bluuebunny/crossref_metadata_2025 huggingface dataset:
Microtexture, Asperities, and Crack Deflection in Al-Li 2090 T8E41
Authors: Haase JD , Guvenilir A , Witt JR , Langøy MA , Stock SR
Published: March 2009

Roughness-induced closure is held to be responsible for very low fatigue crack growth rates observed in certain plate orientations of Al-Li 2090 T8E41, and the geometry of asperities producing this closure correlates with macrotexture. Little work, however, has focused on the role of individual grain orientations (microtexture) or of average orientation within small groups of adjacent grains (mesotexture) on the crack's path through a sample, i.e., on whether the variation in grains' orientations determines where the crack will deflect. This paper reports synchrotron X-ray microbeam diffraction mapping of the three-dimensional microtexture in samples of Al-Li 2090. Groups of adjacent pancake-shaped grains are found to have very similar orientations, producing nearly single-crystal regions approaching thicknesses of 0.3 mm along the sample's S (short-transverse) direction. These near-single-crystal volumes produce large asperities with surfaces having substantial Mode III character, asperities which appear over the range of stress intensity ranges observed (∼5 to ∼25 MPa√m). Results of these experiments suggest not only that this type of mesotexture plays an important role in determining fatigue crack path in compact tension samples of Al-Li 2090 but also that specific orientations of the groups of grains lead to large crack deflections.
Some Observations on Mixed-Mode Fatigue Behavior of Polycrystalline Metals
Authors: Miller KJ , Brown MW , Yates JR
Published: March 2009

Fatigue cracks are almost invariably one of two kinds, either Stage I (shear type) or Stage II (tensile type). The orientation, length, and lifetime duration of these cracks are dependent on: the loading system; the artefact geometry; and the material microstructure.

All of these factors are therefore important in the fatigue analysis of a component or a structure, but especially so when mixed-mode loading (such as tension plus torsion) and mixed-mode cracking occur. However, the prior loading history and/or the history of crack growth is frequently ignored in both theoretical and experimental studies in which a crack or defect of a given size is extended by a given loading system.

This paper records some results and some difficulties in our attempts to understand both mixed-mode loading and mixed-mode crack growth and their interactions in polycrystalline metals. In particular two regimes are considered in this study. Firstly, long crack growth thresholds under both mixed Mode (I and II) and (I and III) as well as fatigue crack propagation rates and branching criteria are discussed. Under such conditions, crack propagation behavior necessarily requires the problem of crack face interference to be addressed. Secondly, short crack behavior under multiaxial loading is studied and it is shown that multi-axial fatigue endurance can be predicted from Stage I and Stage II crack growth laws, which sometimes require crack coalescence to be taken into account.
A Fractographic Study of Load-Sequence-Induced Mixed-Mode Fatigue Crack Growth in an Al-Cu Alloy
Authors: Ashbaugh NE , Porter WJ , Prakash RV , Sunder R
Published: March 2009

Fatigue crack growth behavior was studied on C(T) and SE(T) specimens from Al-Cu alloy 2014 using specially-designed load sequences. The experiments were organized to induce microscopic mixed-mode fatigue crack growth while at the same time reducing or eliminating fatigue crack closure. This was achieved by switching between high-amplitude, low-stress ratio and low-amplitude, high-stress ratio cycles. Retardation effects observed under these conditions are attributed to local crack branching and mixed-mode conditions induced by variable amplitude loading.
Mixed-Mode Static and Fatigue Crack Growth in Central Notched and Compact Tension Shear Specimens
Authors: Shlyannikov VN
Published: March 2009

