bluuebunny/crossref_metadata_2025 huggingface dataset:
Fatigue Prediction Based on Computational Fracture Mechanics
Authors: Chang AT , Nelson NW , Cordes JA , Kim Y-J
Published: March 2009

A general numerical method has been developed to predict fatigue crack initiation and fatigue crack propagation under cyclic loads. Finite element technique is employed to model specimen geometry. The method requires uniaxial stress/strain material data only; empirical fatigue constants are not required. It determines the number of load cycles for crack initiation or for crack propagation based on fracture energy, W, and dissipated energy per cycle, E. Fracture energy is the plastic-strain energy contained in a Dugdale-type cohesive stress zone located near an initial defect. The size of the cohesive zone of a specimen is established by a computational procedure that simulates elastic-plastic fracture under static loading. Dissipated energy per cycle is the plastic-strain energy in a hysteresis loop that exhibits cyclically stable behavior after “shake-down.” The number of cycles to initiate a crack or to propagate a crack to a finite amount is obtained by dividing the fracture energy, W, by the dissipated energy per cycle, E.

Verification examples showing stress amplitude versus number of cycles (S-N diagram) are compared with experimental results. Also, experimental crack growth rate as a function of stress intensity factor (Paris-type equation) is compared with calculated results based on this method. Fatigue constants based on experimental data were not required in the computations. Limited comparison of predicted cycles to experimental results indicates good correlation. This work is significant in that it suggests a new and improved approach to problems of fatigue. Since fatigue behavior can be predicted using only uniaxial material tensile data, it holds out the promise of a reduced need for experimental work in the future.
A Crack-Closure Model for the Fatigue Behavior of Notched Components
Authors: Hou C-Y , Lawrence FV
Published: March 2009

Newman's Dugdale strip-yield model for crack closure was used to estimate the crack-closure behavior of cracks emanating from notches. The estimated effective stress-intensity ratio U(a) = ΔKeff(a)/ΔK(a) obtained using Newman's model did not exhibit the frequently observed “dip” presumably caused by the notch plasticity. To incorporate the effects of notch plasticity into the Newman model, the plastic stretches caused by the notch were superposed on the plastic stretches caused by the crack tip. A finite element procedure was used to calculate the magnitude of the notch plastic stretches. It was found that the magnitude of the notch plastic stretches was much larger than that of the crack-tip plastic stretches and that the notch plastic stretches were the main source of the plastic wake when the crack tip was within the notch plastic zone. The validity of the proposed modification of Newman's Dugdale strip-yield model was verified by comparing the predicted results with the published experimental data for various notched components. The effects of the notch-plastic zone on U(a) were found to be significant for stress ratio R ⩾ 0 but to be unimportant for R ≤ - 1
Multiaxial Plasticity and Fatigue Life Prediction in Coiled Tubing
Authors: Tipton SM
Published: March 2009

Coiled tubing is being used increasingly in the oil well drilling and servicing industry. Continuous steel tubing of structural dimensions (up to 89 mm or 3.5 in. in diameter) is wound onto a large-diameter reel for repeated deployment into and out of a well bore. The bending strain range associated with each wrap-unwrap cycle can exceed 3% with lives well below 100 cycles. During constant internal pressure fatigue testing, tubing has been observed to grow in diameter by as much as 30%.

This paper describes an analytical model to predict the fatigue behavior of coiled tubing subjected to variable pressure service conditions. The approach utilizes standard low-cycle fatigue data but requires additional experimental results from constant pressure fatigue testing. The algorithm is based on estimates of biaxial ratcheting from an incremental plasticity model using a hybrid associated flow rule, a modified kinematic hardening rule with multiple von Mises yield surfaces, and a specialized limit surface concept. An empirical damage parameter was formulated based on constant pressure fatigue data using mean and fluctuating von Mises equivalent strain components occurring throughout the life of a section of tubing. This parameter is used with the Palmgren-Miner definition of cumulative damage to track damage that is accumulating nonlinearly under constant or variable pressure histories. Modifications to standard incremental plasticity components and implementation assumptions used to apply the model are presented and discussed. The predictive capability of the model is demonstrated relative to data generated under constant and variable pressure histories.
Residual Operating Fatigue Lifetime—Estimation of Distribution Function
Authors: Kliman V , Füleky P , Jelemenská J
Published: March 2009

Engineering structures often work under operating conditions in which the character of a load undergoes permanent change (different service-loading regimes). Such an operation is very frequent for transportation equipment, earthmoving machines, airplanes, etc. Considering that the service-loading process (time history of σ(t) stress or ∈(t) strain) at particular operating regimes exhibits in a majority of cases the stochastic nature, the question arises as to how to estimate the fatigue resistance of the structure under such service conditions. For this reason, a method was developed to evaluate such service from the viewpoint of fatigue reliability and to estimate the resulting fatigue lifetime in terms of probability interpretation.

