bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Water and Jet Fuel Absorption on Mode I and Mode II Delamination of Graphite/Epoxy
Authors: Hooper SJ , Subramanian R
Published: March 2009

The environmental effects of water and jet fuel absorption on AS4/3501-6 graphite/epoxy were measured in terms of the changes in the interlaminar fracture toughness. Saturated double cantilever beam (DCB) and end notched flexure (ENF) specimens were tested to determine the environmental effects of moisture sorptions on critical strain energy release rates, GIc and GIIc, respectively. These data are compared with baseline data obtained for specimens tested in the room temperature dry (RTD) condition. This phase of the study revealed that moisture absorption of either water or jet fuel tends to toughen this material and that this change is more significant for Mode I than for Mode II.

This paper also presents the results of a fractographic analysis of the fracture surfaces developed in these unidirectional specimens. These results document the changes in the surface morphology due to the environmental conditions studied. Significant differences were observed for Mode I fracture surfaces. SEM fractographs of delamination surfaces for DCB specimens reveal that the moisture absorption condition results in fracture surfaces where the fibers are covered with resin. This corresponds to the Mode I toughening, which is quantified in terms of an increase in GIc. The differences in the fractographs of saturated and room temperature dry ENF specimens are less profound, with the hackle formation in the former case being more deformed than the latter.
Model Composites: A Novel Approach for the Evaluation of Micromechanical Behavior
Authors: Carman GP , Lesko JJ , Razvan A , Reifsnider KL
Published: March 2009

Microlevel damage events in composite materials are extremely important issues when addressing the remaining strength and life of the system. These events include fiber fractures, matrix cracks, and fiber end effects. Development of accurate representations of these phenomena at the local level is difficult and until this time unverifiable. An experimental procedure is suggested which involves a macromodel composite to generate data that are utilized to validate (or invalidate) current and proposed micromechanical analysis of composite materials containing damage. Quantitative experimental data on the perturbed strain field are measured with embedded strain gages and internal Fabry Perot fiber optic strain sensors, while qualitative measurements are accomplished with data obtained from a birefringent matrix. The majority of the tests described herein are representative of continuous fiber composites containing a fiber fracture. A controlled fiber fracture is achieved in the model composite at a predetermined location. Stress concentration and ineffective length (i.e. size of the perturbed stress field) measurements are experimentally determined for different fiber volume fractions and interphase coatings (i.e. on fiber). Initial results suggest that crack propagation plays a significant role in the stress redistribution which occurs in the vicinity of a fiber fracture. We suggest that the interphase region affects the formation and propagation of cracks in the composite. The methodology described in this paper allows a researcher to systematically study the effect of various physical parameters, such as fiber volume fraction, constituent properties, and interphases, on the local stress state in a material system.
Micromechanics of Tensile Strength in Composite Systems
Authors: Gao Z , Reifsnider KL
Published: March 2009

This paper deals with the tensile strength of composites. The fibers of the composites are considered to have a statistical distribution of flaws of the Weibull type. The tensile strength formulation developed explicitly contains the Weibull shape factor, m, stress concentration factors due to fiber fractures, Ci, and the ineffective length, δ. Here, m describes the uniformity of the fiber strength. The stress concentration factors, Ci, characterize the disturbance of stress in neighboring unbroken fibers when i numbers of fiber have broken together. They are related directly to the subsequent event of fracture propagation perpendicular to the fiber direction, which is an important failure mechanism in ceramic composites. The ineffective length, δ, hinges on the concept of “the weakest link” along the fiber direction and is the dominant factor in the bundle strength representation of the strength of polymer-based composites. There is a lack of systematic analysis addressing the competition of these two important failure mechanisms and their influence on the tensile strength of the composites. In this paper, we combine the theory of bundle strength with the mechanics of local stress concentration to develop a micromechanical model for tensile strength, and study the influence of micromechanical properties on the tensile strength of composites and optimal design for tensile-controlled loading applications.

Using the strength formulas developed in this paper, we study how tensile strength is influenced by the changes of the micromechanical properties, m, Ci, and δ, and also by the properties of the fiber and matrix, Ef and Gm. It is shown that the existence of stress concentrations due to fiber fractures increases the chance of local crack propagation and, therefore, decreases the strength values predicted by the bundle strength theory. Stress concentrations play a dominate role in determining the magnitudes of tensile strength, especially for large values of m. However, since the stress concentrations remain virtually constant, for composites with different Ef and Gm and regularly spaced fibers, the variation of the strength curve as a function Ef and Gm is still controlled by the changes of the ineffective length, δ. The predictions of strength from the present model are compared to experimental measurements to validate the accuracy of the formulation.

