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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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At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Parylene C Aging Studies

Parylene C is used in a device because of its conformable deposition and other advantages. Techniques to study Parylene C aging were developed, and "lessons learned" that could be utilized for future studies are the result of this initial study. Differential Scanning Calorimetry yielded temperature ranges for Parylene C aging as well as post-deposition treatment. Post-deposition techniques are suggested to improve Parylene C performance. Sample preparation was critical to aging regimen. Short-term (%7E40 days) aging experiments with free standing and ceramic-supported Parylene C films highlighted "lessons learned" which stressed further investigations in order to refine sample preparation (film thickness, single sided uniform coating, machine versus laser cutting, annealing time, temperature) and testing issues ("necking") for robust accelerated aging of Parylene C.

Achyuthan, Komandoor↗
NASA NTRS1981

The viscoelastic response of a graphite/epoxy laminate

An accelerated characterization procedure has been developed for time-dependent compliances and strengths of resin-matrix composites. The procedure, which is based on a time-temperature-stress superposition principle, is applied to a graphite/epoxy laminate T300/934. Creep rupture analytical models are discussed, and model predictions are compared with experimental data. Master curves for the creep compliances are given.

Brinson, H. F.↗
NASA NTRS1991

Grease Inhibits Stress-Corrosion Cracking In Bearing Race

Coating with suitable grease found to inhibit stress-corrosion cracking in bore of inner race of ball-bearing assembly operating in liquid oxygen. Protects bore and its corner radii from corrosion-initiating and -accelerating substances like moisture and contaminants, which enter during assembly. Operating life extended at low cost, and involves very little extra assembly time.

Beatty, Robert F.↗
DOE OSTI2016

LONG TERM OPERATION ISSUES FOR ELECTRICAL CABLE SYSTEMS IN NUCLEAR POWER PLANTS

Nuclear power plants contain hundreds of kilometers of electrical cables including cables used for power, for instrumentation, and for control. It is essential that safety-related cable systems continue to perform following a design-basis event. Wholesale replacement of electrical cables in existing plants facing licensing period renewal is both impractical and cost-prohibitive. It is therefore important to understand the long term aging of cable materials to have confidence that aged cables will perform when needed. It is equally important in support of cable aging management to develop methods to evaluate the health of installed cables and inform selective cable replacement decisions. The most common insulation materials for electrical cables in nuclear power plants are cross-linked polyethylene and ethylene-propylene rubber. The mechanical properties of these materials degrade over time in the presence of environmental stresses including heat, gamma irradiation, and moisture. Mechanical degradation of cable insulation beyond a certain threshold is unacceptable because it can lead to insulation cracking, exposure of energized conductors, arcing and burning or loss of the ability of the cable system to function during a design-basis accident. While thermal-, radiation-, and moisture-related degradation of polymer insulation materials has been extensively studied over the last few decades, questions remain regarding the long-term performance of cable materials in nuclear plant-specific environments. Identified knowledge gaps include an understanding of the temperature-dependence of activation energies for thermal damage and an understanding of the synergistic effects of radiation and thermal stress on polymer degradation. Many of the outstanding questions in the aging behavior of cable materials relate to the necessity of predicting long-term field degradation using accelerated aging results from the laboratory. Materials degrade faster under more extreme conditions, but extension of behavior to long term degradation under more mild conditions, such as those experienced by most installed cables in nuclear power plants, is complicated by the fact that different degradation mechanisms may be involved in extreme and mild scenarios. The discrepancy in predicted results from short term, more extreme exposure and actual results from longer term, more mild exposures can be counter intuitive. For instance, due to the attenuation of oxidation penetration in material samples rapidly aged through exposure to high temperatures, the bulk of the samples may be artificially protected from thermal aging. In another example, simultaneous exposure of cable insulation material to heat and radiation may actually lead to less damage at higher temperatures than may be observed at lower temperatures. The Light Water Reactor Sustainability program of the United States (US) Department of Energy Office (DOE) of Nuclear Energy is funding research to increase the predictive understanding of electrical cable material aging and degradation in existing nuclear power plants in support of continued safe operation of plants beyond their initial license periods. This research includes the evaluation and development of methods to assess installed cable condition.

