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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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18 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Sulfur Impurities and the Microstructure of Alumina Scales

The relationship between the microstructure of alumina scales, adhesion, and sulfur content was examined through a series of nickel alloys oxidized in 1100 to 1200 deg. C cyclic or isothermal exposures in air. In cyclic tests of undoped NiCrAl, adhesion was produced when the sulfur content was reduced, without any change in scale microstructure. Although interfacial voids were not observed in cyclic tests of NiCrAl, they were promoted by long-term isothermal exposures, by sulfur doping, and in most exposures of NiAl. Two single crystal superalloys, PWA 1480 and Rene' N5, were also tested, either in the as-received condition or after the sulfur content had been reduced to less than 1 ppmw by hydrogen annealing. The unannealed alloys always exhibited spalling to bare metal, but interfacial voids were not observed consistently. Desulfurized PWA 1480 and Rene' N5 exhibited remarkable adhesion and no voidage for either isothermal or cyclic exposures. The most consistent microstructural feature was that, for the cases where voids did form, the scale undersides exhibited corresponding areas with ridged oxide grain boundaries. Voids were not required for spallation nor were other microstructural features essential for adhesion. These observations are consistent with the model whereby scale spallation is controlled primarily by interfacial sulfur segregation and the consequent degradation of oxide-metal bonding.

Smialek, James L.↗
NASA NTRS2011

Effect of Environmental Exposures on Fatigue Life of P/M Disk Superalloys

As the temperature capability of Ni-base superalloy powder metallurgy disks is steadily increased, environmental resistance and protection of advanced nickel-based turbine disk components are becoming increasingly important. Localized surface hot corrosion attack and damage from oxidation have been shown to impair disk fatigue life and may eventually limit disk operating temperatures. NASA Research Announcement (NRA) contracts have been awarded to GE Aviation and Honeywell Aerospace to separately develop fatigue resistant metallic and ceramic coatings for corrosion resistance and the corrosion/fatigue results of selected coatings will be presented. The microstructural response of a bare ME3 disk superalloy has been evaluated for moderate (704 C) and aggressive (760-816 C) oxidizing exposures up to 2,020 hours. Cross section analysis reveals sub-surface damage (significant for aggressive exposures) that consists of Al2O3 "fingers", interfacial voids, a recrystallized precipitate-free layer and GB carbide dissolution. The effects of a Nichrome corrosion coating on this microstructural response will also be presented.

Draper, Susan↗
NASA NTRS2026

Mechanical Properties of Carbon Fiber Reinforced Composites Exposed to Cryogenic Conditions and Space Radiation via Simulation and Testing

As NASA missions extend beyond low Earth orbit, increasing reliance is placed on carbon fiber reinforced polymer (CFRP) composites for spacecraft structures where mass efficiency, durability, and long-term reliability are critical. In service, these materials are subjected to a combination of ultraviolet radiation, vacuum, ionizing radiation, atomic oxygen, and extreme thermal excursions under sustained mechanical loading. Flight systems such as the Boeing Starliner and SpaceX Dragon employ external composite structures that will experience these environments for extended durations. Although prior spaceflight and ground studies have reported limited changes in bulk mechanical properties, the synergistic effects of these environments on composite microstructure, particularly at the fiber matrix interphase, remain insufficiently characterized and represent a potential qualification and reliability risk. This study investigates the effects of short-term cryogenic exposure on a radiation shielding carbon epoxy composite, SC2020, as a ground-based analog for space relevant thermal extremes. The SC2020 material system has previously flown on the International Space Station under the Materials International Space Station Experiment (MISSE) program. Composite specimens were exposed to liquid nitrogen for 6 and 24 hours and evaluated using a multiscale characterization framework that combined ASTM D3039 tensile testing, Atomic Force Microscopy (AFM) based interphase analysis, and helium gas permeability measurements. Tensile testing showed no statistically significant or permanent degradation in global strength or modulus following cryogenic exposure. In contrast, AFM measurements revealed reductions in interphase modulus, weakened adhesion, and increased nanoscale heterogeneity, indicating localized degradation mechanisms not captured by conventional bulk testing. Gas permeability measurements showed a progressive increase in helium diffusion with exposure duration, consistent with micro-void formation or partial interfacial debonding. The results indicate that cryogenic exposure initiates degradation at the fiber matrix interphase while leaving global mechanical properties largely unchanged over short durations. These findings underscore the importance of multiscale diagnostics for identifying early-stage damage mechanisms that may influence long term performance and qualification margins for spaceflight composite structures. The data presented establish a cryogenic baseline for comparison with forthcoming MISSE flight exposure results and support ongoing NASA Established Program to Stimulate Competitive Research (EPSCoR) efforts aimed at improving composite qualification methodologies, risk assessment, and reliability prediction for space environments.