The strength consideration is more complicated when the existence of irregular fiber spacing and the lack of quantitative descriptions of the spacing are taken into account. We show that, even for polymeric composites, optimal strength (when material properties Ef and Gm are taken as design variables) can be achieved due to certain fiber spacing variations. Therefore, the fiber spacing of composites, although difficult to control in the manufacturing process and hard to determine for a specimen, must be considered carefully to eliminate, isolate, or correctly represent its effects on strength and other macromechanical properties of engineering importance.
Fatigue Damage in Thick, Cross-Ply Laminates with a Center Hole
Authors: Wolterman RL , Kennedy JM , Farley GL
Published: March 2009

An experimental investigation was conducted to characterize the response of three composite material systems to long-term cyclic loading. Thick cross-ply laminates of uniwoven AS/4 carbon fabric were produced using a brittle matrix with and without stitching and a tough-matrix material. Quasi-static tension and compression tests were conducted on specimens with and without circular holes to determine strength, modulus, and failing strain. These tests showed that the measured static mechanical properties were insensitive to the type of matrix material because the laminate response was dominated by the 0° fibers. The stitched specimens had significantly lower static compressive strengths.

A series of fatigue tests (tension-tension, compression-compression, and tension-compression) were conducted to study the influence of an open hole on damage and residual strength. These tests showed that the matrix material and stitching influenced the fatigue behavior of the composite. The tough-matrix material developed less damage than the corresponding brittle-matrix specimens and survived one million cycles of fatigue loading at all stress levels. The stitched composite developed less longitudinal damage than the unstitched materials, but it developed more transverse damage. Both the brittle-matrix and the stitched materials failed prior to one million cycles of tension-compression fatigue loading. The residual strength of the materials was affected by the matrix material and the damage state in the specimen.
Local Delamination in Laminates with Angle Ply Matrix Cracks, Part I: Tension Tests and Stress Analysis
Authors: O'Brien TK , Hooper SJ
Published: March 2009

Quasi-static tension tests were conducted on AS4/3501-6 graphite epoxy (02/θ2/-θ2)s laminates, where θ was 15, 20, 25, or 30°. Dye penetrant-enhanced X-radiography was used to document the onset of matrix cracking in the central - θ° plies and the onset of local delaminations in the θ/-θ interface at the intersection of the matrix cracks and the free edge. Edge micrographs taken after the onset of damage were used to verify the location of the matrix cracks and local delaminations through the laminate thickness.

A quasi-3D finite element analysis was conducted to calculate the stresses responsible for matrix cracking in the off-axis plies. Laminate plate theory indicated that the transverse normal stresses were compressive. However, the finite element analysis yielded tensile transverse normal stresses near the free edge. Matrix cracks formed in the off-axis plies near the free edge where in-plane transverse stresses were tensile and had their greatest magnitude. The influence of the matrix crack on interlaminar stresses is also discussed.
Analysis of Thermomechanical Fatigue of Unidirectional Titanium Metal Matrix Composites
Authors: Mirdamadi M , Johnson WS , Bahei-El-Din YA , Castelli MG
Published: March 2009

Thermomechanical fatigue (TMF) data have been generated for a Ti-15V-3Cr-3Al-3Sn (Ti-15-3) material reinforced with SCS-6 silicon carbide fibers for both in-phase and out-of-phase testing. Significant differences in failure mechanisms and fatigue life are noted for the in-phase and out-of-phase testing. The purpose of the research reported in this paper is to apply a micromechanics model to analysis of the data. The analysis predicts the stresses in the fiber and in the matrix material during the thermal and mechanical cycling by calculating both the thermal and mechanical stresses and their time-dependent behavior. The rate-dependent behavior of the matrix was characterized and was used to calculate the constituent stresses in the composite. The predicted 0° fiber stress range was used to explain the composite failure. It was found that for a given condition, temperature, loading frequency, and time at temperature, the 0° fiber stress range may control the fatigue life of the unidirectional composite.
High-Temperature Tension-Compression Fatigue Behavior of a Unidirectional Tungsten Copper Composite
Authors: Verrilli MJ , Gabb TP
Published: March 2009

The high-temperature fatigue behavior of a [0]12 tungsten fiber-reinforced copper matrix composite was investigated. Specimens having fiber volume percentages of 10 and 36 were fatigued under fully reversed, strain-controlled conditions at both 260 and 560°C. The fatigue life was found to be independent of fiber volume fraction because fatigue damage preferentially occurred in the matrix. Also, the composite fatigue lives were shorter at 560°C as compared to 260°C, due to changes in mode of matrix failure. On a total strain basis, the fatigue life of the composite at 560°C was the same as the life of unreinforced copper, indicating that the presence of the fibers did not degrade the fatigue resistance of the copper matrix in this composite system. Comparison of strain-controlled fatigue data to previously generated load-controlled data revealed that the strain-controlled fatigue lives were longer because of mean strain and mean stress effects.
Radiation Embrittlement of Spanish Nuclear Reactor Pressure Vessel Steels
Authors: Bros J , Ballesteros A , López A
Published: March 2009

Commercial pressurized water reactor (PWR) and boiling water reactor (BWR) nuclear power plants contain a series of pressure vessel steel surveillance capsules as the principal means of monitoring radiation effects on the pressure vessel. Changes in fracture toughness are more severe in surveillance capsules than in reactor vessel materials because of their proximity of the reactor core. Therefore, it is possible to predict changes in fracture toughness of the reactor vessel materials.

