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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Assessment of Long Term Creep Using Strain Rate Matching From the Stepped Isostress Method

Creep testing is an ongoing need, particularly with the development of new candidate alloy systems for advanced energy systems. The conventional creep test (CT) is regarded as a proven method to gather creep data however, the test is impractical due to being real-time: lasting up to 105 hours to characterize the service of long-lived turbomachinery components. Accelerated methods to gather the long-term creep properties of materials are needed to reduce the time to qualification of new materials. The time-temperature-stress-superposition principle (TTSSP) and the derivative time-temperature superposition principle (TTSP), time-stress superposition principle (TSSP), stepped isothermal method (SIM), and stepped isostress method (SSM) are accelerated creep tests (ACT) commonly used to predict the long-term creep behaviors of polymers and composites. The TTSP and TSSP tests require multiple specimen tested at various temperatures/stresses whereas the SIM and SSM tests employ a single specimen where temperature/stress are periodically step increased until rupture. The stepped creep deformation curve can then be time and strain shifted to produce a master creep curve. While these ACTs are useful tools to predict long-term creep, the drawback is the lack of mathematical laws to determine the virtual start time and time shift factors, especially for different materials. In this paper, a new self-calibration approach is developed and compared to existing SSM data for Kevlar 49. This new approach focuses on matching the creep strain rates between stress steps and fitting the data to a master curve using a modified theta projection model. This is performed using a MATLAB code consisting of five subroutines. The first subroutine takes the stress, time, and creep strain from SSM/SIM tests, and segregates the data intro arrays corresponding to each stress level. The second subroutine finds the constants for the modified theta projection model for each stress level. The third subroutine performs a time shift adjustment using creep strain rate matching. The fourth subroutine calculates the accelerated time of rupture. The last subroutine generates accelerated creep versus time plots. Kevlar 49 SSM data is gathered from literature and run through the MATLAB code. The master curves generated from the MATLAB are compared to the conventional creep curve of Kevlar 49 as well as the master curve gathered from literature in order to validate the feasibility of this new approach. The goal of this project is to vet if the self-calibration approach can produce results similar to the reference calibration approach.

36 MATERIALS SCIENCE↗
NASA NTRS1972

Interactions between creep, fatigue and strain aging in two refractory alloys

The application of low-amplitude, high-frequency fatigue vibrations during creep testing of two strain-aging refractory alloys (molybdenum-base TZC and tantalum-base T-111) significantly reduced the creep strength of these materials. This strength reduction caused dramatic increases in both the first stage creep strain and the second stage creep rate. The magnitude of the creep rate acceleration varied directly with both frequency and A ratio (ratio of alternating to mean stress), and also varied with temperature, being greatest in the range where the strain-aging phenomenon was most prominent. It was concluded that the creep rate acceleration resulted from a negative strain rate sensitivity which is associated with the strain aging phenomenon in these materials. (A negative rate sensitivity causes flow stress to decrease with increasing strain rate, instead of increasing as in normal materials). By combining two analytical expressions which are normally used to describe creep and strain aging behavior, an expression was developed which correctly described the influence of temperature, frequency, and A ratio on the TZC creep rate acceleration.

Sheffler, K. D.↗
DOE OSTI2025

A Finite Element Study of Stress Reduction Techniques in REBCO HTS Conductor on a Round Cable (CORC) Cable

ReBCO high-temperature superconducting (HTS) tape is critical for achieving the high magnetic fields needed in next-generation particle accelerators. Enhancing the mechanical performance of ReBCO tape increases its critical current by reducing internal stress, especially in the superconducting layer. A finite element study examined how copper layer properties affect stress in ReBCO conductor on a round core (CORC) cables. The cable was modeled as a doubly supported beam under uniform compressive stresses."cable was modeled as a doubly supported beam under uniform load to simulate bending. A staged modeling approach from a single tape to a six-layer stack enabled validation and efficient parameter studies. Increasing the yield strength and Young s modulus of the copper layers reduced peak stress in the ReBCO layer. These results support development of improved tape stacks for high-field accelerator magnets

