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NASA NTRSDate not supplied

LES Simulation of Cooling Airflow of High-Pressure Turbine Using the Source Term Approach

In this study, the Open National Combustion code (OpenNCC) is applied to simulate the airflow inside the high-pressure turbine (HPT) of the energy efficient engine (EEE). The main objective of this study is to validate the proposed methodology of a three-dimensional unsteady calculation (LES) of the HPT with a moving mesh capability to capture a relative motion at the stator and rotor interface. In our previous study, the adiabatic wall assumption was made at all the solid surfaces of HPT (i.e., the cooling airflows were not included.) To relax this assumption, we implement the surface source team approach, in which we impose the source term at a specific area of a cooling airflow exit at the solid surfaces by specifying an injection angle, temperature, turbulent intensity, and mass flowrate of each cooling airflow. The beneficial feature of this approach is that it does not require a mesh for each hole. For the validation of the model, the data for EEE model is obtained from the General Electric (GE) test campaign, and the full scale warm-air rig test condition representing a test point close to the integrated core/low spool design condition is considered.

CFD Hot-streaks↗

DOE OSTI2022

A Square Pulse Thermoreflectance Technique for the Measurement of Thermal Properties

We report on a laser-based square pulse thermoreflectance (SPTR) technique for the measurement of thermal properties for a wide range of materials. SPTR adopts the pump-probe thermoreflectance principle to monitor the evolution of local temperature after square pulse excitation. The technique features a compact setup, high spatial resolution, and fast data collection. By comparing the acquired SPTR signals with a continuum heat transfer model, material thermal properties can be obtained. Taking advantage of various spot sizes and modulation frequencies, SPTR can measure both the thermal diffusivity and thermal conductivity of poorly to moderately conductive materials and the thermal conductivity of conductive materials with satisfactory accuracy, with potential to be applied to more conductive materials. The technique was validated on three materials: fused silica, single crystal CaF2 and single crystal nickel (with conductivities ranging from 1 W·m -1 ·K -1 to 100 W·m -1 ·K -1 ) with typical measurement errors of 5 % to 20 %. The leading sources of error have been identified by Monte Carlo simulations, and the primary limitations of SPTR are discussed. The compact, fiberized platform we describe here will allow instruments based on this methodology to be deployed in complex, multi-analytical environments for the type of high-throughput correlative analyses that are key to materials design and discovery.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

NASA NTRS2004

Subsonic Aircraft With Regression and Neural-Network Approximators Designed

At the NASA Glenn Research Center, NASA Langley Research Center's Flight Optimization System (FLOPS) and the design optimization testbed COMETBOARDS with regression and neural-network-analysis approximators have been coupled to obtain a preliminary aircraft design methodology. For a subsonic aircraft, the optimal design, that is the airframe-engine combination, is obtained by the simulation. The aircraft is powered by two high-bypass-ratio engines with a nominal thrust of about 35,000 lbf. It is to carry 150 passengers at a cruise speed of Mach 0.8 over a range of 3000 n mi and to operate on a 6000-ft runway. The aircraft design utilized a neural network and a regression-approximations-based analysis tool, along with a multioptimizer cascade algorithm that uses sequential linear programming, sequential quadratic programming, the method of feasible directions, and then sequential quadratic programming again. Optimal aircraft weight versus the number of design iterations is shown. The central processing unit (CPU) time to solution is given. It is shown that the regression-method-based analyzer exhibited a smoother convergence pattern than the FLOPS code. The optimum weight obtained by the approximation technique and the FLOPS code differed by 1.3 percent. Prediction by the approximation technique exhibited no error for the aircraft wing area and turbine entry temperature, whereas it was within 2 percent for most other parameters. Cascade strategy was required by FLOPS as well as the approximators. The regression method had a tendency to hug the data points, whereas the neural network exhibited a propensity to follow a mean path. The performance of the neural network and regression methods was considered adequate. It was at about the same level for small, standard, and large models with redundancy ratios (defined as the number of input-output pairs to the number of unknown coefficients) of 14, 28, and 57, respectively. In an SGI octane workstation (Silicon Graphics, Inc., Mountainview, CA), the regression training required a fraction of a CPU second, whereas neural network training was between 1 and 9 min, as given. For a single analysis cycle, the 3-sec CPU time required by the FLOPS code was reduced to milliseconds by the approximators. For design calculations, the time with the FLOPS code was 34 min. It was reduced to 2 sec with the regression method and to 4 min by the neural network technique. The performance of the regression and neural network methods was found to be satisfactory for the analysis and design optimization of the subsonic aircraft.