This paper describes the status of the reactor vessel surveillance programs relating to Spanish nuclear power plants. To date, twelve capsules have been removed and analyzed from seven of the nine Spanish reactors in operation. The results obtained from the analysis of these capsules are compared with the predictions of the Nuclear Regulatory Commission (NRC) Regulatory Guide 1.99, Rev. 2, by means of measured and expected increase of the nil-ductility transition reference temperature (RTNDT). The comparison is made considering the different variables normally included in the studies of radiation response of reactor pressure vessel materials, such as copper content of steel, level of neutron fluence above 1 MeV, base metal or weld metal, and so forth. The surveillance data have been used for determining the adjusted reference temperatures and upper shelf energies at any time. The results have shown that the seven pressure vessels are in excellent condition to continue operating with safety against brittle fracture beyond the design life, without the need to recuperate the degraded properties of the materials by annealing of the vessel.
Surveillance Data on Neutron Embrittlement of Modern Low-Sensitivity Steel
Authors: Najžer M , Glumac B , Loose A , Remec I , Trandafir A
Published: March 2009

The reactor pressure vessel of the Krško Nuclear Power Plant, a two-loop 632-MWe pressurized water reactor, is made of SA 533 grade B class 1 steel plate welded by wire type B-4 Linde 0091. Copper content of both metals is low. Results of impact testing of reactor pressure vessel specimens from the first two withdrawn surveillance capsules are reported. Neutron exposure of the capsules was near 1 and 3 ⋅ 1019 ncm-2 E > 1 MeV, respectively. The exposure rate was around 1.5 ⋅ 1011 n s-1 cm-2 E > 1 MeV. Measured shifts in nil ductility temperature and impact upper shelf energy are compared with predictions of Nuclear Regulatory Commission (NRC) Regulatory Guide RG 1.99 Rev. 2. Nil ductility temperature shifts are in good agreement for base metal, whereas they are underpredicted for weld. Small or negligible upper shelf energy shifts were observed. Instrumented impact testing data are analyzed. Results are compared with a recent analysis of instrumented data from the power reactor embrittlement database.
A Utility Perspective on Management of Reactor Pressure Vessel Embrittlement
Authors: Perrin JS
Published: March 2009

A utility perspective on evaluating and managing radiation embrittlement of pressure vessels is presented in this paper. Included are observations and experiences gained from pressure vessel surveillance capsule programs, and activities on flux reduction through the use of low-leakage core-loading patterns. Also included are pressure vessel embrittlement considerations relating to plant rerating and plant life extension.
An Overview of Radiation Embrittlement Modeling for Reactor Vessel Steels
Authors: Pavinich WA , Griesbach TJ , Server WL
Published: March 2009

The process by which reactor pressure vessel steels respond to neutron radiation exposure is very complex. Not only are the nuclear vessel materials varied in chemistry and processing history, but the embrittlement accumulation process is a function of many different variables, some of which we can only guess at their contributions. The nuclear industry has sponsored many theoretical studies, mechanical properties test programs, and microstructural examinations to understand better the embrittlement process. Mechanistic models have been developed by several people leading to a fairly sophisticated understanding of radiation embrittlement, but a comprehensive tool for direct prediction of transition temperature shift and upper shelf toughness reduction is still lacking. This paper reviews previous work in mechanistic modeling and identifies areas where more research activities are needed. A plan to improve the predictive capabilities of mechanistic models is discussed, which centers on real vessel materials. The goal of such models is to understand the interactions of microstructure, processing history, chemical composition (for example, copper, nickel, phosphorus), radiation temperature, time at temperature, possible thermal annealing response, neutron flux, flux spectrum, and flux attenuation through the vessel wall.
B&W Owners Group Master Integrated Reactor Vessel Surveillance Program
Authors: Fyfitch S , Gross LB , Lowe AL , Moore KE , Yoon KK
Published: March 2009

A Master Integrated Reactor Vessel Surveillance Program (MIRVP) was designed to provide the data required to ensure the continued licensability of 17 reactor vessels including 8 Babcock & Wilcox (B&W)-designed 177-fuel assembly (FA) plants and 9 Westinghouse-designed plants with B&W-fabricated reactor vessels. This program is an extension of the integrated surveillance program developed for the B&W 177-FA plants. The program combines 17 separate surveillance programs with supplementary weld metal capsules and high fluence weld metal capsules designed to meet the needs for license renewals. In addition, capsules are included to address the requirements of reactor vessel annealing and to study the fluence spectrum effects of the two plant designs. Design details are provided for the new high fluence surveillance capsules.
Surveillance Dosimetry of the French 900-MW Pressurized Water Reactors (PWRs): Results, Uncertainties, and Reactor Series Effects
Authors: Lloret R
Published: March 2009

Experimental neutron characterization (or dosimetry) of surveillance capsule irradiation is a main step in the surveillance program of irradiation effects on pressurized water reactor (PWR) vessels. Recently in France, results were obtained with 50 capsules removed from 29 900-MW (electric) reactors and analyzed using a standard method implementing, among others, the TRIPOLI Code to determine the perturbations caused by structural parts (stiffeners) and the neutron spectra in the capsules with their variance-covariance matrices.