The result of life calculation is the distribution function (DF) of the residual fatigue lifetime for the given loading regime. This DF reflects the loading history (sequence and nature of the previous loading regimes) and the random character of the interacting material and loading parameters. The DF obtained is in a good agreement with the experimental results. Finally, illustrative examples that reflect practical applications of this theory are presented.
Comparison Between Natural Weathering and Fluorescent UV Exposures: UVA-340 Lamp Test Results
Authors: Fedor GR , Brennan PJ
Published: March 2009

Samples of different polymers were exposed in Florida, Arizona and Ohio for two years. The same samples were exposed in a laboratory weathering chamber using two irradiance levels and two moisture cycles. The artificial and naturally weathered samples were then compared. With the exception of mildew growth, the laboratory exposures with moisture degraded the materials in a similar manner as seen outdoors. for a significant number of the samples, the lab exposures which included a condensation cycle gave better correlation than the cycles that were UV light only. For 9 of the 15 materials tested, the rate of degradation in the artificial weathering chamber was increased by increasing the light intensity level. In addition, the study confirms that acceleration factors are highly material dependent.
Estimation of Durability of GFRP Laminates Under Stress-Corrosive Environments Using Acoustic Emission
Authors: Fujii Y , Ramakrishna S , Hamada H
Published: March 2009

The objective of this investigation was to estimate the creep life of glass fiber reinforced plastic (GFRP) materials subjected to stress-corrosive environments using acoustic emission (AE). The laminates were fabricated using combinations of rigid bisphenolic polyester resin (LP-1), flexible vinylester resin (R806), random fiber mat and woven cloth. The creep tests were conducted in 5% nitric acid (HNO3) environment. The rigid matrix composites displayed higher AE count rate than the flexible matrix composites. For given creep testing conditions, the woven cloth reinforced specimens displayed higher number of AE counts than the random mat reinforced specimens. The creep life decreased with increasing creep stress, whereas the AE count rate increased with increasing creep stress. A linear relationship was found between the creep life and the AE count rate.
Microstructure of Oxide Films Formed during the Waterside Corrosion of the Zircaloy-4 Cladding in Lithiated Environment
Authors: Pêcheur D , Godlewski J , Billot P , Thomazet J
Published: March 2009

Zircaloy-4 cladding materials have been oxidized in a lithiated environment in autoclave and in out-of-pile loop tests. In such oxidation tests, a strong enhancement of the oxidation rate can occur depending on the water chemistry conditions and on the oxidation time. In this work, in order to improve our understanding of the detrimental effect of lithium on the corrosion behavior of the Zircaloy-4 cladding, the microstructure of oxide films has been characterized by TEM. Simultaneously, the lithium profiles and concentrations in the oxide layers have been determined using the SIMS technique, special attention being paid to the metal-oxide interface.

These analyses revealed the existence of a correlation between the oxidation rate, the oxide microstructure, and the lithium profiles in the oxide films. Before the occurrence of the strong acceleration of the oxidation rate, the whole oxide layer is mainly composed of columnar grains and the lithium has no access to the metal-oxide interface. On the contrary, after the occurrence of such a strong accelerated corrosion rate, the inner part of the oxide layer is composed mainly of equiaxed grains (up to the metal-oxide interface), lithium then having access to the metaloxide interface.

These experimental results are described extensively and analyzed, and their contribution to the understanding of the influence of lithium on the oxidation process of Zircaloy-4 cladding material is discussed as a function of an existing thin inner barrier layer at the metal-oxide interface.
PWR Zircaloy Fuel Cladding Corrosion Performance, Mechanisms, and Modeling
Authors: Cheng B , Gilmore PM , Klepfer HH
Published: March 2009

Oxide axial profiles for Zircaloy-4 cladding measured for 17 fuel rods irradiated from 50 to 60 GWD/t in three different pressurized water reactors (PWRs) were studied. Computer simulation of the magnitude and shape of the various profiles involved five contributing interrelated effects: (1) thermal feedback, (2) radial thermal redistribution of corrosion hydrogen, (3) metallurgical variables (tin content and radiation-induced changes in intermetallic particle size distribution), (4) lithium hydroxide exposure history, and (5) radiation effects in the oxide film. Published research results supporting each separate effect are reviewed, and the selection of a predictive simulation for each phenomenon and the effect of varying its relative weight are reported. The effect on cladding corrosion of hydrogen thermal redistribution to the oxide-metal interface, coupled with thermal feedback, was found to be a predominant factor in predicting the magnitude and shape of cladding oxide profiles at high burnups for PWR fuel rods. Tin content, coupled with thermal feedback, was also found to have a very important effect on the predicted magnitude of oxide profiles at all burnups. Radiation-induced dissolution of fine Zr(Cr,Fe)2 intermetallic particles in the Zircaloy-4 cladding primarily affected the predicted shape of the oxide profiles at the colder inlet region of the fuel rods. Lithium hydroxide (LiOH) exposure history and radiation effects in the oxide film appeared to play non-negligible secondary roles in determining the oxide profiles at all burnups.
Investigation of In-Pile Grown Corrosion Films on Zirconium-Based Alloys
Authors: Gebhardt O , Hermann A , Bart G , Blank H , Garzarolli F , Ray ILF
Published: March 2009