Patnaik, Surya N.↗

DOE OSTI2024

Effect of sintering temperature on feature resolution and flexural strength of ceramics fabricated through vat photopolymerization additive manufacturing

Although ceramic additive manufacturing (AM) could be used to fabricate complex, high-resolution parts for diverse, functional applications, one ongoing challenge is optimizing the post-process, particularly sintering, conditions to consistently produce geometrically accurate and mechanically robust parts. This study aims to investigate how sintering temperature affects feature resolution and flexural properties of silica-based parts formed by vat photopolymerization (VPP) AM. Test artifacts were designed to evaluate features of different sizes, shapes and orientations, and three-point bend specimens printed in multiple orientations were used to evaluate mechanical properties. Sintering temperatures were varied between 1000°C and 1300°C. Deviations from designed dimensions often increased with higher sintering temperatures and/or larger features. Higher sintering temperatures yielded parts with higher strength and lower strain at break. Many features exhibited defects, often dependent on geometry and sintering temperature, highlighting the need for further analysis of debinding and sintering parameters. To the best of the authors’ knowledge, this is the first time test artifacts have been designed for ceramic VPP. This work also offers insights into the effect of sintering temperature and print orientation on flexural properties. These results provide design guidelines for a particular material, while the methodology outlined for assessing feature resolution and flexural strength is broadly applicable to other ceramics, enabling more predictable part performance when considering the future design and manufacture of complex ceramic parts.

36 MATERIALS SCIENCE↗

NASA NTRS2008

Structural Benchmark Creep Testing for the Advanced Stirling Convertor Heater Head

The National Aeronautics and Space Administration (NASA) has identified the high efficiency Advanced Stirling Radioisotope Generator (ASRG) as a candidate power source for use on long duration Science missions such as lunar applications, Mars rovers, and deep space missions. For the inherent long life times required, a structurally significant design limit for the heater head component of the ASRG Advanced Stirling Convertor (ASC) is creep deformation induced at low stress levels and high temperatures. Demonstrating proof of adequate margins on creep deformation and rupture for the operating conditions and the MarM-247 material of construction is a challenge that the NASA Glenn Research Center is addressing. The combined analytical and experimental program ensures integrity and high reliability of the heater head for its 17-year design life. The life assessment approach starts with an extensive series of uniaxial creep tests on thin MarM-247 specimens that comprise the same chemistry, microstructure, and heat treatment processing as the heater head itself. This effort addresses a scarcity of openly available creep properties for the material as well as for the virtual absence of understanding of the effect on creep properties due to very thin walls, fine grains, low stress levels, and high-temperature fabrication steps. The approach continues with a considerable analytical effort, both deterministically to evaluate the median creep life using nonlinear finite element analysis, and probabilistically to calculate the heater head s reliability to a higher degree. Finally, the approach includes a substantial structural benchmark creep testing activity to calibrate and validate the analytical work. This last element provides high fidelity testing of prototypical heater head test articles; the testing includes the relevant material issues and the essential multiaxial stress state, and applies prototypical and accelerated temperature profiles for timely results in a highly controlled laboratory environment. This paper focuses on the last element and presents a preliminary methodology for creep rate prediction, the experimental methods, test challenges, and results from benchmark testing of a trial MarM-247 heater head test article. The results compare favorably with the analytical strain predictions. A description of other test findings is provided, and recommendations for future test procedures are suggested. The manuscript concludes with describing the potential impact of the heater head creep life assessment and benchmark testing effort on the ASC program.