The method is briefly explained, emphasizing some choices that were made concerning the uncertainty evaluations. Then the results of neutron fluxes and fluences are discussed, and the mean measured value for each reactor series is compared with the computed one.

In France, PWRs in operation are highly standardized. There are only 2 models of 900- MW reactors, called “CPO” and “CPY.” The 50 capsules examined were taken from these 2 series of reactors and can be arranged into 6 classes only. Some classes have as many as 18 elements, as, for example the “U” capsule class, irradiated at a 20° position in the “CPY” series. In each class, the observed standard deviation and estimated uncertainty are discussed. Because some classes have a strong statistical weight, the averaged results are highly significant.

These reactors are constructed and operated very close to a standard model in power, flux, and dosimetry measurements. Therefore, results are weakly scattered. They confirm the calculations and indicate that the uncertainty we find for capsule fluence is reasonable. The best is only 6.1% (2 σ). Knowledge of the vessel fluence (and of its uncertainty, which is being evaluated) is an essential factor for plant life anticipation.
Radiation Effects on the Mechanical Properties of SA 508 Cl.3 Forging
Authors: Milella PP , Pini A , Marschall CW
Published: March 2009

This paper presents preliminary results of an ongoing research program that the Italian Committee for Research and Development of Nuclear Energy and Alternative Energies (ENEA) started in the framework of the previous Nuclear Energy Plan.

A total of 25 tension tests, 162 Charpy tests, 56 KId tests, 30 KIc (or JIc) tests, and 17 crack-arrest tests have been performed on unirradiated and irradiated materials. Results are reported in this paper.

Additional tests are planned for irradiated specimens, including 19 tensile tests. 20 KId tests, 50 KIc tests, and 9 crackarrest tests.

This paper also reports on comparisons among transition-temperature shifts measured and expected on the basis of some of the best-known empirical prediction equations.
Interactions of Defects with Dislocations in Reactor Pressure Vessel Steels
Authors: Munier A , Schaller R , Mercier O , Waeber WB
Published: March 2009

The radiation embrittlement of reactor pressure vessel steels may be partially explained by the creation of irradiation-induced defects that interact with the moving dislocations during plastic deformation. This research studies the nature of the interaction mechanisms and the microstructural properties and the mobility of the dislocations.

Internal friction measurements, torsional plastic deformation tests, and transmission electron microscopy observations have been performed in ferritic steels used for reactor pressure vessels, more precisely the JRQ (Japanese Reference Quality) steel (International Atomic Energy Agency [IAEA] correlation monitoring material). The influence of neutron irradiation and of annealing treatments is discussed.
Influence of Neutron Irradiation on the Microstructure and Mechanical Properties of 15Kh2MFA Steel
Authors: Páv T , Kočík J , Keilová E
Published: March 2009

The mechanical properties and microstructural changes in 15Kh2MFA steel after neutron irradiation of the base metal and the weld metal have been studied. The surveillance specimens of a nuclear power plant reactor with a low copper (Cu) content (Cu < 0.08 wt% for the base metal, Cu < 0.06 wt% for the weld metal) were used for the investigation. The characteristic radiation-induced defects were found by transmission electron microscopy (TEM) starting from a neutron fluence of 4 × 1023 n/m2 (E > 1 MeV). “Black dots” concentrating to dislocation lines coexist with small resolvable dislocation loops at higher neutron fluences. Refinement of the vanadium carbide (VC) particles has been demonstrated also in the ferritic grains of the weld metal.

The mechanical properties of both the base metal and the weld metal result in a high radiation stability. The observed microstructural changes correspond to the measured values of the radiation embrittlement and strengthening, but the increase in the flow stress cannot be explained on the basis of the visible radiation damage alone.
Recovery of the Transition Temperature of Irradiated WWER-440 Vessel Metal by Annealing
Authors: Amayev AD , Kryukov AM , Sokolov MA
Published: March 2009