composite reliability↗
NASA NTRS1997

Comparison of the Effects of Debonds and Voids in Adhesive Joints

An analytical model is developed to compare the effects of voids an debonds on the interfacial shear stresses between the adherends and the adhesive in simple lap joints. Since the adhesive material above the debond may undergo some extension (either due to applied load or thermal expansion or both), a modified shear lag model, where the adhesive can take an extensional as well as shear deformation, is used in the analysis. The adherends take on only axial loads and act as membranes. Two coupled nondimensional differential equations are derived, and in general, five parameters govern the stress distribution in the overlap region. As expected, the major differences between the debond and the void occur for the stresses near the edge of the defect itself. Whether the defect is a debond or a void, is hardly discernible by the stresses at the overlap region. If the defect occurs precisely at or very close to either end of the overlap, however, differences of the order of 20 percent in the peak stresses can be obtained.

Rossettos, J. N.↗
NASA NTRS2018

Simulating the Effects of Carbon Nanotube Continuity and Interfacial Bonding on Composite Strength and Stiffness

Molecular dynamics simulations of carbon nanotube (CNT) composites, in which the CNTs are continuous across the periodic boundary, overestimate the experimentally measured mechanical properties of CNT composites along the fiber direction. Since the CNTs in these composites are much shorter than the composite dimensions, load must be transferred either directly between CNTs or through the matrix, a mechanism that is absent in simulations of effectively continuous CNTs. In this study, the elastic and fracture properties of high volume fraction discontinuous carbon nanotube/amorphous carbon composite systems were compared to those of otherwise equivalent continuous CNT composites using ReaxFF reactive molecular dynamics simulations. These simulations were used to show how the number of nanotube-matrix interfacial covalent bonds affect composite mechanical properties. Furthermore, the mechanical impact of interfacial bonding was decomposed to reveal its effect on the properties of the CNTs, the interfacial layer of matrix, and the bulk matrix. For the composites with continuous reinforcement, it was found that any degree of interfacial bonding has a negative impact on axial tensile strength and stiffness. This is due to disruption of the structure of the CNTs and interfacial matrix layer by the interfacial bonds. For the discontinuous composites, the modulus was maximized between 4%-7% interfacial bonding and the strength continues to increase up to the highest levels of interfacial bonding studied. Areas of low stress and voids were observed in the simulated discontinuous composites at the ends of the tubes, from which fracture was observed to initiate. Experimental carbon nanotube yarn composites were fabricated and tested. The results were used to illustrate knockdown factors relative to the mechanical performance of the tubes themselves.

Benjamin D Jensen↗
NASA NTRS1990

The effect of weak interface on transverse properties of a ceramic matrix composite

Experimental studies conducted at NASA Lewis on SiC reaction-bonded Si3N4 composite system showed that transverse stiffness and strength were much lower than those predicted from existing analytical models based on good interfacial bonding. It was believed that weakened interfaces were responsible for the decrease in tranverse properties. To support this claim, a two-dimensional FEM analysis was performed for a transverse representative volume element. Specifically, the effect of fiber/matrix displacement compatibility at the interface was studied under both tensile and compressive transverse loadings. Interface debonding was represented using active gap elements connecting the fiber and matrix. The analyses show that the transverse tensile strength and stiffness are best predicted when a debonded interface is assumed for the composite. In fact, the measured properties can be predicted by simply replacing the fibers by voids. Thus, it is found that little or no interfacial bonding exists in the composite, and that an elastic analysis can predict the transverse stiffness and strength.

