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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Intraplate deformation and closure of the Australia-Antarctica-Africa plate circuit

Plate motion data along the Southeast, Southwest, and Central Indian ridges have been reduced to 67 spreading rates, 38 transform fault azimuths, and 135 earthquake slip vectors in order to study the current motion between the Australian, Antarctic, and African plates and to investigate whether this plate circuit obeys closure. Magnetic profiles are modeled to determine rates consistently over a 3-m.y. time-averaging interval, and the new rates are shown to differ from published rates by as much as 5 mm/yr. The results indicate that Indian Ocean plate circuit nonclosure and the deformation that it suggests are much smaller than previously supposed, and support a model in which the significant deformation occurs in a diffuse plate boundary along the equatorial Indian Ocean between the Central Indian Ridge and the Sumatra Trench.

Demets, Charles↗
NASA NTRS1988

Beyond plate tectonics - Looking at plate deformation with space geodesy

The requirements that must be met by space-geodetic systems in order to constrain the horizontal secular motions associated with the geological deformation of the earth's surface are explored. It is suggested that in order to improve existing plate-motion models, the tangential components of relative velocities on interplate baselines must be resolved to an accuracy of less than 3 mm/yr. Results indicate that measuring the velocities between crustal blocks to + or - 5 mm/yr on 100-km to 1000-km scales can produce geologically significant constraints on the integrated deformation rates across continental plate-boundary zones such as the western United States.

Jordan, Thomas H.↗
NASA NTRS1988

Mechanisms of elevated-temperature deformation in the B2 aluminides NiAl and CoAl

A strain rate change technique, developed previously for distinguishing between pure-metal and alloy-type creep behavior, was used to study the elevated-temperature deformation behavior of the intermetallic compounds NiAl and CoAl. Tests on NiAl were conducted at temperatures between 1100 and 1300 K while tests on CoAl were performed at temperatures ranging from 1200 to 1400 K. NiAl exhibits pure-metal type behavior over the entire temperature range studied. CoAl, however, undergoes a transition from pure-metal to alloy-type deformation behavior as the temperature is decreased from 1400 to 1200 K. Slip appears to be inherently more difficult in CoAl than in NiAl, with lattice friction effects limiting the mobility of dislocations at a much higher tmeperature in CoAl than in NiAl. The superior strength of CoAl at elevated temperatures may, therefore, be related to a greater lattice friction strengthening effect in CoAl than in NiAl.

Yaney, D. L.↗
NASA NTRS1989

The transient behavior of weldpools with a deformed free surface

Transient heat transfer, fluid flow and phase change in a weldpool with a deformed free surface are modeled using a two-dimensional coordinate transfornmation. The theoretical approach makes it possible to accurately model high Marangoni number flows even when the free surface is not horizontal. The method is used to examine the role of surface deformation and surface tension forces in controlling weld penetration during cathode spot welding of steel and aluminum. The calculated flow fields, temperature distributions and weldpool shapes are consistent with experimental findings, and are in reasonable agreement with previously published modeling efforts.

Thompson, M. E.↗
NASA NTRS1989

Deformation and fracture of single-crystal and sintered polycrystalline silicon carbide produced by cavitation

An investigation was conducted to examine the deformation and fracture behavior of single-crystal and sintered polycrystalline SiC surfaces exposed to cavitation. Cavitation erosion experiments were conducted in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (1 mm) to the surface of SiC. The horn frequency was 20 kHz, and the double amplitude of the vibrating disk was 50 microns. The results of the investigation indicate that the SiC (0001) surface could be deformed in a plastic manner during cavitation. Dislocation etch pits were formed when the surface was chemically etched. The number of defects, including dislocations in SiC (0001) surface, increased with increasing exposure time to cavitation. The presence of intrinsic defects such as voids in the surficial layers of the sintered polycrystalline SiC determined the zones at which fractured grains and fracture pits (pores) were generated. Single-crystal SiC had superior erosion resistance to that of sintered polycrystalline SiC.

