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At least 91 records · Page 5

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NASA NTRS1986

Cyclic creep analysis from elastic finite-element solutions

A uniaxial approach was developed for calculating cyclic creep and stress relaxation at the critical location of a structure subjected to cyclic thermomechanical loading. This approach was incorporated into a simplified analytical procedure for predicting the stress-strain history at a crack initiation site for life prediction purposes. An elastic finite-element solution for the problem was used as input for the simplified procedure. The creep analysis includes a self-adaptive time incrementing scheme. Cumulative creep is the sum of the initial creep, the recovery from the stress relaxation and the incremental creep. The simplified analysis was exercised for four cases involving a benchmark notched plate problem. Comparisons were made with elastic-plastic-creep solutions for these cases using the MARC nonlinear finite-element computer code.

Kaufman, A.↗

NASA NTRS1976

Accelerated heat-aging studies on fluororubber in various media

Heat-aging studies were conducted on fluororubber (copolymers of vinylidene fluoride and perfluoropropylene) using N,N-dicinnamylidene-1,6-hexanediamine, a Schiff's base of 1,6-hexanediamine, and MgO as acid acceptor. The principal technique employed was chemical stress relaxation for determining network changes brought about in the heat-aged fluororubber. This technique was backed up by swelling measurements, gel permeation chromatography, and IR spectroscopy. Stress relaxation curves are plotted for a wide range of variation in parameters (time, crosslinking density, state of curing, temperature, intermittent and continuous relaxation).

Kalfayan, S. H.↗

NASA NTRS2001

Compatibility Testing of Non-Metallic Materials for the Urine Processor Assembly (UPA) of International Space Station (ISS)

In the International Space Station (ISS), astronauts will convert urine into potable water with the Urine Processor Assembly (UPA). The urine is distilled, with the concentrated form containing about 15% brine solids, and the dilute form as a blend of pre-treated urine/wastewater. Eighteen candidate non-metallic materials for use with the UPA were tested in 2000 for compatibility with the concentrated and dilute urine solutions for continuous times of at least 30 days, and at conditions of 0.5 psia pressure and 100 F, to simulate the working UPA environment. A primary screening test for each material (virgin and conditioned) was dynamic mechanical analysis (DMA) in the stress relaxation mode, with the test data used to predict material performance for a 10-year use in space. Data showed that most of the candidate materials passed the compatibility testing, although a few significant changes in stress relaxation modulus were observed.

Wingard, Charles Doug↗

NASA NTRS1995

Chemical and Thermal Analysis

During the past six months we have conducted significant research in several domains in order to clarify and understanding the aging and chemical failure mechanism of thermoplastics (PVDF or Tefzel) for pipes. We organized numerous analytical studies with methods including Fourier Transform Infrared Spectroscopy, Dynamic Mechanical Analysis, Differential Scanning Calorimetry, and Stress Relaxation experiments. In addition we have reanalyzed previous thermogravimetric data concerning the rate of deplasticization of Coflon pipe. We investigated a number of aged samples of both Tefzel and Coflon that were forwarded from MERL. We conducted stress relaxation experiments of Coflon pipe at several temperatures and determined an activation energy. We also examined the dynamic mechanical response PVDF during deplasticization and during methanol plasticization. We performed numerous DSC analyses to research the changing crystalline morphology. We have noted significant changes in crystallinity upon aging for both PVDF and Tefzel. Little variation in elemental composition was noted for many of the aged Coflon and Tefzel samples tested.

Bulluck, J. W.↗

NASA NTRS2022

Epoxy Resins with Reduced Viscoelastic Relaxation

The payloads of current space launch systems, e.g. rockets, are constrained to a small size due to aerodynamic, weight, and cost limitations. Deployable structures allow for a folded or stowed configuration during launch and deployment at the mission location. Polymer composites have shown both high weight savings over current metallic structure, as well as improved specific mechanical strengths overall. One issue encountered with polymer composite deployables, however, is stress relaxation of the polymer matrix in the stowed configuration. In this study, a range of different epoxy formulations were evaluated as potential matrix resins for deployable composites. A new multifunctional epoxy resin with a fortifying additive is predicted to show 70% less stress relaxation after 1 year as compared to a state-of-the-art aerospace epoxy matrix.

Ray S. Peterson↗

NASA NTRS1976

Time-temperature-stress dependence of boron fiber deformation

The time-dependent deformation of boron fibers over the temperature range from -190 to 800 C is studied by flexural stress relaxation and flexural internal friction techniques on 203-micron diam specimen fibers commercially produced by chemical vapor deposition (CVD) on a 13-micron tungsten substrate. It is shown that up to at least 800 C all nonelastic behavior observed during axial deformation of CVD boron fibers can be explained solely by anelastic mechanisms and that although creep strains are small, boron fiber anelasticity can produce significant mechanical effects which would otherwise be neglected under the elastic approximation. Relations are obtained to demonstrate the considerable effects of anelasticity on such fiber/composite properties as modulus, creep, creep recovery, stress relaxation, and damping capacity. For an elastic-core/anelastic-sheath model, boron fibers on tungsten substrates are shown to have predictable fracture stresses for time-temperature conditions ranging from impact to long-time stress rupture. Possible techniques for altering these stresses are discussed.