The main results obtained from studying recovery of irradiation embrittlement of reactor vessel steels are presented. Investigations were carried out using WWER-440 vessel materials (base metal, weld metal). The irradiation temperature was 270°C, which corresponded to the operation temperature of the vessel wall. The effect of the annealing temperature and annealing time, neutron fluence, and phosphorus and copper impurity contents on recovery of the ductile-to-brittle transition temperature was studied. Recovery of the transition temperature depends mainly on the annealing temperature. At a phosphorus content exceeding 0.02%, recovery of properties in annealing is complicated. The effect of copper at contents lower than 0.2% has not been observed. At an annealing temperature of 420°C, residual postannealing embrittlement does not depend on neutron fluence. The results obtained in irradiation-annealing-reirradiations are also presented. After annealing, the tendency of the steel to irradiation embrittlement is not increased over that in its initial state.
Sensitivity Analysis of Thermal and Stress Fields in the Core Belt of a Pressurized Water Reactor
Authors: Glumac B , Najžer M
Published: March 2009

Reactor pressure vessel (RPV) steel undergoes significant changes in brittle to ductile transition temperature shift ΔRTNDT when irradiated with fast neutrons leaking from the pressurized water reactor (PWR) core. This means that, according to American Society of Mechanical Engineers (ASME) III-G Code, pressure-temperature limits have to be recalculated periodically. Heatup-cooldown limits require intensive calculations of time-dependent thermal and stress fields, brittle to ductile transition temperature RTNDTdetermination, and so forth, and often with highly scattered material properties data. The error induced by those scattered data is then compensated for with introduction of additional safety factors specified by the ASME code.

By using sensitivity analysis and performing only moderately complicated calculations, one can not only calculate certain quantities, but also their variance-covariance matrixes, based on a known (either measured or modeled) variance-covariance matrix of input parameters (i.e., material data).

This contribution deals with an approach to sensitivity analysis of the calculated timedependent temperature [{Ti(t)}, < δTi(t)∣δTi(t) >] and both hoop and thermal stress profiles [{σi(M)(t)}, < δσi(M)(t)∣δσi(M)(t) >] and [{σi(T)(t) < δσi(T)t)∣δσj(T)(t) >] in the core belt of the pressure vessel wall. The contribution only briefly outlines what has to be done to calculate stress intensity factors together with their variance-covariance information, and, on the other hand, determine adjusted RTNDT and reference stress intensity factor KIR together with their variances.
Critical Plane Approaches for Multiaxial Fatigue Damage Assessment
Authors: Socie D
Published: March 2009

This paper reviews the evolution of the critical plane damage models and traces their origins from the early work such as that of Guest. Physical justification in the form of detailed observations of crack nucleation and early growth are provided for the models. A common feature of all successful models is that they consider both cyclic stresses and strains. Material-dependent failure models are needed to account for the differences in crack nucleation and early growth. Shear strain-based models are appropriate for materials that have substantial Mode II growth. Tensile strain-based models are needed for materials that have predominantly Mode I growth. Problems and inconsistencies in interpreting the damage models for variable amplitude nonproportional loading are discussed. Critical experiments for evaluating and discriminating between proposed damage models are suggested.
A Multiaxial Fatigue Criterion Including Mean-Stress Effect
Authors: Ellyin F , Kujawski D
Published: March 2009

An energy-based criterion for multiaxial fatigue which includes the influence of mean stress in an explicit form is presented. The criterion is applicable to fatigue lives up to initiation of a small size crack and could be classified as a criterion for the initiation of macroscopic cracks. The appropriate fatigue damage variable is defined as a function of the mechanical input of the distortion energy per reversal of loading, the level of tensile mean stress, and the nature of the imposed constraint. A fatigue failure criterion is obtained by plotting the damage variable versus number of reversals to failure to produce a “master” life curve. The effect of mean stress on fatigue life is described in a consistent manner for both high- and low-cycle fatigue. The predictions of the proposed criterion are compared with experimental data, and the agreement is found to be good.
A Method Based on Virtual Strain-Energy Parameters for Multiaxial Fatigue Life Prediction
Authors: Liu KC
Published: March 2009

A new method is proposed for multiaxial fatigue life prediction using correlation parameters based on virtual strain energy as a measure of fatigue damage on critical planes of fracture. The virtual strain-energy parameters are physically associated with two different modes of fatigue fracture planes. The critical plane leading to Mode I fracture is driven by the principal stress and strain, and the other, leading to Mode II fracture, is driven by the maximum shear stress and strain. The mode of crack initiation and propagation depends on material, temperature, strain range, and stress and strain histories, but not on the relative magnitude of the virtual strain-energy parameters. Biaxial fatigue data obtained from the literature were analyzed for Type 304 stainless steel tested at room and elevated temperatures and for SAE 1045 steel tested at room temperature under in-phase and 90° out-of-phase loading conditions. Comparisons are made between experimental data and theoretical predictions to show the effectiveness of the proposed method.
A Proposed Model for Biaxial Fatigue Analysis Using the Triaxiality Factor Concept
Authors: Zamrik SY , Mirdamadi M , Davis DC
Published: March 2009

