bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Grain and Subgrain Boundaries on Void Formation and Growth in Aluminum Irradiated with Fast Neutrons
Authors: Horsewell A , Singh BN
Published: December 2008

High-purity aluminum was irradiated at 120°C to fast neutron fluences between 2 × 1021 and 5 × 1024 n/m2. Variations in void density and void size in the vicinity of grain and subgrain boundaries have been analyzed. The results document that vacancy accumulation is enhanced in a wide zone up to 15 μm from grain boundaries. The magnitude of the enhanced vacancy accumulation varies as a function of distance from the grain boundary. The presence of subgrain boundaries enhances the vacancy accumulation within the subgrain, and the vacancy accumulation varies markedly as a function of subgrain size. The results are discussed in terms of the consequences of an initial vacancy concentration gradient near grain boundaries on further vacancy accumulation. The maintenance and evolution of vacancy supersaturation gradients are considered in terms of transport and screening of interstitials during vacancy accumulation in the peak zone.
Interstitial Dislocation Loop Nucleation and Growth and Swelling Produced By High-Energy Cascades
Authors: Yu J
Published: December 2008

Structural materials for advanced energy sources, such as magnetically and inertially confined thermonuclear reactors, are subjected to a radiation environment that produces high-energy displacement cascades and transmutation products, which includes helium. We address these characteristics and formulate their effect on the microstructural evolution of a material.

We consider that, immediately after a high energy cascade event, a vacancy-rich region exists near the primary event site and that an interstitial-rich zone, formed by collision chains, exists some distance from the primary event site. Transmutation-product helium can diffuse into the vacancy-rich zone and stabilize bubble nuclei that will later grow if sufficient vacancies and helium atoms diffuse to the nucleus. These bubbles are the sinks for excess radiation-produced vacancies. The excess radiation-produced interstitials migrate and bond; if the binding energy is high enough, a di-interstitial is considered to be a stable dislocation-loop nucleus. The loop nuclei grow if they receive more interstitials than vacancies; this represents material swelling since the bubbles do not cause a lattice contraction to offset the dilation caused by the growing dislocation loops.

Consider that the interstitial-loop growth process can be represented by a Langevin equation. If the growth process at time t is assumed to be dependent only on the instantaneous interstitial concentration and instantaneous microstructural state (dislocation, cavities, and grain boundaries) and independent of previous history, the growth process can be considered to be a Markov process. Application of an equivalent Fokker-Planck equation allows deduction of an interstitial-loop nucleation and growth equation, the solution of which yields an interstitial-loop distribution function.

Use of appropriate material parameters gives the incubation and growth regimes of swelling and also allows a determination of the maximum swelling temperature. At a vacancy and interstitial production rate of 1021 m−3 s−1, these results predict a maximum swelling temperature of 117°C for aluminum, 550°C for stainless steel, and 1100°C for molybdenum. These values are in good agreement with experimental results.

Basically, the maximum swelling temperature is found to be a function of only sυ, ευf/ευm, (ρN + ρl), and ˙Np. The maximum swelling temperature increases as (ρN + ρl), ˙Np, and ευf/ευm increase and decreases as increase and decreases as sυ increases. Here, sυ is the vacancy formation entropy, ευf is the vacancy formation energy, ευm is the vacancy mobility energy, ρN is the network dislocation density, ρl is the dislocation loop density, and ˙Np is the production rate of vacancies and interstitials per unit volume under irradiation.
Stability Studies of Radiation Defects in Aluminum Introduced by 600-MeV Protons
Authors: Paschoud F , Gotthardt R , Green SL
Published: December 2008

Transmission Electron Microscopy (TEM) observations of 600-MeV proton irradiated aluminum reveal high concentrations of helium bubbles. Post irradiation annealing experiments show that the size and the distribution of these bubbles remain constant up to a temperature of about 770 K. Migration and coalescence have only been observed when the specimens were irradiated at 450 K and after annealing at 820 K. Electrical resistivity measurements show a strong increase that cannot be explained solely by the helium bubbles. This increase remains constant up to an annealing-temperature of 770 K. After the annealing treatments, no precipitation of transmutation atoms has been observed, which indicates that they are in solid solution or that they agglomerate in submicroscopic precipitates.
Effects of Neutron Irradiation on the Microstructure and Microhardness of Niobium-Base Alloys
Authors: Michel DJ , Smith HH
Published: December 2008

