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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1959

Study of Plastic Deformation in Binary Aluminum Alloys by Internal-Friction Methods

The damping capacity of several aluminum-copper alloys has been investigated during tensile elongation. This damping is shown to depend on strain rate, strain, temperature, alloy content, and heat treatment. A tentative hypothesis, based on the acceleration of solute atom diffusion by deformation-produced vacancies, is proposed to account for the observed behavior. Internal-friction maxima are observed in deformed aluminum and aluminum-copper alloys at -70 deg and -50 deg C. The peaks appear to be relatively insensitive to frequency and alloy content, but they disappear after annealing at temperatures nearing the recrystallization temperature.

Olson, E. C.↗
NASA NTRS1972

Hodographic approach to predicting inelastic strain at high temperature

An experimental study of the effect of continuous and discontinuous changes in strain rate on the relationship among strain rate, strain, and stress is described. Data from Udimet 700 in tension at 925 C were used in order to relate cyclic tensile creep to the monotonic properties of the material by means of the hodograph. The nature of modifications caused to the hodograph by discontinuous variation of the strain rate was determined from tests. Reloading at discontinuous strain rate caused reactivation of primary creep. A simple method, based on monotonic material properties, is proposed for predicting cyclic tensile creep response. Preliminary results of cyclic tests agree with predicted response.

Berkovits, A.↗
NASA NTRS1998

Low-Temperature Friction-Stir Welding of 2024 Aluminum

Solid state friction-stir welding (FSW) has been demonstrated to involve dynamic recrystallization producing ultra-fine, equiaxed grain structures to facilitate superplastic deformation as the welding or joining mechanism. However, the average residual, equiaxed, grain size in the weld zone has ranged from roughly 0.5 micron to slightly more than 10 micron, and the larger weld zone grain sizes have been characterized as residual or static grain growth as a consequence of the temperatures in the weld zone (where center-line temperatures in the FSW of 6061 Al have been shown to be as high as 480C or -0.8 T(sub M) where T(sub M) is the absolute melting temperature)). In addition, the average residual weld zone grain size has been observed to increase near the top of the weld, and to decrease with distance on either side of the weld-zone centerline, an d this corresponds roughly to temperature variations within the weld zone. The residual grain size also generally decreases with decreasing FSW tool rotation speed. These observations are consistent with the general rules for recrystallization where the recrystallized grain size decreases with increasing strain (or deformation) at constant strain rate, or with increasing strain-rate, or with increasing strain rate at constant strain; especially at lower ambient temperatures, (or annealing temperatures). Since the recrystallization temperature also decreases with increasing strain rate, the FSW process is somewhat complicated because the ambient temperature, the frictional heating fraction, and the adiabatic heating fraction )proportional to the product of strain and strain-rate) will all influence both the recrystallization and growth within the FSW zone. Significantly reducing the ambient temperature of the base metal or work pieces to be welded would be expected to reduce the residual weld-zone grain size. The practical consequences of this temperature reduction would be the achievement of low temperature welding. This study compares the residual grain sizes and microstructures in 2024 Al friction-stir welded at room temperature (about 30C and low temperature (-30C).

Benavides, S.↗
NASA NTRS2002

Tensile Yielding of Multi-Wall Carbon Nanotube

The tensile yielding of multiwall carbon nanotubes (MWCNTs) has been studied using Molecular Dynamics simulations and a Transition State Theory based model. We find a strong dependence of the yielding on the strain rate. A critical strain rate has been predicted above/below which yielding strain of a MWCNT is larger/smaller than that of the corresponding single-wall carbon nanotubes. At experimentally feasible strain rate of 1% /hour and T = 300K, the yield strain of a MWCNT is estimated to be about 3-4 % higher than that of an equivalent SWCNT (Single Wall Carbon Nanotube), in good agreement with recent experimental observations.