Dicarlo, J. A.↗

NASA NTRS2011

Factors Influencing Dwell Fatigue Cracking in Notches of Powder Metallurgy Superalloys

The influences of heat treatment and cyclic dwells on the notch fatigue resistance of powder metallurgy disk superalloys were investigated for low solvus high refractory (LSHR) and ME3 disk alloys. Disks were processed to produce material conditions with varied microstructures and associated mechanical properties. Notched specimens were first subjected to baseline dwell fatigue cycles having a dwell at maximum load, as well as tensile, stress relaxation, creep rupture, and dwell fatigue crack growth tests at 704 C. Several material heat treatments displayed a bimodal distribution of fatigue life with the lives varying by two orders-of-magnitude, while others had more consistent fatigue lives. This response was compared to other mechanical properties, in search of correlations. The wide scatter in baseline dwell fatigue life was observed only for material conditions resistant to stress relaxation. For selected materials and conditions, additional tests were then performed with the dwells shifted in part or in total to minimum tensile load. The tests performed with dwells at minimum load exhibited lower fatigue lives than max dwell tests, and also exhibited early crack initiation and a substantial increase in the number of initiation sites. These results could be explained in part by modeling evolution of peak stresses in the notch with continued dwell fatigue cycling. Fatigue-environment interactions were determined to limit life for the fatigue cycles with dwells.

Gabb, T. P.↗

NASA NTRS1991

Viscoelastic properties of addition-cured polyimides used in high temperature polymer matrix composites

The viscoelastic properties of an addition-cured polyimide, PMR-15, were evaluated through dynamic mechanical and stress relaxation testing. Below the glass transition temperature, the dynamic mechanical properties of the composites are strongly affected by the absorbed moisture in the resin. At temperature 20 C and more above the glass transition temperature, the storage modulus increases continuously with time, indicating that additional crosslinking is occurring in the resin. For resin moisture contents less than 2 percent, stress relaxation curves measured at different temperatures can be superimposed using horizontal shifts along the log(time) axis with only small shifts along the vertical axis.

Roberts, Gary D.↗

NASA NTRS1982

The effect of physical aging on the time-dependent properties of carbon-fiber-reinforced epoxy composites

Annealing in nitrogen below the glass transition temperature T(g) is found to affect the matrix-dominated physical properties of postcured, (+ and -45)2s specimens of Thornel 300 carbon fiber/Fiberite 934 epoxy composite as well as neat Fiberite 934 epoxy resin. The damping behavior, stress-relaxation rate and enthalpic state of the matrix material are found to decrease as functions of annealing time. The time-dependent change in physical properties is explained on the basis of free-volume collapse related to the physical aging of the nonequilibrium glassy network of epoxy. Such physical aging in network epoxy has been demonstrated to be thermoreversible through the monitoring of its kinetics by differential scanning calorimetry, stress relaxation tests, and dynamic mechanical analysis.

Kong, E. S. W.↗

NASA NTRS1991

Effects of elevated temperature on the viscoplastic modeling of graphite/polymeric composites

To support the development of new materials for the design of next generation supersonic transports, a research program is underway at NASA to assess the long term durability of advanced polymer matrix composites (PMC's). One of main objectives of the program was to explore the effects of elevated temperature (23 to 200 C) on the constitutive model's material parameters. To achieve this goal, test data on the observed nonlinear, stress-strain behavior of IM7/5260 and IM7/8320 composites under tension and compression loading were collected and correlated against temperature. These tests, conducted under isothermal conditions using variable strain rates, included such phenomena as stress relaxation and short term creep. The second major goal was the verification of the model by comparison of analytical predictions and test results for off axis and angle ply laminates. Correlation between test and predicted behavior was performed for specimens of both material systems over a range of temperatures. Results indicated that the model provided reasonable predictions of material behavior in load or strain controlled tests. Periods of loading, unloading, stress relaxation, and creep were accounted for.

Gates, Thomas S.↗

NASA NTRS1983

Influence of physical aging on the time-dependent properties of network epoxies and epoxy-matrix composites

The matrix-dominated properties of a network epoxy and its carbon fiber-reinforced composite are noted to be affected by sub-glass transition temperature T(g) annealing in a nitrogen atmosphere. The damping behavior, stress relaxation rate, and enthalpic state of the matrix material were found to decrease as functions of sub-T(g) annealing time. This time-dependent change in physical properties is explained on the basis of a free-volume collapse which is related to the recovery process of the nonequilibrium glassy network of the epoxy. The physical aging in network epoxies has been demonstrated to be thermoreversible. Aging kinetics are studied by means of differential scanning calorimetry, stress relaxation tests, dynamic mechanical analysis, and thermomechanical analysis.