Elastic-plastic crack growth under mixed Mode I and II in six types of aluminum alloys and three types of steel were investigated. The experimental study of fatigue crack growth in six types of the aluminum alloys and one type of the steel is performed on biaxially loaded eight-petal specimens (EPS). All specimens for biaxial loading contained inclined through thickness central cracks. Mixed Mode I/II static and fatigue crack growth experiments on the three types of steels and one type of the aluminum alloy used compact tension shear (CTS) specimens. Two approaches are developed for geometrical modeling of crack growth trajectories for the central notched and compact tension shear specimens respectively. The principal feature of such modeling is the determination of crack growth direction and the definition of crack length increment in this direction. On the basis of the analysis of the experimental data for the aluminum alloys and the steels an empirical crack reorientation criterion is suggested for both brittle and ductile materials. The damage process zone size concept is used for calculations of mixed-mode crack path. The influence of specimen geometry, biaxial loading and properties of the aluminum alloys and the steels on both crack growth direction and crack path at the macroscopic scale is discussed.
Amorphous Alloys Containing Cobalt for Orthopaedic Applications
Authors: Tesk JA , Johnson CE , Skrtic D , Tung MS , Hsu S
Published: March 2009

Amorphous metal alloys have properties and structures unlike those of their crystalline counterparts. For example, a multiphase crystalline structure may exhibit poor corrosion resistance while the amorphous structure is corrosion resistant. The high hardnesses of some amorphous alloys may make them useful for wear resistant applications. Cobalt-based, electrodeposited alloys may be particularly compatible for producing desirable surfaces on orthopaedic Co-Cr-Mo alloys. Amorphous Co-20P alloy (A-Co-20P) has an as-deposited Knoop hardness number (HK) of ca 620. The surface of anodized A-Co-20P has been described elsewhere as predominately phosphorus oxide that may react with water to form an adsorbed hypophosphite layer. Others reported corrosion resistance without pitting. Hence, the potential of A-Co-20P for use as an implant coating to induce bony apposition was evaluated. This evaluation consisted of tests of corrosion resistance and solution chemistry. Another alloy, amorphous Co-Cr-C, was found to have an as-deposited HK of ca 690. Heat treatment produced Knoop hardness numbers (HKs) of ca 1350, between the HKs of zirconia and alumina. Wear and corrosion resistance are expected to be good, but adherence needs to be assessed.
Influence of Post Processing on the Mechanical Properties of Investment Cast and Wrought Co-Cr-Mo Alloys
Authors: Berlin RM , Gustavson LJ , Wang KK
Published: March 2009

The mechanical properties of investment cast Co-Cr-Mo alloy (American Society for Testing and Materials F 75) can be affected to varying degrees by post cast processes such as solution treating (ST), hot isostatic pressing (HIP), sintering used to apply porous coatings, repair welding, abrasive blasting, and laser marking. The mechanical properties of the wrought version of the alloy (American Society for Testing and Materials F 1537) can be influenced by mill practices. Thermo-mechanical processing such as forging, will change the properties of mill products depending on forging practices. Post forging processes such as abrasive blasting and laser marking can affect the mechanical properties to varying degrees.

Testing has shown that abrasive blasting has no significant effect on either alloy. Laser marking can reduce the fatigue strength of both alloys. Sintering the cast alloy will reduce the fatigue strength and that HIP will improve the fatigue strength of the sintered cast alloy. Also, the cast alloy can be repair welded with no loss in tensile properties.
Process Metallurgy of Wrought CoCrMo Alloy
Authors: Lippard HE , Kennedy RL
Published: March 2009

This paper will describe the processing involved in the manufacture of low and high carbon grades of ASTM Wrought Cobalt-28 Chromium-6 Molybdenum Alloy for Surgical Implants (F 1537) as bar stock for use in a wide variety of biomedical applications. The effect of rolling temperature, annealing temperature, and cold drawing on the mechanical properties and grain size will be explored. Careful control of the thermomechanical working conditions is required to meet high strength requirements because age hardening and cold working of the alloy significantly degrade the ductility while producing the higher strength levels.
Residual Abrasive Material from Surface Grinding of Metal-Metal Hip Implants: A Source of Third-Body Wear?
Authors: Wang A , Bobyn JD , Yue S , Medley JB , Chan FW
Published: March 2009

The surfaces of fifteen heads of all metal hip implants were investigated using a scanning electron microscope. The heads were manufactured from Co-Cr-Mo alloy, either cast, low carbon wrought or high carbon wrought. Six of the surfaces were investigated prior to hip simulator testing, three after 3 million cycles of simulator testing and six after 6 million cycles of testing. Third-body wear type scratches of varying amounts were observed on all fifteen heads. Residual grinding stone material from the manufacturing process was also found at varying amounts on the surfaces of all fifteen heads. There was a correlation between the extent of this residual material and the extent to which the surfaces were scratched.
An Overview of PVD Coating Development for Co-Based Alloys
Authors: Pellman MA
Published: March 2009