In-pile grown corrosion films on different fuel rod claddings (standard Zircaloy-4, extra low tin Zircaloy (ELS), and Zr2.5Nb) have been studied using a variety of experimental techniques. The aim of the investigations was to find out common features and differences between the corrosion layers grown on zirconium alloys having different composition. Methods applied were scanning and transmission electron microscopy (SEM, TEM), electrochemical impedance spectroscopy (EIS), and electrochemical anodization. The morphology and topography of these oxide layers are, in general, of common nature. However, morphological differences have been observed between the specimens that could explain the irradiation enhancement of corrosion of Zircaloy-4. The features of the compact oxide close to the oxide/metal interface have been characterized by electrochemical methods. The relationship between the thickness of this protective oxide and the overall oxide thickness has been investigated by EIS. It was found that this relation is dependent on the location of the oxide along the fuel rod and on the corrosion rate.
The Influence of Temperature and Yield Strength on Delayed Hydride Cracking in Hydrided Zircaloy-2
Authors: Efsing P , Pettersson K
Published: March 2009

To determine if delayed hydride cracking (DHC) can be the cause of the long axial cracks occasionally found in BWR fuel cladding, a systematic study of DHC in Zircaloy cladding has begun. In the initial stage of the project, a test technique was developed and applied to unirradiated samples of Zircaloy. The present study includes an investigation of the influence of the yield strength and temperature on the crack growth rate and the threshold stress intensity that must be exceeded before cracking begins.

Recrystallized (RXA) Zircaloy-2 has been compared to stress relief annealed (SRA) Zircaloy-2 with similar texture and composition. The results show that the crack propagation rate increases with increasing yield strength at similar stress intensity levels by as much as a decade when the yield strength is tripled. The maximum crack propagation rate measured in this study is ∼6 × 10-7 m/s. The threshold stress intensity, KIH, was found to decrease with increasing yield stress. The measured threshold values are in the range of 13.5 to 7.5 MPa. These figures are close to theoretically derived values using a critical fracture stress criterion of the hydrides as the limiting factor. The incubation period before cracking begins is found to be longer at 200°C than it is at 300°C.
A Model for Analysis of the Effect of Final Annealing on the In- and Out-of-Reactor Creep Behavior of Zircaloy Cladding
Authors: Limbäck M , Andersson T
Published: March 2009

The creep behavior of Zircaloy cladding materials depends on materials texture, degree of recrystallization, and chemical composition. This study is devoted mainly to the analysis of the effect of the final annealing (i.e., the degree of recrystallization) on the creep characteristics. For this purpose, data from a series of thermal creep tests are presented and evaluated. In addition, the in-reactor creep data presented by Franklin et al. are used to evaluate the effect of irradiation on cladding creep performance. The out-of-reactor tests are performed under internal pressurization, and the test matrix covers seven conditions with temperatures from 330 to 400°C and hoop stresses between 80 and 160 MPa. Three lots of Zircaloy-2 claddings and one lot of Zircaloy-4 are considered. The difference between the three Zircaloy-2 lots is in their final annealing conditions. The claddings are either stress relief annealed (SRA), recrystallization annealed (RXA), or partially recrystallization annealed (PRXA). The materials used when fabricating the Zircaloy-2 claddings are from the same ingot, and the chemical compositions of the three types of claddings are almost identical. The Zircaloy-4 cladding included in the test is SRA, and the tin content in this material is similar to that in the Zircaloy-2 materials. The creep data are analyzed by separating the primary (transient) and the secondary (steady-state) creep. In this analysis, the Matsuo creep model, which accounts for both primary and secondary creep, is modified, calibrated, and verified using the new thermal creep data. Based on in-reactor data, the thermal creep model is extended to cover also the creep behavior under irradiation. The claddings considered in the in-reactor test were of both SRA and RXA types, and the experiments were made under external pressure. It is observed that for moderate hoop stresses (<120 MPa) the measured hoop creep rates, both out-of-reactor and in-reactor, decrease with increasing final annealing temperature (increasing degree of recrystallization). However, for higher stresses, the steady state creep rate has a minimum for partially recrystallized claddings. Moreover, the thermal creep tests show that the creep performance of Zircaloy-2 and Zircaloy-4 are similar when the chemical compositions and the fabrication procedures are similar.
Modeling In-Reactor Deformation of Zr-2.5Nb Pressure Tubes in CANDU Power Reactors
Authors: Christodoulou N , Causey AR , Holt RA , Tomé CN , Badie N , Klassen RJ , Sauvé R , Woo CH
Published: March 2009

