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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Residual stress and microstructure in IN718-René41 graded superalloy fabricated by laser blown directed energy deposition

Additively printed Ni-based superalloy with a compositionally graded transition from IN718 to René41 was fabricated by laser blown-powder directed energy deposition (DED/LB-M), with the goals of meeting location-specific temperature capability and reducing component cost for hot gas path turbine components. Residual stress distribution in thin wall specimens with three sets of DED build parameters in the as-built and stress-relieved states was measured by neutron diffraction. For calculating residual stress, the calculated d0 method was found to be more appropriate as stress-free reference than using the lattice spacing measured from the stress-relief heat treated specimens. Longer dwell time (lower interpass temperature), higher energy input, smaller layer thickness resulted in a higher magnitude of tensile residual stresses at edges and compressive residual stresses at center of the specimens. The residual stress results did not show a strong dependence on graded compositions, indicating that the residual stress build-up was more geometry and process dependent. Non-destructive neutron imaging based on the attenuation coefficient qualitatively visualized the compositional variation in the bulk and showed good agreement with quantitative Electron Probe Micro-Analysis (EPMA) measurement. Grain structure, texture, and residual plastic strain along the build direction were characterized by Electron Backscatter Diffraction (EBSD). Long columnar grains with (001) preferred grain orientation were dominant along the build direction. Compositional change did not show an obvious effect on the epitaxial growth of dendrites and the continuation of the columnar grains. Residual plastic strain was relatively low in the as-built specimens.

36 MATERIALS SCIENCE↗
NASA NTRS1997

Model Determined for Predicting Fatigue Lives of Metal Matrix Composites Under Mean Stresses

Aircraft engine components invariably are subjected to mean stresses over and above the cyclic loads. In monolithic materials, it has been observed that tensile mean stresses are detrimental and compressive mean stresses are beneficial to fatigue life in comparison to a base of zero mean stress. Several mean stress models exist for monolithic metals, but each differ quantitatively in the extent to which detrimental or beneficial effects are ascribed. There have been limited attempts to apply these models to metal matrix composites. At the NASA Lewis Research Center, several mean stress models--the Smith-Watson- Topper, Walker, Normalized Goodman, and Soderberg models--were examined for applicability to this class of composite materials. The Soderberg approach, which normalizes the mean stress to a 0.02-percent yield strength, was shown to best represent the effect of mean stresses over the range covered. The other models varied significantly in their predictability and often failed to predict the composite behavior at very high tensile mean stresses. This work is the first to systematically demonstrate the influence of mean stresses on metal matrix composites and model their effects. Attention also was given to fatigue-cracking mechanisms in the Ti-15-3 matrix and to micromechanics analyses of mean stress effects.

Lerch, Bradley↗
NASA NTRS1978

Development of a plasma sprayed ceramic gas path seal for high pressure turbine application

Development of the plasma sprayed graded, layered ZRO2/CoCrAlY seal system for gas turbine engine blade tip seal applications up to 1589 K (2400 F) surface temperature was continued. The effect of changing ZRO2/CoCrAlY ratios in the intermediate layers on thermal stresses was evaluated analytically with the goal of identifying the materials combinations which would minimize thermal stresses in the seal system. Three methods of inducing compressive residual stresses in the sprayed seal materials to offset tensile thermal stresses were analyzed. The most promising method, thermal prestraining, was selected based upon potential, feasibility and complexity considerations. The plasma spray equipment was modified to heat, control and monitor the substrate temperature during spraying. Specimens were fabricated and experimentally evaluated to: (1) substantiate the capability of the thermal prestrain method to develop compressive residual stresses in the sprayed structure and (2) define the effect of spraying on a heated substate on abradability, erosion and thermal shock characteristics of the seal system. Thermal stress analysis, including residual stresses and material properties variations, was performed and correlated with thermal shock test results. Seal system performance was assessed and recommendations for further development were made.

Shiembob, L. T.↗
NASA NTRS2015

Relation Between Residual and Hoop Stresses and Rolling Bearing Fatigue Life

Rolling-element bearings operated at high speed or high vibration may require a tight interference fit between the bore of the bearing and shaft to prevent rotation of the bearing bore around the shaft and fretting damage at the interfaces. Previous work showed that the hoop stresses resulting from tight interference fits can reduce bearing lives by as much as 65 percent. Where tight interference fits are required, case-carburized steel such as AISI 9310 or M50 NiL is often used because the compressive residual stresses inhibit subsurface crack formation and the ductile core inhibits inner-ring fracture. The presence of compressive residual stress and its combination with hoop stress also modifies the Hertz stress-life relation. This paper analyzes the beneficial effect of residual stresses on rolling-element bearing fatigue life in the presence of high hoop stresses for three bearing steels. These additional stresses were superimposed on Hertzian principal stresses to calculate the inner-race maximum shearing stress and the resulting fatigue life of the bearing. The load-life exponent p and Hertz stress-life exponent n increase in the presence of compressive residual stress, which yields increased life, particularly at lower stress levels. The Zaretsky life equation is described and is shown to predict longer bearing lives and greater load- and stress-life exponents, which better predicts observed life of bearings made from vacuum-processed steel.

