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At least 235 records · Page 13

All sources NASA NTRS DOE OSTI

NASA NTRS1991

Stirling engine - Available tools for long-life assessment

A review is presented for the durability approaches applicable to long-time life assessment of Stirling engine hot-section components. The crucial elements are experimental techniques for generating long-time materials property data (both monotonic and cyclic flow and failure properties); analytic representations of slow strain rate material stress-strain response characteristics (monotonic and cyclic constitutive relations) at high temperatures and low stresses and strains; analytic creep-fatigue-environmental interaction life prediction methods applicable to long lifetimes at high temperatures and small stresses and strains; and experimental verification of life predictions. Long-lifetime design criteria for materials of interest are woefully lacking. Designing against failures due to creep, creep-rupture, fatigue, environmental attack, and creep-failure-environmental interaction will require considerable extrapolation. Viscoplastic constitutive models and time-temperature parameters will have to be calibrated for the hot-section materials of interest. Analysis combined with limited verification testing in a short-time regime will be required to build confidence in long-lifetime durability models.

Halford, Gary R.↗

NASA NTRS1998

Laser Heated Oxide Fiber Growth Using Melt Modulation Technique

As a reinforcement for structural composites, single crystal alumina fibers offer low density, high modulus, and high creep resistance. In this study, the laser heated float zone approach was employed to grow c-axis Al2O3 continuous fibers of high purity and high strength. A new melt modulation technique, laser scanning, avoids the formation of surface induced ripples and allows the growth of 50 micro-m diameter sapphire fibers with strengths (approx. 7 GPa) significantly greater than either commercially available fibers grown by the edge-defined film growth process (approx. 2-3 GPa) or laboratory fibers grown by stationary laser heating (approx. 5 GPa). The present work suggests that surface striations are the predominant defects controlling the tensile strength of laser-scanned fibers at room temperature. Several possible mechanisms for inducing surface striations are systematically discussed for several oxide fiber compositions.

Sayir, Ali↗

NASA NTRS2020

Experimental Setup and Parameters for Testing Uncoated and EBC-Coated CMCs under Thermal Gradients Induced by Laser Heating and Backside Air Cooling

An experimental setup to induce through thickness thermal gradients in ceramic matrix composite (CMC) specimens without and with an applied environmental barrier coating (EBC) is described in detail. Components of the experimental set up include a laser for the heat source on the front of the specimen, a cooling air nozzle on the back of the specimen with pyrometers to measure temperatures on both sides, and a full-field thermal image camera to ensure uniformity of the temperature distribution in the gage section of the specimen. Test specimen is loaded in a servohydraulic system, which can superimpose either static or dynamic loads on the specimen to investigate creep and fatigue behaviors of CMCs under through thickness thermal gradient conditions. Mechanical strains on the front and backsides of the test specimen were measured during the thermal gradient tests with superimposed mechanical loading using two contact extensometers. Selected through thickness thermal gradient test results generated with the experimental setup under different nominal front side temperatures and multiple constant laser power levels on uncoated and EBC coated CMCs and procedures used to generate those results are described in this report. Two examples of tests conducted using the test system on uncoated CMCs under creep and sustained-peak, low-cycle fatigue with through thickness thermal gradient conditions are provided.

ceramic matrix composite, environmental barrier co↗

NASA NTRS1990

A creep model for metallic composites based on matrix testing: Application to Kanthal composites

An anisotropic creep model is formulated for metallic composites with strong fibers and low to moderate fiber volume percent (less than 40 percent). The idealization admits no creep in the local fiber direction and assumes equal creep strength in longitudinal and transverse shear. Identification of the matrix behavior with that of the isotropic limit of the theory permits characterization of the composite through uniaxial creep tests on the matrix material. Constant and step-wise creep tests are required as a data base. The model provides an upper bound on the transverse creep strength of a composite having strong fibers embedded in a particular matrix material. Comparison of the measured transverse strength with the upper bound gives an assessment of the integrity of the composite. Application is made to a Kanthal composite, a model high-temperature composite system. Predictions are made of the creep response of fiber reinforced Kanthal tubes under interior pressure.