Changes in shape of internally pressurized tubes caused by operating temperatures and pressures are enhanced by fast neutron irradiation. Lengths and diameters of Zr-2.5Nb pressure tubes in CANada Deuterium Uranium-Pressurized Heavy Water (CANDU-PHW) power reactors and test reactors have been monitored periodically over the past 20 years. Axial and transverse strain rates have been evaluated in terms of operating variables and the crystallographic texture and anisotropic microstructure of the extruded and cold-drawn tubes. The anisotropic deformation occurring during steady-state irradiation creep and growth is described by a self-consistent model that takes into account the presence of intergranular stresses without building up any discontinuities of strain and stress at the grain boundaries. In this model, it is assumed that climb-assisted glide of dislocations on prismatic, basal, and pyramidal planes is the dominant creep mode and that growth occurs by net fluxes of interstitials and vacancies to a non-random distribution of dislocations and grain boundaries. The predictions from a deformation equation based on data from the Pickering and Point Lepreau Nuclear Generating Stations and the WR1, Osiris, DIDO, and NRU test reactors are in good agreement with measurements of pressure tubes in Bruce units. The equation has been employed as a material subroutine in the 3-D finite element code H3DMAP for predicting the detailed shape change of pressure tubes. The prediction from H3DMAP is a more complete description of shape change than that obtained from the closed-form expression.
In Situ Studies of Phase Transformations in Zirconium Alloys and Compounds Under Irradiation
Authors: Motta AT , Faldowski JA , Howe LM , Okamoto PR
Published: March 2009

The High Voltage Electron Microscope (HVEM)/Tandem facility at Argonne National Laboratory has been used to conduct detailed studies of the phase stability and microstructural evolution in zirconium alloys and compounds under ion and electron irradiation. Detailed kinetic studies of the crystalline-to-amorphous transformation of the intermetallic compounds Zr3(Fe1-x,Nix), Zr(Fe1-x,Crx)2, Zr3Fe, and Zr1.5Nb1.5Fe, both as second phase precipitates and in bulk form, have been performed using the in situ capabilities of the Argonne facility under a variety of irradiation conditions (temperature, dose rate). Results include a verification of a dose rate effect on amorphization and the influence of material variables (stoichiometry x, presence of stacking faults, crystal structure) on the critical temperature and on the critical dose for amorphization.

Studies were also conducted of the microstructural evolution under irradiation of specially tailored binary and ternary model alloys. The stability of the ω-phase in Zr-20%Nb under electron and Ar ion irradiation was investigated as well as the β-phase precipitation in Zr-2.5%Nb under Ar ion irradiation. The ensemble of these results is discussed in terms of theoretical models of amorphization and of irradiation-altered solubility.
Influence of Iron in the Nucleation of ⟨c⟩ Component Dislocation Loops in Irradiated Zircaloy-4
Authors: de Carlan Y , Regnard C , Griffiths M , Gilbon D , Lemaignan C
Published: March 2009

Under irradiation, the acceleration of growth of nuclear reactor components made of Zircaloy is clearly correlated to the presence of ⟨c⟩ component dislocation loops. In the early stages, these loops appear to be essentially located close to the intermetallic precipitates. At higher doses, when ⟨c⟩ component dislocation loops are observed all over the microstructure, analysis of the Zr matrix reveals an homogeneous iron content due to the dissolution of the precipitates. Thus, iron may play a significant role in the nucleation of ⟨c⟩ component dislocation loops.

A specific study has been performed on Zircaloy thin foils implanted with increasing amounts of iron. The conditions of nucleation and growth of these ⟨c⟩ component dislocation loops have been followed in a 1-MeV transmission electron microscope. The dose level for ⟨c⟩ component dislocation loop nucleation is weakly Fe-content dependent. However, their size and density increase with increasing iron implantation. These results are compared to direct observations made on irradiated reactor components. The role of iron on accelerated growth is then discussed.
Effects of Extrusion-Billet Preheating on the Microstructure and Properties of Zr-2.5Nb Pressure Tube Materials
Authors: Choubey R , Aldridge SA , Theaker JR , Cann CD , Coleman CE
Published: March 2009

The effects of extrusion temperature and pre-heat soak time for billets on the mechanical properties of Zr-2.5Nb pressure tubes for CANDU reactors have been examined. The β-quenched billets from a quadruple-melted ingot containing approximately 1200 ppm of oxygen were extruded at 780, 815, and 850°C with pre-heat soak times of 15 to 300 min. The extruded hollows were finished by cold drawing (with a 28% reduction in area) and then stress relieving at 400°C. The α-phase grain structure, tensile strength, and fracture toughness properties were found to vary with the pre-heat temperature and soak time. All the materials were tough because embrittling impurities were absent. The tubes with 780°C preheat had a very fine and uniform α-grain structure, giving high strength and toughness at all soak times. The opposite was true for the 850°C soaks; the grain structure was coarse and inhomogeneous and the materials tended to be less strong and less tough. The tubes with the 815°C soaks showed intermediate values of strength and toughness. These variations in mechanical properties are discussed in terms of α-grain refinement and oxygen enrichment.
Zircaloy-2 Lined Zirconium Barrier Fuel Cladding
Authors: Williams CD , Marlowe MO , Adamson RB , Wisner SB , Rand RA , Armijo JS
Published: March 2009

The introduction of Zr-lined “barrier” fuel clad tubing by GE in the early 1980s to counter the pellet-clad interaction (PCI) failure mechanism in fuel for boiling water reactors provided a major improvement in fuel reliability and operational flexibility. While the frequency of fuel failures has been substantially reduced in the past decade, an increased tendency has been observed for failed fuel rods to exhibit post-failure degradation in the form of longer cracks that allow release of radioactive off-gas and contamination of the reactor coolant circuit with tramp fuel material. One factor involved in this degradation is hydriding of the cladding at a location remote from the initial perforation of the fuel rod. This local hydriding can lead to secondary crack initiation in the cladding when stressed by the fuel expansion accompanying a power increase.

