Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “monotonicity”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Room Temperature Cyclic Deformation Behavior of Cast and Extruded NiAl

The fully reversed, strain controlled fatigue behavior of cast and extruded NiAl was evaluated at room temperature for plastic strain ranges of 0.0006 to 0.0002 to provide baseline data on the fatigue life of NiAl and to investigate whether the low ductility of NiAl would result in poor low-cycle fatigue behavior. Except at the smallest plastic strain range investigated, NiAl work hardened continuously until failure with a final fracture stress under cyclic conditions which was at least 60 percent greater than the monotonic fracture strength of NiAl. Fatigue fracture initiation occurred at large internal tear-shaped pores, and fatigue life was controlled or limited by the presence of these processing-related defects. Even with the processing defects present in this material and the limited ductility of NiAl in general, cast and extruded NiAl exhibited much greater fatigue life at room temperature than comparable B2 ordered compounds when compared on a strain range basis.

Noebe, R. D.↗
NASA NTRS1992

The influence of thermal inertia on temperatures and frost stability on Triton

It is presently argued, in view of (1) a thermal inertia model for the surface of Triton which (like previous ones) predicts a monotonic recession of permanent N2 deposits toward the poles and very little seasonal N2 frost in the southern hemisphere, and (2) new spectroscopic evidence for nonvolatile CO2 on Triton's bright southern hemisphere, that much of that bright southern material is not N2. Such bright southern hemisphere volatiles may allow the formation of seasonal frosts, thereby helping to explain the observed spectroscopic changes of Triton during the last decade.

Spencer, John R.↗
NASA NTRS1992

Nonadiabatic transport features in the outer cusp region

The dayside to nightside circulation of plasma along the magnetopause inside the magnetosphere is examined by means of three-dimensional single-particle codes. It is demonstrated that particles incident upon the outer cusp region experience transient non-adiabatic motions, owing to a localized minimum in the field magnitude. Here, possibly large magnetic moment changes yield injection into the loss cone of fractions of the incoming population or, alternatively, enhanced bouncing motions at high altitudes. It is shown that particles gaining access to the magnetotail over the polar cap are progressively extracted from the weak field region by the large-scale convection electric field. In this latter case, the trajectory simulations suggest an implicit 'entry boundary' into the nightside magnetosphere, which corresponds to the sunward edge of field lines featuring monotonic decrease of the field magnitude along their length.

Delcourt, D. C.↗
NASA NTRS1992

Mechanisms of compressive failure in 3D composites

The present study of angle-interlock woven polymer matrix composite behavior under uniaxial monotonic compression notes these materials to be macroscopically ductile, with compressive strains in failure that can exceed 15 percent. Some of the tests conducted on stitched laminates indicated brittle behavior. Woven composite failure mechanisms have been studied by a combination of optical microscopy, moire interferometry, stereoscopy, and digital image comparisons; the critical failure event was in all cases a kink band formation in the primary load-bearing axial rows.

Cox, B. N.↗
NASA NTRS1992

Effect of uneven wall temperature on local heat transfer in a rotating square channel with smooth walls and radial outward flow

The effect of uneven wall temperature on the local heat transfer coefficient in a rotating square channel with smooth walls and radial outward flow is investigated for Reynolds numbers from 2500 to 25,000 and rotation numbers from 0 to 0.352. Three cases of thermal boundary conditions are studied: (1) four walls uniform temperature, (2) four walls uniform heat flux, and (3) leading and trailing walls hot and two side walls cold. It is shown that the heat transfer coefficients on the leading surface are much lower than that of the trailing surface due to rotation. For case 1, the leading surface heat transfer coefficient decreases and then increases with increasing rotation numbers, and the trailing surface heat transfer coefficient increases monotonically with rotation numbers. The trailing surface heat transfer coefficients, as well as those for the side walls, for case 2 are higher than for case 1, and the leading surface heat transfer coefficients for cases 2 and 3 are significantly higher than for case 1.

Han, J.-C.↗
NASA NTRS1992

Elastoplastic notch root strains - Measurements versus finite-element predictions

A study intended to experimentally and computationally probe the nature of the elastoplastic strain fields created by notches with various levels of constraint is presented. An interferometric strain/displacement gage is used to measure both the axial and lateral strain at the center of a machined and polished notch. The monotonic response of various notches is determined using 3D finite-element calculations.

