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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Microstructural Evaluation of Forging Parameters for Superalloy Disks

Forgings of nickel base superalloy were formed under several different strain rates and forging temperatures. Samples were taken from each forging condition to find the ASTM grain size, and the as large as grain (ALA). The specimens were mounted in bakelite, polished, etched and then optical microscopy was used to determine grain size. The specimens ASTM grain sizes from each forging condition were plotted against strain rate, forging temperature, and presoak time. Grain sizes increased with increasing forging temperature. Grain sizes also increased with decreasing strain rates and increasing forging presoak time. The ALA had been determined from each forging condition using the ASTM standard method. Each ALA was compared with the ASTM grain size of each forging condition to determine if the grain sizes were uniform or not. The forging condition of a strain rate of .03/sec and supersolvus heat treatment produced non uniform grains indicated by critical grain growth. Other anomalies are noted as well.

Falsey, John R.↗
NASA NTRS1993

Tensile and stress-rupture behavior of hafnium carbide dispersed molybdenum and tungsten base alloy wires

The tensile strain rate sensitivity and the stress-rupture strength of Mo-base and W-base alloy wires, 380 microns in diameter, were determined over the temperature range from 1200 K to 1600 K. Three molybdenum alloy wires; Mo + 1.1w/o hafnium carbide (MoHfC), Mo + 25w/o W + 1.1w/o hafnium carbide (MoHfC+25W) and Mo + 45w/o W + 1.1w/o hafnium carbide (MoHfC+45W), and a W + 0.4w/o hafnium carbide (WHfC) tungsten alloy wire were evaluated. The tensile strength of all wires studied was found to have a positive strain rate sensitivity. The strain rate dependency increased with increasing temperature and is associated with grain broadening of the initial fibrous structures. The hafnium carbide dispersed W-base and Mo-base alloys have superior tensile and stress-rupture properties than those without HfC. On a density compensated basis the MoHfC wires exhibit superior tensile and stress-rupture strengths to the WHfC wires up to approximately 1400 K. Addition of tungsten in the Mo-alloy wires was found to increase the long-term stress rupture strength at temperatures above 1400 K. Theoretical calculations indicate that the strength and ductility advantage of the HfC dispersed alloy wires is due to the resistance to recrystallization imparted by the dispersoid.

Yun, Hee Mann↗
DOE OSTI2023

Integrated modeling of cracking during deep penetration laser welding of nickel alloys

During deep penetration laser welding of nickel alloys, interactions between composition and processing conditions can lead to the formation of defects. Inconel 740H, for example, has demonstrated a susceptibility to horizontal fusion zone cracking at locations between 70% and 80% of the weld depth during laser welding at powers above 5 kW. Coupling three-dimensional heat transfer, fluid flow, and stress modeling tools allowed that both the strain rate and stress normal to the solidification direction to be calculated. In the Inconel 740H welds, cracks formed at locations where the strain rate and stress simultaneously reached critical levels. No cracking was observed in the Inconel 690 welds, since the strain rate and stress did not simultaneously reach these critical levels.

36 MATERIALS SCIENCE↗
DOE OSTI2020

AA2024/SiC metal matrix composites simultaneously improve ductility and cracking resistance during elevated temperature deformation