Wei, Chenyu↗
NASA NTRS1996

Effect of chemistry and turbulence on NO formation in oxygen-natural gas flames

The effects of chemistry and turbulence on NO formation in oxygen-natural turbulent diffusion flames gas flames have been investigated. The chemistry of nitric oxides has been studied numerically in the counterflow configuration. Systematic calculations with the GRI 2.11 mechanism for combustion of methane and NO chemistry were conducted to provide a base case. It was shown that the 'simple' Zeldovich mechanism accounts for more than 75% of N2 consumption in the flame in a range of strain-rates varying between 10 and 1000 s-l. The main shortcomings of this mechanism are: 1) overestimation (15%) of the NO production rate at low strain-rates because it does not capture the reburn due to the hydrocarbon chemistry, and 2) underestimation (25%) of the NO production rate at high strainrates because it ignores NO production through the prompt mechanism. Reburn through the Zeldovich mechanism alone proves to be significant at low strain-rates. A one-step model based on the Zeldovich mechanism and including reburn has been developed. It shows good agreement with the GRI mechanism at low strain-rates but underestimates significantly N2 consumption (about 50%) at high strain-rates. The role of turbulence has been assessed by using an existing 3-D DNS data base of a diffusion flame in decaying turbulence. Two PDF closure models used in practical industrial codes for turbulent NO formation have been tested. A simpler version of the global one-step chemical scheme for NO compared to that developed in this study was used to test the closure assumptions of the PDF models, because the data base could not provide all the necessary ingredients. Despite this simplification, it was possible to demonstrate that the current PDF models for NO overestimate significantly the NO production rate due to the fact that they neglect the correlations between the fluctuations in oxygen concentration and temperature. A single scalar PDF model for temperature that accounts for such correlations based on laminar flame considerations has been developed and showed excellent agreement with the values given by the DNS.

Samaniego, J. -M.↗
NASA NTRS1992

An investigation of the thermoviscoplastic behavior of a metal matrix composite at elevated temperatures

This research investigates the response of a fiberless 13 layer hot isostatically pressed Ti-15-3 laminate to creep, constant strain rate, and cyclic constant strain rate loading at temperatures ranging from 482C to 649C. Creep stresses from 48 to 260 MPa and strain rates of .0001 to .01 m/m/sec were used. Material parameters for three unified constitutive models (Bodner-Partom, Miller, and Walker models) were determined for Ti-15-3 from the experimental data. Each of the three models was subsequently incorporated into a rule of mixtures and evaluated for accuracy and ease of use in predicting the thermoviscoplastic response of unidirectional metal matrix composite laminates (both 0 and 90). The laminates were comprised of a Ti-15-3 matrix with 29 volume percent SCS6 fibers. The predicted values were compared to experimentally determined creep and constant strain rate data. It was found that all three models predicted the viscoplastic response of the 0 specimens reasonably well, but seriously underestimated the viscoplastic response of the 90 specimens. It is believed that this discrepancy is due to compliant and/or weak fiber-matrix interphase. In general, it was found that of the three models studied, the Bodner-Partom model was easiest to implement, primarily because this model does not require the use of cyclic constant strain rate tests to determine the material parameters involved. However, the version of the Bodner-Partom model used in this study does not include back stress as an internal state variable, and hence may not be suitable for use with materials which exhibit a pronounced Baushinger effect. The back stress is accounted for in both the Walker and Miller models; determination of the material parameters associated with the Walker model was somewhat easier than in the Miller model.

Rogacki, John R.↗
NASA NTRS1992

Search for subgrid scale parameterization by projection pursuit regression

The dependence of subgrid-scale stresses on variables of the resolved field is studied using direct numerical simulations of isotropic turbulence, homogeneous shear flow, and channel flow. The projection pursuit algorithm, a promising new regression tool for high-dimensional data, is used to systematically search through a large collection of resolved variables, such as components of the strain rate, vorticity, velocity gradients at neighboring grid points, etc. For the case of isotropic turbulence, the search algorithm recovers the linear dependence on the rate of strain (which is necessary to transfer energy to subgrid scales) but is unable to determine any other more complex relationship. For shear flows, however, new systematic relations beyond eddy viscosity are found. For the homogeneous shear flow, the results suggest that products of the mean rotation rate tensor with both the fluctuating strain rate and fluctuating rotation rate tensors are important quantities in parameterizing the subgrid-scale stresses. A model incorporating these terms is proposed. When evaluated with direct numerical simulation data, this model significantly increases the correlation between the modeled and exact stresses, as compared with the Smagorinsky model. In the case of channel flow, the stresses are found to correlate with products of the fluctuating strain and rotation rate tensors. The mean rates of rotation or strain do not appear to be important in this case, and the model determined for homogeneous shear flow does not perform well when tested with channel flow data. Many questions remain about the physical mechanisms underlying these findings, about possible Reynolds number dependence, and, given the low level of correlations, about their impact on modeling. Nevertheless, demonstration of the existence of causal relations between sgs stresses and large-scale characteristics of turbulent shear flows, in addition to those necessary for energy transfer, provides important insight into the relation between scales in turbulent flows.