Krause, David L.↗

NASA NTRSDate not supplied

Boundary-Layer Flow Simulations Over Ablating Woven Thermal Protection System Material

Spallation is the mechanical removal of small chunks of material gets removed typically due to high shear conditions of the flow field. This reduces the ability of the thermal protection system (TPS) material to protect the spacecraft as well as cause turbulence in the flow causing higher heating rates. In this work, we focus on the material removal through ablation and high shear flow within the boundary layer region of woven TPS material. Woven TPS (WTPS) material is the latest class of material developed by NASA, to be used within the next generation of space flights. They are complex interlocked weaves designed to create a rigid structure that is highly resistant to heat and can be easily designed and tailored for a wide variety of entry environments. Due to material removal resulting from chemical degradation, the structural integrity of TPS material is affected. Spallation occurs when this structurally compromised material is exposed to the high shear flow conditions within the boundary layer. In order to understand the spallation mechanism within WTPS material, we first perform the material removal simulations which occur primarily through oxidation to obtain the microstructure at various stages of degradation. These simulations are performed using the Porous Microstructure Analysis (PuMA) software developed at NASA Ames. The micro-structure geometry used within these simulations were generated artificially to be similar to the 3D weave architecture of MSR-EEV (Mars Sample Return - Earth Entry Vehicle). The various eroded TPS micro-structures are then subjected to the boundary layer flow conditions to obtain critical surface quantities which contribute to the structural failure mechanism such as heat flux, pressure, and shear stress. The direct simulation Monte Carlo (DSMC) methodology is used to perform these simulations in order to accurately capture the strong gradients within the high-temperature boundary layer flow over the intricate geometry of WTPS material. The boundary layer profile is directly taken from the Computational Fluid Dynamics (CFD) simulation and provided as boundary conditions to the DSMC inlet and outlet. Further, the variation of these properties as the microstructure undergoes changes due to oxidation is also investigated. Finally, these quantities are used as input in PuMA to understand the material expansion/compression and strain within the woven TPS geometry and help in developing a comprehensive spallation and structure failure model.

microstructure↗

NASA NTRSDate not supplied

Boundary-Layer Flow Simulations Over Ablating Woven Thermal Protection System Material

microstructure↗

NASA NTRS1996

Products of Dissociative Recombination in the Ionosphere

SRI International undertook a novel experimental measurement of the product states formed by dissociative recombination (DR) of O2(+), NO(+), and N2(+) as a function of both electron energy and reactant ion vibrational level. For these measurements we used a recently developed experimental technique for measuring dissociation product distributions that allows both the branching ratios to be accurately determined and the electronic and rovibrational state composition of the reactant ions to be specified. DR is the dominant electron loss mechanism in all regions of the ionosphere. In this process, electron attachment to the molecular ion produces an unstable neutral molecule that rapidly dissociates. For a molecular ion such as O2(+), the dissociation recombination reaction is (1) O2(+) + e yields O + O + W. The atomic products of this reaction, in this case two oxygen atoms, can be produced in a variety of excited states and with a variety of kinetic energies, as represented by W in Eq. (1). These atoms are not only active in the neutral chemistry of the ionosphere, but are also especially important because their optical emissions are often used to infer in situ concentrations of the parent molecular ion and ambient electron densities. Many laboratory measurements have been made of DR reaction rates under a wide range of electron temperatures, but very little is known about the actual distributions among the final states of the atomic products. This lack of knowledge seriously limits the validity and effectiveness of efforts to model both natural and man-made ionospheric disturbances. Bates recently identified major deficiencies in the currently accepted branching ratios for O2(+) as they relate to blue and green line emission measurements in the nocturnal F-region. During our two-year effort, we partially satisfied our ambitious goals. We constructed and operated a variable pressure, electron-impact ion source and a high pressure, hollow-cathode discharge ion source for O2(+), NO(+), and N2(+) beams. Translational spectroscopy of the products of dissociative charge transfer in Cs vapor was used to accurately assay the composition of the O2(+) and NO(+) beams and to develop a methodology for the vibrationally controlled preparation of the ground state ion beams. Attempts to assay the N2(+) beam revealed a novel two-electron process in the charge transfer reactions. A coaxial electron gun for the DR measurements was constructed following an extensive numerical design of the fields. Tests of the gun, however, found substantial perturbations of the magnetic fields by the soft iron (CMI-C) assembly containing the Langmuir probe that locates the electron beam. Hydrogen annealing of the iron failed to eliminate the field perturbations, necessitating the removal of the probe assembly. During this work on the coaxial electron gun, we discovered that predissociated high Rydberg states of O2 could be produced by subjecting the molecules to a sudden perturbation by an electromagnetic field. This technique allowed a measurement of the product branching to the atomic limits for the lowest seven vibrational levels of O2(+).