Here this study uses the stir-casting technique to combine a semi-solid AA2024 alloy directly with finely-sized β-SiC p embedded as a powder or with mechanically alloyed granules as a delivery agent. Liquid-state primary fabrication tends to form agglomerates of reinforcement particles, whereas rolling better distributes the composite constituents. Sub-micron reinforcements of low volume fractions do not significantly increase the hardness of the composite materials. Uniaxial tensile testing at elevated temperatures over a wide range of strain rates showed simultaneous increases in the ductility and crack resistance of AA2024 + SiC p granules embedded as a powder when compared to the non-reinforced control material at lower strain rates, with the same toughness as the control material. The maximum engineering strain of 252.7 ± 19.2% was observed in AA2024/SiCp at a strain rate of 10 -4 s -1 . This improvement in properties is attributed to grain refinement in the MMCs, leading to pinning events during the straining and ductility increases. The resultant impediments to grain growth and crack propagation allow the fine-sized reinforcements to control dynamic microstructural changes during fatigue. Cube {001}<100> is a dominant texture component in AA2024, whereas the Goss {011}<100> and S {123}<634> components mainly represent the texture of the discontinuously reinforced aluminum matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Interseismic Strain Accumulation between the Colorado Plateau and the Eastern California Shear Zone: Implications for the Seismic Hazard near Las Vegas, Nevada

ABSTRACT Crustal deformation in the central Basin and Range between the Colorado plateau and the Eastern California Shear Zone is active but slow, making it a challenge to assess how strain is distributed and crustal motion transferred. However, knowledge of strain rates is very important, particularly for addressing the seismic hazard for both the Las Vegas urban area and the site of the proposed Yucca Mountain nuclear waste repository, in southern Nevada. Global Positioning System (GPS) data provide important constraints, particularly now that the GPS network in the area has substantially expanded in recent years. However, because deformation is slow, it is important to mitigate any transient tectonic and nontectonic signals to obtain the most accurate long-term interseismic motion and robust estimation of strain rates. We use data from all GPS stations in the region including both long-running continuous and semicontinuous stations. We model and remove postseismic displacements at these stations using source parameters for 41 events, dating back to the 1700 Cascadia megathrust earthquake, which contribute significantly to the deformation field within the central Basin and Range. We also remove correlated noise from the time series with the common-mode component imaging technique. We find that removal of both the postseismic transients and common-mode noise substantially reduces the uncertainties and spatial variation in the velocities. We find east–west extension across the Las Vegas Valley of 0.5–0.6 mm/yr. The interseismic strain rate field, calculated with the final velocities, reveals higher strain rates through southern Nevada than in previous studies, with rates within Las Vegas Valley of 8.5±2.4×10−9 yr−1. Our results also confirm shear along the Pahranagat shear zone, but the estimated amplitude is strongly affected by postseismic relaxation.

Geochemistry & Geophysics↗
DOE OSTI2020

A continuum damage mechanics (CDM) based Wilshire model for creep deformation, damage, and rupture prediction

In this report, a new constitutive model is derived that combines the Wilshire equations with continuum damage mechanics (CDM) to enable the long-term prediction of creep deformation, damage, and rupture. The Wilshire equations have been demonstrated to accurately extrapolate the stress-rupture, minimum-creep-strain-rate, and time-to-strain of various alloys across decade of life. Recently, the time-to-creep-strain equation has been rearranged to predict creep deformation curves; however, the resulting equation is difficult to calibrate and does not track damage. The CDM-based Sinh constitutive model accurately predicts creep deformation, damage, and stress-rupture for a variety of alloys; however, it lacks an explicit description of stress and temperature co-dependence which limits interpolation and extrapolation ability. In this study, the Wilshire and the CDM-based Sinh equations are combined to create the “WCS” model. The WCS model combines the best features of each model while eliminating their deficiencies. Experimental data for alloy P91 is gathered and the WCS model is calibrated to the data. The WCS model accurately predicts the stress rupture, minimum-creep-strain-rate, creep deformation, and damage of alloy P91 across the experimental stress and temperature range as well as consistency in parametric simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Relation Between the Defect Interactions and the Serration Dynamics in a Zr-Based Bulk Metallic Glass

