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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Crustal Deformation and the Seismic Cycle across the Kodiak Islands, Alaska

The Kodiak Islands are located approximately 130 to 250 km from the Alaska-Aleutian Trench where the Pacific plate is underthrusting the North American plate at a rate of about 57 mm/yr. The southern extent of the 1964 Prince William Sound (${M-w}$ = 9.2) earthquake rupture occurred offshore and beneath the eastern portion of the Kodiak Islands. Here we report GPS results (1993-2001) from northern Kodiak Island that span the transition between the 1964 uplift region along the eastern coast and the region of coseismic subsidence further inland. The horizontal velocity vectors range from 22.9 $\pm$ 2.2 mm/yr at N26.3$\deg$W $\pm$ 2.5$\deg$, about 150 km from the trench, to 5.9 $\pm$ 1.3 mm/yr at N65.9$\deg$W $\pm$ 6.6$\deg$, about 190 km from the trench. Near the northeastern coast of Kodiak the velocity vector above the shallow, locked main thrust zone is between the orientation of PCFC-NOAM plate motion (N22$/deg$W) and the trench-normal (N3O$\deg$W). Further west, our geodetic results suggest the accumulation of shear strain that will be released eventually as left-lateral motion on upper plate faults such as the Kodiak Island fault. These results are consistent with the hypothesis that the difference between the Pacific-North American plate motion and the orientation of the down going slab would lead to 4-8 mm/yr of left-lateral slip. Short-term geodetic uplift rates range from 2 - 14 mm/yr, with the maximum uplift located near the axis of maximum subsidence during the 1964 earthquake. We evaluated alternate interseismic models for Kodiak to test the importance of various mechanisms responsible for crustal deformation rates. These models are based on the plate interface slip history inferred from earlier modeling of coseismic and post-seismic geodetic results. The horizontal (trench perpendicular) and vertical deformation rates across Kodiak are consistent with a model that includes the viscoelastic response to : (1) a downgoing Pacific plate interface that is locked at shallow depths,(2) coseismic slip in the 1964 and (3) interseismic creep below the seismogenic zone. The change in orientation of the horizontal velocity vector occurs down-dip from the locked main thrust zone. In southern Kodiak, the coseismic slip in the 1964 earthquake was smaller than in the northern Kodiak region; yet, the horizontal, interseismic velocities as a function of distance from the trench are comparable to those in northern Kodiak. Based on the earthquake history prior to, and following the 1964 earthquake, we hypothesize that the plate interface in southern Kodiak slips in more frequent large earthquakes than in northern Kodiak.

Sauber, Jeanne↗
NASA NTRS1996

Simulation and modeling of the elliptic streamline flow

Direct numerical simulations are performed for the elliptic streamline flow, which is a homogeneous turbulent flow that combines the effects of solid body rotation and strain. Simulations are run over a range of parameters in order to determine the effect of changing rotation and strain separately. For early times the nonlinear cascade is suppressed, but then is re-established at later times. The growth rate of turbulent kinetic energy agrees at early times with the trends from linear theory, but at later times the flow seems to approach an asymptotic state that is independent of the ratio of mean flow rotation rate to strain rate. A comparison with standard Reynolds stress turbulence models is made. It is found that for strong rotation rates, the models predict decay of the turbulence, while the simulations show exponential growth. Close examination of the simulation results shows that they are affected by excessively low Reynolds numbers. Suggestions for reducing low Reynolds number effects in future simulations is given.

Blaisdell, G. A.↗
NASA NTRS1980

Lateral density variations in elastic Earth models from an extended minimum energy approach

Kaula's minimum energy approach was extended to include the nonhydrostatic gravitational potential energy and the density perturbation field was obtained to degree and order eight. The depth profiles for the density perturbation show a stratification with density excesses and deficiencies alternating with depth. The addition of the gravitational potential energy in the minimization process does not change significantly the conclusions based on results for the minimum shear strain energy case, concerning the inability of the mantle to withstand the lateral loading elastically.