The microstructure and microhardness of several niobium-base binary and ternary alloys were investigated following fast neutron irradiation to a fluence of 18 dpa (4.8 × 1022 n/cm2, >0.1 MeV) at 650°C. Transmission electron microscope (TEM) procedures were employed to observe the microstructure of the alloy specimens following neutron irradiation and in the unirradiated condition. Microhardness measurements were made on irradiated and unirradiated specimens to obtain information on the effects of irradiation on mechanical properties. Neutron irradiation of the Nb-1Mo and Nb-5Mo-1Zr alloys produced voids and a high density of dislocation loops. No voids were observed in the irradiated Nb-1Zr alloy. Microhardness measurements of the irradiated alloys indicated that the effect of the fast neutron irradiation was to increase the microhardness of all the niobium-base alloys. The magnitude of the microhardness increase was found to be a function of the alloy composition, with the smallest increase observed in the Nb-5Mo-1Zr alloy. The observations in this study are in agreement with the previous work, which has shown that molybdenum and zirconium additions were effective in suppressing void formation in niobium-base alloys for irradiation temperatures up to ≈1100°C.
Nickel Ion Irradiation Effects on the Microstructure of 2 1/4% Cr 1% Mo Steel and a Comparison with Neutron Irradiation Data
Authors: Kai J-J , Sindelar RL , Plumton DL , Dodd RA , Kulcinski GL
Published: December 2008

A 2 1/4 Cr 1% Mo steel, heat treated to consist of upper and lower bainite and ferrite, has been shown to undergo substantial microstructural modification when irradiated with 14-MeV nickel ions at 500°C to 350 dpa at the peak damage depth. The original cementite (Fe3C) in upper and lower bainite was replaced by Chi phase, M7C3 and M2C in upper bainite, and Chi and M7C3 in lower bainite. The originally precipitate-free ferrite contained a significant fraction of G phase after irradiation. The replacement of Fe3C by M7C3 and M2C is consistent with the fact that M7C3 and M2C form in a quenched and tempered 2 1/4 Cr 1 Mo steel, but at higher tempering temperatures than does Fe3C. On the other hand, Chi and G phases have not been reported to occur in this steel as a result of thermal treatments. The steel is resistant to void swelling, which amounted to 0.2% or less even at the rather high dpa levels employed in this study. The quantitative data for precipitate and void number densities and sizes agree quite well with a companion neutron irradiation study of the same steel possessing an identical microstructure.
The Fine Scale Microstructure in Cast and Aged Duplex Stainless Steels Investigated by Small Angle Neutron Scattering
Authors: Epperson JE , Lin J-S , Spooner SS
Published: December 2008

Small angle neutron scattering (SANS) allows clustering phenomena to be studied in systems for which the constituent atoms do not differ greatly in atomic number. This investigation used SANS to characterize the fine scale microstructure in two cast and aged duplex stainless steels that are used in the nuclear industry; aging times extended up to eight years. The steels differed in ferrite content by about a factor of two. The intensity at the lowest scattering angle was dominated by scattering effects that were largely removed by applying a 4-kilogauss magnetic field and is, therefore, associated with the ferrite phase. In the range of momentum transfer 0.025 ⪝ q ⪝ 0.2 Å−1, additional scattering caused by a precipitate phase, assumed to be rich in nickel and silicon, was observed. This scattering was rather intense, and the volume fraction of precipitate, in the ferrite, was estimated to be 12 to 18% after long time aging. After 70 000 h at 400°C, there were about 1024 precipitate particles per m3, some 50 Å in mean diameter, and they were distributed in a nonrandom manner, that is, spatially, short-range ordered. This investigation suggests that after aging some 70 000 h at 400°C, the precipitate in the ferrite phase is undergoing ripening or coarsening. The present data are insufficient to indicate at what time this ripening process began.
Radiation-Induced Segregation in Light-Ion Bombarded Ni-8% Si
Authors: Packan N , Heatherly L , Kesternich W , Schroeder H
Published: December 2008

Radiation-induced segregation (RIS) is an important area of current research because of potential detrimental effects including embrittlement. In this work, a quantitative assessment is made of radiation-induced silicon segregation to internal grain boundaries compared with that to the external surfaces. Tension specimens of Ni-8 atomic % Si have been irradiated at 750 K (475°C) with either 7-MeV protons or 28-MeV alpha particles to damage levels of 0.1 to 0.3 dpa. Postirradiation tension testing at 300, 720, or 975 K produces ductile failures with elongations > 15%. However specimens that had implantation of 250 to 1000 atomic ppm He at 975 K (700°C) before irradiation did fracture intergranularly when pulled at 975 K inside a scanning Auger microprobe, thereby exposing grain boundary surfaces for depth profile analysis. After correction for preferential sputtering effects, RIS of silicon to outer surfaces is found to give rise to a strongly enriched surface layer, 20 to 30 nm thick, followed by a shallow depletion zone for the next ∼200 nm, in agreement with previous studies in the literature. By contrast, silicon segregation to fracture-exposed grain faces varies considerably from face to face but generally declines more gradually with little or no depletion zone. Observations of silicon profiles and Ni3Si formation at grain boundaries obtained by analytical transmission electron microscopy (TEM) generally confirm the Auger results.
Effect of Irradiation on Carbide Evolution in a Pure Fe-2¼Cr-1Mo Ferritic Alloy
Authors: Brimhall JL , Simonen EP
Published: December 2008