For this study, the effects of thermal annealing and compressive strain rate on the complexity of the serration behavior in a Zr-based bulk metallic glass (BMG) was investigated. Here, as-cast and thermally-annealed (300 °C, 1 week) Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 BMG underwent room-temperature compression tests in the unconstrained condition at strain rates of 2 × 10 -5 s -1 and 2 × 10 -4 s -1 . The complexity of the serrated flow was determined, using the refined composite multiscale entropy technique. Nanoindentation testing and X-ray diffraction characterization were performed to assess the changes in the microstructure and mechanical properties of the BMG that occurred during annealing. The results indicated that the BMG did not crystallize during annealing in the prescribed heating condition. Nanoindentation tests revealed that annealing led to a significant increase in the depth-dependent nanoindentation hardness and Young’s modulus, which were attributed to the structural relaxation in the glass. Furthermore, both annealing and an increased strain rate resulted in a marked enhancement in the complexity of the serrated flow during compression. It was concluded that the increase in the sample entropy with increasing strain rate is related to an increase in the number of defect interactions during the serrated flow.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Transition in helium bubble strengthening of copper from quasi-static to dynamic deformation

Damage from low-temperature irradiation and the subsequent degradation of materials performance pose significant challenges for the storage of radioactive materials and for peripheral components in some nuclear reactor designs. Fully understanding the mechanical behavior of such materials requires test data for strain rates in both the quasi-static (< 10/s) and dynamic (>> 10/s) regimes. While dynamic testing has generally been avoided in the past for neutron irradiated (contamination concerns) and ion irradiated (insufficient volume) materials, surface-sensitive Richtmyer-Meshkov instability (RMI) tests were used in the present work to overcome these limitations. Here, nanopillar compression, nanoindentation, and RMI testing data from a helium implanted surface layer (~10 µm thick) were compiled to explore the effects of helium bubbles on the materials strength of high-purity copper at strain rates of 0.001/s – 10 8 /s. While nano-mechanical testing revealed increases in yield strength and hardness with increasing helium dose from 1000 to 4000 appm He, RMI indicated no significant changes in strength as compared to unimplanted copper. Here, this discrepancy in behavior was rationalized through a combination of recent literature and follow-on molecular dynamics (MD) simulations, leading to the conclusion that the nanoscale helium bubbles acting as dispersed barriers to dislocation motion at quasi-static strain rates collapse under shock loading and cease to be effective barriers at high strain rates.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A large deformation multiphase continuum mechanics model for shock loading of soft porous materials

A large deformation, coupled finite-element (FE) model is developed to simulate the multiphase response of soft porous materials subjected to high strain-rate loading. The approach is based on the theory of porous media (TPM) at large deformations. Simplifications to the one-dimensional regime studied in the numerical simulations follow. An overview of several different time integration schemes is presented for the purpose of solving the nonlinear dynamic coupled balance of momenta (mixture and fluid) and balance of mass of the mixture equations. Numerical examples are presented for (i) verification against closed-form analytical solutions assuming small loads, (ii) demonstrating large deformation effects at high strain-rate, and (iii) showing differences in deformations between a single-phase elastodynamics model with occluded compressible pore fluid and a multiphase poroelastodynamics model at high strain-rate. The multiphase model shows that the relative motion of the pore fluid significantly dampens the deformation response of the solid skeleton as compared to the single-phase model, and makes it possible to extract quantitative values for the stresses of the different constituents, thereby allowing one to form preliminary conclusions about the onset of damage in the solid skeleton. The novelty of the current work is developing a multiphase, large deformation, mixture theory numerical model for high strain-rate loading of soft porous materials. It was discovered that explicit, adaptive time-stepping Runge–Kutta schemes offer high accuracy at relatively low cost when compared to traditional implicit or explicit central difference time-stepping schemes for shock-like loadings. Here, shock viscosity is added to the mixture momentum balance equation to regularize the shock front, and a stabilization term is added to the mixture mass balance equation to stabilize equal order interpolation finite elements for the coupled finite element solution of multiphase materials.