Cosby, Philip↗

NASA NTRS2024

Thermal Modeling and Testing of High-Temperature Refractory Ceramic Insulation Felts

Heat transfer in high-temperature, high-porosity, flexible refractory ceramic fibrous insulation felts is investigated. Heat transfer in these insulation materials consists of combined gas conduction, solid conduction, and radiation modes, with the precise theoretical modeling of the latter two modes being formidable. A semi-empirical model that requires inverse methods and steady-state thermal test data to infer some of the required model parameters is further developed in this study, and applied to five insulation materials for temperatures between 300 K and 1900 K. The steady-state thermal test setup at NASA Langley Research Center with recent modifications to increase its testing capability to 1900 K is discussed. Design considerations to ensure one-dimensional heat transfer in the test setup are described. Test data and corresponding thermal models for alumina and zirconia-based fibrous insulation felts are presented. Furthermore, test data and thermal models on two fibrous insulation samples containing additives to further suppress either radiation or gas conduction modes of heat transfer are presented. Previously published alumina-based insulation data are also re-processed using the updated modeling methodology. The significance of various heat transfer modes in typical insulation samples is discussed and used to provide general guidance on optimum insulation layups.

Kamran Daryabeigi↗

DOE OSTI2023

Demonstrating non-isothermal vacuum membrane air dehumidification for efficient next-generation air conditioning

Vacuum membrane air dehumidification has gained significant interest in recent years as a highly efficient means of air dehumidification. Prior theoretical modeling work by the authors introduced the Active Membrane Energy Exchanger (AMX) concept, which combines active heat exchange and vacuum membrane dehumidification into one non-isothermal system, and found that it could outperform other air conditioning technologies under many conditions. However, no experimental literature exists on combining active heat exchangers and vacuum membrane dehumidification. Here, the goal of the present study is to evaluate the dehumidification (mass transfer) performance of the AMX concept relative to isothermal membrane dehumidification through three main methodologies: (1) membrane material-level characterization, (2) experimental prototype development and testing, and (3) computational fluid dynamics (CFD) simulations. The dense membranes used in this work showed up to a 40% increase in water vapor permeance at cooler temperatures, and the prototype system showed up to a 6% increase in humidity removal when the air was simultaneously cooled. Furthermore, the membrane module-integrated heat exchange tubes provided additional mixing and turbulence, leading to a 4–8% increase in humidity removal. The upper limit coefficient of performance was equal to approximately 2.2, necessitating advanced system designs to improve efficiency. This study is the first to highlight that combining the cooling and vacuum dehumidification processes can improve dehumidification performance.