Binienda, W. K.↗

DOE OSTI2024

Probabilistic Analysis of Long-Term Degradation of Microwave Cavity Flow Sensor

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

DOE OSTI2023

Final Report on Development of Microwave Resonant Cavity Transducer for Fluid Flow Sensing: Development of Microwave Cavity Flow Meter for Advanced Reactor High Temperature Fluids

We are investigating a microwave resonant cavity transducer for flow sensing in the vessel of a high temperature fluid advanced reactor (AR), such as a molten salt cooled reactor (MSCR) or a sodium fast reactor (SFR). This transducer is a hollow metallic cylindrical cavity, with one of the flat walls of the cylinder flexible enough to undergo microscopic deflection due to dynamic fluid pressure. Membrane deflection leads to a shift in the microwave resonant frequency, which can be detected with a spectrum analyzer. We have developed a continuum electromechanics model of the microwave resonant cavity transducer performance. The deflection of the membrane is calculated using equations from the theory of plates and shells for deflection of radially constrained thin circular disk under uniform load. The microwave frequency shift is calculated using the equations from the microwave cavity perturbation theory. The transducer model was validated with proof-of-concept experimental data of water flow sensing with a Brass 360 right circular cylinder with 22.2mm diameter and 203µm thick wall. The cavity was excited through a subwavelength hole in TE 011 mode with resonant frequency f ≈ 17.8GHz. Subsequently, we have performed a preliminary proof-of-principle test of flow sensing in high temperature liquid sodium in environment. For this test, we have developed a cylindrical resonator with the same dimensions as for the Brass 360 cavity. The resonator for high temperature liquid sodium test was machined from stainless steel 316 and electroplated with silver on the interior surfaces. We have also developed and insertion probe consisting of a 50cm WR-42 brass waveguide enclosed in a protective SS316 tube. The cavity was excited through a subwavelength hole on the side of the wall of the cylinder in the TE 011 mode with resonant frequency f ≈ 17.8GHz. The liquid sodium setup consists of a cylindrical vessel with a center feed line, where a transducer inserted through the top cap of the vessel measures velocity of the impinging liquid jet. The sodium flow sensing study was performed in a liquid sodium vessel at 340°C temperature and ambient pressure. The flow rate was changed by varying sodium pump power, and a frequency shift of several hundred kHz was observed. The transducer was removed after 70 days of testing. No visual damage was observed, and the electromagnetic response remained the same. As a calibration experiment, we have assembled a water vessel with a center feed, and with dimensions similar to those of the liquid sodium setup. Water flow sensing was performed before and after liquid sodium test. The responses in both cases are similar and agree with COMSOL computer simulations. Because long-term (multi-year) experimental tests of transducer resilience to harsh environment are not practical, we have developed a probabilistic model of creep to estimate transducer resilience to the harsh environment. The probabilistic model considers diffusion creep under the condition of high temperature and low stress, where the stress and temperature are allowed to be random variables with Gaussian distributions. Using the probabilistic model, we estimate inelastic membrane deflections due to creep for several temperature ranges. We conclude that for temperatures less than 650°C, creep has negligible long-term effect on the transducer performance.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

NASA NTRS1997

Fiber Contraction Approaches for Improving CMC Proportional Limit

The fact that the service life of ceramic matrix composites (CMC) decreases dramatically for stresses above the CMC proportional limit has triggered a variety of research activities to develop microstructural approaches that can significantly improve this limit. As discussed in a previous report, both local and global approaches exist for hindering the propagation of cracks through the CMC matrix, the physical source for the proportional limit. Local approaches include: (1) minimizing fiber diameter and matrix modulus; (2) maximizing fiber volume fraction, fiber modulus, and matrix toughness; and (3) optimizing fiber-matrix interfacial shear strength; all of which should reduce the stress concentration at the tip of cracks pre existing or created in the matrix during CMC service. Global approaches, as with pre-stressed concrete, center on seeking mechanisms for utilizing the reinforcing fiber to subject the matrix to in-situ compressive stresses which will remain stable during CMC service. Demonstrated CMC examples for the viability of this residual stress approach are based on strain mismatches between the fiber and matrix in their free states, such as, thermal expansion mismatch and creep mismatch. However, these particular mismatch approaches are application limited in that the residual stresses from expansion mismatch are optimum only at low CMC service temperatures and the residual stresses from creep mismatch are typically unidirectional and difficult to implement in complex-shaped CMC.

