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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Experimental verification of damping mechanisms in a composite beam

A method of estimating the distributed damping parameters based on the measured modal parameters (frequency and damping ratios) was derived. Three different mathematical models were used to model the damping mechanism of a quasi-isotropic pultruded cantilevered beam. These three models were (1) viscous (air) damping, (2) strain rate damping, and (3) both viscous and strain rate damping. The eigenvalues of the partial differential equation model were found to be uninfluenced by any of the three assumed damping models. Values were obtained for each of the damping models as well as the modulus of elasticity of the beam. It was found that the two-parameter damping model provided the best fit to measured modal data. However, the two-parameter damping model could only reproduce the measured damping ratios to within 85 percent.

Cudney, Harley H.↗
NASA NTRS2006

Dynamic Fracture Initiation Toughness at Elevated Temperatures With Application to the New Generation of Titanium Aluminide Alloys

Recently, a new generation of titanium aluminide alloy, named Gamma-Met PX, has been developed with better rolling and post-rolling characteristics. I'revious work on this alloy has shown the material to have higher strengths at room and elevated temperatures when compared with other gamma titanium aluminides. In particular, this new alloy has shown increased ductility at elevated temperatures under both quasi-static and high strain rate uniaxial compressive loading. However, its high strain rate tensile ductility at room and elevated temperatures is limited to approx. 1%. In the present chapter, results of a study to investigate the effects of loading rate and test temperature on the dynamic fracture initiation toughness in Gamma-Met PX are presented. Modified split Hopkinson pressure bar was used along with high-speed photography to determine the crack initiation time. Three-point bend dynamic fracture experiments were conducted at impact speeds of approx. 1 m/s and tests temperatures of up-to 1200 C. The results show that thc dynamic fracture initiation toughness decreases with increasing test temperatures beyond 600 C. Furthermore, thc effect of long time high temperature air exposure on the fracture toughness was investigated. The dynamic fracture initiation toughness was found to decrease with increasing exposure time. The reasons behind this drop are analyzed and discussed.

Shazly, Mostafa↗
NASA NTRS2011

Turbulence Modeling for Shock Wave/Turbulent Boundary Layer Interactions

Accurate aerodynamic computational predictions are essential for the safety of space vehicles, but these computations are of limited accuracy when large pressure gradients are present in the flow. The goal of the current project is to improve the state of compressible turbulence modeling for high speed flows with shock wave / turbulent boundary layer interactions (SWTBLI). Emphasis will be placed on models that can accurately predict the separated region caused by the SWTBLI. These flows are classified as nonequilibrium boundary layers because of the very large and variable adverse pressure gradients caused by the shock waves. The lag model was designed to model these nonequilibrium flows by incorporating history effects. Standard one- and two-equation models (Spalart Allmaras and SST) and the lag model will be run and compared to a new lag model. This new model, the Reynolds stress tensor lag model (lagRST), will be assessed against multiple wind tunnel tests and correlations. The basis of the lag and lagRST models are to preserve the accuracy of the standard turbulence models in equilibrium turbulence, when the Reynolds stresses are linearly related to the mean strain rates, but create a lag between mean strain rate effects and turbulence when nonequilibrium effects become important, such as in large pressure gradients. The affect this lag has on the results for SWBLI and massively separated flows will be determined. These computations will be done with a modified version of the OVERFLOW code. This code solves the RANS equations on overset grids. It was used for this study for its ability to input very complex geometries into the flow solver, such as the Space Shuttle in the full stack configuration. The model was successfully implemented within two versions of the OVERFLOW code. Results show a substantial improvement over the baseline models for transonic separated flows. The results are mixed for the SWBLI assessed. Separation predictions are not as good as the baseline models, but the over prediction of the peak heat flux downstream of the reattachment shock that plagues many models is reduced.