Kong, E. S. W.↗

NASA NTRS1985

A survey of unified constitutive theories

The state of the art of time temperature dependent elastic viscoplastic constitutive theories which are based on the unified approach werre assessed. This class of constitutive theories is characterized by the use of kinetic equations and internal variables with appropriate evolutionary equations for treating all aspects of inelastic deformation including plasticity, creep, and stress relaxation. More than 10 such unified theories which are shown to satisfy the uniqueness and stability criteria imposed by Drucker's postulate and Ponter's inequalities are identified. The theories are compared for the types of flow law, kinetic equation, evolutionary equation of the internal variables, and treatment of temperature dependence. The similarities and differences of these theories are outlined in terms of mathematical formulations and illustrated by comparisons of theoretical calculations with experimental results which include monotonic stress-strain curves, cyclic hysteresis loops, creep and stress relaxation rates, and thermomechanical loops. Numerical methods used for integrating these stiff time temperature dependent constitutive equations are reviewed.

Chan, K. S.↗

NASA NTRS2003

Compatibility Testing of Polymeric Materials for the Urine Processor Assembly (UPA) of International Space Station (ISS)

In the International Space Station (ISS), astronauts will convert urine into potable water with the Urine Processor Assembly (UPA) by a distillation process. The urine is pre-treated, containing flush water and stabilizers. About 2.5% solids in the urine are concentrated up to 16% brine through distillation. Dynamic mechanical analysis (DMA) in the stress relaxation mode was primarily used to test 15 polymeric UPA materials for compatibility with the pre-treated and brine solutions. There were concerns that chromium trioxide (CrO3), a stabilizer not in the original pre-treat formulation for similar compatibility testing in 2000, could have an adverse effect on these polymers. DMA testing is partially complete for polymeric material samples immersed in the two solutions at room temperature for as long as 200 days. By comparing each material (conditioned and virgin), the stress relaxation modulus (E) was determined for short-term use and predicted for as long as a 10-year use in space. Such a delta E showed a decrease of as much as 79% for a Nylon material, but an increase as much as 454% for a polysulfone material, with increasing immersion time.

Wingard, Charles D.↗

DOE OSTI2026

Understanding failure in austenitic steels: Key considerations for molten salt storage in CSP applications

Owing to their excellent properties, austenitic stainless steels are extensively used in boilers, furnaces, molten salt tanks, and other applications that are subjected to extreme mechanical loads and high-temperature conditions. Their high corrosion and creep resistance make them suitable for high-temperature operating environments. Additionally, good fatigue resistance and favorable mechanical and visual properties are essential. However, the premature failure of several components at elevated temperatures has been previously reported. Although the failure analysis of components in service is complex, processes such as cold work and welding have been identified as contributing factors to the performance degradation of these steels. This study aims to analyze the various documented failure modes and mechanisms in austenitic steels, including creep, cracking, stress relaxation cracking, and fatigue, to better understand the multi-objective design requirements for these alloys as structural materials for high-temperature molten salt tanks in Concentrating Solar Power (CSP) plants. Stabilized austenitic grades, such as AISI 347H, demonstrate superior resistance to creep and corrosion-related degradation when compared to non-stabilized grades like AISI 316L at temperatures relevant to concentrated solar power (CSP) applications. In contrast, nickel-based alloys offer enhanced corrosion resistance, albeit at a higher cost. This review underscores that creep, stress relaxation cracking, and thermo-mechanical fatigue are the predominant long-term failure risks in CSP hot tanks.

14 SOLAR ENERGY↗

NASA NTRS1982

Nonequilibrium material effects on the behavior of polymeric composite matrices and their related composites

The effects of physical aging on the material properties of some linear and network macromolecular glasses are discussed. The free volume concept is used to describe this behavior. The effect of physical aging on properties of some uniaxial graphite/fiber epoxy resin composites is investigated using stress relaxation in both tensile and flexural modes. The matrix polymers used were resins both of which are based on a 4,4-methylenedianiline derivative of epichlorohydrin with diamino diphenyl sulfone (DDS) as the curing agent. The matrix resin, as used in the practical application in composites, not fully cured and the glass transition of the network was dependent on the curing schedule. The physical aging of the bulk crosslinked epoxy was found to depend on the annealing temperature, and the T sub g of the resin. The physical aging of the composite, monitored by the stress relaxation method, was found to be dependent on the testing direction.