Engineering↗
DOE OSTI2022

Quasi-static to Dynamic Response and Microstructure Development of Tantalum-Tungsten Alloys

Lawrence Livermore National Laboratory (LLNL) has a need to understand the quasi-static to dynamic mechanical response and microstructure evolution of tantalum-tungsten (Ta-W) alloys made by conventional wrought processing and additive manufacturing (AM). This two-year work scope will be performed at the Colorado School of Mines (Mines) and will include quasi-static (e.g., 10 -3 s -1 ) mechanical testing in tension and compression, along with high strain rate (Kolsky) pressure bar testing in tension and compression (e.g., 10 3 s -1 ), without and with temperature variations. Gas gun experiments at higher strain rates (e.g., over 10 3 s -1 ) will also be pursued. Complementary microstructure characterization will be performed on undeformed and deformed samples to fundamentally understand the role of processing on the microstructure characteristics and resulting deformation mechanisms that dictate the mechanical response of these alloys with variations in strain rate, temperature, and strain state. Alloys of interest will be Ta-based, containing up to 10 wt.% W. LLNL will supply initial wrought-processed material for sample machining. LLNL will also supply samples built by AM during the project for testing. Arc melting and casting may also be performed at Mines to produce small quantities of different alloys for machining and exploratory testing on an as-needed basis. This project will support multiple postdocs and graduate students at Mines, training them with the knowledge, skills, and abilities needed to study materials under extreme conditions of interest to the National Nuclear Security Agency.

36 MATERIALS SCIENCE↗
NASA NTRS1995

Velocity Fields of Axisymmetric Hydrogen-Air Counterflow Diffusion Flames from LDV, PIV, and Numerical Computation

Laminar fuel-air counterflow diffusion flames (CFDFs) were studied using axisymmetric convergent-nozzle and straight-tube opposed jet burners (OJBs). The subject diagnostics were used to probe a systematic set of H2/N2-air CFDFs over wide ranges of fuel input (22 to 100% Ha), and input axial strain rate (130 to 1700 Us) just upstream of the airside edge, for both plug-flow and parabolic input velocity profiles. Laser Doppler Velocimetry (LDV) was applied along the centerline of seeded air flows from a convergent nozzle OJB (7.2 mm i.d.), and Particle Imaging Velocimetry (PIV) was applied on the entire airside of both nozzle and tube OJBs (7 and 5 mm i.d.) to characterize global velocity structure. Data are compared to numerical results from a one-dimensional (1-D) CFDF code based on a stream function solution for a potential flow input boundary condition. Axial strain rate inputs at the airside edge of nozzle-OJB flows, using LDV and PIV, were consistent with 1-D impingement theory, and supported earlier diagnostic studies. The LDV results also characterized a heat-release hump. Radial strain rates in the flame substantially exceeded 1-D numerical predictions. Whereas the 1-D model closely predicted the max I min axial velocity ratio in the hot layer, it overpredicted its thickness. The results also support previously measured effects of plug-flow and parabolic input strain rates on CFDF extinction limits. Finally, the submillimeter-scale LDV and PIV diagnostics were tested under severe conditions, which reinforced their use with subcentimeter OJB tools to assess effects of aerodynamic strain, and fueVair composition, on laminar CFDF properties, including extinction.

Pellett, Gerald L.↗
NASA NTRS1981

Mechanical characterization of PMR-15 graphite/polyimide bolted joints

Data are presented for the static and viscoelastic performance of Celion 6000/PMR-15 graphite/polyimide composite belted joints over the 21 to 315 C temperature range. Two laminate configurations were obtained by sectioning a panel into subpanels oriented parallel and perpendicular to the laminate principal direction. Three replicates of each specimen geometry were tested at each of the test temperatures for the two laminate configurations, and failure load was defined as the maximum load attained as indicated by the load/deflection curve. Effects of strain-rate on strength and failure mode behavior were assessed at room temperature for strain rates of 0.002, 0.01, 0.10, and 1.00/s, and creep behavior was examined for the case of bearing deformations at temperatures of 21 and 177 C. Results are presented, including: (1) two distinct mechanisms were responsible for bearing failure and related to laminate configuration, (2) the + or - 45 deg plies carried the shearing load in composite bolted joints, and (3) the material exhibited strain-rate sensitivity which varied with laminate configuration.