PVD coatings such as titanium nitride (TiN), zirconium nitride (ZrN), and more recently diamond like carbon (DLC), can improve the corrosion and abrasion resistance of medical devices. The success of these coatings has been based on their ability to improve wear resistance and corrosion resistance. The materials properties, biocompatibility and performance of commercially available PVD coatings are discussed along with applications for orthopedic implants and dental devices. The next generation of coatings for biomaterials applications is in development. These coatings include tetrahedral amorphous carbon and DLC coatings. The differences between these processes and traditional PVD will be reviewed. In addition, preliminary performance and tribological data will be presented.
Spray Droplet Size and Concentration of Metsulfuron-methyl Affects Ethylene Production in Oranges
Authors: Salyani M , Burns JK , Kender WJ
Published: March 2009

Metsulfuron-methyl has been suggested as a new experimental abscission chemical for harvesting Florida citrus fruit. The objective of the research was to determine the effects of spray droplet size and active ingredient (a.i.) concentration on the efficacy of the product.

‘Hamlin’ and ‘Valencia’ orange explants (fruit with peduncle) were sprayed using different droplet sizes or dipped in metsulfuron-methyl solution at three concentrations. Treatment solutions also contained adjuvants Kinetic and Buffer Xtra. The treated explants were stored under constant temperature and humidity conditions and assayed at 3, 5, 7, 9 or 10 days. The metsulfuron-methyl induced internal ethylene was measured by gas chromatography and used as response variable.

Droplet size, a.i. concentration and measurement day had significant effects on ethylene production. Although the response was highly variable, ethylene production tended to increase with droplet size and metsulfuron-methyl concentration. Overall, the variability of response increased at the higher concentrations.
Spray Coverage of Potato Plants Using Various Types of Applicators
Authors: Hofman VL , Panigrahi S , Secor G , Preston D , Gudmestad N
Published: March 2009

Total coverage of potato plants with fungicide usually provides the best protection from disease. A technique has been developed to measure the spray coverage on plant leaves with the use of fluorescent dye, a CCD camera, and a computer with the appropriate software to determine area of coverage. This method is used to compare the coverage efficiency of various types of spray equipment. Several applicators have been studied, including conventional sprayers, air assist sprayers and spray planes. Air-assist sprayers have shown slightly improved application efficiency as compared to conventional sprayers at equal application rates. Most ground sprayers show improved coverage over aircraft, but the application rate for ground sprayers is usually much higher than for aircraft.
Exploring the Role of Formulation and Adjuvant Chemistry in Pesticide Deposit Formation
Authors: Downer RA , Hall FR , Cooper JA , Fox RD
Published: March 2009

Spray retention by foliar targets is influenced by both biological and rheological factors. Plant morphology (leaf surface-structure, degree of pubescence, leaf angle) combined with the spray liquid properties (surface tension, viscosity, droplet sizes, and velocities) all affect spray retention and subsequent deposit formation. Techniques used to explore these phenomena include on-demand droplet generators, high-speed movies and video-imaging. Maximum bubble pressure and oscillating jet technologies allow surface tension measurements to be made at droplet surface ages consistent with the impaction process. Judicious formulating and increased use of adjuvants has created many new opportunities for manipulating spray and deposit formation. However, recently proposed tactics advocating the use of coarse sprays to reduce drift have implications for pesticide targeting and subsequent pest control. This paper focuses on what has been learned about the phenomena of impaction, retention and deposit formation and suggests that while improved retention of pesticides by target plants is both practical and achievable, pesticide use rate reductions may not be so until the relationship between retention and efficacy is better understood.
Improvements in Water Dispersibility and Stability of Starch-Containing Agricultural Formulations Using Vinylpyrrolidone Copolymers
Authors: Goehner RH , Narayanan KS , Curry JF
Published: March 2009