Fifield, Dr Leonard S↗
DOE OSTI2017

High-Field Nb3Sn Cos-theta Dipole with Stress Management

Cost-effective superconducting dipole magnets with operating fields up to 16 T are being considered for the LHC en-ergy upgrade (HE-LHC) and a Future Circular Collider (FCC). To demonstrate feasibility of 15 T accelerator quality dipole mag-nets, FNAL as a part of the US-MDP is developing a single-aper-ture Nb3Sn dipole demonstrator based on a 4-layer graded cos-theta coil with 60 mm aperture and cold iron yoke. In parallel, to explore the limit of the Nb3Sn accelerator magnet technology, op-timize magnet design and performance parameters, and reduce magnet cost, magnet design studies are also being performed to push the nominal bore field to 16 T in a 60-mm aperture cos-theta dipole. Results of these studies are reported and discussed in this paper.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Use of an autonomous sensor to evaluate the biological performance of the advanced turbine at Wanapum Dam

Hydropower is the largest renewable energy resource in the United States and the world. However, hydropower dams have adverse ecological impacts because migrating fish may be injured or killed when they pass through hydroturbines. In the Columbia and Snake River basins, dam operators and engineers are required to make those hydroelectric facilities more fish-friendly through changes in hydroturbine design and operation after fish population declines and the subsequent listing of several species of Pacific salmon under the Endangered Species Act of 1973 . Public Utility District No. 2 of Grant County, Washington, requested authorization from the Federal Energy Regulatory Commission to replace the ten turbines at Wanapum Dam with advanced hydropower turbines designed to improve survival for fish passing through the turbines while improving operation efficiency and increasing power generation. As an additional measure to the primary metric of direct injury and mortality rates of juvenile Chinook salmon using balloon tag-recapture methodology, this study used an autonomous sensor device - the Sensor Fish - to provide insight into the specific hydraulic conditions and physical stresses experienced by the fish as well as the specific causes of fish biological response. We found that the new hydroturbine blade shape and the corresponding reduction of turbulence in the advanced hydropower turbine were effective in meeting the objectives of improving fish survival while enhancing operational efficiency of the dam. The frequency of severe events based on Sensor Fish pressure and acceleration measurements showed trends similar to those of fish survival determined by the balloon tag-recapture methodology. In addition, the new turbine provided a better pressure and rate of pressure change environment for fish passage. Altogether, the Sensor Fish data indicated that the advanced hydroturbine design improved passage of juvenile salmon at Wanapum Dam.

13 HYDRO ENERGY↗
DOE OSTI2011

Test Results of the First 3.7 m Long Nb3Sn Quadrupole by LARP and Future Plans

In December 2009 during its first cold test, LQS01, the first Long Nb{sub 3}Sn Quadrupole made by LARP (LHC Accelerator Research Program, a collaboration of BNL, FNAL, LBNL and SLAC), reached its target field gradient of 200 T/m. This target was set in 2005 by the US Department of Energy, CERN and LARP, as a significant milestone toward the development of Nb{sub 3}Sn quadrupoles for possible use in LHC luminosity upgrades. LQS01 is a 90 mm aperture, 3.7 m long quadrupole using Nb{sub 3}Sn coils. The coil layout is equal to the layout used in the LARP Technological Quadrupoles (TQC and TQS models). Pre-stress and support are provided by a segmented aluminum shell pre-loaded using bladders and keys, similarly to the TQS models. After the first test the magnet was disassembled, reassembled with an optimized pre-stress, and reached 222 T/m at 4.5 K. In this paper we present the results of both tests and the next steps of the Long Quadrupole R and D.