30 DIRECT ENERGY CONVERSION↗

NASA NTRS2018

ASTERIA Operations Demonstrates the Value of Combining the Mission Assurance and Fault Protection Roles on CubeSats

On November 20, 2017, ASTERIA (Arcsecond Space Telescope Enabling Research in Astrophysics), a 6U CubeSat performing a technology demonstration of astrophysical measurements, deployed from the ISS. The technology demonstration goals to achieve precision photometry via arcsecond-level line-of-sight pointing error and highly stable focal plane temperature control were met by February 2018. Extended mission operations are ongoing, with the primary focus on observing nearby stars for transiting exoplanets. Throughout development and operations, the roles of mission assurance and fault protection have proven critical to achieving the primary technical goals and to maintaining a healthy spacecraft through multiple extended missions. Given the budget and schedule constraints typical of a CubeSat, innovative tailoring of processes has been critical to success throughout both development and operations of ASTERIA. Mission assurance plays an important role in identifying and evaluating risk and developing cost-effective mitigations. Flexibility in the fault protection design offers a variety of options for implementing risk mitigations as risks have been uncovered both in pre-delivery testing and in mission operations. This paper will discuss the approach taken on ASTERIA to implement mission assurance and fault protection and the resulting benefits to operational efficiency and success. It will briefly address the advantages of this approach during development, in which the combination of the roles provided mission assurance significant insight to system risks, which feeds back into testing methodologies and directly into fault protection design. Operations will be discussed in detail. During this phase, the roles merge to identify in-flight fault protection updates to efficiently respond to anomalies and improve the likelihood of successful technology demonstrations. The paper will also detail the tools that are used to analyse data, identify anomalies, and develop the updates to uplink to the spacecraft. Finally, the general operational approach will be discussed to highlight the usefulness of the ASTERIA processes and their applicability to future CubeSat missions.

Knapp, Mary↗

NASA NTRS1998

A Formal Approach for Designing and Evaluating Procedures

Operator interaction with modern control systems is a topic of great concern in high-risk industries such as nuclear power and commercial aviation. The issues associated with such systems focus on the ability of the operators (e.g., pilots) to achieve mission goals safely while containing failures. Operators must be able to interact safely and reliably with highly automatic and complex systems across the full spectrum of possible operating conditions, including normal, abnormal, and emergency situations. In environments such as commercial aviation, operator interaction with the machine is specified through a set of standard operating procedures (SOP). A procedure represents a collective agreement on the 'best' way to perform a given task. The intent of this paper is to suggest a formal methodology, for designing and evaluating procedures, that is both reliable and systematic. Our approach involves two major elements: a model of the machine and a list of the operator's task specifications (goals). We use formal modeling paradigms for describing the system and super-imposing on it the operator's tasks. Such paradigms, based on recent frameworks such as Statecharts and Hierarchical Hybrid Machines appear to be adequate methods for analyzing operator interaction with modern control systems. To illustrate this methodology, we model and analyze the sequence of actions for an emergency procedure. The procedure, Irregular Engine Start, for a medium-range aircraft, specifies the sequence of immediate actions that must be performed by the crew to avoid an uncontrolled rise in engine temperature during start-up. A model of engine behavior during a hot start is constructed. It also describes the various actions that can be taken by the crew and the resulting outcomes. The model is then opened up as a tree of all possible action sequences. This action tree allows us to trace the correct sequences necessary to achieve the desired end-goal (secure and shut down of the engine). In conclusion, we argue that the current process of designing and evaluating procedures can be improved. We discuss the implications of this approach for designing and evaluating this and other types of procedures. We conclude with insights about the benefits and limitation of this methodology, and offer suggestions for future research.

