bluuebunny/crossref_metadata_2025 huggingface dataset:
Irradiation Creep and Swelling of Russian Ferritic-Martensitic Steels Irradiated to Very High Exposures in the BN-350 Fast Reactor at 305–335°C
Authors: Konobeev YV , Dvoriashin AM , Porollo SI , Shulepin SV , Budylkin NI , Mironova EG , Garner FA
Published: April 2008

Russian ferritic/martensitic (F/M) steels EP-450, EP-852, and EP-823 were irradiated in the BN-350 fast reactor in the form of gas-pressurized creep tubes. The first steel is used in Russia for hexagonal wrappers in fast reactors. The other steels were developed for compatibility with Pb-Bi coolants and serve to enhance our understanding of the general behavior of this class of steels.

In an earlier paper we published data on irradiation creep of EP-450 and EP-823 at temperatures between 390–520°C, with dpa levels ranging from 20–60 dpa. In the current paper, new data on the irradiation creep and swelling of EP-450 and EP-852 at temperatures between 305–335°C and doses ranging from 61–89 dpa are presented. Where comparisons are possible, it appears that these steels exhibit behavior that is very consistent with that of Western steels. Swelling is relatively low at high neutron exposure and confined to temperatures < 420°C, but it may be camouflaged somewhat by precipitation-related densification. These irradiation creep studies confirm that the creep compliance of F/M steels is about one-half that of austenitic steels.
Inter-Granular Phosphorus Segregation in Ferritic Steels
Authors: Faulkner RG , Flewitt PEJ , Lu Z
Published: April 2008

Progress in modeling irradiation-assisted non-equilibrium and equilibrium inter-granular segregation in steels is discussed. Solute drag and rate theory models for non-equilibrium segregation are compared. Reference is made to recent MD simulation work on irradiated grain boundaries in alpha iron.

Predictions of the models for phosphorus inter-granular segregation are compared with several recently published experimental data from pressure vessel steels. These are as follows. • A summary of results from field emission gun scanning transmission electron microscopy (FEGSTEM) and Auger Electron Spectrometry (AES) for a variety of dose and dose rate conditions. • A systematic survey of results from variable dose rate experiments. • Results from a recent systematic data set produced from C-Mn steel submerged arc welds.

The fits of the predictions to the observations show the strong influence of site competition from carbon on the final phosphorus inter-granular segregation level.
Tensile Property Estimates Obtained Using a Low Compliance Shear Punch Test Fixture
Authors: Toloczko MB , Kurtz RJ , Abe K , Hasegawa A
Published: April 2008

It has been previously shown that for a variety of BCC and FCC metals, shear punch properties correlate well with uniaxial tensile properties from corresponding miniature tensile tests. However, recent studies of the shear punch test technique using finite element modeling have suggested that large amounts of compliance may greatly affect the shape of the observed load versus displacement trace and the shear yield stress determined from the trace. On the assumption that this would lead to shear punch properties that correlate even better with uniaxial tensile properties, a low compliance shear punch test fixture was constructed. Shear punch tests were performed on a variety of unirradiated metals, and the shear punch properties were compared to tensile properties from corresponding miniature tensile tests. These correlations were then compared to previous correlations obtained using an older fixture with much greater compliance.
Revisiting the Use of SiC as a Post Irradiation Temperature Monitor
Authors: Snead LL , Williams AM , Qualls AL
Published: April 2008

Silicon carbide has been used as a post-irradiation temperature monitor since first proposed for this use in 1961. The basic technique has been the repeated measurement of the length of a SiC monitor following isochronal annealing. This technique has been shown to overestimate irradiation temperature by ∼100°C. This paper discusses the use of alternate techniques, including electrical resistivity, to infer irradiation temperature. It is shown that electrical resistivity predicts irradiation temperature within ∼20°C of actual irradiation temperature. Additionally, this technique can be used in the low-temperature (<150°C) amorphization regime, and in irradiation temperatures where irradiation damage is characterized by simple defects in crystalline SiC (<900°C).
New Methodologies for Developing Radiation Embrittlement Models and Trend Curves of the Charpy Impact Test Data
Authors: Wang JA , Rao NSV
Published: April 2008

A new methodology is developed for the prediction of RPV embrittlement that utilizes a combination of domain models and nonlinear estimators including neural networks and nearest neighbor regressions. The Power Reactor Embrittlement Database is used in this study. The results from newly developed nearest neighbor projective fuser indicate that the combined embrittlement predictor achieved about 67.3 % and 52.4 % reductions in the uncertainties for General Electric Boiling Water Reactor plate and weld data compared to Regulatory Guide 1.99, Revision 2, respectively. The implications of irradiation temperature effects to the development of radiation embrittlement models are then discussed. A new methodology that incorporates the chemical compositions into the Charpy trend curve is also developed. The purpose of this new fitting procedure is to generate a new multi-space topography that can properly reflect the inhomogeneity of the surveillance materials and utilize this multi-space trend surface to link and project the surveillance test results to that of reactor pressure vessel steels.
Mechanical and Structural Property Changes of Monolithic SiC and Advanced SiC/SiC Composites due to Low Temperature He+-Ion Irradiation and Post-Irradiation High Temperature Annealing
Authors: Nogami S , Miwa S , Hasegawa A , Abe K
Published: April 2008