43 PARTICLE ACCELERATORS↗
DOE OSTI2011

Measurement of Thermal Dependencies of PBG Fiber Properties

Photonic crystal fibers (PCFs) represent a class of optical fibers which have a wide spectrum of applications in the telecom and sensing industries. Currently, the Advanced Accelerator Research Department at SLAC is developing photonic bandgap particle accelerators, which are photonic crystal structures with a central defect used to accelerate electrons and achieve high longitudinal electric fields. Extremely compact and less costly than the traditional accelerators, these structures can support higher accelerating gradients and will open a new era in high energy physics as well as other fields of science. Based on direct laser acceleration in dielectric materials, the so called photonic band gap accelerators will benefit from mature laser and semiconductor industries. One of the key elements to direct laser acceleration in hollow core PCFs, is maintaining thermal and structural stability. Previous simulations demonstrate that accelerating modes are sensitive to the geometry of the defect region and the variations in the effective index. Unlike the telecom modes (for which over 95% of the energy propagates in the hollow core) most of the power of these modes is located in the glass at the periphery of the central hole which has a higher thermal constant than air ({gamma}{sub SiO{sub 2}} = 1.19 x 10{sup -6} 1/K, {gamma}{sub air} = -9 x 10{sup -7} 1/K with {gamma} = dn/dT). To fully control laser driven acceleration, we need to evaluate the thermal and structural consequences of such modes on the PCFs. We are conducting series of interferometric tests to quantify the dependencies of the HC-633-02 (NKT Photonics) propagation constant (k{sub z}) on temperature, vibration amplitude, stress and electric field strength. In this paper we will present the theoretical principles characterizing the thermal behavior of a PCF, the measurements realized for the fundamental telecom mode (TE{sub 00}), and the experimental demonstration of TM-like mode propagation in the HC-633-02 fiber.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Laser-assisted sol-gel growth and characteristics of ZnO thin films

ZnO thin films were grown on Si(100) substrates by a sol-gel method assisted by laser beam irradiation with a 325 nm He-Cd laser. In contrast to conventional sol-gel ZnO thin films, the surface morphology of the laser-assisted sol-gel thin films was much smoother, and the residual stress in the films was relaxed by laser irradiation. The luminescent properties of the films were also enhanced by laser irradiation, especially, by irradiation during the deposition and post-heat treatment stages. The incident laser beam is thought to play several roles, such as annihilating defects by accelerating crystallization during heat treatment, enhancing the surface migration of atoms and molecules, and relaxing the ZnO matrix structure during crystallization.

36 MATERIALS SCIENCE↗
NASA NTRS1998

Accelerated Strength Testing of Thermoplastic Composites

Constant ramp strength tests on unidirectional thermoplastic composite specimens oriented in the 90 deg. direction were conducted at constant temperatures ranging from 149 C to 232 C. Ramp rates spanning 5 orders of magnitude were tested so that failures occurred in the range from 0.5 sec. to 24 hrs. (0.5 to 100,000 MPa/sec). Below 204 C, time-temperature superposition held allowing strength at longer times to be estimated from strength tests at shorter times but higher temperatures. The data indicated that a 50% drop in strength might be expected for this material when the test time is increased by 9 orders of magnitude. The shift factors derived from compliance data applied well to the strength results. To explain the link between compliance and strength, a viscoelastic fracture model was investigated. The model, which used compliance as input, was found to fit the strength data only if the critical fracture energy was allowed to vary with temperature reduced stress rate. This variation in the critical parameter severely limits its use in developing a robust time-dependent strength model. The significance of this research is therefore seen as providing both the indication that a more versatile acceleration method for strength can be developed and the evidence that such a method is needed.

Reeder, J. R.↗
DOE OSTI2011

Improved Creep-­Fatigue Models on Advanced Materials for SFR Applications (Final Report)