Degani, Asaf↗

NASA NTRS2010

Probabilistic Simulation for Combined Cycle Fatigue in Composites

A methodology to compute probabilistic fatigue life of polymer matrix laminated composites has been developed and demonstrated. Matrix degradation effects caused by long term environmental exposure and mechanical/thermal cyclic loads are accounted for in the simulation process. A unified time-temperature-stress dependent multifactor interaction relationship developed at NASA Glenn Research Center has been used to model the degradation/aging of material properties due to cyclic loads. The fast probability integration method is used to compute probabilistic distribution of response. Sensitivities of fatigue life reliability to uncertainties in the primitive random variables (e.g., constituent properties, fiber volume ratio, void volume ratio, ply thickness, etc.) computed and their significance in the reliability-based design for maximum life is discussed. The effect of variation in the thermal cyclic loads on the fatigue reliability for a (0/+/- 45/90)s graphite/epoxy laminate with a ply thickness of 0.127 mm, with respect to impending failure modes has been studied. The results show that, at low mechanical cyclic loads and low thermal cyclic amplitudes, fatigue life for 0.999 reliability is most sensitive to matrix compressive strength, matrix modulus, thermal expansion coefficient, and ply thickness. Whereas at high mechanical cyclic loads and high thermal cyclic amplitudes, fatigue life at 0.999 reliability is more sensitive to the shear strength of matrix, longitudinal fiber modulus, matrix modulus, and ply thickness.

Chamis, Christos C.↗

NASA NTRS2010

Experimental Creep Life Assessment for the Advanced Stirling Convertor Heater Head

The United States Department of Energy is planning to develop the Advanced Stirling Radioisotope Generator (ASRG) for the National Aeronautics and Space Administration (NASA) for potential use on future space missions. The ASRG provides substantial efficiency and specific power improvements over radioisotope power systems of heritage designs. The ASRG would use General Purpose Heat Source modules as energy sources and the free-piston Advanced Stirling Convertor (ASC) to convert heat into electrical energy. Lockheed Martin Corporation of Valley Forge, Pennsylvania, is integrating the ASRG systems, and Sunpower, Inc., of Athens, Ohio, is designing and building the ASC. NASA Glenn Research Center of Cleveland, Ohio, manages the Sunpower contract and provides technology development in several areas for the ASC. One area is reliability assessment for the ASC heater head, a critical pressure vessel within which heat is converted into mechanical oscillation of a displacer piston. For high system efficiency, the ASC heater head operates at very high temperature (850 C) and therefore is fabricated from an advanced heat-resistant nickel-based superalloy Microcast MarM-247. Since use of MarM-247 in a thin-walled pressure vessel is atypical, much effort is required to assure that the system will operate reliably for its design life of 17 years. One life-limiting structural response for this application is creep; creep deformation is the accumulation of time-dependent inelastic strain under sustained loading over time. If allowed to progress, the deformation eventually results in creep rupture. Since creep material properties are not available in the open literature, a detailed creep life assessment of the ASC heater head effort is underway. This paper presents an overview of that creep life assessment approach, including the reliability-based creep criteria developed from coupon testing, and the associated heater head deterministic and probabilistic analyses. The approach also includes direct benchmark experimental creep assessment. This element provides high-fidelity creep testing of prototypical heater head test articles to investigate the relevant material issues and multiaxial stress state. Benchmark testing provides required data to evaluate the complex life assessment methodology and to validate that analysis. Results from current benchmark heater head tests and newly developed experimental methods are presented. In the concluding remarks, the test results are shown to compare favorably with the creep strain predictions and are the first experimental evidence for a robust ASC heater head creep life.