Wilkes, G. L.↗

NASA NTRS1997

Structure-Property Relationships of Bismaleimides

The purpose of this research was to control and systematically vary the network topology of bismaleimides through cure temperature and chemistry (addition of various coreactants) and subsequently attempt to determine structure-mechanical property relationships. Characterization of the bismaleimide structures by dielectric, rheological, and thermal analyses, and density measurements was subsequently correlated with mechanical properties such as modulus, yield strength, fracture energy, and stress relaxation. The model material used in this investigation was 4,4'-BismaleiMidodIphenyl methane (BMI). BMI was coreacted with either 4,4'-Methylene Dianiline (MDA), o,o'-diallyl bisphenol A (DABA) from Ciba Geigy, or Diamino Diphenyl Sulfone (DDS). Three cure paths were employed: a low- temperature cure of 140 C where chain extension should predominate, a high-temperature cure of 220 C where both chain extension and crosslinking should occur simultaneously, and a low-temperature (140 C) cure followed immediately by a high-temperature (220 C) cure where the chain extension reaction or amine addition precedes BMI homopolymerization or crosslinking. Samples of cured and postcured PMR-15 were also tested to determine the effects of postcuring on the mechanical properties. The low-temperature cure condition of BMI/MDA exhibited the highest modulus values for a given mole fraction of BMI with the modulus decreasing with decreasing concentration of BMI. The higher elastic modulus is the result of steric hindrance by unreacted BMI molecules in the glassy state. The moduli values for the high- and low/high-temperature cure conditions of BMI/MDA decreased as the amount of diamine increased. All the moduli values mimic the yield strength and density trends. For the high-temperature cure condition, the room- temperature modulus remained constant with decreasing mole fraction of BMT for the BMI/DABA and BMI/DDS systems. Postcuring PMR-15 increases the modulus over that of the cured material even though density values of cured and postcured PMR were essentially the same. Preliminary results of a continuous and intermittent stress relaxation experiment for BMI:MDA in a 2:1 molar ratio indicate that crosslinking is occurring when the sample is in the undeformed state. Computer simulation of properties such as density, glass transition temperature, and modulus for the low- temperature cure conditions of BMI/MDA and BMI/DABA were completed. The computer modeling was used to help further understand and confirm the structure characterization results. The simulations correctly predicted the trends of these properties versus mole fraction BMI and were extended to other BMI/diamine systems.

Tenteris-Noebe, Anita D.↗

NASA NTRS1972

Dimensional stability and micromechanical properties of materials for use in an Orbiting Astronomical Observatory.

This paper considers the problem of irreversible dimensional changes of the order of one thousandth of an inch per inch in several candidate materials for orbiting observatories. Although there are a number of potential sources for such dimensional changes, those given major consideration here are applied stresses and residual stresses. Results of room-temperature microyield strength (MYS) and microcreep tests to detect permanent strains associated with both short- and long-duration loading are reported for several engineering materials, including four aluminum alloys (2024, 5456, 6061, and 2014), I400 beryllium, TZM molybdenum, AZ31 magnesium, two titanium alloys (Ti-6Al-4V and Ti-5Al-2.5Sn), 321 stainless steel, a graphite/epoxy composite, and low-expansion glass ceramics. The data indicate that: metallic materials have no true elastic limit; the MYS cannot be predicted reliably from, and is appreciably less than, the conventional yield strength: significant microcreep and stress relaxation can occur at stresses somewhat below the MYS.

Marschall, C. W.↗

DOE OSTI2026

Sculpting 2D Crystals via Membrane Contractions before and during Solidification

When phospholipids crystallize within the otherwise fluid membranes of giant unilamellar vesicles, the resulting molecularly thin “2D” solids exhibit great variety in their morphology evolution. For example, within membranes containing moderate amounts of the crystallizing component, crystals grow with a fixed morphology depending on vesicle size. Conversely for membranes containing large amounts of the crystallizing species, we find small compact crystals on vesicles of all sizes. However, on large vesicles, growing crystals sprout flower petals that lengthen progressively. These behaviors result from two combined mechanisms: first, like other 2D solids, the shear rigidity of phospholipid crystals renders them intolerant to morphologies with nonzero Gaussian curvature. As a result and especially at elevated membrane tension, the cost of bending elasticity is reduced at the expense of line energy by the formation of flowers as opposed to compact crystals. Second, the composition-dependent tension rise during cooling relaxes via water permeation of the membrane with a time constant scaling as R2. The amount of crystal formed for a small decrease in temperature determines this composition-dependent increase in stress from thermal contractions versus solidification. Surface Evolver computations were motivated using the predicted tension evolution to develop a processing space that maps to experimental observations for initial and growing crystal morphology. Important variable groups are identified, including a scaled ratio of bending to line energy, a vesicle-size-independent group for membrane contractions, and a time constant for stress relaxation. Though processing stresses ultimately relax, the crystal morphology persists well beyond the processing window.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