Miyoshi, Kazuhisa↗
NASA NTRS1990

Optical Detection Of Deformations Of An Antenna

Proposed control subsystem generates small aiming-bias signals to correct for deviations of 70-m-diameter reflector of microwave antenna from its ideal shape. Takes optical measurements to determine deformations produced by such environmental factors as wind, gravity, and thermal differentials. Using these measurements, subsystem estimates misalignment of radiation pattern caused by deformations. Signals to correct for estimated misalignment added to angle-command signals of main antenna-aiming system. To measure deviations laser ranging devices placed at base of feed on rigid intermediate reference structure, white retroreflectors placed on parts that deviate from assigned positions relative to intermediate reference structure.

Schumacher, L. L.↗
NASA NTRS1990

Thermomechanical deformation testing and modeling in the presence of metallurgical instabilities

A number of viscoplastic constitutive models were developed to describe deformation behavior under complex combinations of thermal and mechanical loading. Questions remain, however, regarding the validity of procedures used to characterize these models for specific structural alloys. One area of concern is that the majority of experimental data available for this purpose are determined under isothermal conditions. This experimental study is aimed at determining whether viscoplastic constitutive theories characterized using an isothermal data base can adequately model material response under the complex thermomechanical loading conditions typical of power generation service. The approach adopted was to conduct a series of carefully controlled thermomechanical experiments on a nickel-based superalloy, Hastelloy Alloy X. Previous investigations had shown that this material experiences metallurgical instabilities leading to complex hardening behavior, termed dynamic strain aging. Investigating this phenomenon under full thermomechanical conditions leads to a number of challenging experimental difficulties which up to the present work were unresolved. To correct this situation, a number of advances were made in thermomechanical testing techniques. Advanced methods for dynamic temperature gradient control, phasing control and thermal strain compensation were developed and incorporated into real time test control software. These advances allowed the thermomechanical data to be analyzed with minimal experimental uncertainty. The thermomechanical results were evaluated on both a phenomenological and microstructural basis. Phenomenological results revealed that the thermomechanical hardening trends were not bounded by those displayed under isothermal conditions. For the case of Hastelloy Alloy X (and similar dynamic strain aging materials), this strongly suggests that some form of thermomechanical testing is necessary when characterizing a thermoviscoplastic deformation model. Transmission electron microscopy was used to study the microstructural physics, and analyze the unique phenomenological behavior.

Castelli, Michael G.↗
NASA NTRS1990

The effect of hydrogen and microstructure on the deformation and fracture behavior of a single crystal nickel-base superalloy

A study was conducted on the effects of internal hydrogen and microstructure on the deformation and fracture of a single crystal nickel-base superalloy. In particular, room temperature plane strain fracture toughness and tensile tests were performed on hydrogen-free and hydrogen charged samples of PWA 1480. The role of microstructure was incorporated by varying the levels of porosity and eutectic gamma/gamma prime through hot isostatic pressing and heat treatment. The room temperature behavior of PWA 1480 was unusual because precipitate shearing was not the primary deformation mechanism at all strains. At strains over 1 percent, dislocations were trapped in the gamma matrix and an attempt was made to relate this behavior to compositional differences between PWA 1480 and other superalloys. Another unique feature of the tensile behavior was cleavage of the eutectic gamma/gamma prime, which is believed to initiate the failure process. Fracture occurred on (111) planes and is likely a result of shear localization along these planes. Elimination of the eutectic gamma/gamma prime greatly improved the tensile ductility, but pososity had no effect on tensile properties. Large quantities of hydrogen (1.74 at. percent) were gas-phase charged into the material, but surprisingly this was not a function of the amount of porosity or eutectic gamma/gamma prime present. Desorption experiments suggest that the vast majority of hydrogen is at reversible lattice trapping sites. This large, uniform concentration of hydrogen dramatically reduced the tensile strain to failure, but only slightly affected the reduction in area. Available hydrogen embrittlement models were examined in light of these results and it was found that the hydrogen enhanced localized plasticity model can explain much of the tensile behavior. K(IC) fracture toughness tests were conducted, but it was necessary to also perform J(IC) tests to provide valid data.