Lillard, Randolph P.↗
NASA NTRS1983

The dynamic tensile strength of ice and ice-silicate mixtures

The dynamic tensile strength of icy media is measured at strain rates on the order of 10,000/sec to aid in the understanding of impact and cratering phenomena. Compressed samples consisting of ice and ice-silicate mixtures with 5 and 30 wt % sand were impacted at temperatures between 230 and 250 K by projectile plexiglas plates imparting the required strain rates in less than 0.75 microsec. Taking the tensile stress corresponding to the transition from intact to spalled or fragmented samples as the dynamic tensile strength, strengths of 17, 20 and 22 MPa were obtained for the pure ice, 5 wt % sand, and 30 wt % sand specimens, respectively. The values lie considerably above those observed in static testing. A continuum fracturing model is used to obtain relations between tensile strength and stress rate as well as to derive stress and damage histories during tensile loading and the size distribution of icy fragments as a function of strain rate.

Lange, M. A.↗
NASA NTRS2018

Plasticity and Damage Modeling of Stress Asymmetry and Dynamic Behavior of AFS Additive Manufactured Aluminum Alloy 2219

The Solid State Additive Manufacturing (AM) process referred as MELD that fabricated the samples in this study, provides a new path for repairing, coating, joining and additive manufacturing metals and metal matrix composites. This research will be the first application of a physics-based microstructure dependent internal state variable (ISV) plasticity and damage material model to capture the mechanical response of an AM Aluminum Alloy (AA) 2219 via the MELD process. In this research, a microstructure-based internal state variable (ISV) plasticity-damage model was used to capture the mechanical behavior of AFS 2219 aluminum alloy. Aeroprobe Corporation, creator and patent holder for the MELD process, fabricated the material by pushing a solid filler rod of AA2219-T861 material through a hollow rotating tool onto an AA2219 T851 plate substrate. As feedstock, solid or powder precursor metals are pushed through a nonconsumable rotating cylindrical tool. Herein, added layers are deposited and metallurgically bonded to substrate material or previously deposited layers by the heat generated from the rotating tool through plastic deformation of the filler material. Once a layer has been added, the tool height increases, and starts the deposition of the next layer. This process results in beneficial properties such as grain refinement, homogenization and reduced porosity (fully dense). This process will experience temperatures similar to those in the weld nugget zone (WNZ) in friction stir welding (FSW), ranging from 0.6-0.9 Tm, with Tm being the melting point of the material. MELD is highly scalable with AA deposition rates reaching over 1000 cm3/hr, which allows for MELD being used for repairs, coatings, and building components. A motivating factor driving the research for physics-based history dependent material modeling of MELD components is the ability to accurately capture the stress-state and strain rate dependence in the material caused by variations in material microstructure from the MELD processing of new or repaired components. The ISV model incorporates microstructural content and is consistent with continuum level kinematics, kinetics, and thermodynamics. These features allow the ISV model to capture large deformations at the structural scale using the kinematic and isotropic hardening, while microscale damage is obtained from the microstructural features. The benefits of the ISV model arise from the inclusion of structure-property relationships identified from microstructural characterization and experimentation. The Bauschinger effect (BE) is an important concept, vital in the accurate prediction of cyclic stress-strain response of ductile materials such as metals. The ISV model has been successfully used to capture the behavior and damage, and the BE of different aluminum alloys and steels. The ISV model uses kinematic and isotropic hardening to help capture deformations of the material at the macro scale. To understand this hardening relationship, calculating the kinematic and isotropic hardening relationship in the material is warranted for a high-fidelity model. Electron Backscattered Diffraction (EBSD) was used to characterize the as-fabricated microstructure, where a fully-dense equiaxed grain morphology with average grain size of 2.5 μm was observed. Microhardness mapping of the as-built structures, monotonic tension and compression experiments at both quasi-static (0.001/s) strain rates, tension-followed-by-compression and compression-followed-by-tension experiments were performed to obtain the set of plasticity and damage constants necessary to capture strain rate and stress state behavior of this additive material. To calibrate the plasticity-damage model, a single set of constants were determined to capture the different stress states the MELD AA2219. One set of the constants was determined from experimental true stress-strain curves for the tension and compression data. Additionally, microstructural information and data from the open literature were used as the other model constants. This research is a first of its kind for AFS AA2219, includes correlating the ISV model to the monotonic experimental results that capture the isotropic and kinematic plasticity mechanical response.