Ion irradiation was observed to accelerate the kinetics of the carbide evolution in a “pure” Fe-2¼Cr-1Mo steel. The type and structure of the phases were unchanged by irradiation compared to thermal annealing. The transformation kinetics of Fe3C to M7C3 and the independent nucleation of Mo2C were both enhanced by the ion irradiation. The observed irradiation enhancement of the kinetics however did not follow a predicted trend based on a simple radiation enhanced diffusion model. There is no obvious explanation for this discrepancy except that mechanisms in addition to enhanced diffusion influence the complex transformation process.
Phase Stability in Fe-15 Ni-x Cr Alloys Under Ion or Electron Irradiation
Authors: Stubbins JF
Published: December 2008

Three high-purity Fe-Ni-Cr alloys have been irradiated with heavy ions and with electrons at 450, 550, and 650°C to doses up to 60 dpa. The heavy ion irradiations were carried out with Ni6+ ions, and specimens were irradiated both with and without helium preinjection. The electron irradiations were carried out in a high-voltage electron microscope (HVEM) on specimens without helium preinjection. The alloy compositions were Fe-15 Ni-10 Cr, Fe-15 Ni-16 Cr, and Fe-15 Ni-20 Cr, all with low carbon levels. The latter two of these compositions are austenitic at room temperature, whereas the first can include martensite on rapid cooling. According to the computer calculated phase diagrams of Watkins et al, all of these alloy compositions would be metastable at the lowest irradiation temperature in the present study, but the austenitic phase structure should be stable in all of the alloys at the highest irradiation temperature.

Voids were formed in all of these alloys during irradiation at 650°C to 30 dpa, and swelling levels at that dose were independent of the presence or absence of preinjected helium. The more interesting irradiation behavior consisted of irradiation induced phase transformations, predominantly during irradiation at 450°C. The Fe-15 Ni-16 Cr alloy precipitated a fine distribution of M23C6 after 30 dpa at 450°C under heavy ion irradiation. Similar precipitates were not found in either of the two other alloys under similar irradiation conditions. The formation of these precipitates in a low carbon alloy was surprising. The precipitates were nucleated uniformly and were not associated with dislocation loops or other microstructural features.

The more striking irradiation induced phase transformation was the evolution of austenitic regions in the martensitic laths in the Fe-15 Ni-10 Cr alloy during irradiation at 450°C. This martensitic reversion reaction seems to involve a mechanism associated with dislocation loop formation. The martensitic reversion reaction was observed for materials irradiated both by HVEM and by heavy ions. The austenitic precipitates were stable and grew with irradiation at 450°C. They were found to nucleate in all orientations consistent with the Nishiyama-Wassermann relation in the body centered cubic matrix. This paper develops an explanation of this behavior based on dislocation loop morphology and relative phase stability.
Neutron-Induced Spinodal-Like Decomposition of Fe-Ni and Fe-Ni-Cr Alloys
Authors: Garner FA , Brager HR , McCarthy JM
Published: December 2008

Spinodal decomposition is not known to occur in the iron-rich regime of Fe-Ni and Fe-Ni-Cr fcc systems. During neutron irradiations conducted in the range 450 to 600°C, however, a spinodal-like separation occurs in the invar compositional range. Ion irradiation studies show a similar behavior and indicate that the separation process occurs as least as high as 725°C. The spinodal-like process is characterized by compositional microoscillations between ∼25 and ∼50% nickel with temperature-dependent wavelengths on the order of hundreds of nanometers. These oscillations exhibit a dependence on crystallographic orientation and cause significant hardening. Comparison of these oscillations with those produced in the Santa Catharina meteorite (Fe-35Ni) leads to the conclusion that Fe-Ni invar alloys decompose very slowly (many thousands of years) in the absence of radiation. During irradiation, the spinodallike decomposition and its subsequent coarsening are accelerated by enhanced diffusion and at lower irradiation temperatures by segregation of nickel via the inverse Kirkendall effect at radiation-produced sinks.
Compositional Fluctuations and Stability of Alloys Under Radiation
Authors: Krishan K , Abromeit C
Published: December 2008

Composition fluctuations arising from thermal effects are known to influence the spinodal decomposition in concentrated alloys. However, at very low temperatures these effects slow down because of the very small diffusion rates. This paper examines the possibility of having such fluctuations induced by radiation via the fluctuations in the supersaturated defect concentration. Various mechanisms are discussed. In particular, it is shown that the interstitial exchange with lattice atoms combined with diffusion leads to fluctuations that grow linearly with time. An estimate of this effect indicates that it has a significant contribution to the decomposition of an alloy. A detailed mathematical analysis of the phenomena is presented and application to some alloys that show spinodal like decomposition during irradiation is discussed.
Helium Ion Implantation Induced Damage in Metallic Glasses
Authors: Tyagi AK , Nandedkar RV
Published: December 2008