The hardness and elastic modulus of monolithic β-SiC and advanced SiC/SiC composites irradiated to He-concentrations of from 1000 to 20 000 at.ppm at a temperature below 100°C were investigated. Changes in the hardness and elastic modulus were correlated with He release behavior and with irradiation-induced microstructural changes and swelling. The hardness and elastic modulus of the irradiated specimens decreased with He concentration. Amorphized regions were observed in the He-irradiated monolithic β-SiC. The mechanical property changes likely were due to irradiation-induced amorphization. By the annealing at 1200°C, nearly full recovery was achieved. He release peaks from the pyrolitic graphite, monolithic SiC, and the SiC/SiC composites at 250, 700, 1000, and 1150°C, were identified. The correlation between the mechanical property and microstructural changes and the He release behavior are discussed.
Effects of Pu Decay on a Sodalite Based Ceramic Waste Form
Authors: Jue J-F , Frank SM , O'Holleran TP , Sinkler W , Barber TL , Johnson SG , Goff KM
Published: April 2008

Radiation damage from the decay of actinide containing phases in ceramic waste forms can reduce their mechanical integrity and chemical durability. In order to understand the radiation damage and predict long-term degradation behavior of the ceramic waste forms in a repository, a short-lived actinide can be loaded into the ceramic waste form to accelerate the damage process. In the current study, a 238Pu (half-life of 87.7 years) doped ceramic waste form was fabricated by uniaxial hot pressing. The starting materials are 75 volume percent salt occluded zeolite and 25 volume percent binding glass. After consolidation, the final product has sodalite and glass as the major constituents, while halite, nepheline and actinide containing phases are minor phases. After three years, density measurements, scanning electron microscopy, and seven-day product consistency tests all reveal little or no change. Bubbles/voids have been found by transmission electron microscopy. No defect-segregation was observed. Although X-ray diffraction shows a very small unit cell expansion in PuO2 and sodalite, micro-cracking and interface debonding were not observed. Preliminary results show that the electron beam dose required to amorphize sodalite decreases with increasing alpha decay dose.
Creep Deformation in V-4Cr-4Ti in a Low-Oxygen Lithium Environment
Authors: Grossbeck ML
Published: April 2008

Vanadium alloys, especially V-Cr-Ti alloys, are candidates for first wall and blanket structures of fusion reactors. However, a major vulnerability of this class of alloys is the affinity for oxygen and other interstitial solutes. Oxygen, nitrogen, carbon, and hydrogen can severely harden and embrittle vanadium alloys. Oxygen is particularly troublesome because of its prevalence and its relatively high diffusivity. In determining creep behavior, the effects of oxygen must be understood. In the present study, specimens, in the form of pressurized tubes, were exposed to liquid lithium, the coolant of choice for vanadium systems, at temperatures of 665, 765, and 800°C. Under these conditions, oxygen is removed from the vanadium alloy by the lithium. The data produced were compared with data from similar specimens tested in a high vacuum environment, where the oxygen level in the alloy is expected to slowly increase, at Pacific Northwest National Laboratory in order to evaluate the effect of oxygen. The Li-exposure resulted in higher creep rates and shorter rupture times. The effect was most clear at 800°C, where deformations 4–6 times higher were observed for the Li-exposed specimens.
Varying Temperature Irradiation Effects of V-4Cr-4Ti Alloy with Small Addition of Si, Al and Y
Authors: Satou M , Nakayashiki H , Chuto T , Hasegawa A , Abe K
Published: April 2008

To evaluate the effects of temperature varying neutron irradiation of the V-4Cr-4Ti alloys with small addition of Si, Al and Y, microstructure observation and micro Vickers hardness test were carried out after one cycle varying temperature neutron irradiation at the Japan Material Test Reactor (JMTR). On a temperature increase from 200 to 350°C, the hardness of the alloys decreased. It is possible to explain the appearance of a vacancy-rich state after the temperature change and subsequent mutual disappearance of interstitials and the vacancies. Following the temperature change from 375 to 500°C, the hardness of the alloys increased, therefore, the vacancy-rich state might not appear due to stable defect clusters formed at the lower temperature conditions.
Probabilistic Life Prediction Isn't as Easy as It Looks
Authors: Annis C
Published: April 2008

Many engineers effect “probabilistic life prediction” by replacing constants with probability distributions and carefully modeling the physical relationships among the parameters. Surprisingly, the statistical relationships among the “constants” are often given short shrift, if not ignored altogether. Few recognize that while this simple substitution of distributions for constants will indeed produce a nondeterministic result, the corresponding “probabilities” are often woefully inaccurate. In fact, even the “trend” can be wrong, so these results can't even be used for sensitivity studies. This paper explores the familiar Paris equation relating crack growth rate and applied stress intensity to illustrate many statistical realities that are often ignored by otherwise careful engineers. Although the examples are Monte Carlo, the lessons also apply to other methods of probabilistic life prediction, including FORM/SORM (First/Second Order Reliability Method) and related “fast probability integration” methods.
Probabilistic Fatigue: Computational Simulation
Authors: Chamis CC , Pai SS
Published: April 2008