A biaxial fatigue model for axial-torsion strain cycling is proposed which includes elastic and plastic strain life regimes at elevated temperatures. The biaxial fatigue model is based on the concept that the transition from the plastic strain region to the elastic strain region on the bilinear fatigue life curve occurs at a cycle at which the elastic and plastic strain components of the total applied strain are equal. This transition corresponds to the intersection of the elastic and plastic strain lifelines of either a uniaxial or a torsional strain cycling condition. The transition point can combine the fatigue strength and fracture ductility coefficients of the material. The von Mises yield criterion was modified by using the Davis and Connelly triaxiality factor (TF) to account for the effects of the stress state and strain rate on the fracture ductility at elevated temperatures, which decreases as TF increases. Experimental data by the authors as well other available data were analyzed on the basis of the proposed model.
An Incremental Life Prediction Law for Multiaxial Creep-Fatigue Interaction and Thermomechanical Loading
Authors: Yeh N-M , Krempl E
Published: March 2009

An incremental multiaxial life prediction law (IMLP) is proposed which consists of the three-dimensional thermoviscoplasticity theory based on overstress (TVBO) combined with a multiaxial damage accumulation law (MDA) to compute the life-time or cycles-to-crack initiation. Crack growth is not considered in this paper but is needed to ascertain the useful life of a component. The method is intended for application to high-temperature low-cycle fatigue with and without hold times and for triangular and trapezoidal waveforms when creep-fatigue interaction takes place.

The deformation behavior is determined by solving the coupled differential equations of TVBO for the strain variation of interest, i.e., continuous cycling, hold times, or fast/slow or slow/fast loading. Only the cyclic neutral version of TVBO is used here, although cyclic hardening and recovery of state formulations are available.

The incremental damage accumulation law consists of a fatigue and a creep damage rate equation. When the sum of creep and fatigue damage reaches 1, crack initiation is said to occur. The damage accumulation equations assume that the combined actions of stress and inelastic strain rate contribute to damage and that damage evolution does not influence the constitutive equation. Fatigue damage always accumulates, but a negative creep damage rate is possible to allow for healing (creep damage is, however, always positive). In accordance with scarce experimental evidence, the maximum inelastic shear strain rate, a hydrostatic pressure-modified effective stress, as well as a parameter which depends on the multiaxiality of loading are used in each damage rate equation. The multiaxiality loading parameter depends on maximum inelastic shear strain rate for fatigue damage, while it is a function of maximum principal stress for creep damage.

All material constants for TVBO and MDA are determined from isothermal tests on Type 304 Stainless Steel (SS) at 538°C using data of Zamrik [1], Blass and Zamrik [2], and Blass [3]. The damage accumulation law correlates fatigue life under biaxial (tension-torsion) cycling with and without hold times. Only the results of one biaxial test series were available to compare the generally favorable predictions (correlations) with experiments.
Macro-Micro Approach in High-Cycle Multiaxial Fatigue
Authors: Dang-Van K
Published: March 2009

An original methodology for fatigue resistance of mechanical structure is presented. Our approach relies on a “macro-micro” analysis. Here micro stresses in misoriented elements are evaluated when the macroscopic loading path is known. Multiaxial fatigue criteria are then proposed. Some industrial applications are shown.
Multiaxial Low-Cycle Fatigue Evaluations of Pressure Vessel Components
Authors: Chattopadhyay S
Published: March 2009

In this paper analytical methods to evaluate multiaxial low-cycle fatigue lives for pressure vessel components have been appraised and the sources of conservatism highlighted. For this purpose, a sample design problem involving a pressure vessel component fabricated from a coded structural alloy subjected to a specified set of loading conditions has been evaluated for fatigue. The local strain approach has been used as a basis for assessing two methods of fatigue evaluation in the low-cycle fatigue regime: (1) the ASME Code, and (2) the Japanese MITI Code. A possible extension of the code procedures to include hydrostatic effects has been recommended.
Multiaxial Fatigue and Life Prediction of Composite Hip Prostheses
Authors: Liao K , Reifsnider KL
Published: March 2009

The fatigue behavior of hip prostheses made from laminated fiber-reinforced polymeric composite material under biaxial loading was investigated. Two types of prototype specimens with different layups were cyclically tested at several different load levels. X-ray radiography and surface replication were used to record fatigue damage development in the specimens tested. Internal damage features were also examined after sectioning of selected specimens. Damage mechanisms in the composite prostheses are discussed in association with results of the nondestructive and destructive tests. A life prediction model that is capable of predicting the residual strength and life of a composite prosthesis was developed based on a critical-element model for composite laminates. An example is provided to illustrate the application of the life prediction model.
Through-Hardening or Case-Hardening for Tapered Roller Bearings?
Authors: Hengerer F , Brockmüller U , Sörström PO
Published: March 2009

Based on the know-how gained from many years of experience in the manufacturing and application of both through-hardened and case-hardened (carburized) tapered roller bearings, a tailor-made selection of material and heat treatment for each application can be offered.