The precipitation of implanted helium into bubbles, blistering, flaking, and exfoliation in metallic glasses, such as Fe80B20, Fe40Ni40B20, Fe40Ni40P14B6, Fe40Ni38Mo4B18, Ni64Zr36, Ni33Zr67, and Ni60Nb40, are investigated. A systematic study as a function of relevant irradiation and material variables is presented. The effect of projectile energy (50 to 150 keV), total dose (1 × 1016 to 1 × 1019 ions/cm2), dose rate (10 to 100 μÅ/cm2), cold-work (30% thickness reduction), and thermal crystallization has been studied and is presented. The changes in surface topography caused by post-irradiation annealing for 2 h at 473, 673, and 873 K are also examined. The results on metallic glasses are compared with similar results reported for crystalline materials. The similarities and differences in the damage behavior of metallic glasses and crystalline materials are discussed. The main difference observed for metallic glasses is a relatively higher (50 to 100%) resistance against blistering or flaking or both as compared to their crystalline counterparts. Helium trapping, bubble nucleation, and bubble growth in metallic glasses are also briefly described.
Study of Heavy and Light Ion Irradiation Induced Effects in Metallic Glass Samples
Authors: Bhagawat A , Kurup MB , Prasad KG , Sharma RP
Published: December 2008

The effect of Ar+ ion bombardment of a-Fe40Ni40B20 samples at varying fluences and energies, as investigated by conversion electron Mössbauer technique in the near surface region, has revealed an important correlation between the total Mössbauer absorption and the energy density deposited by the incident ions in the material. The shape of this correlation suggests that as the dose of the 150-keV incident Ar+ ions is increased, stresses develop in the material that may be responsible initially for the strengthening and beyond a certain limit for the weakening or breaking of the inter- and intra-molecular bondings in the irradiated region of the amorphous material. Increase in the energy of the incident ions also produces similar effects. This behavior depends greatly on the initial surface condition of the amorphous system. The proton irradiation of the same material has a different effect. It seems to cause the atomic spins to reorient in random directions in the sample. At the same time there is a formation of an additional iron or iron-nickel (Fe-Ni) rich phase in the irradiated samples at doses above 5 × 1015 protons/cm2. In these latter samples the relative susceptibility and hence the bulk magnetization are found to decrease continuously as a function of irradiation dose.
Use of X-radiographic Tracers to Measure Fiber Orientation in Short Fiber Composites
Authors: Denton DL , Munson-McGee SH
Published: December 2008

The degree of fiber orientation is a major factor in determining the mechanical behavior of short fiber composites. A method to determine fiber orientation in short fiber composites such as sheet molding compound (SMC) is presented. A small amount of glass fibers with a high lead content is incorporated into the composite as a tracer. The lead glass tracer fibers mimic the orientation changes that occur during processing. X-radiographs of composites containing the lead glass fibers provide clear, high contrast images of the tracer fibers. Analysis of the X-radiographic image by Fraunhofer light diffraction provides a quantification of the degree of fiber orientation in the composite. Results of fiber orientation and mechanical property measurements for SMC composites molded with different amounts of flow are presented. These techniques are potentially useful for research on fiber orientation, development of prototype parts, optimization of molding conditions, and production quality control.
Characterization of Bolted Joint Behavior in SMC-R50
Authors: Wilson DW
Published: December 2008

Results are reported for an experimental investigation into the fundamental characteristics of bolted joint behavior in SMC-R50. Single fastener, double lap test specimens were employed to evaluate strength and failure mode behavior as a function of clamping pressure, fastener size, fastener edge distance, and fastener half spacing. Although three failure modes were observed — bearing, cleavage, and net tension — the net tension failure mode dominated through a broad range of geometries. This contributed to an observed difference in strength trends observed for the SMC-R50 material systems as compared to continuous fiber laminate systems. The implications of the empirical strength and failure mode findings on design and analysis of mechanical joints in SMC-R50 are discussed.
Thermoelastic Response of the Cylindrically Orthotropic Disk — Numerical and Experimental Evaluation
Authors: Gillespie JW , Pipes RB
Published: December 2008

Transfer molding of axisymmetric bodies with fiber-reinforced molding compounds through gates coincident with the geometric axis of revolution (z) results in fiber orientation distributions which may also be independent of tangential position (ϑ). In the present study, the thermoelastic response of the annular disk is correlated with analytic, numerical integration and finite-element predictions. Methodology based upon microscopic examination of the disk fiber orientation distribution is presented for the material property characterization required by the various solution techniques. Correlation with experimental results indicates that all three techniques adequately predict the in-plane deformation and strain response of the cylindrically orthotropic disk. The approximate elasticity solution of the layered fiber orientation distribution obtained by the finite-element method reveals an interlaminar stress state at tractionfree surfaces which decays within one disk thickness. Consequently, the analytical and numerical integration techniques which assume homogeneity through the disk thickness are not applicable for stress calculations in the vicinity of the free edges.
Using ASTM D 4065-82 for Predicting Processability and Properties
Authors: Driscoll SB
Published: December 2008

Recently ASTM D 4065-82 (Practice for Determining and Reporting Dynamic Mechanical Properties of Plastics) was written to provide important rheological information on thermoplastics and thermosetting resins/reinforced plastics-composites. These procedures have been established for monitoring the subtleties of macromolecular behavior, the effects of processing, and in-service performance.

The significance of ASTM D 4065-82 is the generation of important rheological information that can be used to organize, monitor, and provide the necessary information for the quality control and assurance engineer.