Krause, David L.↗

DOE OSTI2023

Low-Temperature Geothermal Geospatial Datasets: An Example from Alaska

This project is a component of a broader effort focused on geothermal heating and cooling (GHC) with the aim of illustrating the numerous benefits of incorporating GHC and geothermal heat exchange (GHX) into community energy planning and national decarbonization strategies. To better assist private sector investment, it is currently necessary to define and assess the potential of low-temperature geothermal resources. For shallow GHC/GHX fields, there is no formal compilation of subsurface characteristics shared among industry practitioners that can improve system design and operations. Alaska is specifically noted in this work, because heretofore, it has not received a similar focus in geothermal potential evaluations as the contiguous United States. The methodology consists of leveraging relevant data to generate a baseline geospatial dataset of low-temperature resources (less than 150 degrees C) to compare and analyze information accessible to anyone trying to understand the potential of GHC/GHX and small-scale low-temperature geothermal power in Alaska (e.g., energy modelers, communities, planners, and policymakers). Importantly, this project identifies data related to (1) the evaluation of GHC/GHX in the shallow subsurface, and (2) the evaluation of low-temperature geothermal resource availability. Additionally, data is being compiled to assess repurposing of oil and gas wells to contribute co-produced fluids toward the geothermal direct use and heating and cooling resource potential. In this work we identified new data from three different datasets of isolated geothermal systems in Alaska and bottom-hole temperature data from oil and gas wells that can be leveraged for evaluation of low-temperature geothermal resource potential. The goal of this project is to facilitate future deployment of GHC/GHX analysis and community-led programs and update the low-temperature geothermal resources assessment of Alaska. A better understanding of shallow potential for GHX will improve design and operations of highly efficient GHC systems. The deployment and impact that can be achieved for low-temperature geothermal resources will contribute to decarbonization goals and facilitate widespread electrification by shaving and shifting grid loads.

15 GEOTHERMAL ENERGY↗

DOE OSTI2025

Tailoring composition and deformation modes at the microstructural level for next generation low-cost high-strength austenitic stainless steels

The objective of this project is to enable deliberate development of cost-effective, hydrogen resistant alloys by establishing detailed relationships specific to the effects of alloy composition, short-range order (SRO), and microsegregation in the presence of hydrogen on the transition between homogeneous deformation and localized plasticity in shear bands. In collaboration with the International Institute for Carbon-Neutral Energy Research, I2CNER, at Kyushu University in Japan, we conceptualized, designed, and manufactured four austenitic alloys that maintain corrosion resistance and ensure lower cost relative to baseline commercial alloys. The mechanical properties and deformation modes of the novel alloys (KU alloys) were assessed in the presence of hydrogen (H). Correlations between composition and performance revealed that two of the KU alloys are suitable replacements for 316 steel, while another is a viable replacement for 304 steel at room temperature. We found that, in the presence of other austenite stabilizing elements namely Mn and N, replacing Ni with Cu does not lead to martensite formation as has been previously reported.1–3 Furthermore, we found that the addition of Cu leads to an earlier onset of multiple slip resulting in an relative earlier onset of a higher work hardening rate (WHR). Greater understanding of the relationships between alloy composition and SRO required the development of a novel advanced electron diffraction methodology to characterize SRO in complex FCC alloys. This innovative approach, which combines fluctuation and correlation analyses of diffuse-scattering signals, successfully differentiated between SRO and long-range ordering (LRO). Further investigations into annealed austenitic stainless steels could provide insights into manipulating SRO and its effects on material properties. Atomistic simulations provided understanding of SRO behavior that was difficult to capture experimentally. This project created the first spin cluster expansion model that is able to capture and describe SRO effects in Fe-Ni-Cr FCC alloys, accounting for the non-negligible effects of magnetism. An automated computational workflow was established to provide reliable predictions of SRO in Fe-Ni-Cr austenitic alloys, both with and without the presence of H atoms. Analysis of the propensity for SRO in Fe-Ni-Cr alloys revealed that H tends to cluster with specific, well-defined SRO domains. The computational framework is general purpose and can be extended to realistic stainless steels across diverse composition ranges. With confidence that SRO is possible in austenitic stainless steels, we developed a discrete dislocation finite element code to understand the interaction of dislocations with SRO in the presence of H. By incorporating H effects on the dislocation emission and SRO stress field we show that the critical stress for the dislocation pileup to breakthrough the SRO domain decreases in the presence of H, which directly contributes localized deformation at the macroscale. Through the simulation of a uniaxial tension test, we demonstrated that H-induced weakening of SRO stress field and H-enhanced dislocation emission can lead to the onset of shear localization at lower macroscopic strains. As a whole, this project identified three novel alloys that show improvements in performance and cost efficiency for H-facing applications by studying correlations between alloy chemistry and deformation behavior. We also made significant advancements to experimental and computational methodologies necessary to study the chemistry and distribution of SRO across a range of alloys, which in turn allowed us to demonstrate how deformation mechanisms change due to the contributions of SRO in austenitic alloys in the presence of H. The combined advancements in fundamental understanding with novel alloy development in this project has increased the viability of next generation H-technologies for the broader public through accessible low-cost alloys and accelerated development towards future H-infrastructure.