This report provides an update on the materials performance criteria and methodology relevant to sodium-cooled fast reactors (SFRs). The report is the second deliverable (Level 2) in FY11 (M2A11AN040303) under the work package A-11AN040303 “Materials Performance Criteria and Methodology” as part of the Advanced Structural Materials Program for the Advanced Reactor Concepts. The overall objective of the Materials Performance Criteria and Methodology work project is to evaluate the key requirements for the ASME Code qualification and the Nuclear Regulatory Commission (NRC) approval of advanced structural materials in support of the design and performance of sodium-cooled fast reactors. Advanced materials are a critical element in the development of fast reactor technologies. Enhanced materials performance not only improves safety margins and provides design flexibility, but also is essential for the economics of future advanced fast reactors. Qualification and licensing of advanced materials are prominent needs for the development and implementation of advanced fast reactor technologies. Nuclear structural component designs in the U.S. comply with the ASME Boiler and Pressure Vessel (B&PV) Code Section III (Rules for Construction of Nuclear Facility Components), and the NRC grants licensing. As the SFRs will operate at higher temperatures than the current light water reactors (LWRs), the design of elevated-temperature components must comply with ASME Section III Subsection NH (Class 1 Components in Elevated Temperature Service). A number of technical issues relevant to materials performance criteria and high temperature design methodology in the SFR were identified and presented in earlier reports. A viable approach to resolve these issues and the R&D priority were also recommended. The development of mechanistically based creep-fatigue interaction models for life prediction and reliable data extrapolation was chosen to be the central focus in near-term efforts. Our current focus is on the creep-fatigue damage issue in high-strength ferritic/martensitic steels such as mod.9Cr-1Mo (G91) and NF616 (G92) steels. The current ASME creep-fatigue design rule puts severe limits of fatigue and creep loads for G91 steel, the lead structural material for fast reactors. High-strength ferritic/martensitic steels behave fundamentally differently from austenitic stainless steels, for which the current ASME creep-fatigue design rules were developed. The unique deformation and damage characteristics in G91 steel, e.g. cyclic softening, degradation of creep and rupture strength during cyclic service, demands a new creep-fatigue design procedure that explicitly accounts for the material’s unique creep-fatigue behavior. G92 steel, a variant of G91 steel, is the lead candidate in the Advanced Alloy Development program. There is currently no creep-fatigue design rule available for this advanced alloy. To support the predictive model development and resolve the over-conservative issue with the ASME design rule for G91 steel, we recovered stress relaxation data from thirteen creep-fatigue tests conducted in the late 1980s and early 1990s at Oak Ridge National Laboratory, and conducted extensive data analysis in FY10. Based on this database and available literature data, we have developed a Cyclic Softening Model and a Stress Relaxation Model specifically applied to G91 steel. The Cyclic Softening Model describes the cyclic stress variation as a function of cycle number during creep-fatigue loading, and well captures the cyclic softening effect in G91 steel. The Stress Relaxation Model predicts the stress relaxation curve during the hold time of cyclic loading, and its dependence on the cyclic softening effect of G91 steel. Two creep-fatigue test systems have been set up to conduct model-oriented creep-fatigue experiments. Results from three creep-fatigue experiments on Heat 30176 of G91 steel are presented in the report: (1) a test performed at 550°C with total strain amplitude of 0.25% and a hold time of 60 s applied at peak strains in both tension and compression, (2) a test performed at 400°C with total strain amplitude of 0.5% and a tension and compression hold time of 60 s, and (3) a test performed at 20°C with total strain amplitude of 0.5% and a tension and compression hold time of 60 s. Three different test temperatures were chosen for two primary reasons: to determine the transition temperature of creep-fatigue interaction from a plasticity-dominated process to a creep-dominated process; to develop unified creep-fatigue models applicable for a wide range of temperatures. With the detailed information obtained in the creep-fatigue experiments, we have further refined the Cyclic Softening Model and Stress Relaxation Model. The Cyclic Softening Model and Stress Relaxation Model were incorporated into the Bilinear Creep-Fatigue Damage Model, forming a powerful framework for the life prediction by an improved creep-fatigue evaluation methodology. The cyclic softening and consecutive stress relaxation responses in G91 steel can be considered for each cycle of the entire cyclic process in the model calculation. The creep-fatigue damage in G91 steel was evaluated using the improved Bilinear Creep-Fatigue Damage Model without any safety margins. It was found that due to the significant cyclic softening in G91 steel, the unit creep damage in each cycle decreased continuously with increasing number of cycles. This continuous change in creep damage with cycling has significant implication in the evaluation of the total creep-fatigue damage. The total creep damage can be one order of magnitude lower when the unit creep damage at half-life was used in the evaluation in comparison with the calculations using the unit creep damage at cycle 10. An important conclusion was that the accurate assessment of the creep-fatigue damage in G91 steel must consider the history of cycling. This analysis provides a more realistic assessment of creep-fatigue damage and identifies critical factors that must be considered in the development of future advanced models for high-strength ferritic/martensitic alloys. An accelerated creep-fatigue testing methodology is proposed for the design of model-oriented experiments to guide and validate model developments. Future work will focus on creep-fatigue experiments of both G91 and G92 steels, and development of creep-fatigue models applicable to advanced ferritic/martensitic alloys. The Cyclic Softening Model and Stress Relaxation Model will be improved by incorporating microstructural parameters of dislocation density, subgrain structure, and second-phase particles. Collaborative modeling of combined Cyclic Softening Model, Stress Relaxation Model, and Interactive Damage Rate Model will be pursued for life prediction under creep-fatigue loading with extended hold time. Accelerated, well-controlled creep-fatigue tests will be conducted to provide high-quality experimental data to refine key material parameters and validate models.