Dynamic mechanical testing (DMT) of polymeric material systems provides important viscoelastic information for the materials, design, and process engineer. DMT quickly generates the critical information the materials engineer needs for formulating new composite systems, especially as raw material prices change. The design engineer will use DMT results for optimizing functional performance with greater reliability. The processing engineer will use DMT for assessing the effects of processing parameters on functional performance and determining the effect of changes in temperature on modulus, impact, and dimensional stability.

DMT provides critical information on the effects of the environment, such as moisture on long-term durability. Short-term dynamic mechanical characterizations can be used to establish “master curves” for predicting time-dependent behavior.

Representative examples of DMT as a function of frequency (dynamic oscillation), strain, temperature, and time are cited to illustrate how ASTM D 4065-82 is being used as an effective analytical procedure for ensuring quality and for predicting long-term functional performance. The final section of this paper details the use of instrumented impact testing [ASTM D 3763-79 (Test Method for High-Speed Puncture Properties of Rigid Plastics)] for generating additional information on functional performance.
Effect of Temperature Rise and Fall on the Strength and Permeability of Concrete Made With and Without Fly Ash
Authors: Owens PL
Published: December 2008

The effect of temperature rise and fall on concrete at early ages is rarely measured. Concrete cured at elevated temperature has reduced strength at later ages. The changes that occur in the hydration products have been measured in terms of the amount of calcium hydroxide [Ca(OH)2] present. Since permeability is, to some degree, a function of the water/cement ratio and strength, the measurement of permeability was necessary. Concrete is more permeable after it has undergone a temperature cycle similar to that found in practice. The use of Class F fly ash can effectively reduce the permeability of heat-affected concrete.
Effects of Early Heat of Hydration and Exposure to Elevated Temperatures on Properties of Mortars and Pastes with Slag Cement
Authors: Roy DM , White EL , Nakagawa Z
Published: December 2008

The behavior of slag cements combining substantial amounts of separately ground granulated blast-furnace slag is compared with that of portland cements. The effects on mortars and pastes of exposure to elevated temperatures during early-stage hydration and at later ages has been determined. Compressive strength, density, microstructure, permeability, and dimensional change were the primary properties investigated. The effects of long-term exposure at temperatures up to 250°C were evaluated. Very low permeabilities, <10−8 darcys (μm2), were maintained in most of the mortars. The phase changes with time were determined by X-ray diffraction.

Compressive strengths up to 200 MPa and higher were found in some of the mortars. Pore structures by mercury porosimetry were also investigated, revealing very fine pore structures and some changes with elevated temperature.
Use of ASTM Standards in Integrated Manufacturing Systems
Authors: Schilling PE
Published: December 2008

ASTM is the world's largest source of voluntary consensus standards. ASTM Committee E-31 on Computerized Systems has developed standard guides for a six-phase method of developing systems. The phases are project definition, functional requirements, functional design, implementation design, system assembly, and system evaluation. These guides have been applied to many different kinds of systems and application areas. An example shows how ASTM standards are applied in a project that integrates process control and management information systems in an aluminum rolling mill.
EIA and ISO Numerical Control Standards for Automated Manufacturing Systems
Authors: Bacheler AT
Published: December 2008

This paper identifies and describes the numerical control (N/C) standards published by the Electronic Industries Association as EIA or ANSI/EIA standard in the United States and the N/C standards published by the International Organization for Standardization (ISO). The current status of projects and proposals for new N/C standards in both organizations is described, with emphasis on interfaces and communications links related to automated manufacturing. The object of these standards is the establishment of codes, formats, interfaces, protocols, and procedures to facilitate the automation of industrial machines and systems, including machine tools.
Standardization Suggested by the Automated Manufacturing Research Facility (AMRF)—A Research Testbed for the Factory of the Future
Authors: Bloom HM , McLean CR
Published: December 2008

A research effort has been initiated at the National Bureau of Standards to develop a small batch manufacturing system to support study and experimentation in automated metrology and interface standards for the computer-integrated factory of the future. When completed, the resulting testbed system will be made available for fundamental studies in manufacturing technology by scientists and engineers from government, industry, and universities. This paper will report on the progress of major software and hardware subprojects that are being carried out in support of the construction of the automated manufacturing research facility (AMRF).

The basic approach to the research effort is to take the structure of an automated manufacturing facility and decompose the functional elements in a top-down fashion until the elements are no longer divisible. At this level, systems are defined for robots, machine tools, carts, conveyors, and other similar type units. The project will define the processes that occur at each level, how they can be measured using sensors with software support, and the interface specifications between modules.

Projects will address the applications of (1) hierarchical control techniques to implement the factory-command structure, (2) artificial intelligence/expert systems and decision support systems for work on distributed process planning and scheduling functions, (3) distributed data management systems to handle the problem of a wide spectrum of data requirements, and (4) network communications techniques to manage the various data flow requirements through all levels of the factory system.

Other projects will address the issue of software support systems for an automated system. Graphics interfaces for a manufacturing environment will be developed based on human engineering considerations. Simulation and emulation systems are and will continue to be used for the design and testing of automated systems.