Walston, William S.↗
NASA NTRS1989

Connection forces in deformable multibody dynamics

In the dynamic formulation of holonomic and nonholonomic systems based on D'Alembert-Lagrange equation, the forces of constraints are maintained in the dynamic equations by introducing auxiliary variables, called Lagrange multipliers. This approach introduces a set of generalized reaction forces associated with the system generalized coordinates. Different sets of variables can be used as generalized coordinates and accordingly, the generalized reactions associated with these generalized coordinates may not be the actual reaction forces at the joints. In rigid body dynamics, the generalized reaction forces and the actual reaction forces at the joints represent equipollent systems of forces since they produce the same total forces and moments at and about any point on the rigid body. This is not, however, the case in deformable body analyses wherein the generalized reaction forces depend on the system generalized reference and elastic coordinates. In this paper, a method for determining the actual reaction forces at the joints from the generalized reaction forces in deformable multibody systems is presented.

Shabana, A. A.↗
NASA NTRS1989

High temperature inelastic deformation under uniaxial loading - Theory and experiment

The elevated-temperature uniaxial inelastic deformation behavior of an Ni-base alloy, B1900 + Hf, is investigated by performing isothermal tensile, creep, cyclic, stress relaxation, and thermomechanical fatigue tests. The range of strain rates examined is from 10 to the -7th to 100 per sec, while the test temperatures range from 25 to 1093 C. This extensive constitutive data base has been used for evaluating the unified constitutive models of Bodner and Partom (1972) and of Walker (1972) which apply for the small-strain regime. Comparison of test results with independent model predictions indicates good agreement over a broad range of loading conditions, demonstrating the applicability of the unified-constitutive-equation approach for describing the strongly nonlinear and temperature-dependent response of meals under a wide range of deformation and thermal histories. Thus the results give confidence that the unified approach is an effective and efficient approach in which complex history-dependent thermoviscoplastic flow can be represented within a single inelastic strain-rate term.

Chan, K. S.↗
NASA NTRS1990

Aeroelastic analysis of wings using the Euler equations with a deforming mesh

Modifications to the CFL3D three-dimensional unsteady Euler/Navier-Stokes code for the aeroelastic analysis of wings are described. The modifications involve including a deforming mesh capability which can move the mesh to continuously conform to the instantaneous shape of the aeroelastically deforming wing, and including the structural equations of motion for their simultaneous time-integration with the governing flow equations. Calculations were performed using the Euler equations to verify the modifications to the code and as a first-step toward aeroelastic analysis using the Navier-Stokes equations. Results are presented for the NACA 0012 airfoil and a 45 deg sweptback wing to demonstrate applications of CFL3D for generalized force computations and aeroelastic analysis. Comparisons are made with published Euler results for the NACA 0012 airfoil and with experimental flutter data for the 45 deg sweptback wing to assess the accuracy of the present capability. These comparisons show good agreement and, thus, the CFL3D code may be used with confidence for aeroelastic analysis of wings. The paper describes the modifications that were made to the code and presents results and comparisons which assess the capability.

Robinson, Brian A.↗
NASA NTRS1989

Composite deformable mirror

A mathematical model for a spherical hexagon mirror is described along with the model verifications. A flat hexagon epoxy mirror with aluminum core is used in test procedures in addition to a flat aluminum honeycomb mirror. Deformable reflector concepts are discussed and piezoelectric actuators are analyzed. The concept of a deformable mirror can be used to improve the overall RMS error of the mirror as presently designed and/or make necessary on-orbit adjustments.