Wilson, D. W.↗
NASA NTRS1994

Strain accumulation in the Shumagin Islands: Results of initial GPS measurements

Deformation in the Shumagin seismic gap has been monitored with repeated trilateration (EDM) in the 1980-1987 interval and with the Global Positioning System (GPS) in the 1987-1991 interval. The geodetic network extends for 100-km across the Shumagin Islands to the Alaska Peninsula. Results from the GPS surveys are consistent with those previously reported for the EDM surveys: we failed to detect significant strain accumulation in the N30 deg W direction of plate convergence. Using the method of simultaneous reduction for position and strain rates, we found the average rate of extension in the direction of plate convergence to be -25 +/- 25 nanostrain/yr (nstrain/yr) during the 1987-1991 interval of GPS surveys compared with -20 +/- 15 nstrain/yr during the 1981-1987 interval of complete EDM surveys. We found a marginally significant -26 +/- 12 nstrain/yr extension rate in the 1981-1991 interval covered by the combined EDM and GPS surveys. Strain rates are higher, but not significantly so, in the part of the network closest to the trench. Spatial variation in the deformation is observed in the 1980-1991 average station velocities, where three of the four stations closest to the trench have an arcward velocity of a few mm/yr. The observed strain rates are an order of magnitude lower than the -200 nstrain/yr rate predicted by dislocation models.

Larson, Kristine M.↗
DOE OSTI2022

Characterization of Tri-lab β-Tin (Sn)

This report documents details of the microstructure and mechanical properties of -tin (Sn), that is used in the Tri-lab (Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL), Sandia National Laboratories (SNL)) collaboration project on Multi-phase Tin Strength. We report microstructural features detailing the crystallographic texture and grain morphology of as-received -tin from electron back scatter diffraction (EBSD). Temperature and strain rate dependent mechanical behavior was investigated by multiple compression tests at temperatures of 200K to 400K and strain rates of 0.0001 /s to 100 /s. Tri-lab tin showed significant temperature and strain rate dependent strength with no significant plastic anisotropy. A sample to sample material variation was observed from duplicate compression tests and texture measurements. Compression data was used to calibrate model parameters for temperature and rate dependent strength models, Johnson-Cook (JC), Zerilli-Armstrong (ZA) and Preston-Tonks-Wallace (PTW) strength models.

36 MATERIALS SCIENCE↗
NASA NTRS1986

Geodetic measurement of deformation in the central Mojave Desert, California

Data from triangulation and trilateration surveys made during 1934-1982 are used to calculate shear strain rates in the central Mojave Desert of California. For the region between the Helendale and Camp Rock faults the shear strain rate was determined to be 0.16 + or - 0.03 microstrain/yr, with maximum right-lateral shear strain occurring on a plane oriented N41 deg W + or - 5 deg. If this deformation is due to right-lateral motion across the northwest trending local faults, the average shear straining corresponds to a relative displacement of 6.7 + or - 1.3 mm/yr across this portion of the network, accounting for about 12 percent of the predicted 56 mm/yr of relative motion between the North Atlantic and Pacific plates. From the Camp Rock fault eastward across the network there is a transition from significant to very low strain rates. Examination of nine focal mechanisms and their relation to the local geology and the strain data suggests that most of the long-term displacement occurs on the major northwest trending faults oriented nearly along the direction of relative motion between the North American and Pacific plates. Secondary faulting, controlled by a Coulomb-Anderson failure mechanism or by slip on preexisting faults can account for the occurrence of earthquakes on faults of other orientations.

