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At least 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Constitutive Model of Radiation Aging Effects in Filled Silicone Elastomers under Strain

Filled silicone elastomers, an essential component in many technological applications, are often subjected to controlled or unintended radiation for a variety of reasons. Radiation exposure can lead to permanent mechanical and structural changes in the material, which is manifested as altered mechanical response, and in some cases, a permanent set. For unfilled elastomers, network theories developed and refined over decades can explain these effects in terms of chain-scission and cross-link formation and a hypothesis involving independent networks formed at different strain levels of the material. In this work, we expose a filled silicone rubber to gamma radiation while being under finite elongational strain and show that the observed mechanical and structural changes can be quantitatively modeled within the same theoretical framework developed for unfilled elastomers as long as nuances associated with the Mullins effect are accounted for in a consistent manner. In this work, we employ Ogden’s incompressible hyperelastic model within the framework of Tobolsky’s two-network scheme to describe the observed permanent set and mechanical modulus changes as a function of radiation dosage. In the process, we conclude that gamma radiation induces both direct cross-linking at chain crossings (H-links) and main-chain-scission followed by cross-linking (Y-links). We provide an estimate of the ratio of chain-scission to cross-linking rates, which is in reasonable agreement with previous experimental estimate from Charlesby–Pinner analysis. We use density functional theory (DFT)-based quantum mechanical calculations to explore the stability of –Si and –SiO radicals that form upon a radiation-induced chain-scission event, which sheds light on the relative rates of Y-linking and H-linking processes.

36 MATERIALS SCIENCE↗
DOE OSTI2024

The effect of repeated hurricanes on the age of organic carbon in humid tropical forest soil

Abstract Increasing hurricane frequency and intensity with climate change is likely to affect soil organic carbon (C) stocks in tropical forests. We examined the cycling of C between soil pools and with depth at the Luquillo Experimental Forest in Puerto Rico in soils over a 30‐year period that spanned repeated hurricanes. We used a nonlinear matrix model of soil C pools and fluxes (“soilR”) and constrained the parameters with soil and litter survey data. Soil chemistry and stable and radiocarbon isotopes were measured from three soil depths across a topographic gradient in 1988 and 2018. Our results suggest that pulses and subsequent reduction of inputs caused by severe hurricanes in 1989, 1998, and two in 2017 led to faster mean transit times of soil C in 0–10 cm and 35–60 cm depths relative to a modeled control soil with constant inputs over the 30‐year period. Between 1988 and 2018, the occluded C stock increased and δ 13 C in all pools decreased, while changes in particulate and mineral‐associated C were undetectable. The differences between 1988 and 2018 suggest that hurricane disturbance results in a dilution of the occluded light C pool with an influx of young, debris‐deposited C, and possible microbial scavenging of old and young C in the particulate and mineral‐associated pools. These effects led to a younger total soil C pool with faster mean transit times. Our results suggest that the increasing frequency of intense hurricanes will speed up rates of C cycling in tropical forests, making soil C more sensitive to future tropical forest stressors.

14C↗
DOE OSTI2023

Assessing Electrolyte Fluorination Impact on Calendar Aging of Blended Silicon-Graphite Lithium-Ion Cells Using Potentiostatic Holds

Silicon-based lithium-ion batteries have started to meet cycle life metrics, but they exhibit poor calendar life. Here, electrolyte fluorination impact on calendar fade of blended silicon-graphite anodes is explored using a LiPF 6 in EC:EMC:FEC electrolyte vs LiBOB in EC:EMC electrolyte. We utilize a combined experimental-modeling approach applying potentiostatic voltage holds (V-hold) to evaluate electrolyte suitability for calendar life in a shortened testing timeframe (~2 months). Our theoretical framework deconvolutes the irreversible parasitic capacity losses (lithium lost to the solid electrolyte interphase) from the V-hold electrochemical data. Unfluorinated electrolyte (dominant LiBOB reduction) exhibits higher cell resistance as compared to fluorinated electrolyte (dominant FEC reduction). Both systems have similar irreversible capacities during the voltage hold duration with slower rate of parasitic capacity loss for the LiBOB system. Extrapolation of the parasitic losses to end of life capacity fade of 20% shows LiBOB electrolyte outperforming LiPF 6 electrolyte in calendar life. The results demonstrate the applicability of the V-hold protocol as a rapid material screening tool providing semi-quantitative calendar lifetime estimates.