Probabilistic computational simulation of fatigue-life is illustrated in terms of several sample cases that have been generated over the past ten years. The cases are selected to illustrate applications to multi-scale, multi-discipline, and multi-physics. These cases include composite laminate; coupled thermal, mechanical, fatigue, and creep; pressurized tank; engine blades; engine rotor; and composite combustor liner. The fundamentals for probabilistic computational fatigue are briefly described, and general comments are included on what it takes to perform probabilistic computational fatigue and to validate it. Typical results show that fatigue-life can be evaluated for complex components and for complex loadings. Probability of survival curves can be generated, and probabilistic sensitivities influencing fatigue-life can be determined. The paper describes what can be done rather than details of a specific case.
The Prediction of Fatigue Life Distributions from the Analysis of Plain Specimen Data
Authors: Shepherd DP
Published: April 2008

For any structurally critical component subject to fatigue, the safety of the structure depends on an accurate prediction of the life under this failure mode. However, in such circumstances it is insufficient to consider only the mean behavior of the material. To ensure structural integrity, a model for the distribution of life to failure is required, which will allow lives to be assessed relative to acceptable safety levels.

In previous work, a methodology for deriving fatigue life estimates for arbitrary specimen and component geometries from plain specimen data has been developed [4]. The methodology is based on a procedure for developing a model for the initiation behavior of the material from the specimen data and for applying this to an arbitrary material geometry or stress field. In the current paper, this method is further developed to allow for the associated distribution of fatigue lives to be calculated. This involves direct consideration of the statistical relationship between crack initiation and crack propagation, so that the distribution of initiation lives can be derived accurately. However, incorporating these considerations directly into the methodology reveals some inconsistencies in the formulation of the original model. These relate to the fact that, at high stresses, the specimens will fail in tension rather than classical fracture, thus altering the interpretation of the data. It is shown that a more robust model can be developed, but only by including the distribution of tensile strength as an additional variable, and by considering the statistical relationship between this and the other fundamental variables.

The methodology which arises from the incorporation of these considerations into the basic calculation scheme is then developed, including a means for estimating the distribution of life to failure at all points on the stress against cyclic life curve.
Predicting Fatigue Life Under Spectrum Loading in 2024-T3 Aluminum Using a Measured Initial Flaw Size Distribution
Authors: DeBartolo EA , Hillberry BM
Published: April 2008

Measured cracked particle distributions were used in conjunction with a closure-based life prediction package (FASTRAN 3.8 [1]) to predict fatigue life distributions under a spectrum load (FALSTAFF — Fighter Aircraft Loading STAndard For Fatigue [2]) in 2024-T3 aluminum. A single crack model was assumed and no threshold stress intensity factor was used. Simulations were done at maximum stress levels of 175, 205, and 275 MPa. The resulting fatigue life distributions were compared at different levels of probability of failure. At all levels, predicted fatigue lives were within a factor of two or better of experimental fatigue lives. This indicates both that the single-flaw model can be a reasonable approximation for life prediction and that it is not necessary to truncate initial flaw size distributions at some threshold level.
Preliminary Results of the United States Nuclear Regulatory Commission's Pressurized Thermal Shock Rule Reevaluation Project
Authors: Dickson TL , Williams PT , Bass BR , Kirk MT
Published: April 2008

The current federal regulations to insure that nuclear reactor pressure vessels (RPVs) maintain their structural integrity when subjected to transients such as pressurized thermal shock (PTS) events were derived from computational models developed in the early to mid-1980s. Since that time, there have been advancements in relevant technologies associated with the physics of PTS events that impact RPV integrity assessment. Preliminary studies performed in 1999 suggested that application of an improved technology could reduce the conservatism in the current regulations while continuing to provide reasonable assurance of adequate protection to public health and safety. A relaxation of PTS regulations could have profound implications for plant license-extension considerations. Based on the above, the United States Nuclear Regulatory Commission (USNRC) initiated in 1999 a program to re-evaluate the current PTS regulations within the framework established by modem probabilistic risk assessment techniques.

As part of the USNRC PTS project, improved computational models have evolved through interactions between experts in the relevant disciplines of thermal hydraulics, probabilistic risk assessment, human reliability analysis, materials embrittlement effects on fracture toughness (crack initiation and arrest), fracture mechanics methodology, and fabrication-induced flaw characterization. The experts have been from the NRC staff, their contractors, and representatives from the nuclear industry. These improved models have been implemented into the FAVOR (Fracture Analysis of Vessels: Oak Ridge) computer code which is an applications tool for performing risk-informed structural integrity evaluations of embrittled RPVs subjected to transient thermal-hydraulic loading conditions.