A modification of Zr-lined “barrier” fuel clad tubing has been developed to retard post-failure local hydriding while retaining the proven PCI resistance of the high-purity sponge Zr barrier. By adding a thin inner layer of corrosion-resistant Zircaloy-2 bonded to the inner surface of the Zr-barrier tube, the resistance to internal corrosion and hydrogen generation in a perforated fuel cladding tube is made equivalent to that of an all-Zircaloy-2 tube. Tests show that the PCI mitigating capability of the Zr barrier is not compromised by this inner Zircaloy-2 liner. Materials considerations and manufacturing technology used to integrate this optional inner liner with other Zr barrier tubing properties and performance requirements are discussed with a summary of testing experience.
In-BWR and Out-of-Pile Nodular Corrosion behavior of Zry-2/4 Type Melts with Varying Fe, Cr, and Ni Content and Varying Process History
Authors: Ruhmann H , Manzel R , Sell H-J , Charquet D
Published: March 2009

Zircaloy-based materials with constant tin content, a constant sum of Fe and Cr content, and different Fe/Cr ratios were manufactured from small ingots (6 kg) by forging, β- quenching, hot rolling, and cold rolling with two different annealing sequences resulting in two accumulated annealing parameters (A-parameter A = 0.1 E-18 and A = 10 E-18 h). To study the effect of Ni, chromium was substituted by nickel in one alloy. As a reference material standard, ASTM Zry-2 was manufactured in a similar way.

These materials were examined for precipitate size by TEM and for their out-of-pile corrosion behavior in 400°C steam up to 319 days and in 500°C steam up to 24 h in a static and in a refreshed autoclave. Furthermore, samples made from these alloys were irradiated in a commercial BWR for two years.

By comparing results from different out-of-pile tests to the data from in-pile irradiation, the following conclusions can be drawn: 1. The A-parameter influence with respect to corrosion is the dominant factor for out-of-pile and in-pile corrosion behavior. 2. The (Fe+Ni)/Cr ratio influences the corrosion resistance, especially at low values below about 5. The effect is found to be strong for low A-parameter materials under irradiation of two cycles comparable to the behavior found for the high A-parameter materials in out-of-pile tests. The nodular corrosion test reproduces the in-pile behavior found for the high A-parameter materials. 3. Nickel substituted for Cr reduces the effect of the A-parameter on corrosion. 4. Sizes of intermetallic precipitates depend on chemical composition and structure. Significant numbers of Zr3Fe particles are found only at Fe/Cr ratios above 5. These particles are significantly larger than the Laves phases. 5. The influence of the chemical composition of the precipitates to the corrosion behavior cannot be finally evaluated on the two-cycle irradiated samples. Further information is expected when the samples reach higher in-pile exposure.
A Structural Test Facility for In-Plane Biaxial Testing of Advanced Materials
Authors: Bartolotta PA , Ellis JR , Abdul-Aziz A
Published: March 2009

A benchmark structural testing facility has been established at the NASA Lewis Research Center (NASA/LeRC). The primary function of this facility is to verify structural analysis methods under prototypical loading conditions. Further, the facility allows the structural integrity of advanced monolithic and composite materials to be evaluated in subelement form. The intent is to bridge the gap that frequently exists between coupon level response and full-scale component response by conducting tests on subscale structures that have relatively simple geometries (that is, plates, beams, etc.). Unlike actual components, these subelement structures can be mechanically loaded and heated in a controlled and precise manner leading to reliable data. As a first step in this research, two in-plane biaxial plate test systems were recently installed at NASA/LeRC. This paper describes one of these test systems that consists of a 500-kN load frame with four hydraulic actuators, a state-of-the-art digital controller and innovative control software, an advanced quartz lamp radiant furnace, a large environmental chamber, and an automated in-situ crack measurement system.
Adjustable Work Coil Fixture Facilitating the Use of Induction Heating in Mechanical Testing
Authors: Ellis JR , Bartolotta PA
Published: March 2009

Induction heating has been used in fatigue testing for over 30 years. Typically, a work coil is used in conjunction with a workstation and a radio frequency power supply to provide localized specimen heating. One disadvantage of the approach is that the localized nature of the heating leads to temperature gradients over the specimen's length. However, experience has shown that the configuration of work coils can be optimized to give relatively uniform temperature profiles over the specimen's gage section. The variables of interest include: coil diameter and overall length, number of turns, spacing of turns, and grouping of turns. Optimization of these variables by trial and error can be a lengthy and frustrating process. This is particularly the case in multiaxial experiments involving large tubular specimens and correspondingly large work coils. The subject work coil fixture was designed and developed to facilitate the optimization process. The approach adopted was to subdivide the work coil into three segments. Each segment has its own positioning mechanism that allows independent adjustment in both the radial and vertical senses. It was shown that use of this fixture allows temperature profiles within ±1% of the mean to be achieved with minimum difficulty. Such levels of precision were found to be necessary in a recent series of thermomechanical experiments conducted on Hastelloy X.
Additional Hardening Due to Tension-Torsion Nonproportional Loadings: Influence of the Loading Path Shape
Authors: Calloch S , Marquis D
Published: March 2009