Sanchez, B. V.↗
NASA NTRS1978

Computer-aided analysis and design of the shape rolling process for producing turbine engine airfoils

Mild steel (AISI 1018) was selected as model cold rolling material and Ti-6A1-4V and Inconel 718 were selected as typical hot rolling and cold rolling alloys, respectively. The flow stress and workability of these alloys were characterized and friction factor at the roll/workpiece interface was determined at their respective working conditions by conducting ring tests. Computer-aided mathematical models for predicting metal flow and stresses, and for simulating the shape rolling process were developed. These models utilized the upper bound and the slab methods of analysis, and were capable of predicting the lateral spread, roll separating force, roll torque, and local stresses, strains and strain rates. This computer-aided design system was also capable of simulating the actual rolling process, and thereby designing the roll pass schedule in rolling of an airfoil or a similar shape.

Lahoti, G. D.↗
NASA NTRS1989

Electrical properties of materials for high temperature strain gage applications

A study was done on the electrical resistance of materials that are potentially useful as resistance strain gages at high temperatures under static strain conditions. Initially a number of binary alloys were investigated. Later, third elements were added to these alloys, all of which were prepared by arc melting. Several transition metals were selected for experimentation, most prepared as thin films. Difficulties with electrical contacts thwarted efforts to extend measurements to the targeted 1000 C, but results obtained did suggest ways of improving the electrical resistance characteristics of certain materials.

Brittain, John O.↗
NASA NTRS1993

Characterization of Si (sub X)Ge (sub 1-x)/Si Heterostructures for Device Applications Using Spectroscopic Ellipsometry

Spectroscopic ellipsometry (SE) characterization of several complex Si (sub X)Ge (sub 1-x)/Si heterostructures prepared for device fabrication, including structures for heterojunction bipolar transistors (HBT), p-type and n-type heterostructure modulation doped field effect transistors, has been performed. We have shown that SE can simultaneously determine all active layer thicknesses, Si (sub X)Ge (sub 1-x) compositions, and the oxide overlayer thickness, with only a general knowledge of the structure topology needed a priori. The characterization of HBT material included the SE analysis of a Si (sub X)Ge (sub 1-x) layer deeply buried (600 nanometers) under the silicon emitter and cap layers. In the SE analysis of n-type heterostructures, we examined for the first time a silicon layer under tensile strain. We found that an excellent fit can be obtained using optical constants of unstrained silicon to represent the strained silicon conduction layer. We also used SE to measure lateral sample homogeneity, providing quantitative identification of the inhomogeneous layer. Surface overlayers resulting from prior sample processing were also detected and measured quantitatively. These results should allow SE to be used extensively as a non-destructive means of characterizing Si (sub X)Ge (sub 1-x)/Si heterostructures prior to device fabrication and testing.

Sieg, R. M.↗
NASA NTRS1978

Computer-aided analysis and design of the shape rolling process for producing turbine engine airfoils

Mild steel (AISI 1018) was selected as model cold-rolling material and Ti-6Al-4V and INCONEL 718 were selected as typical hot-rolling and cold-rolling alloys, respectively. The flow stress and workability of these alloys were characterized and friction factor at the roll/workpiece interface was determined at their respective working conditions by conducting ring tests. Computer-aided mathematical models for predicting metal flow and stresses, and for simulating the shape-rolling process were developed. These models utilize the upper-bound and the slab methods of analysis, and are capable of predicting the lateral spread, roll-separating force, roll torque and local stresses, strains and strain rates. This computer-aided design (CAD) system is also capable of simulating the actual rolling process and thereby designing roll-pass schedule in rolling of an airfoil or similar shape. The predictions from the CAD system were verified with respect to cold rolling of mild steel plates. The system is being applied to cold and hot isothermal rolling of an airfoil shape, and will be verified with respect to laboratory experiments under controlled conditions.

