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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Closure measurements of naturally initiating small cracks

The initiation and growth of cracks in smooth 2024-T3 aluminum specimens have been investigated using acetate replicas. Crack opening displacements were measured as a function of load at several positions along the crack as it grew across the thickness of the specimen and became a through-the-thickness crack of a few millimeters in length. Tests run for R-ratios of 0.5, 0.0 and -1.0 showed that closure loads were well predicted by Newman's (1984) model. It is noted that small cracks grow slightly faster than would be predicted by long-crack data, and it is suggested that effects other than plasticity-induced closure must be taken into account.

Sharpe, W. N., Jr.↗
NASA NTRS1988

Reynolds stress closure in jet flows using wave models

Ways of implementing the turbulence closure scheme based on modeling the large scale coherent structures as instability waves were sought. The computational tools necessary to apply this scheme to jets of arbitrary geometry were developed. The model, developed earlier, was extended to the shock structure of supersonic jets of arbitrary geometry and multiple jets. It was found that though the qualititate features of the unsteady flow field could be predicted there were always difficulties with some of the quantitative features. This led to the new formation of the closure scheme. The schemes for computations tools which were developed are efficient and represent the application of the very powerful mathematical tools to the problems of practical significance.

Morris, P. J.↗
NASA NTRS1988

Higher order turbulence closure models

Theoretical models are developed and numerical studies conducted on various types of flows including both elliptic and parabolic. The purpose of this study is to find better higher order closure models for the computations of complex flows. This report summarizes three new achievements: (1) completion of the Reynolds-stress closure by developing a new pressure-strain correlation; (2) development of a parabolic code to compute jets and wakes; and, (3) application to a flow through a 180 deg turnaround duct by adopting a boundary fitted coordinate system. In the above mentioned models near-wall models are developed for pressure-strain correlation and third-moment, and incorporated into the transport equations. This addition improved the results considerably and is recommended for future computations. A new parabolic code to solve shear flows without coordinate tranformations is developed and incorporated in this study. This code uses the structure of the finite volume method to solve the governing equations implicitly. The code was validated with the experimental results available in the literature.

Amano, Ryoichi S.↗
NASA NTRS1988

Analysis of crack closure under plane strain conditions

The phenomenon of plasticity-induced crack closure is associated with the development of residual material on the flanks of an advancing fatigue crack. While it is easy to see that this residual material can come from the side faces of a specimen under plane stress conditions, it is difficult to discover the origin of this extra volume of material on the crack flanks when it is assumed that plane deformations occur and plastic flow is incompressible. The purpose of this paper is to determine whether plasticity-induced fatigue crack closure occurs in an elastic-perfectly plastic body under plane strain conditions.

Fleck, Norman A.↗
NASA NTRS1988

The influence of crack closure on fatigue crack growth thresholds in 2024-T3 aluminum alloy

Crack opening loads were determined in load-shedding fatigue crack growth threshold tests on 2024-T3 aluminum alloy at stress ratios R of -2, -1, 0, 0.33, 0.5, and 0.7. The effects of load-shedding procedure and rate were investigated. Values of threshold Delta-K were found to vary significantly with R, whereas values of threshold effective Delta-K did not. That is, the variation of threshold Delta-K with R was almost completely explained by accounting for the measured variation in crack opening load behavior with R. The load-shedding guidelines of ASTM Test Method for Measurement of Fatigue Crack Growth (E 647) produced a threshold Delta-K value for R = 0.7 that was in agreement with the value determined using a procedure that should minimize closure effects. At both R = 0 and R = 0.7, high load-shedding rates produced high values of threshold Delta-K caused by large closure effects.