Li, Meimei↗
DOE OSTI2018

Prediction of residual stresses in a multipass pipe weld by a novel 3D finite element approach

Due to enormous computation cost, current residual stress simulation of multipass girth welds are mostly performed using two-dimensional (2D) axisymmetric models. The 2D model can only provide limited estimation on the residual stresses by assuming its axisymmetric distribution. In this study, a highly efficient thermal-mechanical finite element code for three-dimensional (3D) model has been developed based on high performance Graphics Processing Unit (GPU) computers. Our code is further accelerated by considering the unique physics associated with welding processes that are characterized by steep temperature gradient and a moving arc heat source. It is capable of modeling large-scale welding problems that cannot be easily handled by the existing commercial simulation tools. To demonstrate the accuracy and efficiency, our code was compared with a commercial software by simulating a 3D multi-pass girth weld model with over 1 million elements. Our code achieved comparable solution accuracy with respect to the commercial one but with over 100 times saving on computational cost. Moreover, the three-dimensional analysis demonstrated more realistic stress distribution that is not axisymmetric in hoop direction.

Huang, Hui↗
DOE OSTI2015

CD36 is indispensable for thermogenesis under conditions of fasting and cold stress

Hypothermia can occur during fasting when thermoregulatory mechanisms, involving fatty acid (FA) utilization, are disturbed. CD36/FA translocase is a membrane protein which facilitates membrane transport of long-chain FA in the FA consuming heart, skeletal muscle (SkM) and adipose tissues. It also accelerates uptake of triglyceride-rich lipoprotein by brown adipose tissue (BAT) in a cold environment. In mice deficient for CD36 (CD36{sup −/−} mice), FA uptake is markedly reduced with a compensatory increase in glucose uptake in the heart and SkM, resulting in lower levels of blood glucose especially during fasting. However, the role of CD36 in thermogenic activity during fasting remains to be determined. In fasted CD36{sup −/−} mice, body temperature drastically decreased shortly after cold exposure. The hypothermia was accompanied by a marked reduction in blood glucose and in stores of triacylglycerols in BAT and of glycogen in glycolytic SkM. Biodistribution analysis using the FA analogue {sup 125}I-BMIPP and the glucose analogue {sup 18}F-FDG revealed that uptake of FA and glucose was severely impaired in BAT and glycolytic SkM in cold-exposed CD36{sup −/−} mice. Further, induction of the genes of thermogenesis in BAT was blunted in fasted CD36{sup −/−} mice after cold exposure. These findings strongly suggest that CD36{sup −/−} mice exhibit pronounced hypothermia after fasting due to depletion of energy storage in BAT and glycolytic SkM and to reduced supply of energy substrates to these tissues. Our study underscores the importance of CD36 for nutrient homeostasis to survive potentially life-threatening challenges, such as cold and starvation. - Highlights: • We examined the role of CD36 in thermogenesis during cold exposure. • CD36{sup −/−} mice exhibit rapid hypothermia after cold exposure during fasting. • Uptake of fatty acid and glucose is impaired in thermogenic tissues during fasting. • Storage of energy substrates is reduced in thermogenic tissues during fasting. • CD36 is important for nutrient homeostasis to survive life-threatening challenges.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2023

Computer-controlled finishing via dynamically constraint position-velocity-time scheduler

In a Computer Numerical Controlled (CNC) finishing process, the target material removal from an optical surface is guided by the convolution between the influence function of a machine tool and its dwell time at certain points over the surface. To reduce dynamics stressing and increase machining efficiency, the dwell time must be converted to varying velocities, which are the actual inputs to the machine tool controller. Conventionally, the conversion assumed constant acceleration and relied on linear motion interpolation, which caused discontinuities in velocities. This unsmooth motion affects the material removal distribution, and, thus, the accuracy of the finished surface shape. Many modern CNC machines support the smoother, cubic-polynomial interpolated Position-Velocity-Time (PVT) motion mode; however, the conventional scheduler may fail to provide suitable velocities for the PVT. Here in this study answers this challenge by proposing a novel PVT-based velocity scheduler that achieves smooth motion while considering CNC dynamic limits. Firstly, the principle of the PVT is explained, and the PVT-based velocity scheduler is formulated. Secondly, a quadratic programming is used to optimize the velocities by imposing the CNC dynamic constraints and the C 1 continuities (zeroth and first derivatives are continuous) simultaneously. Thirdly, the smoothness and accuracy of the scheduled velocities are studied on different kinds of tool paths via simulation. Finally, a sub-0.3 nm level surface finishing experiment using ion beam figuring is demonstrated to verify the feasibility of the proposed method. The PVT-based scheduler and simulator code is open-sourced.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Assessment of irradiation embrittlement effect on fatigue life of a pressurized-water reactor pressure vessel using the fracture toughness master curve approach