Kuo, C. P.↗
NASA NTRS1990

Behaviour of plate elements based on the first-order shear deformation theory

A new analytical technique to assess the performance of shear deformable elements is presented, which makes it possible to determine a priori whether a given element will lock when used to model thin structures. The role that shear constraints play in determining the behavior of thin elements was established by comparing the results of key numerical tests with the predictions of element behavior made by studying the form of the shear constraints. Conclusions regarding locking behavior and the effects of reduced integration in thin shear deformable elements are presented, including the findings (1) that singularity of the shear stiffness matrix is not sufficient to avoid locking; (2) that the effect of mesh refinement on an element that contains spurious constraints is two-fold; and (3) that reduced integration does not remove spurious constraints but rather relaxes them. The results of the study are in agreement with previous studies of Mindlin plate elements in regarding Lagrangian elements as superior to serendipity elements when either full or reduced integration is employed.

Averill, R. C.↗
NASA NTRS1990

Deformation and thermal histories of chondrules in the Chainpur (LL3.4) chondrite

The results of optical and TEM studies of chondrules in the Chainpur (LL3.4) chondrite are presented. Results were obtained, using a microprobe, from quantitative microchemical analyses for Mg, Fe, Si, and Ca for the chondrule olivine and pyroxene, showing that chondrules in the Chainpur chondrite experienced varied degrees of annealing and deformation. It is suggested that Chainpur may be an agglomerate of a breccia that experienced little overall deformation or heating during and after the final accumulation and compaction of its constituents.

Ruzicka, Alex↗
NASA NTRS1989

Prediction of thermal-stress and deformations due to phase change in solidifying objects via flux/stress based finite element representations

The paper presents numerical simulations for the prediction of thermal-stress and deformation fields resulting from phase change in solidifying bodies employing new finite element representations. The formulations herein demonstrated provide different perspectives and physical interpretation for the modeling/analysis of thermo-mechanical problems and possess several inherent advantages. In comparison to traditional approaches for solving similar problems, the paper employs new flux/stress based representations to enhance the overall effectiveness. Comparative numerical applications validate applicability of the formulations for predicting the temperature induced deformations and stresses resulting from effects due to phase change.

Tamma, K. K.↗
NASA NTRS1990

Ridge belts - Evidence for regional- and local-scale deformation on the surface of Venus

Models for the formation of the ridge belt fan assemblage on Venus are evaluated through consideration of the orientation, spatial distribution, topographic expression, and wavelengths of observed tectonic surface features. A compressional mechanism is favored for long wavelength deformation corresponding to the spacing of ridge belts (300 - 400 km). However, short wavelength ridges and grooves (10 - 20 km) that are contained within belts and trend parallel to them are likely to be compressional and extensional, and to reflect both regional and local stress fields. It is hypothesized that large-scale (much greater than ridge spacing) early-stage mantle downwelling is the source of regional compression responsible for the establishment of the long wavelength of deformation.

Zuber, M. T.↗
NASA NTRS1990

Seismotectonics of thin- and thick-skinned deformation in the Andean foreland from local network data - Evidence for a seismogenic lower crust

Local network data from San Juan, Argentina, provides new information about crustal seismicity in the Andean foreland above a horizontal segment of the subducted Nazca Plate. Two areas of foreland seismicity are found, one associated with the Sierras Pampeanas basement uplifts, and the other beneath, but not within, the Precordillera foreland fold-thrust belt. The Precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation. In both areas, events are concentrated between 15 and 35 km depth and have volumetric, rather than planar, faultlike distributions. The depth distribution is unusually deep for intraplate earthquakes and suggests a brittle-ductile transition near 30-35 km.

Smalley, Robert, Jr.↗
NASA NTRS1990

Deformation effects of droplet fluctuations on dynamics in an Ising ferromagnetic state

Deformation effects of droplet fluctuations on the dynamics in an Ising ferromagnetic state in two dimensions are studied in the case of an order-parameter-nonconserving system by investigating an equation of motion for a domain boundary. Analytic and numerical studies show that the deformation effects on the survival probability of the droplets simply result in changing the time scale or renormalizing a kinetic coefficient.

Nakanishi, Hiizu↗