Starch can be used as a suitable filler for granules and pellets, or as an encapsulating agent in agricultural formulations. Starch has the appeal of being a safe and biodegradable natural food ingredient. However, improvements can be made in the water dispersibility and stability of starch-containing formulations. Vinylpyrrolidone copolymers are shown to improve the suspensibility of 2% starch in water. Use of cornstarch to encapsulate active ingredients has been described in the literature. In the present study, vinylpyrrolidone copolymers have been used to reduce the temperature needed for gelatinization. Examples of prototype formulations illustrating the use of polymer-modified starch are given. A working model exemplifies the balance of hydrophobic and hydrophilic moieties required in the vinylpyrrolidone copolymers to enhance dispersion.
Modification of Release Rate of Encapsulated A.I. Through Fluid Selection
Authors: Verbelen RA , Lemoine S
Published: March 2009

Microencapsulation is one of the most important ways to control the release rate of a pesticide. Such a technology provides many advantages, such as an adjustable release of the core active ingredient (a.i.), stabilization against degradation of the pesticide, reduced toxicological properties and environmental impact. The release rate of an encapsulated pesticide can be modified, according to the requirements of the application, through fluid selection. Some fluids contribute to accelerate the delivery of liquid or solid a.i., others slow it down.

Several microcapsule formulations have been prepared with a selection of fluids (solvents), while keeping the other capsule parameters constant. The results reported in this paper demonstrate that the fluid, chosen among dearomatized and isoparaffinic hydrocarbons, aromatic hydrocarbons, acetate derivatives and blends of these, significantly influences: - The residual efficacy of Chlorpyrifos on cockroaches — The extent of growth inhibition of fungi by Propiconazole — The diffusion of the a.i. through rat skin.

By matching the biological control patterns with the expected dermal toxicity of a formulation, the formulator can select the specific fluid that better suits his requirements.
A Discussion of the Dependence of Cuticular Mobility on Molecular Volume
Authors: Fowler JD
Published: March 2009

The last few years have seen the publication of experimental cuticular mobilities for diverse chemicals in several plant species. The measurements are made in vitro using either cuticles isolated enzymatically, or using reconstituted wax extracts from the cuticles; it can be shown that these different methods are consistent with each other. Mobility parameters have been measured both in the presence and absence of uptake-enhancing adjuvants. After accounting for partitioning effects, cuticular diffusion coefficients can be calculated and these diffusivities are observed to have a simple dependence on the molar volume of the diffusing chemical.

This paper presents anecdotal evidence of several kinds to suggest that this dependence can be used to predict when an active ingredient will be sensitive to the use of mobility-enhancing adjuvants. Specifically, it is expected that foliar-active herbicides will be sensitive to such adjuvants if the logarithm of their cuticular mobility [m2/s] is between about -16 and -15. We show also that in the presence of an optimal adjuvant for the specific system, the movement of a chemical through the cuticle can conform to the familiar case of diffusion through a fluid, and is governed by the Stokes-Einstein relation when modified to account for the effect of hindered diffusion through the narrow transport paths within the cuticle. The observed dependence of diffusivity on molar volume then implies a very low value for the “free” volume (voidage) that is available for diffusion within the cuticle. In turn this low value implies the presence of structural features within the cuticle, and microscopic studies of the cuticle are consistent with this interpretation. Thus there appears to be a direct connection between in vitro measured cuticular mobilities, the need for adjuvants to promote uptake, and the structure of the plant cuticle.
Interaction Studies with Grafted Butene Vinyl Pyrrolidone Copolymer and Ligninsulfonates
Authors: Jon DI , Narayanan KS , Curry J
Published: March 2009

Interactions between a graft copolymer of polyvinylpyrrolidone and butene (AGRIMER™ AL10-LC)2 and ligninsulfonates were demonstrated by surface tension measurements. Two types of ligninsulfonates: LSA and LSB showed differences in their surface activities. A surface tension lowering effect was observed with the more surface-active ligninsulfonate (LSB) and graft copolymer. However, no such effect was found with the less surface-active ligninsulfonate (LSA). When the mixed system [graft copolymer + LSA] and [graft copolymer + LSB] were stored at 52°C for 4 months, the surface tension of water in the region below the surface saturation region was lowered for LSA and increased for LSB when compared with room temperature samples. There was no apparent change in storage under the above conditions in the macroscopic properties of the components. The change in the surface activity on accelerated storage at high temperature is hypothesized to arise from changes in the conformation of the components in the matrix.