Recently, the FAVOR code was applied to a domestic commercial pressurized water reactor to evaluate the adequacy of the current regulations and to determine if a technical basis can be established to support a change in the current regulations. This paper gives an overview of the improved computational methodology and presents some results of the preliminary analyses.
A Software Framework for Probabilistic Fatigue Life Assessment of Gas Turbine Engine Rotors
Authors: McClung RC , Enright MP , Millwater HR , Leveront GR , Hudak SJ
Published: April 2008

An enhanced life management process based on probabilistic damage tolerance methods has been developed to address material anomalies in titanium rotating components of gas turbine engines. Related methods are being used as tools to investigate the impact of engine monitoring and usage variability on prognosis for field readiness and life management. This paper begins with an overview of the process of probabilistic damage tolerant design, using the DARWIN® computer program to illustrate the interplay between various random variables and the conventional elements of structural design and life prediction. Special attention is then given to the initial distribution of material anomalies, scatter in fatigue crack growth data, and variability in complex mission histories. The significance of each source of variability for different applications is discussed.
Effect of Individual Component Life Distribution on Engine Life Prediction
Authors: Zaretsky EV , Hendricks RC , Soditus SM
Published: April 2008

The effect of individual engine component life distributions on engine life prediction was determined. A Weibull-based life and reliability analysis of the NASA Energy Efficient Engine was conducted. The engine's life at a 95 and 99.9 % probability of survival was determined based on the engine manufacturer's original life calculations and assumed values of each of the component's cumulative life distributions as represented by a Weibull slope. The lives of the high-pressure turbine (HPT) disks and blades also were evaluated individually and as a system in a similar manner. Knowing the statistical cumulative distribution of each engine component with reasonable engineering certainty is a condition precedent to predicting the life and reliability of an entire engine. The life of a system at a given reliability will be less than the lowest-lived component in the system at the same reliability (probability of survival). Where Weibull slopes of all the engine components are equal, the Weibull slope had a minimal effect on engine L0.1 life prediction. However, at a probability of survival of 95% (L5 life), life decreased with increasing Weibull slope.
Principles of Variable Amplitude Fatigue Design and Testing
Authors: Sonsino CM
Published: April 2008

The proper consideration of variable amplitude loading by utilizing service spectra and appropriate Gassner-lines is essential for the design of light-weight components and structures by allowing loads in significant excess of the Woehler-line (S-N curve). This permits higher stresses than under constant amplitude loading and renders reduced component dimensions. Reliable reconstitution and simulation methods for service load-time histories require not only the rainflow matrices, but also information about the order of the cycles described by Markovian matrices, the power spectral density and, for multiaxial applications, the cross-correlations between the particular load directions as well as the phase relations. A major problem in numerical fatigue life assessment is still the fatigue life calculations for spectrum loading, because of the scattering of the real damage sum D over a wide range, which is not entirely understood. These findings demonstrate the need for experimental spectrum tests, which are indispensable for ensuring the safety of parts. With regard to safety and liability requirements, the failure probability resulting from the probability of occurrence of the spectrum, from the scattering of the fatigue strength and from the failure criterion (technical crack or propagation), must be taken into account.
A Framework for a Standardization Effort for Fatigue Crack Growth Testing Under Variable Amplitude Spectrum Loading
Authors: McKeighan PC , McMaster FJ
Published: April 2008

Fatigue crack growth testing under variable amplitude loading conditions is currently not standardized. Without standardization, the methods and techniques applied by different laboratories vary, and as such, lab-based bias can occur readily. The goal of this paper is to discuss in detail the experience developed and lessons learned during years of spectrum crack growth testing. Issues concerned with specimen geometry, crack geometry, precracking methodology, crack length measurement, spectrum content, load magnitude control, and interpretation of results are addressed herein. Although the ASTM Standard Test Method for Measurement of Fatigue Crack Growth Rates, E 647, provides guidance that is useful to this test, the specialized nature of spectrum testing does provide its own unique set of issues different from da/dN versus ΔK testing.
Variable Amplitude Loading on a Resonance Test Facility
Authors: Pöting S , Traupe M , Hug J , Zenner H
Published: April 2008

The uncertainty in fatigue lifetime calculations brings up the necessity of experimental verification. Several efforts have been undertaken in the past to improve and introduce new test methods. An alternative method for variable amplitude loading using resonance test facilities is introduced and discussed. A comparison of test results on a resonance test facility with a new control system and a servo hydraulic test facility shows no significant difference in the test results. The compatibility of the test results to those results of accepted fatigue life tests is proven for three test series of different specimens. The introduced resonance test control offers a new way for fatigue life tests, reducing the cost and time, as well as increasing the safety of products because it permits testing to much longer lives in a relatively short time.
Spectrum Editing for a Full-Scale Fatigue Test of a Fighter Aircraft Wing with Buffet Loading
Authors: Hewitt RL , Weiss JP , Nor PK
Published: April 2008

This paper describes the spectrum editing techniques that were used to derive the test spectrum for the full-scale fatigue test of an F/A-18 wing that is currently underway at the Institute for Aerospace Research, National Research Council Canada. The derivation of the baseline spectrum and the truncation methodology are described, together with details of the validation process and the calculated effects of the truncation levels that were accepted.

