean-squares and angle-dependent mean-squares, may be useful in the design of composite plates subjected to random pressure fields and in the evaluation of their reliability.

Cederbaum, G.↗
NASA NTRS1989

Experiments on structural control of sound transmitted through an elastic plate

Active control of sound transmission through an elastic plate has been experimentally studied. The control was applied directly to the plate by point force vibration inputs, and the error information and performance was measured in the radiated acoustic field by discrete microphones. Test cases were studied in which the error sensors were accelerometers mounted on the plate. A time-domain least-mean-squares adaptive algorithm was used as the basis of the control system design. The results showed an impressive increase in plate transmission loss over a wide range of frequencies with a maximum of two control shakers. The advantages of using error information derived from the radiated acoustic field rather than structural response is demonstrated. The mechanisms behind the efficiency of this control approach are discussed.

Fuller, C. R.↗
NASA NTRS1990

Making More Efficient Use Of Battery-Plate Mass

Improved active material for positive plate of lead/acid electric storage battery made with additional porosity to give electrolyte access to larger plate-surface area. 65 to 68 percent of active mass of plate used to generate electric current. Batteries with new plate material offer extremely long cycle life.

Rowlette, John J.↗
NASA NTRS1990

Innovative design of composite structures: The use of curvilinear fiber format to improve buckling resistance of composite plates with central circular holes

The gains in buckling performance are explored that can be achieved by deviating from the conventional straightline fiber format and considering the situation whereby the fiber orientation in a layer, or a group of layers, can vary from point to point. The particular situation studied is a simply supported square plate with a centrally located hole loaded in compression. By using both a sensitivity analysis and a gradient-search technique, fiber orientation in a number of regions of the plate are selected so as to increase the buckling load relative to baseline straightline designs. The sensitivity analysis is used to determine which regions of the plate have the most influence on buckling load, and the gradient search is used to find the design that is believed to represent the absolute maximum buckling load for the conditions prescribed. Convergence studies and sensitivity of the final design are discussed. By examining the stress resultant contours, it is shown how the curvilinear fibers move the load away from the unsupported hole region of the plate to the supported edges, thus increasing the buckling capacity. The tensile capacity of the improved buckling design is investigated, and it is shown that both tensile capacity and buckling capacity can be improved with the curvilinear fiber concept.

Hyer, M. W.↗
NASA NTRS1990

Modeling of composite beams and plates for static and dynamic analysis

A rigorous theory and corresponding computational algorithms was developed for a variety of problems regarding the analysis of composite beams and plates. The modeling approach is intended to be applicable to both static and dynamic analysis of generally anisotropic, nonhomogeneous beams and plates. Development of a theory for analysis of the local deformation of plates was the major focus. Some work was performed on global deformation of beams. Because of the strong parallel between beams and plates, the two were treated together as thin bodies, especially in cases where it will clarify the meaning of certain terminology and the motivation behind certain mathematical operations.

Hodges, Dewey H.↗
NASA NTRS1989

Modal interaction in postbuckled plates. Theory

Plates can have more than one buckled solution for a fixed set of boundary conditions. The theory for the identification and the computation of multiple solutions in buckled plates is examined. The theory predicts modal interaction (which is also called change in buckle pattern or secondary buckling) in experiments on certain plates with multiple theoretical solutions. A set of coordinate functions is defined for Galerkin's method so that the von Karman plate equations are reduced to a coupled set of cubic equations in generalized coordinates that are uncoupled in the linear terms. An iterative procedure for solving modal interaction problems is suggested based on this cubic form.

Thurston, Gaylen A.↗
NASA NTRS1990

Postbuckling response of long thick isotropic plates loaded in compression including higher order transverse shearing effects

Buckling and postbuckling results for aluminum plates loaded in compression are presented. The buckling results were plotted to show the effects of thickness on the stress coefficient. Buckling results are given for various length-to-width ratios. Postbuckling results for plates with transverse shearing flexibility are compared to results from classical theory for various width-to-thickness ratios. The plates are considered to be long with side edges simply supported, with edges free of stress and the plates are subjected to longitudinal compressive displacement. Characteristic curves indicating the average longitudinal direct stress resultant as a function of the applied displacements are calculated based on four different theories: Classical von Karman, first-order shear deformation, higher-order shear deformation, and three-dimensional flexibility. Present results indicate that the three-dimensional flexibility theory gives the lowest and therefore, most accurate results. The higher-order shear deformation theory has fewer unknowns than the three-dimensional flexibility but is not as accurate. The figures presented show that small differences occur in the maximum stress resultants and the transverse displacements calculated when the effects of transverse shear are included.

Stein, Manuel↗
NASA NTRS1990

Nonlinear analysis of compressively loaded linked-plate structures

A method is presented for the geometrically nonlinear analysis of the equilibrium behavior of a general class of compressively loaded, prismatic, linked-plate structures. The semi-analytical method accounts for geometric shape imperfections, and allows the use of laminated composite plate properties. Buckling eigenfunctions are used as displacement shape functions, and the associated second-order displacement fields are incorporated. The nonlinear plate equilibrium equations are expressed in terms of the assumed displacement form, and the Galerkin method is used to obtain a set of algebraic equations which are nonlinear in the modal amplitudes and the load parameter. Buckling eigensolutions for the class of structures under consideration can be obtained from the VIPASA computer code; thus, subject to some additional restrictions in loading and plate anisotropy, structural configurations which can be analyzed with regard to buckling response by VIPASA can be analyzed with regard to geometrically nonlinear response using the new method.

Stoll, Frederick↗
NASA NTRS1991

High-frequency techniques for RCS prediction of plate geometries

Several different high-frequency methods for modeling the radar cross sections (RCSs) of plate geometries are examined. The Method of Equivalent Currents and a numerically derived corner diffraction coefficient are used to model the RCS of a rectangular, perfectly conducting plate in nonprincipal planes. The Uniform Theory of Diffraction is used to model the RCS of a rectangular, perfectly conducting plate in principal planes. For the soft polarization case, first-order and slope-diffraction terms are included. For the hard polarization case, up to four orders of diffraction are included. Finally, the Uniform Theory of Diffraction for impedance wedges and the Impedance Boundary Condition are used to model the RCS of a coated, rectangular plate in principal planes. In most of the cases considered, comparisons are made between theoretical and experimental results.

Balanis, Constantine A.↗
NASA NTRS1991

High-frequency techniques for RCS prediction of plate geometries

Radar cross section (RCS) prediction of several rectangular plate geometries is discussed using high-frequency techniques such as the Uniform Theory of Diffraction (UTD) for perfectly conducting and impedance wedges and the Method of Equivalent Currents (MEC). Previous reports have presented detailed solutions to the principal-plane scattering by a perfectly conducting and a coated rectangular plate and nonprincipal-plane scattering by a perfectly conducting plate. These solutions are briefly reviewed and a modified model is presented for the coated plate. Theoretical and experimental data are presented for the perfectly conducting geometries. Agreement between theory and experiment is very good near and at normal incidence. In regions near and at grazing incidence, the disagreement between the data vary according to diffraction distances and angles involved. It is these areas of disagreement which are of extreme interest as an explanation for the disagreement will yield invaluable insight into scattering mechanisms which are not yet identified as major contributors near and at grazing incidence. Areas of disagreement between theory and experiment are identified and examined in an attempt to better understand and predict near-grazing incidence, grazing incidence, and nonprincipal-plane diffractions.

Balanis, Constantine A.↗