Fatigue↗
DOE OSTI2021

Strongly Anisotropic Thermomechanical Response to Shock Wave Loading in Oriented Samples of the Triclinic Molecular Crystal 1,3,5-Triamino-2,4,6-trinitrobenzene

All-atom molecular dynamics (MD) simulations were used to study shock wave loading in oriented single crystals of the highly anisotropic triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The crystal structure consists of planar hydrogen-bonded sheets of individually planar TATB molecules that stack into graphitic-like layers. Shocks were studied for seven systematically prepared crystal orientations with limiting cases that correspond to shock propagation exactly perpendicular and exactly parallel to the graphitic-like layers. The simulations were performed for initially defect-free crystals using a reverse-ballistic configuration that generates explicit, supported shocks. Final longitudinal stress components are between ≈8.5 and ≈10.5 GPa for the 1.0 km s –1 impact speed studied. Orientation-dependent properties are reported including shock speeds, stresses, temperatures, compression ratios, and local material strain rates. Spatiotemporal maps of the temperature, stress tensor, material flow, and molecular orientations reveal complicated processes that arise for specific shock directions. Furthermore, the results indicate that TATB shock response is highly sensitive to crystal orientation, with significant qualitative differences for the time evolution of the stress tensor and temperature, elastic/inelastic compression response, defect formation and growth, critical von Mises stress, and strain rates during shock rise that span nearly an order of magnitude. A variety of inelastic deformation mechanisms are identified, ranging from crumpling of graphitic-like layers to dislocation-mediated plasticity to intense shear strain localization. To our knowledge, these are the first systematic MD simulations and analysis of explicit shock wave propagation along nontrivial crystal directions in a triclinic molecular crystal.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effects of sample bias on adhesion of magnetron sputtered Cr coatings on SiC

Swelling of SiC at 300 C due to in-service neutron irradiation causes tensile residual stresses in coatings which are expected to adversely affect the performance of coated SiC composite fuel cladding for light water reactors. Matching the coating swelling with the substrate, a solution common for thermal expansion, is not practical in the case of neutron irradiation. Biasing samples during magnetron sputtering deposition induces compressive residual stress which may counteract this. In this study, chromium coatings were deposited on SiC by DC magnetron sputtering with no external heating at bias voltages of –50V, –75V, and –100V. The effects of the bias voltage on morphology, residual stress, microstrain, texture, and adhesion are shown. The low deposition temperature resulted in the coating microstructure evolution following an energetic particle bombardment dominated trend. Additionally, at the two lower bias voltages knock-on implantation dominated increasing the residual stress and microstrain while at the highest bias voltage, thermal spike migration allowed for defect relaxation. When the knock-on induced compressive residual stress exceeded 0.8 GPa microcrack formation in the SiC substrate decreased coating adhesion. While no microcracks formed at the lowest bias voltage, insufficient atomic mobility during coating growth lead to voids forming in the coating. A balance is needed to form void-free coatings that have high compressive residual stress.

36 MATERIALS SCIENCE↗
NASA NTRS1957

The Combinations of Thermal and Load Stresses for the Onset of Permanent Buckling in Plates

A simple and practical method for evaluating the onset of permanent buckling in plates in the presence of combined thermal and compressive load stresses is outlined. A particular application of the method shows reasonable agreement with tests of 17-7 PH stainless-steel square tubes. The results indicate that the compressive load stress which the plate can support at the onset of permanent buckling is substantially reduced as the temperature difference of the plate and adjoining members increases.

LOADS AND STRESSES, STRUCTURAL - COMPRESSION↗
NASA NTRS1962

Local Circumferential Buckling of Thin Circular Cylindrical Shells

The problem of circumferential buckling of a thin circular cylindrical shell due to compressive hoop stresses which vary in the axial direction is examined. For extremely localised compressive hoop stress distributions resulting from thermal discontinuity effects, or from a uniform, radial line loading, the buckle pattern should also be localised. Simplified analyses into these two types of problem are considered which show that only a limited number of buckle deflection modes needs to be assumed.