Even with aggressive truncation, the test spectrum contained nearly 135 000 unique load conditions. Because of control system limitations and the time required to optimize and check the actuator loads for so many load conditions, a binning process was used to reduce the number of unique load conditions to less than 50 000. The process is detailed and the effects on the calculated life are discussed.
Large Commercial Aircraft Loading Spectra: Overview and State of the Art
Authors: Divenah LL , Beaufils JY
Published: April 2008

The purpose of this paper is to present an overview of an Airbus' approach regarding fatigue spectra for large commercial aircraft. An accurate load representation is essential to the modern aircraft fatigue optimization process. The effort to develop spectra must be consistent with the efforts made to optimize materials and geometries. The main characteristics of large commercial aircraft spectra, both for prediction and testing, are presented. Recent advances are specifically highlighted.
Effect of Transient Loads on Fatigue Crack Growth in Solution Treated and Aged Ti-62222 at -54, 25, and 175°C
Authors: Stephens RR , Stephens RI , Berge SC , Lemm DE , Glancey CD
Published: April 2008

Transient loads consisting of single tensile overloads and single tensile overloads followed by single compressive underloads were applied to Ti-62222 solution treated and aged titanium alloy at -54, 25, and 175°C. Tensile overload ratios were 2.0 and 2.5, and the compressive underload ratio was -0.5. Four reference steady state ΔKss values, using constant ΔK testing at R = 0.1, were investigated at each temperature. Cycles of delay, fatigue crack growth during delay, and minimum fatigue crack growth rate during a transient load were obtained for all tests. Cycles of delay ranged from zero to crack arrest. Higher tensile overloads caused greater delay cycles, and underloads were often detrimental. Low and high temperatures were primarily beneficial to delay cycles relative to those at room temperature. Crack growth delay distance was always greater than the pertinent reversed plastic zone size. Fatigue crack growth life predictions were made using FASTRAN III and AFGROW computer programs and produced both conservative and non-conservative results with more than half the predictions being within ± 2 of the experimental results. Macro- and microfractography revealed surface crack closure, Mode II displacements, crack tip blunting, branching, and tunneling contributed to the transient fatigue crack growth behavior.
On the Causes of Deviation from the Palmgren-Miner Rule
Authors: McEvily AJ , Ishihara S , Endo M
Published: April 2008

Based upon considerations of fatigue crack growth, explanations for the observation that the Palmgren-Miner damage summation often deviates from unity can be established. It is assumed that the fatigue lifetime is taken up entirely in fatigue crack propagation, that is the number of cycles spent in crack initiation is a negligibly small fraction of the total lifetime. This assumption permits the fatigue lifetime to be analyzed in terms of a basic constitutive relation for the rate of fatigue crack growth that is given by: dadN=A(ΔKeff−ΔKeffth)2 where a is the crack length, N is the number of cycles, A is a material-environmental constant, ΔKeff is the effective range of the stress intensity factor, i. e., Kmax - Kop, where Kmax is the maximum value of the stress intensity factor in a loading cycle, Kop is the stress intensity factor at the crack opening level, and ΔKeffth is the effective value of the stress intensity factor at the threshold level.

Three variable amplitude loading conditions are considered: two-step loading, multiple two-step loading, and overload and underload loading. In each case, the cause for deviation from a Palmgren-Miner damage summation of unity is clarified.
Effect of Overloads and Underloads on Fatigue Crack Growth and Interaction Effects
Authors: Romeiro F , de Freitas M , Pommier S
Published: April 2008

Under constant amplitude loading, a single variable (ΔK) is required in crack growth relationships. The transferability of fatigue laws, determined under constant amplitude loading, to variable amplitude fatigue requires at least an additional variable, whose evolution with crack length accounts for the interaction effects between cycles of different types. The crack opening level (Kop) is usually employed for this purpose because it can be determined from the experiments and compared with predictions from models or FEM analyses. This paper presents an analysis of fatigue crack growth on M(T) specimens of medium carbon steel specimens and using FEM analyses. The specimens are subjected to repeated blocks of cycles made up of one or several overloads separated by a variable number of baseline cycles. The experiments are simulated by FEM analyses, taking into account the cyclic plastic behavior of the low carbon steel. The main objective of this study is to better understand the mechanisms at the origin of interactions effects due to the presence of overloads (or underloads) at different locations of the block loading. It is concluded that the interaction effects are closely related to the cyclic plastic behavior of the material and namely to the Bauschinger effect.
Overload Effects in Aluminum Alloys: Influence of Plasticity and Environment
Authors: Ranganathan N , Tougui A , Lacroix F , Petit J
Published: April 2008