Tregoning, R. L.↗
NASA NTRS1993

A solution scheme for the Euler equations based on a multi-dimensional wave model

A scheme for the solution of scalar advection on an unstructured mesh has been developed, tested, and extended to the Euler equations. The scheme preserves a linear function exactly, and yields nearly monotone results. The flux function associated with the Euler scheme is based on a discrete 'wave model' for the system of equations. The wave model decomposes the solution gradient at a location into shear waves, entropy waves and acoustic waves and calculates the speeds, strengths and directions associated with the waves. The approach differs from typical flux-difference splitting schemes in that the waves are not assumed to propagate normal to the faces of the control volumes; directions of propagation of the waves are instead computed from solution-gradient information. Results are shown for three test cases, and two different wave models. The results are compared to those from other approaches, including MUSCL and Galerkin least squares schemes.

Powell, Kenneth G.↗
NASA NTRS1992

Structural optimization for joined-wing synthesis

The differences between fully stressed and minimum-weight joined-wing structures are identified, and these differences are quantified in terms of weight, stress, and direct operating cost. A numerical optimization method and a fully stressed design method are used to design joined-wing structures. Both methods determine the sizes of 204 structural members, satisfying 1020 stress constraints and five buckling constraints. Monotonic splines are shown to be a very effective way of linking spanwise distributions of material to a few design variables. Both linear and nonlinear analyses are employed to formulate the buckling constraints. With a constraint on buckling, the fully stressed design is shown to be very similar to the minimum-weight structure. It is suggested that a fully stressed design method based on nonlinear analysis is adequate for an aircraft optimization study.

Gallman, John W.↗
NASA NTRS1992

Radial profiles of quiet time magnetospheric parameters

The quiet time magnetosphere is studied using ion measurements covering energies from about 1 keV to about 4 MeV from selected passes of the Charge Composition Explorer during low geomagnetic activity. The quiet time plasma pressure in the midnight sector was found to be comparable to that deduced by Spence et al. (1989) and Kan et al. (1992) from inverting the magnetic field models of Tsyganenko and Usmanov (1982) and Tsyganenko (1987). The radial profile of the total plasma pressure exhibited a peak at L = 3-4 and decreased monotonically from L = 4 and L = 9. Contrary to theoretical expectations for the magnetospheric closure of the region 2 field-aligned current system, no large-scale earthward decrease in plasma pressure was found to occur in these outer L shells.

Lui, A. T. Y.↗
NASA NTRS1992

Quasar evolution and the growth of black holes

A 'minimalist' model of AGN evolution is analyzed that links the measured luminosity function to an elementary description of black hole accretion. The observed luminosity function of bright AGN is extrapolated and simple prescriptions for the growth and luminosity of black holes are introduced to infer quasar birth rates, mean fueling rates, and relict black hole distribution functions. It is deduced that the mean accretion rate scales as (M exp -1./5)(t exp -6.7) and that, for the most conservative model used, the number of relict black holes per decade declines only as M exp -0.4 for black hole masses between 3 x 10 exp 7 and 3 x 10 exp 9 solar masses. If all sufficiently massive galaxies pass through a quasar phase with asymptotic black hole mass a monotonic function of the galaxy mass, then it is possible to compare the space density of galaxies with estimated central masses to that of distant quasars.

Small, Todd A.↗
NASA NTRS1993

Streamwise vortex meander in a plane mixing layer

Previous measurements in a two-stream plane mixing layer have shown that the mean streamwise vorticity decays monotonically with downstream distance such that it is absent in the far-field (self-similar) region. It was not clear at that time if this measured decay was a result of actual vortex diffusion or increasing vortex meander within the mixing layer. The present experimental study was conducted in order to determine the existence of streamwise vortex meander in a mixing layer, and if present, its significance on the measured properties. The dependence of the velocity cross-correlation on the fixed probe location is considered a good indicator of the stationarity of the streamwise vortex location. The cross-correlation measurements obtained here indicate that the spanwise meander is negligible while the transverse apparent meander (normal to the plane of the mixing layer) is relatively large.

Leboeuf, Richard L.↗
NASA NTRS1993

Grid-independent upwind scheme for multidimensional flow

Recent advances in the development of a grid-independent finite volume scheme for the Euler equations of gas dynamics are described. In the proposed method, flowfield gradient data are reconstructed locally (on a triangle) using five elementary planar waves, and an upwind numerical flux function for grid-oblique waves is developed to model the effect of the passage of these waves on the data in a cell. Numerical examples for several two-dimensional test problems are included. These results show high wave resolution and nearly monotone strong-wave transitions.