CYLINDRICAL SHELL↗
NASA NTRS2017

Residual Stresses in a NiCrY-Coated Powder Metallurgy Disk Superalloy

Protective ductile coatings will be necessary to mitigate oxidation and corrosion attack on superalloy disks exposed to increasing operating temperatures in some turbine engine environments. However, such coatings must be resistant to harmful surface cracking during service. The objective of this study was to investigate how residual stresses evolve in such coatings. Cylindrical gage fatigue specimens of powder metallurgy-processed disk superalloy LSHR were coated with a NiCrY coating, shot peened, and then subjected to fatigue in air at room and high temperatures. The effects of shot peening and fatigue cycling on average residual stresses and other aspects of the coating were assessed. Shot peening did induce beneficial compressive residual stresses in the coating and substrate. However, these stresses became more tensile in the coating with subsequent heating and contributed to cracking of the coating in long intervals of cycling at 760 C. Substantial compressive residual stresses remained in the substrate adjacent to the coating, sufficient to suppress fatigue cracking. The coating continued to protect the substrate from hot corrosion pitting, even after fatigue cracks initiated in the coating.

superalloy↗
DOE OSTI2022

Mechanisms of Mitigating Chloride-Induced Stress Corrosion Cracking of Austenitic Steels by Laser Shock Peening

This study investigates the effect of laser shock peening (LSP) on the chloride-induced stress corrosion cracking (SCC) of 304 austenitic steels. LSP can induce a high compressive residual stress to a depth of 700 µm and plastic deformation structures of dislocations, deformation twins, and stacking faults. Constant-load SCC tests in MgCl2 solution suggested that LSP can retard the crack initiation and slow the crack growth. LSP-treated subsurface layers experience ductile fracture while the central regions exhibit intergranular SCC. The LSP-induced deformation structures may impede dislocation slips, while the LSP-induced compressive residual stress can lessen the stress intensity factor of crack tips and decrease the local stress for film rupture.

Materials Science↗
NASA NTRS1977

Calculation of three-dimensional compressible turbulent boundary layers

From comparisons of high speed data with low speed closure procedures using variable mean density, there does not appear to be any appreciable influence of compressibility upon turbulent shear stress modeling in compressible turbulent boundary layers, even for extreme cases such as Mach 14 to 20 with a change in density across the layer of up to a factor of 100. Other evidence of apparent lack of compressibility caused new physics which may alter the shear stress for the compressible boundary layer cases including: (1) fluctuation Mach number was generally less than 1; (2) the shear stress distribution through the boundary layer was not a function of Mach number for zero pressure gradient flows; (3) the Morkovin hypothesis was valid up to Mach 5 (based on fluctuation data); (4) profile N power was not a function of Mach number, at least up to Mach 10; and (5) the nondimensional burst period was approximately the same as that for low speed.

Source record↗
DOE OSTI2023

Effects of anisotropic shale creep on the stress and permeability evolution of a geological nuclear waste repository

To ensure long-term safety and performance, geological nuclear waste repositories require low-permeability barriers such as bentonite buffers and/or shale host rock. Shale is not only known for its low permeability but also for its trend to undergo time-dependent deformation (i.e., creep), which could heal damage, but the effects of shale creep on the long-term performance of nuclear waste repositories have not been clearly understood. In particular, the anisotropic nature of shale (i.e., bedding) could have a significant effect on its creep behavior, and consequently, on the long-term performance of nuclear waste repositories. In this research, numerical simulations were carried out with the objective of showing the effects of anisotropic shale creep on the stress and permeability evolution of a generic geological nuclear waste repository in shale. The TOUGH-FLAC simulator, a thermo-hydromechanically (THM) coupled numerical code, was used for the simulations. To achieve the objective, comparisons were performed between the results of anisotropic shale creep simulations and those of different simulation cases, namely, no creep (i.e., elastic), isotropic creep, and long-term creep shale cases. Results of the comparisons show that the elastic and isotropic creep shale cases respectively led to the overestimation and underestimation of stress and permeability in the repository, whereas the long-term creep shale case, which accumulated greater creep in later periods than in earlier periods, helped to keep large shear and tensile stresses from developing while maintaining compressive spherical stress, resulting in consistently low permeability levels. These results indicate that performance assessments with elastic and isotropic creep formation models will provide the upper and lower bound estimates of stress and permeability, while more reasonable estimates will be given by an anisotropic creep formation model, and that shale with long-term creep characteristics will be beneficial in many aspects of the safety and performance of nuclear waste repositories.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
NASA NTRS1997

Overload and Underload Effects on the Fatigue Crack Growth Behavior of the 2024-T3 Aluminum Alloy

Fatigue crack growth tests were conducted on 0.09 inch thick, 3.0 inch wide middle-crack tension specimens cut from sheets of 2024-T3 aluminum alloy. The tests were conducted using a load sequence that consisted of a single block of 2,500 cycles of constant amplitude loading followed by an overload/underload combination. The largest fatigue crack growth life occurred for the tests with the overload stress equal to 2 times the constant amplitude stress and the underload stress equal to the constant amplitude minimum stress. For the tests with compressive underloads, the fatigue crack growth life decreased with increasing compressive underload stress.