This paper presents the behavior of selected aluminum alloys when subjected to a single overload, with an aim to highlight the effects of plasticity and environment. The delaying effect of a single overload is given in terms of number of delay cycles. The delay behavior is systematically compared with the constant amplitude crack growth resistance. It is shown that for most of the studied alloys, a strong parallel can be drawn between the constant amplitude crack growth behavior and the overload induced delay. Delay induced by an overload in vacuum is higher than that in air for the 7075 and the 2024 alloys, compatible with stronger constant amplitude crack growth resistance in vacuum. The influence of the elastic-plastic behavior is stronger in vacuum than in air. In the case of the Aluminum Lithium alloy, delay in air is higher than that in vacuum, which is opposite to the behavior under constant amplitude loading. The basic mechanisms governing crack growth in aluminum alloys and their influence on the delay induced by an overload are revealed. The experimental results are compared with a phenomenological model, taking into account the cyclic plastic behavior of materials.
Load History in Fatigue: Effect of Strain Amplitude and Loading Path
Authors: Aubin V , Quaegebeur P , Degallaix S
Published: April 2008

The low-cycle fatigue behavior of a duplex stainless steel, 60% α — 40% γ, is studied under tension-compression/torsion loading at room temperature and under strain control. It is shown that the duplex stainless steel has an isotropic behavior under cyclic proportional loading. The loading path induces an extra-hardening on cyclic hardening of duplex stainless steel but lower than that on austenitic stainless steels. The effect of loading history is studied in terms of strain amplitude, mean strain, and loading path. It is shown that only histories in strain amplitude and loading path have an effect on the stabilized stress.
Fuzzy Probabilistic Assessment of Aging Aircraft Structures Subjected to Multiple Site Fatigue Damage
Authors: Akpan UO , Rushton PA , Dunbar TE , Koko TS
Published: April 2008

A strategy is developed for fuzzy probabilistic assessment of the fatigue resistance of aging aircraft structures due to multiple site fatigue damage (MSD). The residual strength of an aircraft structure may be significantly reduced by the existence of fatigue damage at multiple locations. Depending on the level of subjectivity and degree of knowledge, MSD-related parameters may be represented as either purely random variables or fuzzy random variables. The membership functions of probabilistic characteristics of fuzzy random variables, namely mean values and standard deviations, are developed. Mechanistic and probabilistic models used to evaluate multi-site fatigue damage are also presented. A probabilistic solution strategy, employing the first order reliability method (FORM), is combined with the response surface-based fuzzy modeling approach to develop possibility distributions of the probabilistic response quantities (namely reliability indices and failure probabilities) for components subjected to multiple site fatigue damage. Instead of providing the traditional single valued, purely probabilistic measure for reliability, the present formulation proves its merit in its ability to combine experimental data with expert knowledge to provide confidence bounds on the structural integrity of aging aircraft. Moreover, the predicted bounds are dependent on the level of knowledge regarding the fuzzy input parameters, with a higher degree of knowledge resulting in more narrow bounds. An example problem is used to demonstrate the advantages of the proposed methodology.
Validation of Complex Wheel/Hub Subassemblies by Multiaxial Laboratory Tests Using Standardized Load Files
Authors: Fischer G
Published: April 2008

For the evaluation of safety components like the wheel/hub subassembly, the operational loading, including special event loading as well as the influence of adjacent components and their interaction, must be taken into account. By using the described test procedure in the Biaxial Test Facility, all service-like deformations can be simulated in accelerated program tests. A reliable time, as well cost, saving validation of new designs, materials, and manufacturing technologies can be carried out.
Modeling Thermomechanical Cyclic Deformation by Evolution of Its Activation Energy
Authors: Wu XJ , Yandt S , Au P , Immarigeon J-P
Published: April 2008

This paper presents a new approach for modeling the deformation response of metallic materials under thermomechanical fatigue loading conditions, based on the evolution of thermal activation energy. In its physical essence, inelastic deformation at high temperatures is a thermally activated process. The thermal activation energy, which controls the time and temperature dependent deformation behavior of the material, generally evolves with the deformation state (γp) of the material, in response to the applied stress τ. In the present approach, the inelastic flow equation is integrated for a deformation range where strain hardening is predominant. The simplified integration version of the model only needs to be characterized/validated by isothermal tensile and fatigue testing, and it offers an explicit description of the TMF behavior in terms of physically defined variables. By identifying the dependence of these variables on the cyclic microstructure, the model may also offer a mechanistic approach for fatigue life prediction.
Cyclic Deformation Behavior of Haynes 188 Superalloy Under Axial-Torsional, Thermomechanical Loading
Authors: Bonacuse PJ , Kallurį S
Published: April 2008

Four types of axial-torsional, thermomechanical fatigue experiments have been performed on Haynes 188 superalloy: mechanically in-phase and thermally in-phase, mechanically in-phase and thermally out-of-phase, mechanically out-of-phase and thermally in-phase, and mechanically out-of-phase and thermally out-of-phase. In this study, representative cyclic deformation behavior of the axial-torsional, thermomechanical experiments is presented and compared with isothermal experiments performed at the two temperature extremes (316°C and 760°C) of the thermal cycle. The evolution of cyclic hardening under axial-torsional, thermomechanical conditions is also presented and compared with the corresponding isothermal data.
Environmental Effects on the Isothermal and Thermomechanical Fatigue Behavior of a Near-γ Titanium Aluminide
Authors: Maier HJ , Fischer FOR , Christ H-J
Published: April 2008