Some projects will address the problem of making the elementary unit, such as robots, more capable of responding to a changing environment. These projects include adding many types of sensors (vision, tactile, force, proximity, etc.) to the robot system to allow for a more sensory interactive real-time control system. A continuing project utilizing sensors on machine tools has demonstrated the ability to improve a machine tool's accuracy. These projects will lead to adaptive control being performed at the equipment level.
Wide-Range Weight Functions for the Strip with a Single Edge Crack
Authors: Orange TW
Published: December 2008

This paper presents a closed-form expression for the weight function for a strip with a single edge crack. The expression is valid for relative crack lengths from zero to unity. It is based on the assumption that the shape of an opened edge crack can be approximated by a conic section. The results agree well with published values for weight functions, stress intensity factors, and crack mouth opening displacements.
Internal Hydrogen Degradation of Fatigue Thresholds in HSLA Steel
Authors: Esaklul KA , Gerberich WW
Published: December 2008

The presence of internal hydrogen through cathodic charging has substantial influence on the near-threshold fatigue behavior of an HSLA steel with an as-received yield strength of 365 MPa. The results of fatigue crack propagation tests in both as-received and heat-treated conditions indicate higher crack propagation rates and lower threshold stress intensities in the presence of internal hydrogen. These effects are dependent on strength, mean stress or R-ratio, and test temperature. The enhancement in the crack propagation process is more severe in the higher strength material and at higher mean stress. An unusual behavior was observed due to decreasing test temperature from 300 to 123 K in which higher thresholds and lower crack propagation rates at 233 and 173 K were obtained in the presence of internal hydrogen. These effects are a result of true intrinsic hydrogen enhanced fracture and deformation processes.
Axial Fracture Toughness Testing of Zr-2.5Nb Pressure Tube Material
Authors: Davies PH , Stearns CP
Published: December 2008

A complete flaw evaluation of a longitudinal defect in a CANDU (CANada Deuterium Uranium) reactor pressure tube requires knowledge of the axial fracture toughness of the material. This paper addresses the problem of measuring fracture toughness of thin pressure tube material. Compact tension specimens (∼4 mm thickness) are prepared from flattened and stress-relieved Zr-2.5Nb pressure tube material. Axial crack growth resistance curves (J-Δa) and crack opening displacement (COD) are determined at room temperature and 150°C using the unloading compliance method. The latter is shown to give excellent estimates of crack extension for side-grooved specimens (≤2% variation) and for plane specimens at room temperature (≤7% variation for Δa > 1.5 mm). However, for plane specimens at 150°C and short crack extensions (Δa < 1.5 mm), the compliance calculated crack length underestimates the true crack extension by approximately 20%. The J-Δa curves and COD are shown to be in good agreement with previous results of Simpson.
Characterization of Crack Closure
Authors: Sehitoglu H
Published: December 2008

Crack opening and closure levels are determined for small cracks growing from sharp and blunt notches during cyclic loading. Crack tip opening displacement measurements as low as 0.1 μm are reported. Experimental crack opening levels are compared with analytical crack opening levels for various stress intensity ranges, crack lengths, and R-ratios.

The crack growth rate behavior in the vicinity of blunt and sharp notches as well as in smooth specimens is studied. The high crack growth rates observed for small cracks growing from blunt notches are interpreted based on the variation of crack opening level with the crack length. Effective stress intensity range is defined to account for crack closure effects. The crack growth rate correlated well with the effective stress intensity range obtained from sharply and bluntly notched specimens of 1070 steel.
Crack Arrest Behavior of a High-Strength Aluminum Alloy
Authors: Lin C , Hoagland RG
Published: December 2008

This paper reports on a series of crack arrest tests on two lots of 7075-T651 high strength aluminum alloy. The tests were performed and the data analyzed in accordance with procedures developed for high toughness, medium strength steels. Both KID and KIa are reported and the results show that these crack arrest parameters behave considerably differently than in the case of medium strength steels. Problems with specimen size requirements, fracture mode transitions, and generation of unbroken ligaments were not factors affecting the present results; their absence may, at least in part, account for these differences. Also, the strain-rate sensitivity of the flow strength of this aluminum alloy is considerably less than that of ferritic steels and this may also be important.
In Situ SEM Observation of Fracture Processes in Short Glass Fiber Reinforced Thermoplastic Composite
Authors: Sato N , Kurauchi T , Sato S , Kamigaito O
Published: December 2008

To study the fracture mechanism of a short glass fiber reinforced thermoplastic composite, the surface of the composite under stress was examined by a scanning electron microscope. The fracture processes, including crack initiation and propagation, could be observed in situ. The fracture was proved to proceed through five stages: (1) occurrence of a cluster of microcracks and voids in the matrix located at the fiber ends; (2) propagation of the interfacial microcracks in the matrix located on the fiber sides; (3) occurrence of bands of microcracks in the matrix, accompanied by the formation of shear bands in the matrix; (4) crack openings and slow crack propagation in the shear bands, followed by yielding and necking; and (5) catastrophic crack propagation, leading to the failure of the composite.
Stress Intensity Factors for Surface Cracks with Arbitrary Shapes in Plates and Shells
Authors: Miyoshi T , Shiratori M , Tanabe O
Published: December 2008

In order to analyze stress intensity factors for surface cracks with arbitrary shapes in plates and shells, the authors have developed a new finite element scheme based upon the line spring model. In this scheme Ahmad's thick shell elements are combined with the newly developed line spring elements with quadratic shape functions. This line spring method has more potential of application than a previously reported method where flat shell elements were combined with the line spring elements with linear shape functions.