Mueller, Scott [Northern Illinois U.]↗
DOE OSTI2023

Comparative Analysis of Hotspot Stress Endurance in Pristine and Thermal Cycled Prestressed Glass–Glass Photovoltaic Modules

Hotspots pose a significant long-term reliability challenge in photovoltaic (PV) modules that can have a detrimental impact on the efficiency, safety, and financial viability of a PV system. This paper aims to evaluate the endurance of hotspot stress in pristine and prestressed glass–glass (GG) modules. The accelerated prestressing was conducted for 600 thermal cycles (TC600) to represent decades of field exposure. GG modules are quickly becoming an alternative to the traditional glass–backsheet (GB) modules that have been the industry standard. Unlike other conventional studies that subject only pristine modules to hotspot stress, this paper evaluates the performance of an accelerated/simulated field-aged GG module (using TC600) and a pristine GG module. Pre- and post-characterizations were performed before and after each test to determine changes in electrical performance and observe any defects in GG modules. During the hotspot test, an approximately 200 °C maximum cell temperature was observed with a cell shading of 25% (the worst-case shading ratio). After the hotspot test, electroluminescence imaging indicated that most cells in the prestressed GG module exhibited severe damage whereas no significant defects were evident in the pristine GG module where the prestressed GG module degraded 8.2% and the pristine GG module degraded 1.5% in maximum power. These findings are critical for the industry, considering that GG bifacial modules will dominate the market.

14 SOLAR ENERGY↗
DOE OSTI2019

Effect of Prestrain on the Fatigue Life of Superelastic Nitinol

Three types of fatigue testing are performed to elucidate the effects of prestraining superelastic Nitinol on its subsequent fatigue lifetime: rotary bending and tension–tension testing of wire, and beam bending using diamond-shaped specimens fabricated from tubing. Results show that local plastic deformation during prestraining induces residual stresses that have a pronounced effect on fatigue performance, enhancing performance when the fatigue duty cycle is of the same sense as the prestraining (tensile prestraining followed by a tensile duty cycle, for example), and decreasing fatigue lifetime when the sense of the duty cycle is opposite to that of prestraining. This provides an avenue to increasing fatigue lifetime, but more importantly it highlights the need to fully understand the nature of the duty cycle: for example, prestraining a stent by crimping it into a delivery catheter induces favorable residual stresses with respect to subsequent pulsatile fatigue, but might accelerate fracture in other modes, such as axial or crush fatigue. Caution is also advised when trying to apply data from “constant life diagrams” derived from the literature (Ref 1, 2 for example) that may not properly reflect the strain history of the device being analyzed.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Neutron radiation hardness testing of 650V / 7.5 A GaN power HEMT

In this work, we have performed neutron radiation hardness testing for commercially available 650 V/7.5 A GaN power HEMT. The devices were tested at Los Alamos Neutron Science Center (LANSCE) inside Irradiation of Chips Electronics, ICE house-1 at a flux density of 10(6) n/cm(2) -s and energy level above 1 MeV, which is approximately 10(6) times higher than neutron radiation at airliner flight height (30,000 ft). During the experiments, the devices were stressed electrically, thermally and irradiated with neutron radiation. We have observed degradation of AlGaN layer due to accelerated neutron radiation (1.54 x 10(10) n/cm(2) above 1.5 MeV neutron energy) under electrical stress condition. We have performed parametric defect analysis through step-by-step application of neutron irradiation, thermal stress and electrical bias, and observed highest degradation of threshold voltage (0.14 V) for combined electrical stress and neutron irradiation.