Meneveau, C.↗
NASA NTRS1976

Plastic deformation of polycrystalline zirconium carbide

The compressive yield strength of arc-melted polycrystalline zirconium carbide has been found to vary from 77 kg per sq mm at 1200 C to 19 kg per sq mm at 1800 C. Yield drops were observed with plastic strain-rates greater than 0.003/sec but not with slower strain rates. Strain-rate change experiments yielded values for the strain-rate sensitivity parameter m which range from 6.5 at 1500 C to 3.8 at 1800 C, and the product dislocation velocity stress exponent times T was found to decrease linearly with increasing temperature. The deformation rate results are consistent with the Kelly-Rowcliffe model in which the diffusion of carbon assists the motion of dislocations.

Darolia, R.↗
NASA NTRS1986

High temperature tensile and creep behaviour of low pressure plasma-sprayed Ni-Co-Cr-Al-Y coating alloy

The high temperature tensile and creep behavior of low pressure plasma-sprayed plates of a typical Ni-Co-Cr-Al-Y alloy has been studied. From room temperature to 800 K, the Ni-Co-Cr-Al-Y alloy studied has nearly a constant low ductility and a high strength. At higher temperatures, it becomes weak and highly ductile. At and above 1123 K, the behavior is highly dependent on strain rate and exhibits classic superplastic characteristics with a high ductility at intermediate strain rates and a strain rate sensitivity of about 0.5. At either higher or lower strain rates, the ductility decreases and the strain rate sensitivities are about 0.2. In the superplastic deformation range, the activation energy for creep is 120 + or - 20 kJ/mol, suggesting a diffusion-aided grain boundary sliding mechanism. Outside the superplastic range, the activation energy for creep is calculated to be 290 + or - 20 kJ/mol.

Hebsur, M. G.↗
NASA NTRS1999

Effects of Internal and External Hydrogen on Inconel 718

Internal hydrogen embrittlement (IHE) and hydrogen environment embrittlement (HEE) tensile and bend crack growth tests were performed on Inconel 718. For the IHE tests, the specimens were precharged to approximately 90 ppm hydrogen by exposure to 34.5 MPa H2 at 650 C. The HEE tests were performed in 34.5 MPa H2. Parameters evaluated were test temperature, strain rate for smooth and notch specimen geometries. The strain rate effect was very significant at ambient temperature for both IHE and HEE and decreased with increasing temperatures. For IHE, the strain rate effect was neglible at 260'C, and for HEE the strain rate effect was neglible at 400 C. At low temperatures, IHE was more severe than HEE, and at high temperatures HEE was more severe than IHE with a cross over temperature about 350 C. At 350 C, the equilibrium hydrogen concentration in Inconel 718 is about 50% lower than the hydrogen content of the precharged IHE specimens. Dislocation hydrogen sweeping of surface absorbed hydrogen was the likely transport mechanism for increasing the hydrogen concentration in the HEE tests sufficiently to produce the same degree of embrittlement as that of the more highly hydrogen charged IHE specimens. The main IHE fracture characteristic was formation of large, brittle flat facets, which decreased with increasing test temperature. The IHE fracture matrix surrounding the large facets ranged between brittle fine faceted to microvoid ductility depending upon strain rate, specimen geometry as well as temperature. The HEE fractures were characteristically fine featured, transgranular and brittle with a significant portion forming a "saw tooth" crystallographic pattern. Both IHE and HEE fractures were predominantly along the {1 1 1) slip and twin boundaries. With respect to embrittlement mechanism, it was postulated that dislocation hydrogen sweeping and hydrogen enhanced localized plasticity were active in HEE and IHE for concentrating hydrogen along (1 1 1) slip and twin planes. Final brittle failure occurred by hydrogen induced planer decohesion.

Walter, R. J.↗
NASA NTRS1995

Natural Strain

The purpose of this paper is to present a consistent and thorough development of the strain and strain-rate measures affiliated with Hencky. Natural measures for strain and strain-rate, as I refer to them, are first expressed in terms of of the fundamental body-metric tensors of Lodge. These strain and strain-rate measures are mixed tensor fields. They are mapped from the body to space in both the Eulerian and Lagrangian configurations, and then transformed from general to Cartesian fields. There they are compared with the various strain and strain-rate measures found in the literature. A simple Cartesian description for Hencky strain-rate in the Lagrangian state is obtained.