DiCarlo, James A.↗

DOE OSTI2022

Cavitation-resistant intergranular precipitates enhance creep performance of θ'-strengthened Al-Cu based alloys

Tensile and compressive creep properties of a quaternary Al-Cu-Mn-Zr (ACMZ) alloy and its commercial counterpart (Al-Cu-Mn-Zr with Ni, Co and Sb additions, RR350) are investigated at 300°C. At low stresses up to 30 MPa where diffusional creep dominates, creep resistance is the same in tension and compression and RR350 deforms more slowly than ACMZ, consistent with RR350 alloy's larger linear fraction of intergranular precipitates (Al7Cu2(NiFe) and Al9FeNi for RR350 vs. θ-Al2Cu for ACMZ) and a reduced fraction of precipitate-free zones near grain boundaries. At stresses between 30 and 80 MPa, dislocation creep with a stress exponent n ~ 3 becomes rate-limiting in compression, which is expected to be controlled by θ' precipitates within the grain bulk. By contrast, in tension, enhanced creep rate and higher apparent stress exponents are measured, consistent with cavitation at intergranular precipitates becoming increasingly dominant as the stress increases. In the dislocation creep regime, RR350 alloy is again more creep resistant than ACMZ alloy, which is related to three mechanisms (i) a reduced fraction of softer precipitate-free zones, (ii) more effective load transfer to intergranular precipitates, and (iii) reduced cavitation. A model for cavitation is applied to calculate tensile creep rates from compressive creep rates and the model successfully predicts the improved tensile creep resistance of the RR350 alloy. Overall, the present investigation underscores the importance of intergranular grain boundary precipitates, in addition to strengthening θ' precipitates, in enhancing the creep resistance of Al-Cu alloys.

36 MATERIALS SCIENCE↗

NASA NTRS1987

Preliminary study of creep thresholds and thermomechanical response in Haynes 188 at temperatures in the range 649 to 871 C

The following conclusions were drawn from this study of creep thresholds and thermomechanical response: (1) creep threshold can be determined using the latest electrohydraulic test equipment, providing that test durations are short and relatively large accumulations of creep strain are used in defining the threshold; (2) significant creep strains were measured under monotonic loading as stress levels as low as 4 ksi at temperatures predicted for solar receiver service; and (3) the material exhibited creep ratchetting during simulated service cycles, a result not predicted by analysis using current constitutive models for Haynes 188.

Ellis, J. R.↗

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 NTRS2000

Damage Resistance of Titanium Aluminide Evaluated

As part of the aviation safety goal to reduce the aircraft accident rate, NASA has undertaken studies to develop durable engine component materials. One of these materials, g-TiAl, has superior high-temperature material properties. Its low density provides improved specific strength and creep resistance in comparison to currently used titanium alloys. However, this intermetallic is inherently brittle, and long life durability is a potential problem. Of particular concern is the material s sensitivity to defects, which may form during the manufacturing process or in service. To determine the sensitivity of TiAl to defects, a team consisting of GE Aircraft Engines, Precision Cast Parts, and NASA was formed. The work at the NASA Glenn Research Center at Lewis Field has concentrated on the fatigue response to specimens containing defects. The overall objective of this work is to determine the influence of defects on the high cycle fatigue life of TiAl-simulated low-pressure turbine blades. Two types of defects have been introduced into the specimens: cracking from impact damage and casting porosity. For both types of defects, the cast-to-size fatigue specimens were fatigue tested at 650 C and 100 Hz until failure.

Lerch, Bradley A.↗

DOE OSTI2023

Tailoring the γ-γ'-γ" dual superlattice microstructure of INCONEL® 725 by high temperature aging and Nb/Ta additions for superior creep properties

Next-generation energy systems require superior resistance to creep deformation due to the considerably prolonged exposure times at operating stress and temperature. To improve the elevated temperature properties of INCONEL® 725 (IN725), a corrosion resistant alloy, several variants with different Ti/Al ratio and judicious amounts of Nb and Ta were made. Furthermore, a high temperature aging (HTA) heat treatment, designed to promote favorable precipitate phase formation, was explored. These adjustments allowed to tailor the amount and type of precipitate strengthening which led to significant increases in time to failure. The Ti/Al ratio was used to favor the formation of γ' or γ" precipitates. Compact morphology precipitates, consisting of γ' precipitates surrounded by a γ" shell, were formed in alloys with a low Ti/Al ratio. The HTA increased the creep life of various alloy formulations up to a maximum improvement of 371% as compared to the standard aging heat treatment. The Nb and Ta additions had a similar effect on increasing creep life by promoting and stabilizing γ" precipitation. The positive effect of the additions was even more pronounced when coupled with the HTA. A phase stability study with up to 10,000 h exposure at 700°C revealed that the compact morphology helped in slightly reducing coarsening of the γ' precipitates, although the effect on creep was not significant. The findings of this study enable design of dual superlattice alloys through microstructural engineering that yields superior performance and can be applied to a wide range of alloys in the IN718 and derivatives family.