Phillips, Edward P.↗
NASA NTRS1988

Fatigue crack closure behavior at high stress ratios

Fatigue crack delay behavior at high stress ratio caused by single peak overloads was investigated in two thicknesses of 7475-T731 aluminum alloy. Closure measurements indicated no closure occurred before or throughout the overload plastic zones following the overload. This was further substantiated by comparing the specimen compliance following the overload with the compliance of a low R ratio test when the crack was fully open. Scanning electron microscope studies revealed that crack tunneling and possibly reinitiation of the crack occurred, most likely a result of crack-tip blunting. The number of delay cycles was greater for the thinner mixed mode stress state specimen than for the thicker plane strain stress state specimen, which is similar to low R ratio test results and may be due to a larger plastic zone for the mixed mode cased.

Turner, C. Christopher↗
NASA NTRS1988

Simulations using a 3D-moment turbulence closure in a two-dimensional cloud model - Comparisons with observations

A method for deep cumulus cloud simulations is presented, in which the plumes or clouds are simulated using a two-dimensional axisymmetric numerical cloud model with third-moment turbulence closure. This method is based on the assumption that the only uncertainty in the turbulence closure is in the turbulent length scale specification. Therefore, the length scale is varied, and, using different length scales, the simulated plumes are compared to the observed ones, making it possible to optimize the length scale prescription.

Krueger, Steven K.↗
NASA NTRS1990

Analytical methods for the development of Reynolds stress closures in turbulence

Analytical methods for the development of Reynolds stress models in turbulence are reviewed in detail. Zero, one and two equation models are discussed along with second-order closures. A strong case is made for the superior predictive capabilities of second-order closure models in comparison to the simpler models. The central points are illustrated by examples from both homogeneous and inhomogeneous turbulence. A discussion of the author's views concerning the progress made in Reynolds stress modeling is also provided along with a brief history of the subject.

Speziale, Charles G.↗
NASA NTRS1990

Through-the-thickness fatigue crack closure behavior in an aluminum alloy

The variation in fatigue crack closure behavior across the thickness of aluminum alloy specimens was investigated. The specimen geometries examined were the middle crack tension M(T) and compact tension C(T). The fatigue crack closure behavior was determined using remote displacement and strain gages, near tip strain gages, and fatigue striations. A hybrid experimental/numerical method was also used to infer the crack opening loads. The results indicate a variation in crack opening load, of 0.2 in the specimen interior to 0.4 to 0.5 at the surface.

Dawicke, D. S.↗
NASA NTRS1989

Overload retardation due to plasticity-induced crack closure

Experiments are reported which show that plasticity-induced crack closure can account for crack growth retardation following an overload. The finite element method is used to provide evidence which supports the experimental observations of crack closure. Finally, a simple model is presented which predicts with limited success the retardation transient following an overload.

Fleck, N. A.↗
NASA NTRS1990

Simulation of crack growth and crack closure under large cyclic plasticity

The propagation of a crack during cyclic loading with large plasticity was simulated using a finite element method. The numerical procedure included the determination of crack closing and opening by examining the contact condition of the crack surfaces in the plasticity iteration. The method was applied to the growth of cracks in Alloy 718 single edge notch specimens at 538 C subject to strain cycling under both nominally elastic and elastoplastic conditions. The results show, among other things, that the crack closure and opening stresses become more compressive as the loading becomes more plastic, and that, under a given control condition, the closure and opening stresses become less compressive as the crack propagates.

Kim, K. S.↗
NASA NTRS1991

Quick-Actuating Closure And Handling System

Quick-actuating closure and handling system developed for containers, particularly for high-pressure vessels or shells entered frequently. Features simplicity in design and operation, saving of time, monitoring of leakage, and safety. Another significant feature, use of conical surfaces to engage effectively shear blocks in locked position to provide necessary preload and to transmit internal-pressure load directly to pressure vessel. Designed to impart proper radial movement to shear blocks, based on space available for longitudinal travel of screwjacks. Adaptable to many applications; used for such containers as autoclaves, vacuum chambers, and wide variety of specialized vessels requiring frequent entry and closure.