In most applications, endurance life or wear resistance, or both, are the decisive parameters. Through-hardened and case-hardened tapered roller bearings perform equally well in such applications, provided the requirements on hardness, microstructure, and surface integrity are fulfilled.

If high-dimensional stability is required, through-hardening is advantageous particularly in the austempered modification.

For applications requiring high-structural strength in particular with heavy shock loads, carburizing is preferable, if the ratio of case depth and the relevant cross-section is less than about 20%.

In applications with extreme hoop stresses, austempering or case hardening are heat treatment methods to be considered.
Residual Stresses and Material Behavior of M50NiL and RBD
Authors: Böhmer H-J
Published: March 2009

Nowadays requirements for bearings in aircraft gas turbines are a high challenge for the material. Increased temperature, starved lubrication, high rotational speed with high hoop stresses, and tight shaft fit result in a complex loading condition.

To bear and resist these requirements the new caburizing bearing steel M50NiL was developed. Material and part properties of M50NiL are presented and compared with other hightemperature bearing steels. Hints are given according to the correct design in case depth and core toughness for aerospace bearings.

Particular attention is given to the role of residual stresses, especially in carburized bearing parts. Material properties and production parameters that affect the development of the desired compressive residual stresses at the near surface region are discussed.
Finite Element Modeling of Carburizing for Alloy Steels
Authors: Dupen BM , Morral JE , Law CC
Published: March 2009

A finite element program has been developed to predict the distribution of carbon in carburized gears and bearings made from alloy steels. The finite element method was used because complicated shapes could be considered and because the same computer models could be applied to future studies on the formation of residual stresses during heat treatment.

The program was developed using COSMOS/M, a commercial finite element code, on a Sun 3/60 workstation. The program considers the variation of carbon diffusivity with concentration and it accounts for carbide precipitation in a unique way. In addition, it can be applied to both conventional and vacuum carburizing.

The program was tested by comparing its predictions with experimental measurements. The experiments involved carburizing an M50NiL specimen and measuring profiles of carbon concentration versus depth by electron microprobe. Profiles of total carbon (i.e., matrix + carbide) were obtained below both flat and curved surfaces, while below a flat surface a profile of matrix carbon was measured as well. The computer predictions involved first developing a one-dimensional model. The model was based on physical property data and two parameters that were adjusted to fit the profile measured below the flat surface. Then a two-dimensional model was developed using the same parameters, and compared with experimental profiles. The results illustrate the accuracy of the model at internal and external corners.
High-Hardness, High-Density Powder Metal Bearing Applications
Authors: Chidester AJ , Green WB , Corbo K
Published: March 2009

A unique, assembled camshaft, where the lobes are made from high-density powder metal 4680 steel, has been developed for automotive applications. The development was motivated by weight reduction, reduced costs, and the increased stress levels created by the use of tappet roller followers. We will present rolling contact fatigue test data on the lobe material using a ball-on-rod tester and a Unisteel-type flat washer tester. Density levels from 7.6 g/cc to 7.8 g/cc were tested in this program. Other properties such as yield strength, tensile strength, elongation, fracture toughness, fatigue crack growth rate, and rotating beam fatigue life of high-density 4680 powder metal hardened and tempered to HRC 58-62 will be presented. Other powder metal bearing applications requiring high hardness and density, such as slider blocks for automotive air conditioners and square linear bearings for automotive constant velocity joints, will be discussed.
A Comparison of Spindle Ball Bearings with Steel or Ceramic Balls for Very High Speed Applications
Authors: Buchner K
Published: March 2009

The object of the study was to replace oil mist lubrication which involves high physical complexity in the case of spindle ball bearings for very high speeds by grease lubrication. This has been achieved by the use of balls made of the ceramic material silicon nitride in conjunction with a tribologically matched grease grade.

On the one hand, the low density of this material compared with that of steel distinctly reduces the centrifugal force acting on the balls while, on the other hand, its high modulus of elasticity increases contact rigidity and Hertzian contact pressure. Moreover, there is a smaller contact ellipse which as a favorable effect on such tribological quantities as rolling and sliding friction, lubricant stressing, and starvation.

The high material hardness, which is of the order of DPH 1600, distinctly reduces wear on solid contact under conditions of short-time defective lubrication.

The experiments carried out in the spindle ball bearings test rigs at different speeds and for different lengths of time revealed the advantages of the ceramic balls in the temperature and wear behavior.
Cryogenic Turbopump Bearing Material Development Program
Authors: Spitzer RF , Chin HA , Haluck DA
Published: March 2009

Cryogenic rolling element bearings operate in a very hostile environment. Materials used in this environment require a unique combination of characteristics. AISI 440C is the normally selected alloy because of its high chromium content (17%) and resultant corrosion resistance. However, AISI 440C has very poor cryogenic fracture toughness (KIc = 11 MPa√m (10 ksi √in.) at - 267°C (- 450°F) and is prone to stress corrosion cracking (SCC). Alternate materials like AISI 9310 or M50NiL can provide the requisite cryogenic fracture toughness (KIc = 27.5 MPa√m (25 ksi √in.) at - 267°C (-450°F) and SCC resistance, but neither provide general corrosion resistance due to their low chromium content.