Antonov, Stoichko↗

NASA NTRS2004

Effects of Initial Powder Size on the Mechanical Properties and Microstructure of As-Extruded GRCop-84

GRCop-84 was developed to meet the mechanical and thermal property requirements for advanced regeneratively cooled rocket engine main combustion chamber liners. It is a ternary Cu- Cr-Nb alloy having approximately 8 at% Cr and 4 at% Nb. The chromium and niobium constituents combine to form 14 vol% Cr2Nb, the strengthening phase. The alloy is made by producing GRCop-84 powder through gas atomization and consolidating the powder using extrusion, hot isostatic pressing (HIP) or vacuum plasma spraying (VPS). GRCop-84 has been selected by Rocketdyne, Ratt & Wlutney and Aerojet for use in their next generation of rocket engines. GRCop-84 demonstrates favorable mechanical and thermal properties at elevated temperatures. Compared to NARloy-Z, the currently used inaterial in the Space Shuttle, GRCop-84 has approximately twice the yield strength, 10-1000 times the creep life, and 1.5-2.5 times the low cycle fatigue life. The thermal expansion of GRCop-84 is 7515% less than NARloy-Z which minimizes thermally induced stresses. The thermal conductivity of the two alloys is comparable at low temperature but NARloy-Z has a 20-50 W/mK thermal conductivity advantage at typical rocket engine hot wall temperatures. GRCop-84 is also much more microstructurally stable than NARloy-Z which translates into better long term stability of mechanical properties. Previous research into metal alloys fabricated by means of powder metallurgy (PM), has demonstrated that initial powder size can affect the microstructural development and mechanical properties of such materials. Grain size, strength, ductility, size of second phases, etc., have all been shown to vary with starting powder size in PM-alloys. This work focuses on characterizing the effect of varying starting powder size on the microstructural evolution and mechanical properties of as- extruded GRCop-84. Tensile tests and constant load creep tests were performed on extrusions of four powder meshes: +140 mesh (great3er than l05 micron powder size), -140 mesh (less than or equal to 105 microns), -140 plus or minus 270 (53 - 105 microns), and - 270 mesh (less than or equal to 53 microns). Samples were tested in tension at room temperature and at 500 C (932 F). Creep tests were performed under vacuum at 500 C using a stress of 111 MPa (16.1 ksi). The fracture surfaces of selected samples from both tests were studied using a Scanning Electron Microscope (SEM). The as-extruded materials were also studied, using both optical microscopy and SEM analysis, to characterize changes within the microstructure.

Okoro, Chika L.↗

NASA NTRS1984

Creep-rupture behavior of candidate Stirling engine alloys after long-term aging at 760 deg C in low-pressure hydrogen

Nine candidate Stirling automotive engine alloys were aged at 760 C for 3500 hr in low pressure hydrogen or argon to determine the resulting effects on mechanical behavior. Candidate heater head tube alloys were CG-27, W545, 12RN72, INCONEL-718, and HS-188 while candidate cast cylinder-regenerator housing alloys were SA-F11, CRM-6D, XF-818, and HS-31. Aging per se is detrimental to the creep rupture and tensile strengths of the iron base alloys. The presence of hydrogen does not significantly contribute to strength degradation. Based percent highway driving cycle; CG-27 has adequate 3500 hr - 870 C creep rupture strength and SA-Fll, CRM-6D, and XF-818 have adequate 3500 hr - 775 C creep rupture strength.