Rivera, O. G.↗
DOE OSTI2020

Deformation behavior of annealed Cu64Zr36 metallic glass via molecular dynamics simulations

In the present work, we investigate the isothermal annealing process of Cu 64 Zr 36 metallic glass (MG) by means of molecular dynamics (MD) simulations, and characterize the compression properties under different temperatures and compressive rates. The pair distribution functions, the bond-orientational order, and the cluster-type index method are modeled and analyzed to characterize changes in the structure. Results of the modeling and analysis reveal that the MgZn 2 -type Laves, MgCu 2 -type Laves and five-fold local symmetry structures can be formed during the isothermal holding at 950 K. Study on the compression properties shows that at a fixed strain rate, the yield strength and yield drop all increase with decreasing temperature. At a fixed temperature, however, the increase of the strain rate leads to a noticeable increase in the yield strength at 600 K, but has little effect on yield strength at 10 K. Moreover, the modeling and analysis of the structure at a temperature of 10 K and strain-rate of 5 × 10 7 s -1 demonstrate that the order-disorder transformation initiates the shear band.

36 MATERIALS SCIENCE↗
NASA NTRS1982

On finite deformation elasto-plasticity

Lee (1969) has proposed a theory based on the decomposition of the total deformation gradient to an elastic and plastic part, and from it has concluded that the additive decomposition of the strain rates holds only approximately. Lubarda and Lee (1981) have declared that Lee's 'exact finite-deformation kinematics shows the almost universal assumption that the total velocity strain or rate of deformation is the sum of elastic and plastic rates to be in error'. Hence questions are raised regarding the validity of essentially all finite deformation elasto-plasticity theories. The present investigation is concerned with these questions. It is shown that the additive decomposition of the strain rates follows from all common finite elasto-plasticity concepts. Lee's theory is examined, and it is shown that this theory also leads to an additive strain rate decomposition, and therefore, his conclusion stems from misinterpretation. It is found that the elastic and the plastic strain rates considered by Lee do not correspond to the same configuration. They are, therefore, not compatible measures.

Nemat-Nasser, S.↗
NASA NTRS2002

Dynamics of Diffusion Flames in von Karman Swirling Flows Studied

Von Karman swirling flow is generated by the viscous pumping action of a solid disk spinning in a quiescent fluid media. When this spinning disk is ignited in an oxidizing environment, a flat diffusion flame is established adjacent to the disk, embedded in the boundary layer (see the preceding illustration). For this geometry, the conservation equations reduce to a system of ordinary differential equations, enabling researchers to carry out detailed theoretical models to study the effects of varying strain on the dynamics of diffusion flames. Experimentally, the spinning disk burner provides an ideal configuration to precisely control the strain rates over a wide range. Our original motivation at the NASA Glenn Research Center to study these flames arose from a need to understand the flammability characteristics of solid fuels in microgravity where slow, subbuoyant flows can exist, producing very small strain rates. In a recent work (ref. 1), we showed that the flammability boundaries are wider and the minimum oxygen index (below which flames cannot be sustained) is lower for the von Karman flow configuration in comparison to a stagnation-point flow. Adding a small forced convection to the swirling flow pushes the flame into regions of higher strain and, thereby, decreases the range of flammable strain rates. Experiments using downward facing, polymethylmethacrylate (PMMA) disks spinning in air revealed that, close to the extinction boundaries, the flat diffusion flame breaks up into rotating spiral flames (refs. 2 and 3). Remarkably, the dynamics of these spiral flame edges exhibit a number of similarities to spirals observed in biological systems, such as the electric pulses in cardiac muscles and the aggregation of slime-mold amoeba. The tail of the spiral rotates rigidly while the tip executes a compound, meandering motion sometimes observed in Belousov-Zhabotinskii reactions.

Nayagam, Vedha↗
NASA NTRS1994

The effects of interstitial content and annealing on the flow and fracture behavior on polycrystalline beta-NiAl

The strain behavior of three polycrystalline NiAl alloys has been investigated at temperatures between 300 and 1200 K. Yield stress plateaus, yield stress transients upon a ten-fold increase in strain rate, work hardening peaks, and dips in the strain rate sensitivity (SRS) have been observed between 700 and 800 K. These observations are indicative of dynamic strain aging (DSA) and are discussed in terms of conventional strain aging theories.