36 MATERIALS SCIENCE↗
NASA NTRS1993

New results on the realizability of Reynolds stress turbulence closures

The realizability of Reynolds stress models in homogeneous turbulence is critically assessed from a theoretical standpoint. It is proven that a well known second-order closure formulated by Shih and Lumley using the strong realizability constraints of Schumann is, in fact, not a realizable model. The problem arises from the failure to properly satisfy the necessary positive second time derivative constraint when a principal Reynolds stress vanishes - a fatal flaw that becomes apparent when the non-analytic terms in their model are made single-valued as required on physical grounds. It is furthermore shown that the centrifugal acceleration generated by rotations of the principal axes of the Reynolds stress tensor can make the second derivative singular at the most extreme limits of realizable turbulence. This previously overlooked effect appears to make it impossible to identically satisfy the strong form of realizability in any version of the present generation of second-order closures. On the other hand, models properly formulated to satisfy the weak form of realizability - wherein states of one or two component turbulence are not accessible in finite time are found to be realizable. However, unlike the simpler and more commonly used second order closures, these models can be ill-behaved near the extreme limits of realizable turbulence due to the way that higher-degree nonlinearities are often unnecessarily introduced to satisfy realizability. Illustrative computations of homogeneous shear flows are presented to demonstrate these points which can have important implications for turbulence modeling.

Speziale, Charles G.↗
NASA NTRS2010

Experimental Validation of FE/BEM Dynamic Strain Model Under Diffuse Acoustic Field Loading

Structural finite element (FE) models naturally output displacement or acceleration response data. However, they can also be used to compute stress, internal forces, and strain response. When coupled with a boundary element model (BEM) of the fluid surrounding the structure, a fully coupled analysis can be performed. Modeling a diffuse acoustic field in the BEM fluid provides an excitation like that found when the structure is placed in a reverberation chamber. Fully coupling the structural FE model to the acoustic BEM model provides a means to predict not only the acceleration response of the panel to diffuse field loading, but also the ability to predict the dynamic stress and strain response. This type of model has been available with current predictive tools, but experimental validation of the prediction of dynamic stress or strain is difficult to find. An aluminum panel was instrumented with accelerometers and strain gages and hung in a reverberation room and subjected to a diffuse acoustic field. This paper presents the comparison of the experimental and predicted results.

Tsoi, W. Ben↗
NASA NTRS1977

The effect of increased gravitational stress on bone

A group of 34 adult male chickens were chronically accelerated over an 18-week period; for the last 4 weeks the surviving animals were subjected to a 3-g field. Males of a similar weight and age were used as static controls. The objective was to evaluate the effects of an altered gravitational state on the physical properties of the tibia bone tested for torsional fracture. Of the 34 initial animals, 15 survived for the entire period and were subjected to analysis. The results suggest that the altered morphology produced by increased gravitational fields does not materially affect bone strength, at least in torsion. Decreased bone diameters were accompanied by increased cortical thickness without change in the bone resistance to torsion. The findings of increased cortical thickness with decreased bone diameter suggest reversal of the usual cellular dynamics of adult bone. Data on bone ash and density failed to reveal any substantial changes in bone mineral or organic content. Histological examination of the cortical bone did not disclose any evidence of pathology.

Riggins, R. S.↗
DOE OSTI2018

Stress Testing of Organic Light- Emitting Diode Panels and Luminaires

This report builds on previous DOE efforts with OLED technology by updating information on a previously benchmarked OLED product (the Chalina luminaire from Acuity Brands) and provides new benchmarks on the performance of Brite 2 and Brite Amber OLED panels from OLEDWorks. During the tests described here, samples of these devices were subjected to continuous operation in stress tests at elevated ambient temperature environments of 35°C or 45°C. In addition, samples were also operated continuously at room temperature in a room temperature operational life test (RTOL). One goal of this study was to investigate whether these test conditions can accelerate failure of OLED panels, either through panel shorting or an open circuit in the panel. These stress tests are shown to provide meaningful acceleration of OLED failure modes, and an acceleration factor of 2.6 was calculated at 45°C for some test conditions. In addition, changes in the photometric properties of the emitted light (e.g., luminous flux and chromaticity maintenance) was also evaluated for insights into the long-term stability of these products compared to earlier generations. Because OLEDs are a lighting system, electrical testing was also performed on the panel-driver pairs to provide insights into the impact of the driver on long-term panel performance.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2018