25 ENERGY STORAGE↗
DOE OSTI2025

Consideration of Decabromodiphenyl Ether Flame Retardant in Thermal and Radiation Aging of Crosslinked Polyethylene Cable Insulation

Decabromodiphenyl ether (decaBDE) has been used as a flame-retardant additive in nuclear-grade electrical cable insulation. However, decaBDE has been identified as a persistent, bioaccumulative and toxic (PBT) substance, leading to regulatory scrutiny. On January 6, 2021, the Environmental Protection Agency (EPA) published a final rule to phase out decaBDE. The 2021 rule set a two-year compliance deadline for “processing and distribution in commerce of decaBDE for use in wire and cable insulation in nuclear power generation facilities.” In recognition of industry concerns following a sudden discontinuation of decaBDE-containing Class 1E wire and cable essential for nuclear power operations and the time needed for qualifying the individual components using the alternative insulation technology, an extended compliance deadline was set in the finalized amendments to the 2021 rule as published by the Environmental Appeals Board on November 12, 2024. The 2024 rule set the compliance deadline for processing and distributing decaBDE-containing wire and cable insulation until the end of the service life of these materials. Since decaBDE has long been relied upon as the flame retardant in one of the most common cross-linked polyethylene (XLPE) nuclear cable insulation formulations, RSCC Firewall III insulation, questions have naturally arisen regarding whether changes in cable performance might be expected for XLPE containing a decaBDE alternative, especially for safety-related cables that must perform their safety function in a design basis event such as a loss of coolant accident.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
NASA NTRS1991

Federal Aviation Administration aging aircraft nondestructive inspection research plan

This paper highlights the accomplishments and plans of the Federal Aviation Administration (FAA) for the development of improved nondestructive evaluation (NDE) equipment, procedures, and training. The role of NDE in aircraft safety and the need for improvement are discussed. The FAA program participants, and coordination of activities within the program and with relevant organizations outside the program are also described.

Aging Aircraft↗
NASA NTRS1991

Ageing airplane repair assessment program for Airbus A300

Summary as a result of a commitment between the FAA and ATA the Airworthiness Assurance Task Force (AATF) steering committee requested the transport aircraft manufacturers to develop a program to assess the repairs on in-service aircraft since recent Supplemental Structural Inspection Programs (SSIP) had not been addressed to assessing repairs. The intent of the assessment program is to remove all existing ambiguities and to achieve a sufficient level of safety across the fleet by applying actual fatigue and damage tolerance criteria defined by FAR 25.571 to repaired structures. A general three stage program has been developed with major aircraft manufacturers and airlines cooperation including area/component classification, repair categorization and inspection/removal requirement establishment which is the basis for model specific repair assessment programs. This paper describes the current status of the repair categorization activities and includes all details about the methodologies developed for determination of the inspection program for the skin on pressurized fuselages. For inspection threshold determination two methods are defined based on fatigue life approach, i.e. a simplified and a detailed method. The detailed method considers 15 different parameters to assess the influences of material, geometry, size, location, aircraft usage and workmanship on the fatigue life of the repair and the original structure. For definition of the inspection intervals a general method is developed which applies to all concerned repairs. For this the initial flaw concept is used by considering 6 parameters and the detectable flaw sizes depending on proposed nondestructive inspection methods. An alternative method is provided for small repairs allowing visual inspection with shorter intervals.

Aging Aircraft↗
NASA NTRS1994

Metaphase yields from staphylococcal enterotoxin A stimulated peripheral blood lymphocytes of unirradiated and irradiated aged rhesus monkeys

The mitogen phytohemagglutinin (PHA) works well in both human and cynomolgus monkey (Macaca fascicularis) lymphocyte cultures to stimulate T cell proliferation. T cells from rhesus monkeys (Macaca mulatta) are less responsive than human cells, producing few metaphases when thousands are required, e.g. in biological dosimetry studies. We show that staphylococcal enterotoxin A (SEA), one of the most potent mitogens known, at a concentration of 0.5 microgram/ml stimulated peripheral lymphocytes to grow with a mitotic index (MI) averaging 0.13 metaphases/cell in old, irradiated rhesus macaques. This was significantly greater (p < 0.001) than that produced by PHA (MI < 0.01) in lymphocytes from the same animals. Whole blood was cultured for 96, 120 and 144 h for five irradiated individuals and for two controls. All cells cultured with SEA produced a high MI with a peak response at 120 h whereas the same cultures showed low MI for each PHA stimulated culture.

Non-NASA Center↗