In 1988, MRC Bearings and Pratt and Whitney (P&W) initiated a combined research and development program aimed at developing a cryogenic corrosion resistant, fracture tough bearing steel. The approach was to case harden by carburization a fracture tough class 3 super 12 stainless steel. This paper discusses the successful carburization process developed by MRC and P&W on an experimental carburizing stainless steel bearing alloy developed by Carpenter Technology, designated EX98. Finally, data derived on the resultant material properties will also be discussed.
Evaluation of the Continuous Casting Method for Bearing Steel Production—SKF Experience
Authors: Hengerer F , Beswick J , Kerrigan A
Published: March 2009

It has been the ambition of numerous steel producers to gain acceptance for the continuous casting method of bearing steel production. The motivation for this is obvious in the light of the general developments in economical steelmaking processes.

At one time the concept of applying the continuous casting method to bearing steel production was seriously questioned but currently many thousands of tons per year of continuously cast AISI 52100 grade bearing steel are being used worldwide with good results. Some of the metallographic parameters associated with continuous casting of high-carbon steel have been the subject of detailed research, in particular macro and micro inclusions and carbide segregation. The results of these studies are described in respect to defining the microstructure-mechanical property relationships for AISI 52100 steel produced by the continuous casting route.

The continuous casting process requirements, including secondary steelmaking metallurgy, for bearing steel production have been evaluated, as well as examination of the quality systems. Before acceptance was given to a particular continuous casting method, laboratory studies were undertaken followed by sample and pilot deliveries in the bearing manufacturing units.

In this paper, details of the general processing requirements for AISI 52100 steel produced by the continuous casting route, conforming to the SKF bearing steel quality standard, are described and data given on the quality of pilot and long-term deliveries of continuous cast AISI 52100 steel.
A Perspective on Gaseous Impact Tests: Oxygen Compatibility Testing on a Budget
Authors: Werley BL
Published: March 2009

Gaseous impact testing has been accomplished in assorted ways dating back to at least the 1950s. ASTM Committee G-4 grappled with disparate views and melded them into ASTM G 74 in 1982. Criticized for being both too sensitive and too insensitive, recent data has unfortunately also led to calls for the test's abandonment. A historical review of the test is presented, speculation on desirable elements in an improved G 74 test are presented, and several arguments for preserving the test are presented. An attempt to analyze the test dynamic is offered. The principal virtue of the test is argued to be its potential simplicity and low cost implementation which may enable compatibility testing by smaller laboratories previously forced to rely on the data of others. A possibly unique ability to study aging effects in polymers is also cited. The greatest need in restructuring the test is argued to be the optimization of geometry to allow data to not only rank materials but to reflect worst-case real-world exposures and perhaps allow inference about materials autoignition temperatures.
A Test Method for Measuring the Minimum Oxygen Concentration to Support an Intraluminal Flame
Authors: Sidebotham GW , Cross JA , Wolf GL
Published: March 2009

A test method for measuring the minimum concentration to support an intraluminal flame is described. An oxidizer is flowed through a plastic tube, and an ignition source is applied to the free end of the tube. If the oxygen concentration is sufficient, a flame propagates along the inner surface of the tube, termed an intraluminal flame. The minimum concentration of oxygen, or limiting oxygen index (LOI), that will support such a flame is determined. Results are reported for polyvinyl chloride tubing and compared to those using the standard ASTM D 2863, the candle-type flammability test. In the intraluminal test, the LOI with helium as the diluent gas is lower than that with nitrogen as the diluent, in contrast to results using ASTM D 2863. The effects of buoyancy are reduced in the intraluminal test, and the results may therefore be more applicable to low gravity environments.
An Assessment of the Flammability Hazard of Several Corrosion Resistant Metal Alloys
Authors: Bryan CJ , Stoltzfus JM , Gunaji MV
Published: March 2009

The Space Shuttle launch environment at the John F. Kennedy Space Center is extremely corrosive. Recent corrosion studies have identified Hastelloy B-2, Haynes Alloy C-230, Hastelloy C-22, Hastelloy C-276, Hastelloy G-30, and Inconel 625 as being very resistant to this launch environment. Since several corrosion related problems have been encountered in the launch support oxygen systems, especially the expansion bellows in the liquid oxygen vacuum jacketed lines, it was decided to evaluate the flaramability of these alloys in oxygen. Therefore, the NASA White Sands Test Facility was requested to evaluate the ignition susceptibility of these alloys using particle impact, frictional heating, and promoted combustion tests. These tests indicated that the corrosion resistant alloys were more ignition resistant than the materials in the existing systems. Hastelloy C-22 expansion bellows installed in the Orbiter Fuel Cell Servicing System have performed satisfactorily since installation in 1989.