Shimansky, R. A.↗
NASA NTRS1983

An investigation of adhesive/adherend and fiber/matrix interactions. Part B: SEM/ESCA analysis of fracture surfaces

Adhesion was studied with emphasis on the characterization of surface oxide layers, the analysis of fracture surfaces, and the interaction of matrices and fibers. A number of surface features of the fractured lap shear samples were noted in the SEM photomicrographs including the beta phase alloy of the Ti 6-4 adherend, the imprint of the adherend on the adhesive failure surface, increased void density for high temperature samples, and the alumina filler particles. Interfacial failure of some of the fractured lap shear samples is invariably characterized by the appearance of an ESCA oxygen photopeak at 530.3 eV assigned to the surface oxide layer of Ti 6-4 adherend. The effect of grit blasting on carbon fiber composites is evident in the SEM analysis. A high surface fluorine concentration on the composite surface is reduced some ten fold by grit blasting.

Beck, B.↗
NASA NTRS2001

Ultrasonic Resonance Spectroscopy of Composite Rings for Flywheel Rotors

Flywheel energy storage devices comprising multilayered composite rotor systems are being studied extensively for utilization in the International Space Station. These composite material systems were investigated with a recently developed ultrasonic resonance spectroscopy technique. The system employs a swept frequency approach and performs a fast Fourier transform on the frequency spectrum of the response signal. In addition. the system allows for equalization of the frequency spectrum, providing all frequencies with equal amounts of energy to excite higher order resonant harmonics. Interpretation of the second fast Fourier transform, along with equalization of the frequency spectrum, offers greater assurance in acquiring and analyzing the fundamental frequency, or spectrum resonance spacing. The range of frequencies swept in a pitch-catch mode was varied up to 8 MHz, depending on the material and geometry of the component. Single and multilayered material samples, with and without known defects, were evaluated to determine how the constituents of a composite material system affect the resonant frequency. Amplitude and frequency changes in the spectrum and spectrum resonance spacing domains were examined from ultrasonic responses of a flat composite coupon, thin composite rings, and thick composite rings. Also, the ultrasonic spectroscopy responses from areas with an intentional delamination and a foreign material insert, similar to defects that may occur during manufacturing malfunctions, were compared with those from defect-free areas in thin composite rings. A thick composite ring with varying thickness was tested to investigate the full-thickness resonant frequency and any possible bulk interfacial bond issues. Finally, the effect on the frequency response of naturally occurring single and clustered voids in a composite ring was established.

Harmon, Laura M.↗
NASA NTRS2021

Experimental and Computational Investigation of Flow Boiling in Microgravity

This study explores use of Computational Fluid Dynamics (CFD) to predict near-saturated flow boiling of FC-72 in microgravity. The computational method employs transient analysis to predict detailed interfacial behavior and heat transfer characteristics along a rectangular channel heated along two opposite walls. Predicted results are validated against experimental temperature measurements and high-speed video images captured during a series of parabolic aircraft maneuvers for three sets of operating conditions which include variations of both mass velocity and wall heat flux. The computational method is based on the multi-phase volume of fluid (VOF) model, which is combined with appropriate phase change and turbulence models, and accounts for both shear-lift force on bubbles and conjugate heat transfer along the heating walls. A key advantage of the CFD method is ability to capture details that are very difficult to measure experimentally, including detailed spatial variations of bubble shape, void fraction, mixture fluid temperature, liquid velocity, and vapor velocity, results for which are presented for each of the three test cases. Different flow regimes predicted along the heated length exhibit a number of dominant mechanisms including bubble nucleation, bubble growth, coalescence, vapor blankets, interfacial waviness, and residual liquid sub-layer, all of which agree well with experiment. Vapor velocity is shown to increase appreciably along the heated length because of increased void fraction, while liquid velocity experiences large fluctuations. Non-equilibrium effects are accentuated with increasing mass velocity, contributing minor deviations of fluid temperature from simulations compared to those predicted by the analytical method. Predicted wall temperature is fairly uniform in the middle of the heated length but increases in the entrance region, due to sensible heat transfer in the subcooled liquid, and decreases toward the exit, mostly because of flow acceleration resulting from increased void fraction.