The reactor pressure vessel (RPV) is a critical structural component in pressurized-water reactors, and it is designed to withstand extreme conditions, such as high pressures, elevated temperatures, and prolonged radiation exposure. Ensuring RPV integrity is essential for the safe and reliable long-term operation of nuclear power plants, especially as aging mechanisms such as fatigue and irradiation embrittlement pose increased risks. Fatigue, caused by cyclic thermal and mechanical loading, can lead to crack initiation in localized high-stress regions. Simultaneously, neutron irradiation, particularly in the beltline region, progressively reduces fracture toughness, increasing susceptibility to brittle fracture. These combined effects of fatigue and irradiation embrittlement potentially impact the RPV structural integrity, necessitating fitness-for-service assessments. This study applies the fracture toughness master curve approach to evaluate the impact of irradiation embrittlement on RPV fatigue life. A coupled thermo-mechanical stress analysis identifies critical stress locations under normal service transients, pinpointing regions most vulnerable to fatigue crack initiation and growth. Stress intensity factors for postulated flaws at these locations are calculated, enabling an assessment of fatigue life under irradiated and unirradiated conditions. The results indicate that neutron irradiation embrittlement accelerates the conditions in which a critical crack can form and lead to failure, particularly at lower temperatures. The failure occurs where reduced fracture toughness limits the material’s resistance to crack growth. Axial cracks at nozzle corners are the most life limiting without irradiation, while circumferential cracks demonstrate longer fatigue lives. The findings highlight the importance of incorporating irradiation effects into fatigue life predictions to ensure the long-term structural integrity of RPVs.

22 - GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2019

Effects of Electron Irradiation on Candidate Materials for Target Window in Accelerator-Driven Molybdenum-99 Production

NorthStar Medical technologies is planning to produce Mo-99 through the photo-nuclear reaction Mo-100 (γ,n) Mo-99 utilizing multiple high-power electron accelerators. In this approach, the production target consists of multiple thin disks of metal Mo-100 cooled by helium gas at high (300–400 psi) pressure [4]. The cost of enriched Mo-100 is on the order of $500 - 1,000 per gram, which makes a smaller target desirable and requires achieving the highest possible electron flux on the target. The heat deposition in the entrance window limits the maximum flux because the window separates the vacuum in the accelerator beam line from the high-pressure helium in the target and is only cooled on one side by the helium flow. Because the window is continuously under stress from the helium pressure and the heat from the electron beam, it is desirable to estimate a lifetime for the window and develop a replacement schedule for the target holder. Such an estimate requires the ability to predict changes in the window material properties under electron irradiation. Candidate materials for the Mo-99 production target window are Inconel 718, 250 maraging steel, and beryllium. These materials encompass a wide range of thermal conductivity, thermal expansion coefficients, Young’s modulus, tensile strength and tensile ductility. The Mo-99 target window is subjected to internal pressure from the helium coolant and thermal stresses. The operating conditions and relative advantages of each material have been described previously [1]. The current work investigates the effects of electron irradiation on the tensile properties of these materials. Axial tensile tests were conducted with electron-irradiated samples and results were compared to baseline results generated for as-fabricated materials.

36 MATERIALS SCIENCE↗
NASA NTRS1979

Effect of moisture on the fatigue behavior of graphite/epoxy composite laminates

The form of the moisture distribution in the specimen (gradient and flat profile) was considered to establish the influence of accelerated moisture conditioning on fatigue behavior. For the gradient specimens having an average moisture content of 1.4 percent, fatigue life was reduced by a factor of 8 at all stress levels investigated. Corresponding reduction in fatigue life for the flat moisture profile specimens at the same average moisture content was comparatively smaller, being about a factor of 5 from the value in dry specimens. X-ray radiographic analysis of damage accumulation in compression-compression fatigue revealed interlaminar cracking to be the dominant mode of failure responsible for the observed enhanced cyclic degradation of moisture-conditioned specimens. This finding was corroborated by the observed systematic reduction in interlaminar shear strength as a function of moisture content, which, in turn, increased the propensity for delamination under cyclic compressive loads. Residual strength measurements on cycled specimens indicated significant strength reductions at long lives, particularly in moisture conditioned specimens.

Ramani, S. V.↗