In the analysis of K-values for a single semielliptical surface crack in tension, it has been found that the method has nearly the same accuracy as the three-dimensional finite element method, while the computing time is one order of magnitude less. The method has been applied to the problem of interaction of twin semielliptical cracks in tension. The estimation scheme for K-values by ASME Boiler and Pressure Vessel Code has been found to give conservative K-values for most of the separated twin cracks, except for very deep cracks. For overlapping twin cracks, K-values at the intersecting points become very large; for example, the highest value is about three times larger than the maximum value for a single crack with the same size as one of the twin cracks. Therefore caution must be used in dealing with these cracks.

Single rectangular and triangular cracks have been analyzed. It has been found that the ASME Code gives conservative K-values for triangular cracks but underestimates values for rectangular cracks.
Elastic-Plastic Failure Analysis of Pressure Vessel Tests
Authors: Sciammarella CA
Published: December 2008

The fracture under internal pressure of twelve vessels working in the upper shelf region is analyzed by means of elasto-plastic criteria. The vessels were manufactured from steels with yield stresses ranging from 295 to 861 N/mm2 and toughnesses between 4.1 and 9.2 kN/mm3/2. The vessels were designed to operate at about σys/1.5 and consequently had different thicknesses. Artificial defects were introduced in the base material and in the heat-affected zone. Some of the vessels were tested in the as-welded condition, some in the stress-relieved condition. Critical crack lengths ranged from 110 to 340 mm. The JR method of Paris and a modified version of the two-parameter method of the Central Electricity Generating Board (United Kingdom) were applied. Predicted burst pressures are in good agreement with measured values, the JR method yielding errors within the 3% range and the two-parameter method giving errors ranging from 3 to 8%, always on the conservative side.
Application of Maximum Load Toughness to Defect Assessment in a Ductile Pipeline Steel
Authors: Willoughby AA , Garwood SJ
Published: December 2008

It is generally considered that, when a material behaves in a fully ductile manner, the most accurate method of fracture analysis is that based on the R-curve/driving force concept. Other, simpler, methods involve the use of a single value of toughness, for instance, that at initiation of tearing or at the maximum load point determined on a suitable small-scale test piece. Maximum load toughness has been used extensively in conjunction with the crack tip opening displacement (CTOD) design curve assessment method. To assess its general applicability, the results from an experimental program involving burst tests, wide plate tests, and numerous small-scale tests on a ductile pipeline steel conforming to API 5LX 56 were analyzed by various methods. These included the CTOD design curve, the CEGB R6 method, the method of Kiefner et al for axial flaws in pipelines, together with a pseudo linear elastic fracture mechanics (LEFM) R-curve approach and the Paris stability analysis. Initiation toughness, maximum load toughness, and R-curves were all used as input, where applicable. It was found that the use of initiation toughness gave very conservative assessments, whereas the R-curve techniques were complex and not always conservative. Maximum load toughness with any of the relevant analyses gave conservative and simple assessments.
Current Understanding of the Mechanisms of Stress Corrosion and Corrosion Fatigue
Authors: Ford FP
Published: December 2008

Failure of components operating in various environments due to cracking remains a safety and economic problem despite the effort that has been devoted to understanding the phenomena of stress corrosion, hydrogen embrittlement, and corrosion fatigue. This article reviews our current understanding of the mechanisms of stress corrosion and corrosion fatigue in ductile alloy/aqueous environment systems, with emphasis placed on the application of this mechanistic knowledge to the formulation of remedial actions, design criteria, and relevant testing techniques.

It is concluded that many of the recent arguments associated with, for example, the applicability of the slip-dissolution or hydrogen-embrittlement mechanisms of crack advancement under static and dynamic loading conditions may be of minor importance from a practical view, since the same rate-determining steps in crack propagation can operate in these mechanisms and modes of cracking. Bearing in mind that crack advance in ductile structural-alloy/aqueous-environment systems is primarily an electrochemically related process, our understanding of the cracking mechanism suggests that, at a given potential, the propagation rate is controlled by reaction rates (for example, dissolution, hydrogen-ion reduction) on the crack tip and sides and how these are affected by the passivation rate, liquid diffusion rate, and strain rate at or near the crack tip. Knowledge of the interactions between these controlling processes leads to a quantitative understanding of the spectrum of behavior between stress corrosion and corrosion fatigue and of the criteria that dictate whether crack propagation will arrest or accelerate.
Stress Corrosion Evaluation of Titanium Alloys Using Ductile Fracture Mechanics Technology
Authors: Anderson DR , Gudas JP
Published: December 2008