Expanding the breeder’s toolbox for perennial grasses

With the increase in world energy demand and the environmental impacts associated with continued fossil fuel use, there is a great need for the development of alternative and sustainable fuel sources. Switchgrass ( Panicum virgatum L. ), a perennial grass native to North America, is poised to become one of the first dedicated biomass crops deployed on a large scale in the United States in large part due to its inherent high yield, adaptability and perennial nature. Current ‘cultivars’ of switchgrass are essentially undomesticated wild selections from natural populations. Thus, there is considerable potential for improvement of this emerging energy crop. Traditional breeding approaches in switchgrass have been hampered by its self-incompatibility and sensitivity to inbreeding, therefore there is a significant need to develop new tools for improved switchgrass breeding. The proposed research uses a systems biology approach to accelerate switchgrass breeding, to develop its biotechnological improvement through the generation of stress-tolerant varieties, and to establish ways to minimize the potential impact of transgenes on ecosystems. In this proposal, we will develop new breeding tools for perennial grasses and apply these tools toward improving switchgrass. Specifically, we will employ two complementary approaches to overcome the difficulty of breeding a large, polyploid, self-incompatible plant. The first approach will accelerate conventional breeding by developing a method to rapidly create doubled haploid (homozygous) lines for breeding. This approach is based on altering CENH3, a centromere-specific histone, leading to the production of haploid offspring when the centromere mutant is crossed to wild type plants. Subsequent genome doubling results in fully homozygous individuals and overcomes one of the major limitations in breeding self-incompatibile crops. These homozygous lines would allow switchgrass breeders to apply a variety of breeding approaches that have been previously impossible. The second approach is to develop tools and reagents to accelerate the biotechnological improvement of switchgrass including: modules for improved stress tolerance, a test bed to evaluate transgenes in a perennial model grass and a robust gene containment system. In order to maintain a favorable energy balance and avoid competition with food crops, dedicated biomass crops must be grown with minimal inputs (fertilizer and water) on marginal lands that are unsuited to the traditional high input agriculture necessary to produce food. Thus, abiotic stress tolerance and efficient water and nutrient use are of paramount importance. Considerable basic research has been directed at these areas and this knowledge has been used to develop transgenic approaches to greatly improve water and nutrient use efficiency. By applying these approaches to switchgrass we will rapidly increase drought tolerance and nutrient use efficiency. This approach can rapidly introduce these traits in different cultivars allowing breeders to add these traits to their elite germplasm. Since biotechnologically improved switchgrass must ultimately be grown in open fields, they must be designed to minimize the potential impact of the transgenes on non-target ecosystems. The best way to do this is to minimize gene flow from the transgenic plants. Thus, we will develop technology to introduce sterility into the transgenic plants. We will then combine this technology with the stress resistance modules to create switchgrass lines with enhanced resistance to abiotic stresses.