High-temperature fatigue behavior of a near-γ titanium aluminide was studied over the temperature range from 500–750 °C under both isothermal and thermomechanical fatigue (TMF) conditions. Cyclic stress-strain (CSS) response was observed to change drastically with test temperature. In isothermal tests conducted below the ductile-to-brittle transition temperature (DBTT) initial cyclic hardening occurred. By contrast, in isothermal fatigue tests performed above the DBTT the material displayed pronounced cyclic saturation throughout the test. Still, modeling of CSS behavior under TMF conditions based solely on isothermal input data is feasible, as corresponding microstructures evolve in both types of tests. Environmental degradation plays a key role in this material, and the effects were assessed based on fatigue tests conducted in air and high-vacuum, respectively. Moreover, the material displays large mean stress effects below the DBTT, and thus, out-of-phase TMF tests were found to be most detrimental, when the temperature range encompassed the DBTT.
Collaborative Research on Thermo-Mechanical and Isothermal Low-Cycle Fatigue Strength of Ni-Base Superalloys and Protective Coatings at Elevated Temperatures in The Society of Materials Science, Japan (JSMS)
Authors: Okazaki M , Take K , Kakehi K , Yamazaki Y , Sakane M , Arai M , Sakurai S , Kaneko H , Harada Y , Itoh A , Okuda T , Nonaka I , Fujiyama K , Nanba K
Published: April 2008

Results of collaborative research by “Subcommittee on Superalloys and Coatings” in The Society of Materials Science, Japan (JSMS), are presented, which cover the thermo-mechanical fatigue (TMF) and high temperature isothermal low-cycle fatigue (ILCF) strengths of Ni-Base superalloy, substrate alloys and the protective coatings. Three kinds of Ni-base superalloys were selected as the substrate: a single crystal alloy, CMSX-4; a directionally solidified alloy, CM247LC; and a polycrystalline alloy, IN738LC. On these substrate specimens the CoNiCrAlY alloy was coated by 250 μm in thickness by low pressure plasma spraying, and then aluminized. This process was managed and undertaken by one of member companies of the Subcommittee. The round robin TMF and ILCF tests were carried out under a strain ratio of -1 at temperature ranged between 400 and 900°C. In the former the tests were performed under the out-of-phase and diamond phase conditions in which the phase difference between strain and temperature were 180° and 90°, respectively. It is shown by the round-robin tests that the TMF lives, as well as the ILCF, were strongly dependent on the substrate alloys. Many important, or noteworthy results were also found: e.g., the TMF fracture behavior of the coated specimens revealed some unique characteristics that were hardly deduced from that of the bare specimens. The effect of coating on the ILCF life was strongly dependent on the temperature. It was not reasonable, or difficult to try to estimate the TMF life of the coatings from the ILCF test results. Based on the observations on the crack initiation, propagation and the fracture surface, the effects of the substrate alloy and the coating on the TMF and ILCF lives were discussed.
Thermomechanical Response of Single Crystal Nickel-base Superalloy CM186 SX
Authors: Kong CN , Bullough CK , Smith DJ
Published: April 2008

CM186 SX is a second generation Rhenium (Re) containing single crystal nickel-base superalloy, which exhibits improved creep and oxidation performance compared with its first generation counterparts. In this paper, the thermomechanical (TMF) response of coated and uncoated <001> single crystal nickel-base superalloy, CM186 SX is presented. In-phase (IP) and Out-of-Phase (OP) cycles were examined using coated and uncoated hollow dumbbell test pieces. Linear heating and cooling with a temperature rate of 6°C/s between 350 and 950°C was applied. The tests were mechanical strain controlled with strain ranges between 0.5 to 1.0%. Detailed examination of the experimental results revealed that variations in wall thickness of the hollow test pieces played a significant role in the set-up of the experiments and interpretation of the results. Finite element analysis was also performed to investigate the influence of the variation in wall thickness and its relevance to the experimental results. Overall, the results showed that the coating reduced the TMF life, particularly for OP cycles. Brittle cracking of the coating, especially at low temperatures, occurred at low strains. For the IP cycle the coating appeared to have less of an influence on life than for OP cycles. The results are compared with those from an earlier program using solid rectangular test pieces.
Thermomechanical Fatigue Analysis of Cast Aluminum Engine Components
Authors: Su X , Zubeck M , Lasecki J , Sehitoglu H , Engler-Pinto CC , Tang C-Y , Allison JE
Published: April 2008

Cast aluminum engine components are increasingly being used in the automobile industry. Due to high engine operating temperature, thermal fatigue during start-stop cycles is a very important design consideration. In this paper we will describe the application of newly developed analysis methods to analyze these complex problems. A user material subroutine for viscoplasticity stress analysis using the commercial FEA code ABAQUS has been developed. Stress and strain of engine heads during thermal cycle are thus analyzed and this code verified. The thermomechanical fatigue model developed by Neu and Sehitoglu, a damage accumulation model including fatigue, oxidation and fatigue damage, has been extended to three-dimensional state of stress by using the concept of critical planes. The enhanced damage model has been implemented in a FEA postprocessor for engine component life analysis. The predictions of thermal strains and fatigue life based on 3D FEA analysis are compared with measurements of engine component tests. Three different thermal loading cycles are considered. The thermal fatigue life predictions are within 30% error of actual component test data.
Acoustic Emission Analysis of Damage Accumulation During Thermal and Mechanical Loading of Coated Ni-Base Superalloys
Authors: Vougiouklakis Y , Hähner P , de Haan F , Stamos V , Peteves SD
Published: April 2008