This paper is concerned with the mechanical behavior of Type 316 austenitic stainless steel under cyclic multiaxial loadings and particular attention is paid to the additional hardening due to the loading path shape. A 90° out-of-phase tension-torsion loading is usually considered in most of phenomenological models as the highest hardening cyclic path. An experimental investigation has been carried out in order to check this hypothesis, and it is shown that other loading paths lead to a higher hardening.
An Axial-Torsional, Thermomechanical Fatigue Testing Technique
Authors: Kalluri S , Bonacuse PJ
Published: March 2009

A technique for conducting strain-controlled, thermochemical, axial-torsional fatigue tests on thin-walled tubular specimens was developed. Three waveforms of loading, namely, the axial strain waveform, the engineering shear strain waveform, and the temperature waveform were required in these tests. The phasing relationships between the mechanical strain waveforms and the temperature and axial strain waveforms were used to define a set of four axial-torsional, thermomechanical fatigue (AT-TMF) tests. Real-time test control (three channels) and data acquisition (a minimum of seven channels) were performed with a software program written in C language and executed on a personal computer. The AT-TMF testing technique was used to investigate the axial-torsional thermomechanical fatigue behavior of a cobalt-base superalloy, Haynes 188. The maximum and minimum temperatures selected for the AT-TMF tests were 760 and 316°C, respectively. Details of the testing system, calibration of the dynamic temperature profile of the thin-walled tubular specimen, thermal strain compensation technique, and test control and data acquisition schemes, are reported. The isothermal, axial, torsional, and in- and out-of-phase axial-torsional fatigue behaviors of Haynes 188 at 316 and 760°C were characterized in previous investigations. The cyclic deformation and fatigue behaviors of Haynes 188 in AT-TMF tests are compared to the previously reported isothermal axial-torsional behavior of this superalloy at the maximum and minimum temperatures.
Plane Stress Crack Resistance Curves of an Inclined Crack Under Biaxial Loading
Authors: Donne CD , Döker H
Published: March 2009

Elastic-plastic stable crack growth under Mixed-Mode I and II loading in thin cruciform specimens of aluminum, Al2024-T3, and structural steel, StE 550, was investigated using a biaxial test rig. Two cruciform specimen types were considered, one containing an inclined through thickness crack, the other with inclined short cracks emanating from a hole (precrack length to hole radius ratio, á0/R = 0.286). It is shown that predominant Mode II loading drove the stable crack in the direction almost parallel to the fatigue pre-crack (maximum shear strain criterion). High (steel) or moderate (aluminum) Mode I crack-tip-opening components caused a crack path deviation, that is, the stable crack grew normal to the maximum tensile stress. The mixed-mode crack resistance curves are presented in the form of the magnitude of a crack-tip-displacement vector, δs = (δI2 + δII2)0.5, and compared with conventional R-curves of standard compact-tension (C(T)) and center-cracked-tension (M(T)) specimens. The effect of the load biaxiality is discussed with regard to previous experiments with Mode I loaded cruciform specimens.
The Energy Dissipation Rate—A New Tool to Interpret Geometry and Size Effects
Authors: Kolednik O , Shan G , Fischer DF
Published: March 2009

This presentation is devoted to geometry and size effects on the crack growth resistance of CT specimens. With a simple numerical model, the main influence factors on these effects can be separated. These are the effects of the in-plane and out-of-plane constraint on the deformation behavior and the locally varying fracture toughness properties of the material along the crack front. Crack growth is modeled for different geometries and sizes assuming that the critical values of the crack tip opening displacement, CODi, and the critical crack tip opening angle, CTOAc, remain constant for all geometries and sizes. Crack growth resistance curves are plotted in terms of the J-integral and the energy dissipation rate, D. Two types of materials are considered, a low-strength and a high-strength steel. For smooth specimens having a/W ratios between 0.4 and 0.7, the slopes of the J-Δa curves change due to the varying global out-of-plane constraint. For plane strain conditions, the J-Δa curves remain nearly unaffected by geometry and size (except for the smallest specimens of the high-strength steel). On the contrary, the energy dissipation rate shows a very strong size and geometry effect. It is possible to describe these effects qualitatively and quantitatively. Estimates of D are presented that are valid for bend-type specimens under small-scale yielding and large-scale or general yielding conditions. It is shown that a knowledge of D is essential for instability considerations. The quantitative estimate of D provides a prediction of the onset of unstable fracture. Materials with a low ratio CTOAc/∈y show a strong tendency to instability (∈y is the yield strain). An engineering estimation is given to determine the specimen size for a given material below which instability is impossible. The advantages and disadvantages of the toughness parameters J and D are discussed. It is demonstrated that local (in-plane) constraint parameters, like the Q stress or the local stress triaxiality, are not sufficient to explain the experimentally observed geometry effects.
An Experimentally Verified Finite Element Study of the Stress-Strain Response of Crack Geometries Experiencing Large-Scale Yielding
Authors: Panontin TL , Sheppard SD
Published: March 2009