Freed, Alan D.↗
NASA NTRS1972

Force-Strain Characteristics and Rupture-Load Capability of Viking-Type Suspension-Line Material Under Dynamic Loading Conditions

A series of tests has been conducted to investigate the elastic behavior of Viking-type suspension-line material under dynamic loading conditions. Results indicate that there is a decrease in both rupture-load capability and elongation at rupture as the test strain rate is increased. Preliminary examination of force-strain characteristics indicates that, on the average, the material exhibits some type of viscous effect which results in a greater force being produced, for a particular value of strain, under dynamic loading conditions than that produced under quasi-static loading conditions. A great deal of uncertainty exists in defining a priori the tensile properties of viscoelastic materials, such as nylon or dacron, under dynamic loading conditions. Additional uncertainty enters the picture when woven configurations such as suspension,line material are considered. To eliminate these uncertainties, with respect to the Viking parachute configuration, a test program has been conducted to obtain data on the tensile properties of Viking-type suspension-line material over a wide range of strain rates. Based on preliminary examination of these data, the following conclusions can be drawn: 1. Material rupture-load capability decreases as strain-rate is increased. At strain rates above 75 percent/sec, no rupture loads were observed which would meet the minimum tensile strength specification of 880 pounds. 2. The material, on the average, exhibits some type of viscous effect which, for a particular value of strain, produces a greater load under dynamic loading conditions than that produced under quasi-static loading conditions.

Poole, Lamont R.↗
NASA NTRS1979

Endochronic theory of transient creep and creep recovery

Short time creep and creep recovery were investigated by means of the endochronic theory of viscoplasticity. It is shown that the constitutive equations for constant-strain-rate stress-strain behavior, creep, creep recovery, and stress relaxation can all ber derived from the general constitutive equation by imposing appropriate constraints. In this unified approach, the effect of strain-hardening is naturally accounted for when describing creep and creep recovery. The theory predicts with reasonable accuracy the creep and creep recovery behaviors for Aluminum 1100-0 at 150 C. It was found that the strain-rate history at prestraining stage affects the subsequent creep. A critical stress was also established for creep recovery. The theory predicts a forward creep for creep recovery stress greater than the critical stress. For creep recovery stress less than the critical stress, the theory then predicts a normal strain recovery.

Wu, H. C.↗
NASA NTRS1995

Radiant extinction of gaseous diffusion flames

The absence of buoyancy-induced flows in microgravity significantly alters the fundamentals of many combustion processes. Substantial differences between normal-gravity and microgravity flames have been reported during droplet combustion, flame spread over solids, candle flames, and others. These differences are more basic than just in the visible flame shape. Longer residence time and higher concentration of combustion products create a thermochemical environment which changes the flame chemistry. Processes such as flame radiation, that are often ignored under normal gravity, become very important and sometimes even controlling. This is particularly true for conditions at extinction of a microgravity diffusion flame. Under normal-gravity, the buoyant flow, which may be characterized by the strain rate, assists the diffusion process to transport the fuel and oxidizer to the combustion zone and remove the hot combustion products from it. These are essential functions for the survival of the flame which needs fuel and oxidizer. Thus, as the strain rate is increased, the diffusion flame which is 'weak' (reduced burning rate per unit flame area) at low strain rates is initially 'strengthened' and eventually it may be 'blown-out'. Most of the previous research on diffusion flame extinction has been conducted at the high strain rate 'blow-off' limit. The literature substantially lacks information on low strain rate, radiation-induced, extinction of diffusion flames. At the low strain rates encountered in microgravity, flame radiation is enhanced due to: (1) build-up of combustion products in the flame zone which increases the gas radiation, and (2) low strain rates provide sufficient residence time for substantial amounts of soot to form which further increases the flame radiation. It is expected that this radiative heat loss will extinguish the already 'weak' diffusion flame under certain conditions. Identifying these conditions (ambient atmosphere, fuel flow rate, fuel type, etc.) is important for spacecraft fire safety. Thus, the objective is to experimentally and theoretically investigate the radiation-induced extinction of diffusion flames in microgravity and determine the effect of flame radiation on the 'weak' microgravity diffusion flame.

Atreya, Arvind↗
NASA NTRS1982

The stress-corrosion behavior of Al-Li-Cu alloys: A comparison of test methods

Two powder metallurgy processed (Al-Li-Cu) alloys with and without Mg addition were studied in aqueous 3.5% NaCl solution during the alternate immersion testing of tuning fork specimens, slow crack growth tests using fracture mechanics specimens, and the slow strain rate testing of straining electrode specimens. Scanning electron microscopy and optical metallography were used to demonstrate the character of the interaction between the Al-Li-Cu alloys and the selected environment. Both alloys are susceptible to SC in an aqueous 3.5% NaCl solution under the right electrochemical and microstructural conditions. Each test method yields important information on the character of the SC behavior. Under all conditions investigated, second phase particles strung out in rows along the extrusion direction in the alloys were rapidly attacked, and played principal role in the SC process. With time, larger pits developed from these rows of smaller pits and under certain electrochemical conditions surface cracks initiated from the larger pits and contributed directly to the fracture process. Evidence to support slow crack growth was observed in both the slow strain rate tests and the sustained immersion tests of precracked fracture mechanics specimens. The possible role of H2 in the stress corrosion cracking process is suggested.