Acoustic emission (AE) allows for the in-situ monitoring of damage during the thermal and mechanical solicitation of a material and can be used to identify various damage mechanisms and interpret the way they interact, in particular, in multi-material systems. In the present work AE activity was monitored during the step-wise tensile testing and during the thermomechanical fatigue (TMF) of Ni-base superalloys (in single-crystalline and directionallysolidified form), coated with oxidation protection coatings. The classification analysis of the AE signals revealed that AE monitoring provides complementary and specific information about the surface-, subsurface-, and bulk damage processes. In combination with the mechanical response and the surface damage analysis by video imaging, it represents a powerful means for non-destructive damage assessment and it offers possibilities for the lifing of coated material systems.
Two Specimen Complex Thermal-Mechanical Fatigue Tests on the Austenitic Stainless Steel AISI 316 L
Authors: Rau K , Beck T , Löhe D
Published: April 2008

Start-stop and load change processes of combustion engines result in inhomogenous and instationary temperature fields, which induce cyclic mechanical loadings, e.g., in solid and cooled turbine blades and vanes. The present study shows results of so-called complex thermal-mechanical fatigue“ (CTMF) tests carried out with a two specimen testing system in order to simulate the interaction of the “hot“ outer and the “cold“ inner side of a cooled turbine blade and the influence of superimposed mechanical loadings. The test pieces were made from the austenitic steel AISI 316 L. Specimen 1 represents the “hot“ and specimen 2 the “cold“ side of the component, respectively. Both specimens were loaded by individual thermal cycles. The mechanical interaction of the “hot“ and the “cold“ side is simulated firstly by keeping the total strains of both specimens at identical values. Secondly, in order to simulate external loadings such as centrifugal forces at turbine blades, the sum of the forces at both specimens is given as a function of time. The influence of dwell times at the maximum temperatures as well as of superimposed forces on the cyclic deformation and the cyclic creep behavior of the two specimen system is presented and discussed.
Effects of Application Technique and Temperature on Characteristics of Modified Bitumen Membrane Assemblies
Authors: Chernotowich K , Brzozowski KJ , Sanicky PJ
Published: April 2008

Results are presented and discussed of a laboratory study that measures T-peel interply adhesion of polymer modified bituminous membranes assembled using typical field application techniques and a wide range of asphalt temperatures. This evaluation includes both standard and polymer modified asphalts, and additionally examines factors such as: a) interply thickness, b) tendencies for the inclusion of blisters, and c) the effects of dark oven heat aging at 70°C (158°F) of the completed assemblies. The findings indicate that the use of elevated asphalt application temperatures, conventionally proposed to improve adhesion, has little to no effect on interply T-peel strengths. The results also illustrate patterns of interply thickness as it relates to asphalt application temperature and/or application technique. Additional indications reveal a tendency to develop blisters that are dependent on application technique and asphalt temperature. The study also proposes that if interply adhesion is considered to be a primary criterion for improved performance of polymer modified bituminous sheet systems, the use of polymer modified interply asphalts must be given due consideration for use in this application.
The Origin and Behavior of a Flood Capping Layer Deposited on Contaminated Sediments of the Saguenay Fjord (Quebec)
Authors: Pelletier E , Desrosiers G , Locat J , Mucci A , Tremblay H
Published: April 2008

The upper section of the Saguenay Fjord was impacted by a catastrophic flood in July 1996. Contaminated sediments were capped by a layer of clean silty post-glacial sediments with background levels of trace metals and polycyclic aromatic hydrocarbons (PAHs). The capping layer was characterized by geotechnical and geochemical methods and its biological recolonization was monitored by annual sampling of the macrofauna. The strong dominance of surface deposit feeders Cirratulidae and Ampharetidae was observed at most stations in the first 2–3 years followed by carnivorous annelids such as Lumbrineridae and Nephtidae species indicating a well recolonized benthic habitat in the Baie des Ha!Ha!. The presence of benthic fauna was a major factor in modifying the density of sediments by physical mixing and irrigation, and in changing the surface roughness. The slope stability of the capping layer is considered as very good except in limited deltaic sectors at the head of the Baie des Ha!Ha! The new layer showed a good efficiency to isolate contaminated sediments from the sediment/water interface. Although manganese and iron were remobilized as the new layer became anoxic, mercury, arsenic and PAHs showed a very limited mobility through the flood layer which allowed geochemists to calculate the present fluxes of toxicants to the Baie des Ha!Ha! without interferences from older contaminated sediments trapped below the flood layer. The present flux of PAHs to bay (0.9 ng.cm-2.g-1) is about 300 times lower than the 1974 flux and 8 times lower than the 1986 flux.