Lahoti, G. D.↗
NASA NTRS1986

Cenozoic thermal, mechanical and tectonic evolution of the Rio Grande rift

Two areas of New Mexico which exhibit complex but similar Cenozoic histories of extensional tectonism are analyzed. The first study area is the Basin and Range province and southern Rio Gande rift in southern New Mexico; the second study area is the central Rio Grande rift in central and northern New Mexico, the southern San Luis basin, the Espanola basin, and the Albuquerque basin. Two phases of extension were identified: the first phase which began in mid-Oligocene was characterized by local high-strain extension events, low-angle faulting, and the development of broad, shallow basins, all indicating an approximately NE-SW + or - 25 deg extension direction; the later phase which occurred primarily in the late Miocene, was characterized by synchronous, high-angle faulting, resulting in large vertical strains which produced the modern Rio Grande rift morphology. Extension direction was approximately E-W. Geotherms were estimated and lithospheric strength curves were calculated for these two phases of extension. A high geotherm was deduced for the early phase resulting in a shallow crustal brittle transition, and insignificant mantle strength. The lithosphere subsequently cooled, resulting in a significant zone of mantle strength beneath the Moho. It is concluded that the interrelationship among regional and local prerifting, synrifting, and postrifting events in the Rio Grande rift attests to the fact that the rifting (in the region studied) should be considered in the context of other geologic events.

Morgan, P.↗
NASA NTRS1980

Some useful concepts for the designer in treating cumulative fatigue damage at elevated temperatures

The paper concentrates on three subjects: (1) further development of the double-linear damage rule for cumulative fatigue damage summation in the sub-creep temperature range; (2) a new consideration of the effect of strain reversals in the creep range of materials, whereby reversals that occur later in the histogram can influence the degree of damage caused by earlier loading events; and (3) the application of the minimum-commitment time-temperature parameter to the treatment of creep rupture of multiheats wherein considerable differences in properties of individual heats may occur. Numerical examples and plots are given.

Manson, S. S.↗
NASA NTRS1981

The preferred-mode coherent structure in the near field of an axisymmetric jet with and without excitation

Visualization and phase-locked hot-wire measurements have been used to investigate the preferred-mode coherent structure in the near field of an axisymmetric jet for different Reynolds numbers and for the two limiting asymptotic states of the exit boundary layer: laminar and fully developed turbulent. Distributions of phase-averaged longitudinal and lateral velocities, coherent vorticity, coherent and incoherent turbulence Reynolds stresses, and coherent strain rate and production have been obtained over the spatial extent of the coherent structure for a Strouhal number of 0.3. It is found that these properties are independent of the initial conditions, but weakly dependent on the Reynolds number up to about 100,000. The naturally occurring coherent structure in the unexcited jet, educed via an optimized conditional sampling scheme, shows no clear dependence on the jet Reynolds number up to 800,000.

Hussain, A. K. M. F.↗
NASA NTRS1989

Strained-layer InGaAs/GaAs/AlGaAs single quantum well lasers with high internal quantum efficiency

Low threshold current density strained-layer In(0.2)Ga(0.8)As/GaAs/AlGaAs single quantum well lasers, emitting at 980 nm, have been grown by molecular beam epitaxy. Contrary to what has been reported for broad-area lasers with pseudomorphic InGaAs active layers grown by metalorganic chemical vapor deposition, these layers exhibit a high internal quantum efficiency (about 90 percent). The maximum external differential quantum efficiency is 70 percent, limited by an anomalously high internal loss possibly caused by a large lateral spreading of the optical mode. In addition, experimental results supporting the theoretically predicted strain-induced reduction of the valence-band nonparabolicity and density of states are presented.

Larsson, Anders↗
NASA NTRS1980

Stresses in adhesively bonded joints: A closed form solution

The plane strain of adhesively bonded structures which consist of two different orthotropic adherents is considered. Assuming that the thicknesses of the adherends are constant and are small in relation to the lateral dimensions of the bonded region, the adherends are treated as plates. The transverse shear effects in the adherends and the in-plane normal strain in the adhesive are taken into account. The problem is reduced to a system of differential equations for the adhesive stresses which is solved in closed form. A single lap joint and a stiffened plate under various loading conditions are considered as examples. To verify the basic trend of the solutions obtained from the plate theory a sample problem is solved by using the finite element method and by treating the adherends and the adhesive as elastic continua. The plate theory not only predicts the correct trend for the adhesive stresses but also gives rather surprisingly accurate results.

Delale, F.↗
NASA NTRS1988

An exactly solvable model for calculating critical misfit and thickness in epitaxial superlattices. II - Layers of unequal elastic constants and thicknesses

The theoretical model developed by van der Merwe and Jesser (1988) is extended to permit the exact determination of the critical misfit and critical thickness of an epitaxially grown superlattice made up of layers with differing elastic constants and thicknesses. Lateral force balances are maintained by means of an explicit relationship between the homogeneous misfit strains and the thicknesses and moduli. A number of simplifying approximations are introduced, and sample numerical results are presented in graphs.