Titran, R. H.↗

DOE OSTI2021

A probabilistic creep model incorporating test condition, initial damage, and material property uncertainty

Uncertainty is prevalent in the creep resistance of alloys, where at elevated temperature and low pressure, rupture can range across logarithmic decades. In this study, a probabilistic continuum-damage-mechanics (CDM)-based model is derived to capture the uncertainty of creep resistance. To meet this objective, creep data for alloy 304 Stainless Steel is gathered. A constitutive model, “Sinh”, is calibrated deterministically to determine the statistical variability of the material properties. Three sources of uncertainty are injected into the model: test condition (stress and temperature), initial damage, and material properties. Probabilistic simulations are carried out by (a) calibrating probability distribution functions (pdfs) for each source of uncertainty (b) randomly sampling the pdfs using Monte Carlo methods and (c) executing simulations to replicate the uncertain creep behavior. A sensitivity analysis is performed to evaluate the relative effect of each source of uncertainty. In full probabilistic simulations, the cumulative uncertainty of creep behavior is evaluated. The probabilistic model accurately predicts the creep deformation and rupture of the available experiments. The probabilistic model is validated for interpolation but lacks extrapolation ability. Several future works are proposed to further improve the model.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Uniaxial stress increases layer stacking disorder in calcium silicate hydrates with low calcium content

The long-term stress response of concrete is controlled by the intergrain and intragrain interactions of calcium-silicate-hydrates (C-S-H), nanocrystalline forms of tobermorites. To investigate both types of interactions, we examined the deviatoric stress response of 11 Å tobermorite powder with X-ray diffraction, finding that its layer stacking (i.e., sliding) disorder significantly increases. A similar experiment was performed on nanocrystalline C-S-H powders before and after applying a 360 MPa uniaxial stress. Layer stacking disorder (in the form of intragrain deformation) increases when there is low water content and when the basal spacing is large. We also find that grains with a smaller thickness-to-diameter aspect ratio are first to develop preferred orientation (in the form of intergrain sliding), facilitated by gel water. Furthermore, these findings show that intragrain deformations in C-S-H can occur at relatively low stresses and provide insights into the creep mechanism of C-S-H.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Advanced Materials & Manufacturing Technology (AMMT): Process understanding for qualifying LPBF 316H SS

Investigations were conducted in fiscal years (FY) 2023 and 2024 to gather relevant data sets addressing challenges related to qualifying 316H stainless steel (SS) for use in future nuclear reactors. This work was a collaborative effort involving researchers from Idaho National Laboratory (INL), Argonne National Laboratory (ANL), and Oak Ridge National Laboratory (ORNL). Key outcomes included: • Development of process-structure-property data sets to better understand the relationships between manufacturing processes, material structure, and performance characteristics. • Establishment of an in-situ monitoring system to link these various data sets together. • Detailed characterization of the raw material feedstock to further strengthen the understanding of process-structure and process-property relationships. Building on this foundation, in FY25 there was interest in exploring the behavior of additively manufactured 316H SS under different test conditions to support the overall material qualification process. Los Alamos National Laboratory (LANL) was tasked with providing specimen samples to the collaborating labs, who then conducted a round-robin study examining factors like selective heat treatment, low-cycle fatigue (LCF), and tensile-creep (T-C) properties. Additionally, high-temperature differential scanning calorimetry (DSC) was performed by LANL to better understand how the material's thermal characteristics change as it is heated up to the melting point. This provided a more comprehensive understanding of the material's behavior. The combined results from these investigations could potentially be used to support the inclusion of 316H stainless steel in Section III, Division 5 of the relevant codes and standards, allowing its use in future nuclear reactor applications.

36 MATERIALS SCIENCE↗

NASA NTRS1996

Structures Division 1994 Annual Report

The NASA Lewis Research Center Structures Division is an international leader and pioneer in developing new structural analysis, life prediction, and failure analysis related to rotating machinery and more specifically to hot section components in air-breathing aircraft engines and spacecraft propulsion systems. The research consists of both deterministic and probabilistic methodology. Studies include, but are not limited to, high-cycle and low-cycle fatigue as well as material creep. Studies of structural failure are at both the micro- and macrolevels. Nondestructive evaluation methods related to structural reliability are developed, applied, and evaluated. Materials from which structural components are made, studied, and tested are monolithics and metal-matrix, polymer-matrix, and ceramic-matrix composites. Aeroelastic models are developed and used to determine the cyclic loading and life of fan and turbine blades. Life models are developed and tested for bearings, seals, and other mechanical components, such as magnetic suspensions. Results of these studies are published in NASA technical papers and reference publication as well as in technical society journal articles. The results of the work of the Structures Division and the bibliography of its publications for calendar year 1994 are presented.

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