White, Dorsey E.↗
NASA NTRS1991

Navier-Stokes computation of compressible turbulent flows with a second order closure

The objective was the development of a complete second order closure for wall bounded flows, including all components of the dissipation rate tensor and a numerical solution procedure for the resulting system of equations. The main topics discussed are the closure of the pressure correlations and the viscous destruction terms in the dissipation rate equations and the numerical solution scheme based on a block-tridiagonal solver for the nine equations required for the prediction of plane or axisymmetric flows.

Dingus, C.↗
NASA NTRS1991

Calculations of particle-laden flows by second-moment closures

A differential second-moment closure is applied to calculate nonswirling turbulent particle-laden jet flow. The closure is effected by extending constant-density single-phase turbulence models to the continuous (carrier) phase of two-phase flow. The dispersed (discrete) phase is treated by stochastic-Lagrangian technique. To reduce the effects of numerical (false) diffusion on the predicted results, a higher order differencing scheme, namely the flux-spline, is employed. In addition, an experimental study is conducted to provide data of good quality, especially near the inlet, for model assessment. The experiment consists of a confined coaxial jet flow of air with glass beads. The beads' diameter is 105 microns and is injected from the inner jet with a mass loading ratio of 0.9. The computed results are in good agreement with experimental data, especially for the carrier phase. However, the predicted turbulence fluctuations associated with the discrete phase are lower than the data at the downstream region of the flow field.

Nikjooy, M.↗
NASA NTRS1991

Megaregolith insulation and the duration of cooling to isotopic closure within differential asteroids and the moon

Global cooling of the moon and large (R = 40-250 km) asterodis, was modeled, starting at or near the solidus. A crucial factor in determining the prevailing interval (Ic) of cooling between igenous crystallization and isotopic closure, for any given depth in the crust, is the extent to which the body is insulated by a regolith/megaregolith layer of porous, fragmental impact debris. Given plausible assumptions regarding the thicknesses of such layers on the moon and the eucrite parent asteroid (and regarding the radius of the eucrite asteroid), the results indicate that deep-crustal regions tend to remain above the Nd and Sr isotopic closure temperature for intervals that are long in comparison to the precision of modern Nd- and Sr-based age measurements, and in comparison to suggested chronologic scenarios of global differentiation. Ic intervals of as long as 100 my may be common among available samples of primordial, deep-crustal cumulates from both bodies. Chronologies for the gross solidification of the moon and the eucrite asteroid should allow for the possibility that any single age for a coarse-grained 'plutonic' or cumulate-textured rock might be many tens of millions of years younger than the igneous crystallization age.

Warren, Paul H.↗
NASA NTRS1991

Current collection and current closure in the Tethered Satellite System

Current collection and closure-path modeling are examined analytically with respect to the Tethered Satellite System (TSS). A particle-in cell code is compared with a one-dimensional unmagnetized fluid code to model the behavior of a positively charged satellite in the ionosphere. The morphology of the sheath and the sheath-region processes are thus examined, and the influence of ions leaving the sheath region is found to cause the attraction of an electron current that is 40 times greater than the steady state value. The enhancement is transient and enhances the acceleration of the electrons in the sheath. A set of modified MHD equations, including those for ion inertia, quasineutrality, and electron drift, is employed to model TSS current closure. Whistler modes are found to exist and can be excited as the TSS passes through the ionosphere. Important conclusions include a significant fluctuation level in the steady state sheath, an ion void which affects the electron population, and some long-lived electrons trapped in the settled sheath with respect to two directions.

Drobot, Adam↗
NASA NTRS1992

Comments on the present state of second-order closure models for incompressible flows

Second-order closure models account for history and nonlocal effects of the mean velocity gradients on the Reynolds stress tensor. Turbulent flows involving body forces or curvature, Reynolds stress relaxational effects, and counter-gradient transport are usually better described. The topics are presented in viewgraph form and include: (1) the Reynolds stress transport equation; (2) issues in second-order closure modeling; and (3) near wall models.

Speziale, Charles G.↗