Adsorption of the graft copolymer, LSA and LSB individually and in combination onto an active ingredient (chlorothalonil) was also studied via surface tension concentration profiles. The results are consistent with preferential adsorption of the graft copolymer in the presence of ligninsulfonate on chlorothalonil.
The Phytotoxicity of Solvents to Seeds
Authors: Frisch PD
Published: March 2009

Seed treatment is a rapidly growing segment of the agricultural chemical industry. Two factors have contributed to this. First, the technology of building crop protection and/or specific attributes into the genetic makeup of the plant has led to a significant increase in the cost of the seed. Second, seed exposure has increased because it is being planted earlier into cold, often hostile, no-till soils that provide a refuge for soil borne diseases and insects. Seed treatments provide a way to protect the farmers' investment by controlling specific diseases that affect seeds prior to, during and after germination.

Historically, seed treatments are applied as water-based flowable formulations but these water-based systems have deficiencies. This study examines the use of hydrocarbon and oxygenated solvents as possible alternatives to water. Solvent-based systems offer added flexibility to the formulation of seed treatment products. If solvents are to be used in this application, a critical first question, which must be addressed, is their phytotoxic effect on the seed. Different types of hydrocarbon and oxygenated solvents of differing molecular weights have been screened with sensitive varieties of major crop seeds (corn, soybean, wheat, cotton). Rates of germination will be discussed as they are influenced by the type of solvent, its volatility and the treatment rate.

It will be shown that selected solvents can be safely used in a seed treatment program and that, within a given solvent type, the key to solvent selection is to minimize contact time with the seed by using a more volatile solvent or a lower treatment rate.
The Formation of a New ASTM E35.22 Task Group to Address Drift Management Adjuvants
Authors: Hewitt AJ , Stern AJ , Bagley WE , Dexter R
Published: March 2009

Drift control agents are often used in pesticide drift management. Their effect is usually intended to be through increasing the droplet size produced in an application by reducing the evaporation rate of droplets and/ or other mechanisms, such as active ingredient encapsulation, or electrostatic charging. Such effects can be measured; however, standard techniques and guidelines are not currently available for many of these assessments specifically for drift control agents. The need for standard guidelines has given rise to the formation of a new task group within the American Society for Testing and Materials (ASTM). This task group falls under the jurisdiction of ASTM Subcommittee E35.22, Pesticide Application and Formulation Systems. The present paper presents a review of the main mechanisms by which drift control agents may operate, and lists the standards and test guidelines that are currently available for assessing some of these mechanisms. This review is intended to serve as a starting point for the task group to define its mission statement, purpose and scope; and to identify current resources that are available for describing drift control agents, while showing the needs for additional resources.
Validation of Filters Used for Sterilizing Liquids
Authors: Martin JM , Cole S , Geerligs G , Howard G , Jaenchen R
Published: March 2009

Validation of sterilizing grade filters in biopharmaceutical production is necessary to assure the filter's efficiency or performance as part of a process, and to ensure the safety and reliability of both the process and its end product. A review of different aspects of validating filters used for sterilizing liquids, taking into account potential influences of process conditions and product formulations is presented.
Points to Consider Before Validating a Liquid Nitrogen Freezer
Authors: Simione FP , Karpinsky JZ
Published: March 2009

The use of liquid nitrogen freezers for low-temperature storage of cells and cell substrates is a common practice in cell banking. Assurance of proper storage temperatures in the vapor phase of liquid nitrogen requires not only adequately designed cryogenic dewars, but verification of liquid levels and resulting working temperatures. Validation of liquid nitrogen freezers requires attention to several aspects of the storage system: (1) design of the freezers and inventory systems, (2) verification of the relationship of the liquid level to working temperature, (3) calibration of all testing equipment, and temperature and liquid level display units on the freezers, and (4) calibration and verification of all alarm systems. Validation procedures must be clearly delineated and followed, and should also include training of personnel and attention to safety issues.
Sensitivity Considerations in Hydrophobic Filter Integrity Testing
Authors: Steere WC , Scheer LA , Hubbert M
Published: March 2009