The objective of this investigation was to assess the feasibility of using J-integral fracture mechanics test techniques to quantify the stress corrosion characteristics of a titanium alloy in seawater. A series of elastic compliance J-integral tests were performed with 1TCT (modified compact tension) specimens of Ti-7Al-2Cb-1Ta in air and under conditions where the crack tip was exposed to seawater. A complementary series of single edge notched cantilever beam tests were performed in natural flowing seawater. Results showed that the stress corrosion susceptibility of this alloy was clearly identifiable in J-integral tests using the JI-R curve extrapolation method. The linear elastic crack initiation thresholds obtained with compact specimens under monotonically increasing load bracketed test results obtained with dead-weight-loaded cantilever beam specimens. For this alloy, the stress corrosion cracking microfracture process was identical in stagnant and fresh-flowing seawater.
Effect of Material and Environmental Variables on SCC Initiation in Slow-Strain-Rate Tests on Type 304 Stainless Steel
Authors: Andresen PL
Published: December 2008

The effects of low-level environmental impurities, test temperature, sensitization heat treatment, and surface condition on crack initiation have been examined in Type 304 stainless steel in constant extension rate stress corrosion cracking tests. Tests were conducted at 288, 200 and 125°C in pure water environments containing 0.2, 0.2, and 1.8 ppm of dissolved oxygen, respectively. At 288°C in highly sensitized specimens, cracks initiated at less than 20% of life in pure water, with impurities accelerating initiation. The bulk pH affected initiation and failure strains, while increasing conductivity at neutral pH affected only strain to failure. At lower temperatures or lesser degrees of sensitization, crack initiation occurred later in life and impurity additions had a stronger impact. Shot-peening reduced the strain to initiation in constant extension rate tests, most noticeably at 125°C. The minimal data available for comparison support the findings of this study.
Evaluation of SCC Test Methods for Inconel 600 in Low-Temperature Aqueous Solutions
Authors: Newman RC , Roberge R , Bandy R
Published: December 2008

In late 1981, widespread leakage was encountered in Alloy 600 steam generator tubing at the Three Mile Island Unit 1 nuclear power plant. The phenomenon was identified as low-temperature intergranular stress-corrosion cracking (SCC) initiated from the inner surfaces of the tubes exposed to the primary coolant. A testing program was initiated to examine the material and environmental factors relevant to these failures, which were found to be associated with sensitization of the material and contamination of the coolant by air and sodium thiosulfate. The test solutions contained 1.3% boric acid with various additions of sulfur compounds and lithium hydroxide. Constant extension rate testing was used as the primary tool to examine environmental effects such as the inhibition of cracking by lithium hydroxide. Important effects of crack initiation frequency on the specimen potential (and therefore crack velocity) are demonstrated. In some of these tests, straining was interrupted following crack initiation, and the effects of further environmental additions were monitored by following the load decay at constant total strain. This procedure generally resulted in a different ranking of the environments. U-bend tests showed that the ranking of different sulfur-containing environments was also highly dependent on the metal heat treatment.
Threshold-Stress Determination Using Tapered Specimens and Cyclic Stresses
Authors: Fessler RR , Barlo TJ
Published: December 2008

Traditional methods of determining the threshold stress for stress-corrosion cracking involve testing a number of specimens that are loaded to various levels of constant initial stress. In research sponsored by the Pipeline Research Committee of the American Gas Association, it was shown that low-frequency, small-amplitude cyclic loads superimposed on a constant load can reduce the threshold stress substantially. Since such loading can occur on real structures, it was felt that the threshold stress should be determined under those more severe and realistic conditions. Furthermore, by using a specimen with a tapered gage section, a range of stresses could be evaluated in a single test, making it possible, in principle, to determine the threshold stress with one or two specimens. Test procedures involving tapered specimens and cyclic loads superimposed upon a constant load are described, and representative data for various line-pipe steels are presented. The test environment consisted of a water solution of sodium carbonate and sodium bicarbonate, and the temperature was controlled at 352 K (175°F). The potential was controlled potentiostatically at the center of the range in which stress-corrosion cracking occurs. Test durations typically were 1 to 2 weeks.
Computer-Controlled Fatigue Crack Growth Rate Testing on Bend Bars in a Corrosive Environment
Authors: Fabis TR , Liaw PK , Ceschini LJ , Leax TR , Landes JD
Published: December 2008

Automated fatigue crack growth experiments were conducted on a 304 stainless steel in air and wet hydrogen at 654 kPa. The tests were performed using three-point bend specimens in the frequency range from 0.1 to 10 Hz.

Results show that the fatigue crack propagation rate in the hydrogen gas environment can increase with decreasing frequency. For ΔK-values higher than 22 MPam, the crack growth rate data in wet hydrogen at 10 Hz are 40% higher than those encountered in the ambient environment. The crack growth rates developed in the constant ΔK tests are in good agreement with those in the decreasing and increasing ΔK tests in both air and wet hydrogen environments.