08 HYDROGEN↗

NASA NTRS1996

Effect of Cyclic Thermo-Mechanical Loads on Fatigue Reliability in Polymer Matrix Composites

A methodology to compute probabilistic fatigue life of polymer matrix laminated composites has been developed and demonstrated. Matrix degradation effects caused by long term environmental exposure and mechanical/thermal cyclic loads are accounted for in the simulation process. A unified time-temperature-stress dependent multi-factor interaction relationship developed at NASA Lewis Research Center has been used to model the degradation/aging of material properties due to cyclic loads. The fast probability integration method is used to compute probabilistic distribution of response. Sensitivities of fatigue life reliability to uncertainties in the primitive random variables (e.g., constituent properties, fiber volume ratio, void volume ratio, ply thickness, etc.) computed and their significance in the reliability- based design for maximum life is discussed. The effect of variation in the thermal cyclic loads on the fatigue reliability for a (0/+/- 45/90)(sub s) graphite/epoxy laminate with a ply thickness of 0.127 mm, with respect to impending failure modes has been studied. The results show that, at low mechanical cyclic loads and low thermal cyclic amplitudes, fatigue life for 0.999 reliability is most sensitive to matrix compressive strength, matrix modulus, thermal expansion coefficient, and ply thickness. Whereas at high mechanical cyclic loads and high thermal cyclic amplitudes, fatigue life at 0.999 reliability is more sensitive to the shear strength of matrix, longitudinal fiber modulus, matrix modulus, and ply thickness.

Shah, A. R.↗

DOE OSTI2026

Multiscale study of helium diffusion in Ni-Cr alloys: Short-range trapping versus long-range channeling

Ni-Cr alloys are widely employed as structural materials in fast nuclear reactors but are vulnerable to high-temperature helium (He) embrittlement (HTHE) under fast neutron irradiation. A comprehensive understanding of He diffusion in Ni-Cr alloys, which governs the kinetics of HTHE, is therefore essential for developing resilient materials and preventing failure. In this work, we reveal the underlying mechanisms of He diffusion in pure Ni and Ni-Cr alloys by integrating density functional theory (DFT) with atomic kinetic Monte Carlo (AKMC) simulations. Our findings uncover a non-monotonic dependence of He diffusivity on Cr concentration, contradicting the monotonic trends predicted by DFT-parameterized theories. At low Cr concentrations, He diffusion is dominated by short-range trapping, characterized by multiple trapping sites and a distinct mechanism within the first nearest neighbor of Cr, differing from that in pure Ni. At high Cr concentrations, these local traps become interconnected, forming long-range fast diffusion channels that enhance He mobility. The competition between localized trapping and extended channeling results in a diffusivity that first decreases, then increases with rising Cr content. These atomic-scale insights offer critical guidance for the design of radiation-tolerant Ni-based alloys. Moreover, the combined DFT-AKMC methodology and the concept of random walker diffusion through interconnected energy basins present a broadly applicable framework for studying transport phenomena in disordered systems.

36 - MATERIALS SCIENCE↗