Fifteen hydrophobic cartridge vent filters are integrity tested using diffusion and intrusion (water breakthrough) methodologies. Several filters are deliberately punctured. All are then re-tested to determine the ability of each method to detect the loss in sterilizing capability. Bacterial challenge per HIMA guidelines of selective filters is performed as a confirmation of sterilizing integrity. Data obtained do indicate an effectiveness of both the diffusion and the water intrusion test method to identify losses in integrity. However, water intrusion or breakthrough results show a limitation in ability to consistently identify those filters with compromised sterilizing capability. Inconsistent fill volumes are addressed as a source of error in breakthrough testing. Inherent advantages and disadvantages of the two methodologies are presented. Though the convenience of breakthrough testing is offset by an inability to identify a loss in membrane integrity, there remain circumstances where breakthrough testing may have a place.
Designing Validation into Chromatography Processes
Authors: Sofer GK
Published: March 2009

Chromatographic processes are used to obtain the purity required for biotechnology products. To avoid excessive costs, validation planning is optimally begun during process development. Designing validation into the Chromatographic process requires training researchers in the importance of validation. Once the process is defined, it can be successfully scaled up to prepare clinical trial material. SOPs must be written for buffer preparation, operation and maintenance of the columns and systems, storage, cleaning, and criteria for acceptability of a column for initial use and lifetime determination. If research has defined the key elements, these SOPs can be written and implemented without further excessive experimentation that leads to regulatory delays.
A New Methodology for Quality Control Testing of Biological and Recombinant Products
Authors: Zaslavsky BY , Chaiko DJ
Published: March 2009

Manufacturing of biological therapeutic agents such as growth hormone, tissue plasminogen activator, alpha interferon, and PEGylated proteins requires new analytical methodologies for characterization and quality control of the products.

A new methodology based on the technique of partitioning in aqueous polymer two-phase systems is recommended for quality control testing. The advantages of the methodology are that it provides unique information that is related to the biological potency, and it is rugged and simple. Here we describe the basics of the technique and the quantitative information it provides and give examples of how the technique can be applied for characterization and quality control of biological and recombinant products.
A New Method for Predicting Extensive Ductile Tearing Using Finite Element Analysis
Authors: Gentilcore ML , Roberts RH
Published: March 2009

A new finite-element model is proposed that can predict the relationship between load, displacement, and crack length for high-toughness, thin plate structural steel members. These types of members exhibit stable tearing for large amounts of crack extension, providing residual capacity for deformation well beyond the point of maximum load. This behavior must be accurately predicted in order to take full advantage of high-toughness plate steels in structural design. The new model uses a strength of materials approach to simulate plastic collapse of the material ahead of the crack tip, allowing the effect of localized necking to be simulated without the need for large-deformation analysis. A simple engineering stress-strain curve is used to define material properties throughout the model. No special crack tip elements or detailed mesh refinement is required. Predictions made using the model are compared to experimental data from an HSLA-80 center-cracked plate specimen and a full-scale I-beam specimen that featured over 150 mm of crack extension.
Elastic-Plastic Crack Growth Simulation and Residual Strength Prediction of Thin Plates with Single and Multiple Cracks
Authors: Chen C-S , Wawrzynek PA , Ingraffea AR
Published: March 2009