two-phase cooling; flow boiling; CFD; microgravity↗
NASA NTRS2003

Investigation of Critical Heat Flux in Reduced Gravity Using Photomicrographic Techniques

Experiments were performed to examine the effects of body force on flow boiling critical heat flux (CHF). FC-72 was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface just prior to CHF. High-speed video imaging techniques were used to identify dominant CHF mechanisms corresponding to different flow orientations and liquid velocities. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed significant sensitivity to orientation for flow velocities below 0.2 m/s, where extremely low CHF values where measured, especially with downward-facing heated wall and downflow orientations. High flow velocities dampened the effects of orientation considerably. The CHF data were used to assess the suitability of previous CHF models and correlations. It is shown the Interfacial Lift-off Model is very effective at predicting CHF for high velocities at all orientations. The flooding limit, on the other hand, is useful at estimating CHF at low velocities and for downflow orientations. A new method consisting of three dimensionless criteria is developed for determining the minimum flow velocity required to overcome body force effects on near-saturated flow boiling CHF. Vertical upflow boiling experiments were performed in pursuit of identifying the trigger mechanism for subcooled flow boiling CHF. While virtually all prior studies on flow boiling CHF concern the prediction or measurement of conditions that lead to CHF, this study was focused on events that take place during the CHF transient. High-speed video imaging and photomicrographic techniques were used to record the transient behavior of interfacial features from the last steady-state power level before CHF until the moment of power cut-off following CHF. The video records show the development of a wavy vapor layer which propagates along the heated wall, permitting cooling prior to CHF only in wetting fronts corresponding to the wave troughs. Image analysis software was developed to estimate void fraction from the individual video images. The void fraction records for subcooled flow boiling show the CHF transient is accompanied by gradual lift-off of wetting fronts culminating in some maximum vapor layer mean thickness, following which the vapor layer begins to thin down as the transition to film boiling ensues. This study proves the Interfacial Lift-off Model, which has been validated for near-saturated flow boiling CHF, is equally valid for subcooled conditions.

Mudawar, Issam↗
NASA NTRS1979

Effects of isothermal and cyclic exposures on interface structure and mechanical properties of FPalpha-Al2O3/aluminum composites

The microstructural and interface stability of FPalpha-Al203/Al-Li composites are investigated as a function of isothermal exposure at 500 C or thermal cycling between 140 and 500 C with hold time at Tmax. Interfacial morphology, growth kinetics, crystal structure, and composition of interfacial reaction products are characterized. Strength is monitored in the transverse orientation, and fracture mechanics is analyzed in terms of interface reaction products. The interfacial reaction product in FP/Al is Li2O.5Al2O3. Significant fiber-matrix reaction occurs during fabrication. The number of thermal cycles rather than total time at Tmax is the determining factor in strength degradation, thermal cycling giving rise to voids at the fiber-matrix interface. Extensive interface failures occur at composite fracture stresses below about 128 MPa; above this stress level failure is attributed to ductile matrix fracture.

Kim, W. M.↗
NASA NTRS1992

Flexible multiply towpreg

This invention relates to an improved flexible towpreg and a method of production therefor. The improved flexible towpreg comprises a plurality of towpreg plies which comprise reinforcing filaments and matrix forming material; the reinforcing filaments being substantially wetout by the matrix forming material such that the towpreg plies are substantially void-free composite articles, and the towpreg plies having an average thickness less than about 100 microns. The method of production for the improved flexible towpreg comprises the steps of spreading the reinforcing filaments to expose individually substantially all of the reinforcing filaments; coating the reinforcing filaments with the matrix forming material in a manner causing interfacial adhesion of the matrix forming material to the reinforcing filaments; forming the towpreg plies by heating the matrix forming material contacting the reinforcing filaments until the matrix forming material liquifies and coats the reinforcing filaments; and cooling the towpreg plies in a manner such that substantial cohesion between neighboring towpreg plies is prevented until the matrix forming material solidifies.