Sauber, Jeanne↗
NASA NTRS1992

Inter- and intra-plate deformation at North American plate boundaries

A geodetic network which spans the region between San Francisco and Lake Tahoe has been measured 5 times completely with triangulation in 1880, 1922, 1929, 1943, 1963. A resurvey with the Global Positioning System (GPS) in 1991 allows the formation of 1 coseismic and 4 interseismic epochs. The data from this network provide a unique opportunity to examine the temporal and spatial evolution of the strain field associated with the 1906 San Francisco earthquake in particular and with the Pacific-North American plate boundary in general. Calculations of strain rate from the network data lead to the following conclusions. (1) There is no resolvable (greater than 0.05 microradians/yr) strain in between Sutter Buttes and the Sierra Nevada. (2) Throughout the time since the 1906 earthquake, a region extending at least as far east as the westernmost Great Valley has been undergoing deformation related to Pac:Nam interaction and the associated earthquake cycle. (3) In the time and space of overlap, our results agree with those from the United States Geological Survey (USGS) trilateration data. Both data sets indicate that strain must be accumulating to the east of Vaca. (4) The San Andreas discrepancy cannot be accommodated in the Great Valley at the 1 sigma level of our results. It is possible to absorb it in that region at the 2 sigma level. (5) Strain rate is elevated in the years following the earthquake and decays slowly with time. It is possible that the rate in the Coast Ranges increases until around 1950 and then decays. With the exception of one epoch, strain rate in the Coast Ranges is consistently fault parallel, shows no sign changes, and is consistent with monotonic strain accumulation.

Beavan, John↗
NASA NTRS1977

Machine for use in monitoring fatigue life for a plurality of elastomeric specimens

An improved machine is described for use in determining the fatigue life for elastomeric specimens. The machine is characterized by a plurality of juxtaposed test stations, specimen support means located at each of the test stations for supporting a plurality of specimens of elastomeric material, and means for subjecting the specimens at each of said stations to sinusoidal strain at a strain rate unique with respect to the strain rate at which the specimens at each of the other stations is subjected to sinusoidal strain.

Fitzer, G. E.↗
DOE OSTI2021

DEM analysis of the effect of interface transition zone on dynamic splitting tensile behavior of high-strength concrete based on multi-phase model

Highlights: • Crushable aggregates with the real geometry shape are generated by using clump-cluster method. • Flat-Joint Model is adopted as the constitutive law. • The tensile strength is almost not affected by ITZ strength when its strength exceeds 0.8 times that of mortar. • A modified formula that can reflect the functional relationship between the ITZ strength and the TDIF is proposed. The tensile strength is one of the main parameters in the design of concrete structures and has an important influence on the cracking performance and durability of concrete materials. As the weak region in concrete, the strength of the interface transition zone (ITZ) directly affects the dynamic mechanical response of concrete. Meanwhile, considering that the tensile performance of concrete is far weaker than its compressive performance, the study of the strength of ITZ is of great significance to the dynamic tensile fracture characteristics and mechanical properties of concrete. In this paper, a method combining 3D scanning technology and Clump-Cluster was proposed to generate a real geometry shape model of crushable aggregate. On this basis, the 3D discrete element model (DEM) of high-strength concrete was constructed using the Flat-Joint Model (FJM). The effects of the ITZ strength on the failure mode, number of microcracks, tensile strength and strain rate effect of high-strength concrete under strain rates from 0.01/s to 100/s were studied by splitting tensile test. The simulation results indicate that the number of microcracks generated decreases with the increase of ITZ strength, and the proportion of ITZ cracks decreases obviously. When the ITZ strength exceeds the critical value of 0.8 times that of mortar, the tensile strength is almost not affected by ITZ strength. In addition, based on the numerical simulation results, a modified formula that can reflect the functional relationship between the ITZ strength and the dynamic increasing factor of the concrete tensile strength (TDIF) is proposed, and the rationality of the TDIF modified formula is verified.

36 MATERIALS SCIENCE↗