Weaver, M. L.↗
DOE OSTI2022

A continuum damage approach to spallation and the role of microinertia

Spall failure is of interest due to its prevalence in high strain rate problems in which the spallation is driven by the interaction of release waves. In this article, a porosity-based damage model that includes microinertial effects is used to examine spall failure. The model is successfully calibrated to plate impact-driven experiments and then used to evaluate experimental conditions producing more extreme strain rate conditions, such as those in laser-driven experiments. The incorporation of microinertia allows us to better understand the increase in apparent macroscopic spall strength seen at high strain rates. Correspondingly, we conclude that the incorporation of microinertial effects improves the model’s predictive capabilities. Microinertial effects result in more severe local tensile stresses that affect the damage evolution, and microinertia can play a significant role in the approach to the material’s ideal strength at extreme loading rates. Additionally, a preliminary parametric study is also carried out to investigate the role of microstructural aspects such as nucleation volume fraction and initial pore radius. One counter-intuitive result from the microinertial effects is that, for a given nucleation site volume fraction, having larger initial pore nucleation sites can lead to an improved macroscopic spall strength.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Stress-controlled medium-amplitude oscillatory shear (MAOStress) of PVA–Borax

We report the first-ever complete measurement of MAOStress material functions, which reveal that stress can be more fundamental than strain or strain rate for understanding linearity limits as a function of Deborah number. The material used is a canonical viscoelastic liquid with a single dominant relaxation time: polyvinyl alcohol (PVA) polymer solution cross-linked with tetrahydroborate (Borax) solution. We outline experimental limit lines and their dependence on geometry and test conditions. These MAOStress measurements enable us to observe the frequency dependence of the weakly nonlinear deviation as a function of stress amplitude. The observed features of MAOStress material functions are distinctly simpler than MAOStrain, where the frequency dependence is much more dramatic. The strain-stiffening transient network model was used to derive a model-informed normalization of the nonlinear material functions that accounts for their scaling with linear material properties. Moreover, we compare the frequency dependence of the critical stress, strain, and strain-rate for the linearity limit, which are rigorously computed from the MAOStress and MAOStrain material functions. While critical strain and strain-rate change by orders of magnitude throughout the Deborah number range, critical stress changes by a factor of about 2, showing that stress is a more fundamental measure of nonlinearity strength. This work extends the experimental accessibility of the weakly nonlinear regime to stress-controlled instruments and deformations, which reveal material physics beyond linear viscoelasticity but at conditions that are accessible to theory and detailed simulation.

Mechanics↗
DOE OSTI2022

High-Temperature Tensile and Creep Test Results on Thin Wall Tube Specimens of ODS Alloys 14YWT and OFRAC

High-temperature tensile and strain rate jump (SRJ) creep tests were conducted on axial specimens fabricated from thin wall tubes of the oxide dispersion strengthened (ODS) alloys 14YWT and OFRAC. The dimensions of the 14YWT thin wall tube was 10.7 mm OD, 0.5 mm WT and 0.92 m and of the OFRAC thin wall tube was 8.5 mm OD + 0.5 mm WT and 1.78 m in length. The axial specimens fabricated from the thin wall tubes were based on a dual-gauge design with gauge dimensions of 4 mm long and 2 mm wide. The tensile tests were conducted in air at 800ºC, 900ºC and 1,000ºC and the SRJ creep tests were conducted in air at 800ºC. Two axial specimens were used in each tensile and SRJ creep tests. The results of the tensile tests showed much lower yield stress (YS) compared to ultimate tensile strength (UTS) indicative of large work hardening properties. The results showed the UTS of OFRAC was higher than that of 14YWT by ~35 MPa at 800ºC. At 900ºC, the UTS of OFRAC and 14YWT was similar at 900ºC and at 1000ºC, the UTS of 14YWT was higher than that of OFRAC by ~10 MPa. The ductility properties of OFRAC were greater than that of 14YWT with higher values of uniform elongation (UE) and total elongation (TE) at all tensile test temperatures. The creep properties of 14YWT and OFRAC were evaluated by the strain rate jump test method at 800ºC. The stress exponent (n) for creep was obtained from the SRJ test data by plotting the values of the log strain rate against the log stress, which is measured from the stress-strain curve generated during the SRJ test. The results showed the highest stress exponents of n = 19.8 and n = 35.0 were measured from the two SRJ tests on 14YWT. The stress exponent measured from one successful SRJ test on OFRAC was n = 13.3, which was lower than that of 14YWT. Nevertheless, these high values of stress exponent are consistent with ODS ferritic alloys such as 14YWT and OFRAC since they indicate dislocation-particle interactions are the dominant creep mechanism for specific ranges of temperature and stress. The results obtained from the tensile and SRJ creep tests conducted on axial specimens in this study indicate that 14YWT and OFRAC retain high strengths at temperatures up to 1,000ºC and good creep performance at 800ºC.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Using Particle Image Velocimetry to Determine Turbulence Model Parameters