Dawicke, David S.↗
DOE OSTI2022

Damage Tolerance Assessment of Laser Clad Repairs of Coarse Grain Ti-6Al-4V

Background: Laser cladding offers a promising approach to repairing aviation components, with the ability to restore the geometric shape and the static and fatigue strength. Objective: The damage tolerance of a coarse grain Ti-6Al-4 V material is assessed via three sets of samples: (i) baseline, (ii) laser clad repaired, and (iii) laser cladding followed by heat treatment. Method: The specimens were subjected to fatigue loading using a marker band technique, in which cracks initiated from pre-defined laser slots and the small crack growth rates were measured by quantitative fractography. Additional characterization was performed on these samples, including hardness measurements, microstructure analysis, and multimodal residual stress analyses. Results: The region of the specimen undergoing laser cladding exhibited higher hardness values and a compressive residual stress, based on the slitting method. The samples subjected to heat treatment after cladding displayed negligible differences in grain size and minimal residual stress relaxation. Conclusions: The results demonstrated significant variability, with data demonstrating that cracks in the laser clad region (exhibiting the largest compressive residual stress) grew faster than cracks in the substrate region of the sample or in the baseline cases. Finally, these counterintuitive results are rationalized with additional analyses from synchrotron-based energy dispersive X-ray diffraction and high energy X-ray diffraction, demonstrating significant spatial variability in the residual elastic strains in the millimeter-sized prior β grains within the material.

36 MATERIALS SCIENCE↗
NASA NTRS1942

Tensile and pack compressive tests of some sheets of aluminum alloy, 1025 carbon steel, and chromium-nickel steel

Tensile and compressive stress-strain curves, stress-deviation curves, and secant modulus-stress curves are given for longitudinal and transverse specimens of 17S-T, 24S-T, and 24S-RT aluminum-alloy sheet in thicknesses from 0.032 to 0.081 inch, 1025 carbon steel sheet in thicknesses of 0.054 and 0.120 inch, and chromium-nickel steel sheet in thicknesses form 0.020 to 0.0275 inch. Significant differences were found between the tensile and the compressive stress-strain curves, and also the corresponding corollary curves; similarly, differences were found between the curves for the longitudinal and transverse directions. These differences are of particular importance in considering the compressive strength of aircraft structures made of thin sheet. They are explored further for the case of compression by giving tangent modulus-stress curves in longitudinal and transverse compression and dimensionless curves of the ratio of tangent modulus to Young's modulus and of the ratio of reduced modulus for a rectangular section to Young's modulus, both plotted against the ratio of stress to secant yield strength.

Atchison, C S↗
NASA NTRS1946

Effect of Brake Forming on the Strength of 24S-T Aluminum-alloy Sheet

Tests were made to determine the effect of brake forming on the strength of 24S-T aluminum alloy sheet that had been formed to an inside bend radius of three times the sheet thickness. The results for both directions of the grain of the material showed that the compressive yield stresses were appreciably increased, that the tensile yield stresses were moderately increased, that the ultimate tensile stresses were only slightly increased, that the elongations were considerably reduced, and that the shapes of the tensile and compressive stress-strain curves were markedly changed.

Heimerl, George J↗
NASA NTRS1951

Buckling of thin-walled cylinder under axial compression and internal pressure

An investigation was made of a thin-walled cylinder under axial compression and various internal pressures to study the effect of the internal pressure on the compressive buckling stress of the cylinder. A theoretical analysis based on a large-deflection theory was also made. The theoretically predicted increase of compressive buckling stress due to internal pressure agrees fairly well with the experimental results. (author)

Lo, Hsu↗
NASA NTRS1990

Tidal disruption of solid bodies

The problem of stress, strain, and breakup in solid satellites and stray bodies subject to tidal perturbations is presently addressed in view of three novel considerations. After presenting a new analytic solution for the stress tensor in a homogeneous and compressible elastic sphere, where the inclusion of compressibility alters stresses by several percent, realistic failure criteria are noted to demonstrate the general failure of such ductile bodies as iron meteoroids by plastic shear, while brittle ice bodies fail by either tensile or shear fracture. A reexamination of crack propagation after initial failure allows the diverse breakup criteria to be reconciled.

Dobrovolskis, Anthony R.↗