Rizzo, P. P.↗
NASA NTRS2001

Quantitative Studies on the Propagation and Extinction of Near-Limit Premixed Flames under Normal- and Micro-gravity

Strained laminar flames have been systematically studied, as the understanding of their structure and dynamic behavior is of relevance to turbulent combustion.. Most of these studies have been conducted in opposed-jet, stagnation-type flow configurations. Studies at high strain rates are important in quantifying and understanding the response of vigorously burning flames and determine extinction states. Studies of weakly strained flames can be of particular interest for all stoichiometries. For example, the laminar flame speeds, S(sup o)(sub u), can be accurately determined by using the counterflow technique only if measurements are obtained at very low strain rates. Furthermore, near-limit flames are stabilized by weak strain rates. Previous studies have shown that near-limit flames are particularly sensitive to chain mechanisms, thermal radiation, and unsteadiness. The stabilization and study of weakly strained flames is complicated by the presence of buoyancy that can render the flames unstable to the point of extinction. Thus, the use of microgravity (mu-g) becomes essential in order to provide meaningful insight into this important combustion regime. In our past studies the laminar flame speeds and extinction strain rates were directly measured at ultra-low strain rates. The laminar flame speeds were measured by having a positively strained planar flame undergoing a transition to a negatively strained Bunsen flame and by measuring the propagation speed during that transition. The extinction strain rates of near-limit flames were measured in mu-g. Results obtained for CH4/air and C3H8/air mixtures are in agreement with those obtained by Maruta et al.

Egolfopoulos, F. N.↗
NASA NTRS1996

Verification of Experimental Techniques for Flow Surface Determination

The concept of a yield surface is central to the mathematical formulation of a classical plasticity theory. However, at elevated temperatures, material response can be highly time-dependent, which is beyond the realm of classical plasticity. Viscoplastic theories have been developed for just such conditions. In viscoplastic theories, the flow law is given in terms of inelastic strain rate rather than the inelastic strain increment used in time-independent plasticity. Thus, surfaces of constant inelastic strain rate or flow surfaces are to viscoplastic theories what yield surfaces are to classical plasticity. The purpose of the work reported herein was to validate experimental procedures for determining flow surfaces at elevated temperatures. Since experimental procedures for determining yield surfaces in axial/torsional stress space are well established, they were employed -- except inelastic strain rates were used rather than total inelastic strains. In yield-surface determinations, the use of small-offset definitions of yield minimizes the change of material state and allows multiple loadings to be applied to a single specimen. The key to the experiments reported here was precise, decoupled measurement of axial and torsional strain. With this requirement in mind, the performance of a high-temperature multi-axial extensometer was evaluated by comparing its results with strain gauge results at room temperature. Both the extensometer and strain gauges gave nearly identical yield surfaces (both initial and subsequent) for type 316 stainless steel (316 SS). The extensometer also successfully determined flow surfaces for 316 SS at 650 C. Furthermore, to judge the applicability of the technique for composite materials, yield surfaces were determined for unidirectional tungsten/Kanthal (Fe-Cr-Al).

Lissenden, Cliff J.↗
NASA NTRS1993

A mechanism for crustal recycling on Venus

Entrainment of lower crust by convective mantle downflows is proposed as a crustal recycling mechanism on Venus. The mechanism is characterized by thin sheets of crust being pulled into the mantle by viscous flow stresses. Finite element models of crust/mantle interaction are used to explore tectonic conditions under which crustal entrainment may occur. The recycling scenarios suggested by the numerical models are analogous to previously studied problems for which analytic and experimental relationships assessing entrainment rates have been derived. We use these relationships to estimate crustal recycling rates on Venus. Estimated rates are largely determined by (1) strain rate at the crust/mantle interface (higher strain rate leads to greater entrainment); and (2) effective viscosity of the lower crust (viscosity closer to that of mantle lithosphere leads to greater entrainment). Reasonable geologic strain rates and available crustal flow laws suggest entrainment can recycle approximately equal 1 cu km of crust per year under favorable conditions.

Lenardic, A.↗