The primary parameter of a standard k-ϵ model, C μ , was calculated from stereoscopic particle image velocimetry (PIV) data for a supersonic jet exhausting into a transonic crossflow. This required the determination of turbulent kinetic energy, turbulent eddy viscosity, and turbulent energy dissipation rate. Image interrogation was optimized, with different procedures used for mean strain rates and Reynolds stresses, to produce useful turbulent eddy viscosity fields. The eddy viscosity was calculated by a least-squares fit to all components of the three-dimensional strain-rate tensor that were available from the PIV data. This eliminated artifacts and noise observed when using a single strain component. Local dissipation rates were determined via Kolmogorov’s similarity hypotheses and the second-order structure function. The eddy viscosity and dissipation rates were then combined to determine C μ . Considerable spatial variation was observed in C μ , with the highest values found in regions where turbulent kinetic energy was relatively low but where turbulent mixing was important, e.g., along the high-strain jet edges and in the wake region. This suggests that use of a constant C μ in modeling may lead to poor Reynolds stress predictions at mixing interfaces. A data-driven modeling approach that can predict this spatial variation of C μ based on known state variables may lead to improved simulation results without the need for calibration.

42 ENGINEERING↗
DOE OSTI2022

Elucidating size effects on the yield strength of single-crystal Cu via the Richtmyer–Meshkov instability

Capturing the dynamic response of a material under high strain-rate deformation often demands challenging and time consuming experimental effort. While shock hydrodynamic simulation methods can aid in this area, a priori characterizations of the material strength under shock loading and spall failure are needed in order to parameterize constitutive models needed for these computational tools. Moreover, parameterizations of strain-rate-dependent strength models are needed to capture the full suite of Richtmyer–Meshkov instability (RMI) behavior of shock compressed metals, creating an unrealistic demand for these training data solely on experiments. Herein, we sweep a large range of geometric, crystallographic, and shock conditions within molecular dynamics (MD) simulations and demonstrate the breadth of RMI in Cu that can be captured from the atomic scale. In this work, yield strength measurements from jetted and arrested material from a sinusoidal surface perturbation were quantified as Y RMI = 0.787 ± 0.374 GPa, higher than strain-rate-independent models used in experimentally matched hydrodynamic simulations. Defect-free, single-crystal Cu samples used in MD will overestimate Y RMI , but the drastic scale difference between experiment and MD is highlighted by high confidence neighborhood clustering predictions of RMI characterizations, yielding incorrect classifications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1993

Time dependent turbulence modeling and analytical theories of turbulence

By simplifying the direct interaction approximation (DIA) for turbulent shear flow, time dependent formulas are derived for the Reynolds stresses which can be included in two equation models. The Green's function is treated phenomenologically, however, following Smith and Yakhot, we insist on the short and long time limits required by DIA. For small strain rates, perturbative evaluation of the correlation function yields a time dependent theory which includes normal stress effects in simple shear flows. From this standpoint, the phenomenological Launder-Reece-Rodi model is obtained by replacing the Green's function by its long time limit. Eddy damping corrections to short time behavior initiate too quickly in this model; in contrast, the present theory exhibits strong suppression of eddy damping at short times. A time dependent theory for large strain rates is proposed in which large scales are governed by rapid distortion theory while small scales are governed by Kolmogorov inertial range dynamics. At short times and large strain rates, the theory closely matches rapid distortion theory, but at long times it relaxes to an eddy damping model.