09 BIOMASS FUELS↗
DOE OSTI2018

Expanding the Breeder’s Toolbox for Perennial Grasses

With the increase in world energy demand and the environmental impacts associated with continued fossil fuel use, there is a great need for the development of alternative and sustainable fuel sources. Switchgrass (Panicum virgatum L.), a perennial grass native to North America, is poised to become one of the first dedicated biomass crops deployed on a large scale in the United States in large part due to its inherent high yield, adaptability and perennial nature. Current ‘cultivars’ of switchgrass are essentially undomesticated wild selections from natural populations. Thus, there is considerable potential for improvement of this emerging energy crop. Traditional breeding approaches in switchgrass have been hampered by its self-incompatibility and sensitivity to inbreeding, therefore there is a significant need to develop new tools for improved switchgrass breeding. The proposed research uses a systems biology approach to accelerate switchgrass breeding, to develop its biotechnological improvement through the generation of stress-tolerant varieties, and to establish ways to minimize the potential impact of transgenes on ecosystems. In this proposal, we will develop new breeding tools for perennial grasses and apply these tools toward improving switchgrass. Specifically, we will employ two complementary approaches to overcome the difficulty of breeding a large, polyploid, self-incompatible plant. The first approach will accelerate conventional breeding by developing a method to rapidly create doubled haploid (homozygous) lines for breeding. This approach is based on altering CENH3, a centromere-specific histone, leading to the production of haploid offspring when the centromere mutant is crossed to wild type plants. Subsequent genome doubling results in fully homozygous individuals and overcomes one of the major limitations in breeding self-incompatibile crops. These homozygous lines would allow switchgrass breeders to apply a variety of breeding approaches that have been previously impossible. The second approach is to develop tools and reagents to accelerate the biotechnological improvement of switchgrass including: modules for improved stress tolerance, a test bed to evaluate transgenes in a perennial model grass and a robust gene containment system. In order to maintain a favorable energy balance and avoid competition with food crops, dedicated biomass crops must be grown with minimal inputs (fertilizer and water) on marginal lands that are unsuited to the traditional high input agriculture necessary to produce food. Thus, abiotic stress tolerance and efficient water and nutrient use are of paramount importance. Considerable basic research has been directed at these areas and this knowledge has been used to develop transgenic approaches to greatly improve water and nutrient use efficiency. By applying these approaches to switchgrass we will rapidly increase drought tolerance and nutrient use efficiency. This approach can rapidly introduce these traits in different cultivars allowing breeders to add these traits to their elite germplasm. Since biotechnologically improved switchgrass must ultimately be grown in open fields, they must be designed to minimize the potential impact of the transgenes on non-target ecosystems. The best way to do this is to minimize gene flow from the transgenic plants. Thus, we will develop technology to introduce sterility into the transgenic plants. We will then combine this technology with the stress resistance modules to create switchgrass lines with enhanced resistance to abiotic stresses.

09 BIOMASS FUELS↗
NASA NTRS2022

Useful Life of Tantalum Capacitors

Currently, the major reliability characteristic of tantalum capacitors is the failure rate, which is a constant used to characterize random failures of electronic components. However, contemporary MnO2 and polymer tantalum capacitors have both infant mortality (IM) and wear-out (WO) failures. This requires assessments of the useful life of the parts that is limited by the time of inception of WO failures that can be determined using an adequate reliability model and results of highly accelerated life testing (HALT). In this presentation, a modified time dependent dielectric breakdown (TDDB) model is used to describe both IM and WO failures during HALT. Specifics of physical processes resulting in degradation and failures in polymer and MnO2 tantalum capacitors are discussed, and the increase of IM failures with the level of stress is explained. The model justifies exponential dependence of the acceleration factors on voltage and predicts higher values of the voltage acceleration constant for IM compared to WO failures.

tantalum capacitors↗
DOE OSTI2017

The MARS Code System User’s Guide Version 15(2016)

This paper is a user’s guide to the current version of the MARS15 Monte Carlo code. MARS allows inclusive and, in many cases, exclusive simulations of three-dimensional hadronic and electromagnetic cascades and modeling of heavy ion, muon and low energy neutron-photon transport in accelerator, detector, spacecraft and shielding components. Particles can have energies from a fraction of an electronvolt up to about 100 TeV. The code has undergone substantial improvements since the last documented version MARS13(95) and since previous releases of MARS15; all these as well as other specific features of the MARS code system are explained in detail. Descriptions of general input and output with commentary and recommendations are given. Examples are given for running the program with distributed sources, complex compounds, arbitrary geometries, and magnetic fields. A built-in graphical-user interface allows the visualization and detailed analysis of the geometry description. Enhanced options are described for the MARS code being coupled with: the DPMJET hadron event generator, the MCNP code for low-energy neutron transport, the ANSYS system for thermal and stress analysis, physics analysis and graphics packages, the MAD accelerator lattice description via a universal MAD-MARS beam line builder, the STRUCT program for tracking particles in accelerator lattices with beam loss recording, the DETRA code for nuclide decay and transmutation calculations, the EGS5 code for precise modeling of low-energy electromagnetic showers, and the ROOT system for description and visualization of arbitrary geometries. Use of the code in a multistage mode, coupled with event generators (DPMJET), with the STRUCT program for tracking particles in accelerator lattices with beam loss recording, and with physics analysis and graphics packages is demonstrated with typical input and output examples.