Parpia, Ijaz H.↗
NASA NTRS1991

Deformation and failure mechanisms in metal matrix composites

An investigation was undertaken to determine the key deformation mechanisms and their interaction leading to failure of both 0 degree and 90 degree Ti 15-3/SCS-6 laminae under monotonic loading. The experimental results suggest that inelastic deformation in the 0-degree lamina is dominated by plastic deformation and that in the 90-degree lamina is dominated by both fiber-matrix debonding and plasticity. The loading-unloading response, monitoring of Poisson's ratio and microscopy were utilized to identify the key deformation mechanisms. The sequence of deformation mechanisms leading to failure are identified for both the 0 and the 90-degree specimens. The threshold strains for plasticity or damage which are referred to as 'microdeformation' in the 0 deg and 90 deg laminae are approximately 0.004 and 0.002, respectively, at room temperature. These strain levels may be considered critical in initiation based structural design with these composites.

Newaz, G.↗
NASA NTRS1992

Behavior of surface and corner cracks subjected to tensile and bending loads in a Ti-6Al-4V alloy

The behavior of part-through flaws with regard to failure under monotonic loading and their growth under fatigue loading was investigated experimentally and analytically. The objective of this paper is to present comparisons of experimental values of toughness obtained using surface- and corner-cracked specimens with those obtained using standard test specimens, and also to compare experimental growth cycles with numerical predictions using the NASA/FLAGRO computer program. Tests were conducted on various types of surface and corner cracks under tensile and bending loads. Room temperature laboratory air provided the test environment. The material used in this investigation was the Ti-6Al-4V alloy in the solution treated and aged (STA) and stress-relieved condition. Detailed tabulation of the fracture toughness data and results of life prediction using the NASA/FLAGRO program are presented. Fatigue crack growth rates for the part-through-cracked specimens are compared with a base curve fitted from the data obtained using standard specimens. The fatigue loading used in the crack growth testing was of constant-amplitude sinusoidal type.

Forman, Royce G.↗
NASA NTRS1993

Viscoplasticity with Creep and Plasticity Bounds

A viscoplastic theory is developed that reduces analytically to creep theory under steady-state conditions and becomes plasticity theory at its rate-independent bound. A viscoplastic model is then constructed by defining material functions that have close ties to the physics of inelasticity. As a consequence, this model is characterized easily - only steady-state creep data, saturated hysteresis loops, and monotonic stress/strain curves are required. The general applicability of the model is demonstrated by the characterization of three f.c.c. metals. A variety of validation experiments is also provided.

Freed, Alan D.↗
NASA NTRS1992

Radial expansion of an ideal MHD configuration and the temporal development of the magnetic field

We study the free radial expansion of a 3-component magnetic configuration. The emphasis of this paper is on the behavior of a field undergoing non-self-similar expansion. Comparing our results with the evolution of a magnetic configuration expanding self-similarly, we find that self-similar expansion appears as the asymptotic limit (with time) of the general case. Using a model field we show that a non-self-similar velocity profile need not have a strict monotonic decrease with time.

Farrugia, C. J.↗
NASA NTRS1993

Double layer propagation in experiments with electron beam injection

Electron beam injection into a plasma is investigated using the analytical inverted Bernstein-Green-Kruskal method. Particle number and momentum conservation laws are applied to evaluate the propagation velocity and potential drop on the leading edge of the beam. Electric potential is supposed to be monotonic, thus the leading front has a double-layer-like structure. For the case of cold particles, analytical expressions for the double layer velocity and potential drop are obtained. It is pointed out that double layer velocity differs from the initial electron speed: even for weak beams a noticeable deceleration takes place. Strong beams are found incapable of penetrating into plasma - their propagation velocity is very small. Ambient electrons undergo a considerable acceleration forming a return current which neutralizes the injector. Possible instability of the distribution functions is discussed.

Bruskin, L. G.↗
NASA NTRS1992

Orbital period change of the low-mass X-ray binary EXO 0748-676

The transient low-mass X-ray binary, EXO 0748-676, discovered with EXOSAT, is known to exhibit eclipses of a 492-s duration with a 3.82-hr period, intensity dips at pre-eclipse phases and type-I X-ray bursts. We observed this source with Ginga in 1989 March, 1990 December, 1991 January, and 1991 August and determined nine eclipse center times. Combining these eclipse center times with the previous result of the EXOSAT observations, we find that the orbital period of this source is not decaying monotonically, contrary to the previously reported suggestion. Instead, it shows a more complex behavior. A quadratic fit to the eclipse data yields a positive rate of change in orbital period with an approximate rate of 0.9 x 10 exp 7/yr, although the EXOSAT observations made in 1985 do not fit this trend. A sinusoidal function gives a better fit to the observed orbital period changes with a period of about 12 yr and an amplitude of about 44 lt-s, although the period is much longer than the observation interval of about 6.5 yr. Possible mechanisms for the orbital period change are discussed.

Asai, Kazumi↗