Large-strain, three-dimensional finite element analyses with incremental plasticity were performed for a variety of crack geometries to study local crack-tip stress-strain fields and associated global fracture parameters under conditions of large-scale yielding. The geometries analyzed include thin, single-edge crack tension, single-edge crack bending, and center-crack tension fracture specimens with varying crack depth (a/W) ratios. Two materials were investigated: a high-hardening, low-strength steel and a moderate-hardening, high-strength steel. Mesh refinement studies were performed to ensure convergence of the finite element predictions. The studies examine the effects of in-plane crack-tip element size, initial crack-tip radius size, and number of through-thickness layers on predicted distributions of crack-tip stress and plastic strain and predicted values of the J-integral and CTOD. In addition, the finite element predictions of specimen behavior were verified experimentally by direct measurements, namely load displacement, load longitudinal strain, and load CTOD, made during and following testing of the fracture specimens.

Representative results of the finite element analyses are presented and compared to previously published data where pertinent. Results from the mesh refinement studies and the verification testing are shown. Predicted trends among the specimens and materials in local distributions of crack-tip plastic strain, triaxiality, and opening stress as well as in global parameters, J-integral and m-factor, are discussed.

The finite element analysis results show that the deformation at the crack tip remains strongly three dimensional, with crack-tip stresses demonstrating near plane strain behavior on the midplane and near plane stress on the free surface, even in relatively thin specimens of finite size. Micromechanical models of ductile fracture based on plastic strain and triaxiality would predict that fracture begins straight ahead of the crack tip on the midplane and that the ductile fracture process occurs relatively close to the crack tip (e.g., within one CTOD). The J-integral was not found to decline monotonically from midplane to free surface but to increase near the free surface in most specimens.
Analysis of Warm Prestress Data
Authors: Macdonald BD , Embley GT , Irizarry-Quinones H , McAfee WJ , McCabe DE , Smith PD , Wuthrich JW
Published: March 2009

Loading a cracked structure at elevated temperature, or warm prestressing (WPS), enhances its fracture resistance at a lower temperature. Five data sets, comprising 119 unclad pressure vessel steel specimens, were combined to derive correlations for WPS-enhanced fracture toughness (KIfrac) in the absence of ductile tearing. New WPS test results for 27 surface-flawed specimens, eight subclad-flawed specimens, and five strain-aged specimens are discussed. KIfrac exceeded non-WPS fracture toughness, KIc, for all experiments. The WPS data showed that no specimens failed while K was decreasing, and that at least an additional 7% additional reloading from the minimum value of applied KI took place prior to final fracture. The data included complete and partial unloading after WPS prior to final fracture. Crack tip three-dimensional elastic-plastic finite element (3DEPFE) analysis was performed to support statistical analysis of the data. Regression models were compared with the Chell WPS model.

Crack tip 3DEPFE analysis indicated that partially unloaded and completely unloaded data should be treated separately, and that the amount of unloading is unimportant for partially unloaded data. The regression models, which use KI at WPS (KIwps) and KIc as independent variables, better represented the WPS benefit than did the more complicated Chell model. An adequate accounting was made for constraint in the WPS experiments. The subclad flaw data support the use of the partial unload regression model provided that some care is taken to represent the effect of intact cladding if present. The effect of strain aging at or below 260°C (500°F) on the WPS benefit was of no consequence for the pressure vessel steels and WPS temperatures used to derive the regression models. The presence of ductile tearing precludes the use of the regression models. The regression model for partial unloading accurately predicted the behavior of full-scale pressure vessel WPS experiments. All but one of the 174 experiments considered lie above the lower 2σ estimate of the regressions. The experiments all supported Type I WPS, i.e., there was no fracture during cooling until reloading occurred. However, the regression equations apply to the reload and are inapplicable to Type I WPS.
How Fatigue Cracks Grow, Interact with Microstructure, and Lose Similitude
Authors: Davidson DL
Published: March 2009

This paper reviews the processes by which fatigue cracks grow and interact with applied load and microstructure. Fatigue crack growth processes are remarkably similar irrespective of microstructure, crack size, or nature of the loading. Large strains at fatigue crack tips applied over repeated cycles severely alter, or homogenize, microstructures, followed by crack advance. Microstructure affects fatigue crack growth kinetics more than growth processes. But, under marginal conditions, fatigue crack growth rates are also affected by microstructural features. Examples are small cracks growing under low stresses or large cracks growing near threshold.

The prediction of safe lifetimes for machine parts, such as gas turbine components, requires that laboratory-generated fatigue crack growth rate data be transferred to field-operating conditions. This transfer depends on the maintenance of similitude: microstructurely, mechanically, and environmentally. However, for many industrially important conditions, similitude with large fatigue crack growth is lost, partially because of changes in fatigue crack closure. The effect of closure on similitude is discussed.