43 PARTICLE ACCELERATORS↗
NASA NTRS1972

Interactions between creep, fatigue, and strain-aging in two refractory metal alloys.

The application of low-amplitude, high-frequency fatigue vibrations during creep testing of two strain-aging refractory alloys (molybdenum-base TZC and tantalum-base T-111) significantly reduced the creep strength of these materials. This strength reduction caused dramatic increases in both the first-stage creep strain and the second-stage creep rate. The magnitude of the creep rate acceleration varied directly with both frequency and A ratio (ratio of alternating to mean stress), and also varied with temperature, being greatest in the range where the strain-aging phenomenon was most prominent. It was concluded that the creep rate acceleration resulted from a negative strain rate sensitivity which is associated with the strain-aging phenomenon in these materials. (A negative rate sensitivity causes flow stress to decrease with increasing strain rate, instead of increasing as in normal materials.)

Sheffler, K. D.↗
NASA NTRSDate not supplied

Infant Mortality and Wear-Out Failures in Polymer and MnO2 Tantalum Capacitors

Reliability testing of modern tantalum capacitors shows the presence of both types, infant mortality (IM) and wearout (WO) failures. To assure reliable operation, the probability of IM failures should be reduced to below the specified level, and the time of WO inception should be greater than the required useful life of the parts at operation conditions. In this work, several types of MnO2 and polymer cathode tantalum capacitors have been tested at highly accelerated conditions to assess voltage acceleration factors, determine adequate burn-in conditions, and assess the useful life. A modified time dependent dielectric breakdown (TDDB) model has been suggested to explain both types of failure, and a physical mechanism of degradation that is based on migration and reactions of oxygen vacancies explains increase of the defect related, IM failures with the level of stress.

tantalum capacitors↗
DOE OSTI2023

An examination of the precipitation behavior of proton irradiated dual phase 308L weldment filler materials

Voids, G phase particles, and Ni-Si rich clusters in proton irradiated dual phase 308L groove filler of a SA508–304 L dissimilar metal weldment are analyzed using advanced characterization techniques. These weldments are often used in light water nuclear reactors and are subject to enhanced corrosion and associated stress corrosion cracking (SCC). Radiation damage is known to accelerate SCC. Ni-Si enriched clusters were observed in proton irradiated γ austenite, while G phase M 6 Ni 16 Si 7 (where M transition metal element) precipitates were observed in proton irradiated δ ferrite. Compositional analysis of the G phase precipitates and Ni-Si clusters from STEM-EDS and APT are compared. Unlike G phase particles in proton irradiated δ ferrite, Ni-Si clusters in proton irradiated γ austenite are not rich in Mn. Both STEM-EDS and APT line-scan profiles of the Ni-Si clusters show that the Fe and Cr concentration gradient between matrix γ austenite and the Ni-Si clusters is not as sharp as those between matrix δ ferrite and G phase. Further, HR-STEM imaging indicates that the lattice parameter of the Ni-Si clusters is commensurate with γ austenite and the clusters do not represent the precipitation of a second phase. Finally, our analysis demonstrates the density and volume fraction of G phase particles and the density of voids scales with proton irradiation damage and the energy to recoils.

308L groove filler↗