Jesser, W. A.↗
NASA NTRS1990

Fabrication of structural components from commercial aluminum alloys using superplastic forming

SPF technology was used to fabricate structural components from the 7475 Al and 8090 Al-Li commercial alloys. Gas-pressurization cycles were established for SPF three-hat stiffener configurations on the basis of uniaxial data and component-geometry considerations. It is established that higher forming rates than the optimum strain rates selected from the uniaxial data for each alloy could be used in the later stages of forming without reducing SPF components' dimensional conformity. Cavitation was precluded through the use of back pressure during forming.

Hales, S. J.↗
NASA NTRS1977

Stiffness and damping characteristics of aluminum in creep

Tensile creep tests conducted at 200 C were performed on annealed commercially pure aluminum specimens in order to measure the dominant elevated temperature dislocation processes. Testing consisted of applying small lateral loads to measure flexural stiffness, and vibrating the specimens laterally in order to measure dynamic modulus and internal damping. It was concluded that (1) the strain hardening increased static stiffness and decreased internal damping during early creep, and (2) the dynamic modulus remained essentially constant at the elastic value during creep. These results imply that primary creep may constitute a mechanism of recovery of dislocatory disorder induced by yielding the material during loading, and that the inelastic modulus utilized as a mathematical concept in several creep buckling theories is not a directly measurable material property.

Berkovits, A.↗
NASA NTRS1962

A REPORT ON THREE SERIES OF EXPERIMENTS AND THE DESCRIPTION OF A SIMPLIFIED MODEL OF THE THIN WALL CYLINDER AND CONE BUCKLING MECHANISM

Three series of experiments were performed to investigate the nature of the buckling phenomenon. They were: a) influence of an increasing dimple on the buckling load of a cylinder; b) investigation of the strain distribution on one cylinder, after one or more waves were imposed on it; c) measurements of lateral deflections at buckling. The results of such experiments give some support to the idea of a buckling model, which can be of eventual interest to the thin wall cylinder and cone buckling problem, where a non-linear system (the buckled band) is considered in series with a linear system (the unbuckled portion of the cylinder, plus the testing-machine )

CYLINDER↗
NASA NTRS1985

Intraplate deformation due to continental collisions: A numerical study of deformation in a thin viscous sheet

A model of crustal deformation from continental collision that involves the penetration of a rigid punch into a deformable sheet is investigated. A linear viscous flow law is used to compute the magnitude and rate of change of crustal thickness, the velocity of mass points, strain rates and their principal axes, modes of deformation, areal changes, and stress. In general, a free lateral boundary reduces the magnitude of changes in crustal thickening by allowing material to more readily escape the advancing punch. The shearing that occurs diagonally in front of the punch terminates in compression or extension depending on whether the lateral boundary is fixed or free. When the ratio of the diameter of the punch to that of the sheet exceeds one-third, the deformation is insenstive to the choice of lateral boundary conditions. When the punch is rigid with sharply defined edges, deformation is concentrated near the punch corners. With non-rigid punches, shearing results in deformation being concentrated near the center of the punch. Variations with respect to linearity and nonlinearity of flow are discussed.

Cohen, S. C.↗
NASA NTRS1974

Investigations to the space shuttle orbiter 2A configuration 0.015-scale model in the NASA Ames Research Center 3.5-foot hypersonic wind tunnel at Mach numbers 5, 7 and 10 (OA11B)

The results of a wind tunnel test to determine the force, moment, and hinge-moment characteristics of the Configuration 2A Space Shuttle Vehicle Orbiter at Mach numbers 5, 7 and 10 are presented. The model was an 0.015-scale representation of the Orbiter Configuration 2A used in test 0A11A and later tests. Six-component aerodynamic force and moment data were recorded from a 1.50-inch internal strain-gage balance, and base pressures were taken for axial and drag force corrections. Hinge-moment data were obtained for the rudder and the inboard and outboard elevon panels of the starboard wing.

Mellenthin, J. A.↗