New data are presented to illustrate the loss of similitude between applied loading and crack tip strain response. The resulting strain rates of material within the process zone are unexpected. Environmentally influenced fatigue crack growth rates are likely to be influenced by these strain rates.
Effect of Absorbed Hydrogen on the Microstructure in the Vicinity of Near-Threshold Fatigue Cracks in Low-Alloy Steel
Authors: Heldt J , Kaesche H
Published: March 2009

The influence of a dry hydrogen environment on near-threshold crack growth propagation rates of fatigue cracks in a low-alloy spheroidized steel was investigated. for separation of environmental and mechanically induced effects, fatigue tests in an ultra-high vacuum (UHV) environment were taken as reference. On a macroscopic scale, we found a significant acceleration of the propagation rates of cracks exposed to a dry hydrogen atmosphere compared to tests in an inert environment. The electron-microscopic characterization of the microstructure in the vicinity of cracks revealed that the acceleration of fatigue propagation by hydrogen can be rationalized by a hydrogen-involved fatigue damage mechanism. Furthermore, it was concluded that hydrogen enhances the dislocation mobility. This effect aids in the formation of dislocation cellular structure in the cyclic plastic zone. The mode of fracture itself is ductile transgranular and appears to be unchanged by absorption of hydrogen at the crack tip.
S-N Curve for Crack Initiation and an Estimate of Fatigue Crack Nucleus Size
Authors: Yang CY , Liaw PK , Palusamy SS , Ren W
Published: March 2009

A study of fatigue life prediction was made for ASTM A533 Grade B nuclear pressure vessel steel. The objectives of the study were to predict the S-N curve, representing crack initiation, and to estimate the average crack nucleus size using an engineering approach. The plastic replica method was used to monitor crack initiation and growth from well-polished specimens under uniaxial tension-tension stressing.

Two methods were used to estimate crack nucleus size: (1) backcalculating crack length via the da/dN versus ΔK relationship, and (2) evaluating an assumed relationship between the endurance limit and the threshold stress intensity factor range. Crack nucleus size estimated by these two methods are fairly consistent when the effects of crack closure and plastic zone correction are taken into account.
Fracture Testing of Large-scale Thin-sheet Aluminum Alloy
Authors: deWit R , Fields RJ , Low SR , Harne DE , Foecke T
Published: March 2009

A series of fracture tests on large-scale, pre-cracked, aluminum alloy panels was carried out to examine and to characterize the process by which cracks propagate and link up in this material. Extended grips and test fixtures were specially designed to enable the panel specimens to be loaded in tension in a 1780-kN-capacity universal testing machine. Ten panel specimens, each consisting of a single sheet of bare 2024-T3 aluminum alloy, approximately 4 m high, 2.3 m wide, and 1 mm thick, were fabricated with simulated through-cracks oriented horizontally at mid-height. Using existing information, a test matrix was set up to explore regions of failure controlled by failure mechanics, with additional tests near the boundary between plastic collapse and fracture. In addition, a variety of multiple site damage (MSD) configurations were included to distinguish between various proposed linkage mechanisms. All tests but one used anti-buckling guides. Three specimens were fabricated with a single central crack, six others had multiple cracks on each side of the central crack, and one had a single crack but no anti-buckling guides. The results of each fracture event were recorded on various media: film, video, computer, magnetic tape, and occasionally optical microscopy. The video showed the crack tip with a load meter in the field of view, using motion picture film for one tip and super VHS video tape for the other. The computer recorded the output of the testing machine load cell, the stroke, and the twelve strain gages at 1.5-s intervals. A wideband FM magnetic tape recorder was used to record data from the same sources. The data were analyzed by two different procedures: (1) the plastic zone model based on the residual strength diagram, and (2) the R-curve. The first three tests were used to determine the basic material properties, and these results were then used in the analysis of the subsequent tests with MSD cracks. There is fairly good agreement between measured values and results obtained from the models.
Fatigue Response of Perforate Titanium for Application in Laminar Flow Control
Authors: Miller JL , Newman JC , Johnson WS
Published: March 2009

The room temperature tensile and fatigue response of non-perforated and perforated titanium for laminar flow control application was investigated both experimentally and analytically. Results showed that multiple perforations did not affect the tensile response, but did reduce the fatigue life. A two-dimensional finite element stress analysis was used to determine that the stress fields from adjacent perforations did not influence one another. The stress fields around the holes did not overlap one another, allowing the material to be modeled as a plate with a center hole. Fatigue life was predicted using an equivalent initial flaw size approach to relate the experimental results to microstructural features of the titanium. Predictions using flaw sizes ranging from 1 to 15 (μm correlated within a factor of 2 with the experimental results by using a flow stress of 260 MPa. By using two different flow stresses in the crack closure model and correcting for plasticity, the experimental results were bounded by the predictions for high gross section stresses. Further analysis of the complex perforation geometry and the local material chemistry is needed to further understand the fatigue behavior of the perforated titanium.