Muzzy, John D.↗
NASA NTRS1992

Flexible multiply towpreg and method of production therefor

This invention relates to an improved flexible towpreg and a method of production therefor. The improved flexible towpreg comprises a plurality of towpreg plies which comprise reinforcing filaments and matrix forming material; the reinforcing filaments being substantially wetout by the matrix forming material such that the towpreg plies are substantially void-free composite articles, and the towpreg plies having an average thickness less than about 100 microns. The method of production for the improved flexible towpreg comprises the steps of spreading the reinforcing filaments to expose individually substantially all of the reinforcing filaments; coating the reinforcing filaments with the matrix forming material in a manner causing interfacial adhesion of the matrix forming material to the reinforcing filaments; forming the towpreg plies by heating the matrix forming material contacting the reinforcing filaments until the matrix forming material liquefies and coats the reinforcing filaments; and cooling the towpreg plies in a manner such that substantial cohesion between neighboring towpreg plies is prevented until the matrix forming material solidifies.

Muzzy, John D.↗
NASA NTRS1986

Interfaces and failure mechanisms in Al-SiC composites

Aluminum alloys reinforced with silicon carbide whiskers exhibit significantly higher strength and modulus than the unreinforced alloys. However, the composites also exhibit low ductility and poor fracture toughness for reasons which are not well-understood. In this study, high resolution and conventional TEM techniques are used to analyze interface microstructure and failure mechanisms in P/M 2124 and 6061 aluminum alloys reinforced with silicon carbide whiskers. Interfacial oxides are typical of both composite materials, often distributed in clusters or in a discrete layer 2-5 nm thick along the whisker-matrix interface. Highly deformed regions beneath bulk tensile fracture surfaces reveal possible fracture nucleation centers as well as sites of stress concentration where intense plastic strain has occurred. Observations of such highly deformed regions include (1) void initiation at whisker ends, (2) interface decohesion involving the thin oxide layer, (3) transverse cracks in whiskers, and (4) cracks in large constituent particles. TEM results are presented and discussed in relation to mechanisms of composite failure.

Nutt, S. R.↗
NASA NTRS1998

Hybrid IR-Gas Heater for Automated Tow Placement

A hybrid infrared-hot gas heat source has been developed and tested for the NASA Langley Tow Placement Facility. The IR heat source provides supplemental heat to the nip region. The additional heat is intended to reduce the need for conduction heating by the compaction roller, which causes (he roller to stick to the panel surface. Initial bench scale testing was performed to identify the most effective means of focusing IR energy into the nip region. A compact lamp and reflector that placed the lamp as close to the nip point as possible was found to deliver the highest heat flux in the nip region. A prototype heater was installed on the NASA Langley tow placement robot. Panels placed with a 400 C (sticking) compaction roller gave DCB initiation toughness numbers comparable to those reported for autoclave processed panels but were found to have unexpectedly high void contents. Placement with compaction roller temperatures that prevented roller sticking resulted in mode I fracture toughness approximately 70% that reported for autoclave processed panels. The variability in strength among specimens placed with reduced roller temperature was found to be greatly reduced, which implies that use of supplemental nip point heat may improve the robustness of the tow placement process. Use of the IR heat source permitted placement with a compaction roller temperature that would have resulted in negligible interfacial strength with the hot gas torch alone. The roller temperature reductions eliminated the need for the robot operator to attend placement operations.

Grenoble, Ray W.↗
NASA NTRSDate not supplied

Flow Boiling & Condensation Experiment (FBCE): Flow Boiling in Earth Gravity and Onboard the International Space Station