Rubinstein, R.↗
NASA NTRS1996

Interferometric Radar Observations of Glaciar San Rafael, Chile

Interferometric radar observations of Glaciar San Rafael, Chile, were collected in October 1994 by NASA's Spaceborne Imaging Radar C (SIR-C) at both L- (24cm) and C-band frequency (5.6cm), with vertical transmit and receive polarization. The C-band data did not yield good geophysical products, because the temporal coherence of the signal was significantly reduced after 24h. The L-band data were, however, successfully employed to map the surface topography of the icefield with a 10m uncertainty in height, and measure ice velocity with a precision of 4 mm/d or 1.4 m/a. The corresponding error in strain rates is 0.05/a at a 30 m horizontal spacing. The one-dimensional interferometric velocities were subsequently converted to horizontal displacements by assuming a flow direction and complemented by feature-tracking results near the calving front. The results provide a comprehensive view of the ice-flow dynamics of Glaciar San Rafael. The glacier has a core of rapid flow, 4.5 km in width and 3.5 degrees in average slope,surrounded by slower moving ice, not by rock. Ice velocity is 2.6 m/d or 0.95 km/a near the equilibrium line altitude (1200m), increasing rapidly before the glacier enters the narrower terminal valley, to reach 17.5 m/d or 6.4 km/a at the calving front. Strain rates are dominated by lateral shearing at the glacier margins (0.4-0.7/a), except for the terminal-valley section, where longitudinal strain rates average close to 1/a. This spectacular longitudinal increase in ice velocity in the last few kilometers may be a fundamental feature of tidewater glaciers.

Rignot, Eric↗
NASA NTRS1993

The viscoplastic behavior of Hastelloy-X single crystal

A viscoplastic constitutive model for simulating the behavior of Hastelloy-X single crystal material was derived based on crystallographic slip theory. To determine the appropriate constitutive model constants and to test the predictions of the model, tests on Hastelloy-X crystals were carried out, including the rate sensitivity, cyclic hardening, nonproportional hardening, relaxation, and strain rate dip tests. It was found necessary to include cube slip in the model in order to correlate the uniaxial behavior of the single crystal, to incorporate the interaction effects in both the hardening and the dynamic recovery evolution equations for the drag stress, and to successfully capture correct strain rate sensitivity under biaxial tension-torsion loading conditions.

Jordan, Eric H.↗
DOE OSTI2025

Meshfree simulation and prediction of recrystallized grain size in friction stir processed 316L stainless steel

Friction stir processing (FSP) is a promising solid-phase microstructural modification technique that can repair and enhance damaged stainless steel surfaces exposed to harsh environments. The quality of the repaired material is closely correlated to the recrystallized grain size in the stir zone (SZ), which is influenced by the thermomechanical conditions dictated by FSP process parameters. Thus, establishing a reliable relationship between these parameters and recrystallized grain size in the SZ is crucial for optimizing repair quality. However, existing experimental approaches often rely on indirect temperatures measured far from the SZ, along with rough strain rate estimations, which are imprecise and time-consuming. Meanwhile, existing mesh-based modeling methods usually face numerical challenges when dealing with the large material deformations inherent in FSP. Here, to address these issues, this study introduces a meshfree process model for FSP based on the smoothed particle hydrodynamics (SPH) method, aimed at predicting process conditions under different parameters. The model is validated using experimental data from 11 combinations of tool traverse and rotation speeds on 316 L stainless steel. Correlations between process parameters, material flow, temperature, strain, strain rate, and recrystallized grain size are revealed through SPH simulations and electron backscatter diffraction (EBSD) imaging. The results show that in situ SZ temperatures range from 1071 to 1322°C, which exceed the tool temperature by over 300°C. Furthermore, SZ temperature, strain rate, and grain size increase monotonically with higher tool temperature and faster traverse speed. A relationship is then established between the model-predicted Zener-Hollomon parameter and the recrystallized grain size based on EBSD data, expressed as ln(d) = -0.364 ln(Z) + 14.673. Finally, this relationship exhibits satisfactory accuracy with errors of less than 26.9% in predicting grain sizes at various SZ locations, which offers valuable insights for optimizing FSP repair processes for 316 L stainless steel.

316L stainless steel↗