Two phase thermal management systems that capitalize on both latent and sensible heats of the working fluid can yield orders of magnitude enhancements in flow boiling and condensation heat transfer coefficients and reduce size and weight of future space systems. Because the understanding of microgravity influences on two-phase flow and heat transfer is quite limited, there is presently an urgent need for a new experimental microgravity facility to enable investigators to perform long-duration flow boiling and condensation experiments in pursuit of reliable databases. This presentation will discuss results from the Flow Boiling and Condensation Experiment (FBCE), a collaborative effort between Purdue University and NASA Glenn Research Center. Experiments have been performed using the final system with the Flow Boiling Module (FBM) in vertical orientation in Earth gravity (Mission Sequence Tests, MST) and in microgravity onboard the International Space Station (ISS). High-speed-video flow visualization, heat transfer, and critical heat flux (CHF) results from the MST are presented for both subcooled liquid and saturated liquid-vapor inlet conditions. CHF predictions made using the Interfacial Lift-off Model are compared with a consolidated database made by compiling FBM datasets obtained in prior years for different orientations in Earth gravity and on parabolic flights. New explicit correlations for CHF and subcooled flow boiling heat transfer coefficient are developed and shown to be excellent in their predictive accuracies against consolidated experimental databases. Computations are performed for flows in microgravity and horizontal flows in Earth gravity, the results of which show a good predictive accuracy for both void fraction and wall temperature. Finally, similar preliminary results from the recent ISS experiments are presented.

Issam Mudawar↗
NASA NTRS1983

Chemical bonding technology for terrestrial photovoltaic modules

Encapsulated photovoltaic modules must hold together for 20 years, reliably resisting delamination and separation of any of the component materials. Delamination of encapsulation materials from each other, or from solar cells and interconnects, can create voids for accumulation of water, promoting corrosive failure. Delamination of silicone elastomers from unprimed surfaces was a common occurrence with early modules, but the incidences of silicone delamination with later modules decreased when adhesion promoters recommended by silicone manufacturers were used. An investigation of silicone delamination from unprimed surfaces successfully identified the mechanism, which was related to atmospheric oxygen and moisture. This early finding indicated that reliance on physical bonding of encapsulation interfaces for long life in an outdoor environment would be risky. For long outdoor life, the material components of a module must therefore be held together by weather-stable adhesion promoters that desirably form strong, interfacial chemical bonds.

Coulter, D. R.↗
NASA NTRS2000

A Study of Bubble and Slug Gas-Liquid Flow in a Microgravity Environment

The influence of gravity on the two-phase flow dynamics is obvious.As the gravity level is reduced,there is a new balance between inertial and interfacial forces, altering the behavior of the flow. In bubbly flow,the absence of drift velocity leads to spherical-shaped bubbles with a rectilinear trajectory.Slug flow is a succession of long bubbles and liquid slug carrying a few bubbles. There is no flow reversal in the thin liquid film as the long bubble and liquid slug pass over the film. Although the flow structure seems to be simpler than in normal gravity conditions,the models developed for the prediction of flow behavior in normal gravity and extended to reduced gravity flow are unable to predict the flow behavior correctly.An additional benefit of conducting studies in microgravity flows is that these studies aide the development of understanding for normal gravity flow behavior by removing the effects of buoyancy on the shape of the interface and density driven shear flows between the gas and the liquid phases. The proposal calls to study specifically the following: 1) The dynamics of isolated bubbles in microgravity liquid flows will be analyzed: Both the dynamics of spherical isolated bubbles and their dispersion by turbulence, their interaction with the pipe wall,the behavior of the bubbles in accelerated or decelerated flows,and the dynamics of isolated cylindrical bubbles, their deformation in accelerated/decelerated flows (in converging or diverging channels), and bubble/bubble interaction. Experiments will consist of the use of Particle Image Velocimetry (PIV) and Laser Doppler Velocimeters (LDV) to study single spherical bubble and single and two cylindrical bubble behavior with respect to their influence on the turbulence of the surrounding liquid and on the wall 2) The dynamics of bubbly and slug flow in microgravity will be analyzed especially for the role of the coalescence in the transition from bubbly to slug flow (effect of fluid properties and surfactant), to identify clusters that promote coalescence and transition the void fraction distribution in bubbly and slug flow,to measure the wall friction in bubbly flow. These experiments will consist of multiple bubbles type flows and will utilize hot wire and film anemometers to measure liquid velocity and wall shear stress respectively and double fiber optic probes to measure bubble size and velocity as a function of tube radius and axial location.

McQuillen, J.↗