This paper presents numerical simulations of elastic-plastic, Mode I crack growth in thin aluminum plates. Two series of tests are simulated, M(T) specimens, and specimens containing multiple co-planar cracks. Elastic-plastic, shell finite element analysis is used to predict the residual strength of the tested plates. The crack tip opening angle (CTOA) measured at a specific distance behind the crack tip is the fracture criterion to characterize stable crack growth under conditions of general yielding. For small M(T) specimens, the limit load is attained due to net section yielding after the plastic zone reaches a free edge. In contrast, the residual strength of large specimens is achieved after some amount of crack growth with the plastic zone well-confined by the elastic region. Results of predicted residual strength are comparable to experimental measurements. However, as the width of the panel increases, the relative difference between experimental measurements and numerical predictions increases. The same CTOA fracture criterion is used to simulate crack growth and link-up in the multiple crack specimens. Predicted link-up load and residual strength are in good agreement with experimental measurements. A loss of residual strength due to the presence of multiple small cracks is observed.
A Numerical Investigation of Loading Rate Effects in Pre-Cracked CVN Specimens
Authors: Koppenhoefer KC , Dodds RH
Published: March 2009

Specimen size and loading rate effects on cleavage fracture and R-curve response remain important technological impediments to industrial application of the pre-cracked Charpy specimen. This investigation employs plane strain and 3-D, nonlinear finite element analyses to conduct an evaluation of crack front stress triaxiality and resistance to ductile tearing for static and impact-loaded, pre-cracked CVN specimens. For cleavage fracture, crack front conditions are characterized in terms of the Weibull stress. To examine impact loading rate effects on resistance to ductile tearing, this investigation employs the computational cell methodology to analyze stable extension of a macroscopic crack in ductile metals under quasi-static and impact loading.
Evaluation of Stress-Intensity Factors Using General Finite-Element Models
Authors: Smith SA , Raju IS
Published: March 2009

Finite-element methods are commonly used to evaluate cracked solids. Post-processing methods are used to extract Mode I stress-intensity factor values from finite-element analyses. These methods include the Crack-Opening-Displacement (COD) method, the Force method, the Virtual Crack Closure Technique (VCCT) and the Equivalent Domain Integral (EDI) method. The COD method, Force method and the VCCT appear to require that the finite-element mesh intersect the crack front in an orthogonal manner in order to obtain accurate stress-intensity factor values. The EDI does not appear to require this orthogonality with the crack front to obtain accurate stress intensity factor values. The objectives of this study are to determine if accurate stress intensity factor values can be obtained from finite-element models that lack orthogonality with the crack front and, if accurate values cannot be obtained, to modify the extraction methods so that accurate stress-intensity factor values can be obtained from models without orthogonality at the crack front.

Models of a through-the-thickness crack in plane strain, an embedded elliptic crack, and a semi-elliptic surface crack were created with and without orthogonality at the crack front. The models with orthogonality at the crack front were termed orthogonal models while the models without orthogonality the crack front were termed general models. Stress-intensity factors were evaluated for both the orthogonal and general models using the COD method, Force method, VCCT, and EDI method. The stress-intensity factor values obtained from these models were compared to accepted reference solutions to determine their accuracy. The COD method, Force method, and VCCT were not able to extract accurate stress-intensity factor values from general models. The EDI method was not affected by the lack of orthogonality at the crack front.

The COD method, Force method, and VCCT were modified for application to general models. These new formulations do not require the finite-element mesh to be orthogonal to the crack front. The values of the stress-intensity factors obtained from the general models using the new formulations of the extraction methods are of the same accuracy as those obtained from the models with orthogonality at the crack front.
A Simplified Transformation Approach to Obtain Structural Calibration Functions
Authors: Cruz JRB , Landes JD
Published: March 2009

In a ductile fracture methodology developed by Landes et al., the result from a laboratory test is used to predict the behavior of a structural component. Within the framework of this methodology, a critical step consists in obtaining the calibration function for the structural component. For those cases where the limit load solution for the structural component is known, a transformation procedure has been used to get the calibration function for the structure directly from that of a fracture toughness specimen. Although the transformation procedure does not involve complex calculations, it does require the user to follow several steps where point to point computations are necessary. This makes the whole process laborious and time consuming. It will be shown in this paper that, with an additional assumption and without loss of accuracy, the transformation procedure can be greatly simplified. In the alternative procedure proposed here, the coefficients of the calibration function for the structure are obtained by simply scaling then counterparts for the fracture toughness specimen. This is accomplished with the use of two factors: a load factor and a deformation factor. Some examples are presented to demonstrate the convenience of this new procedure.
A New Theoretical Framework for Inelastic Fracture Processes
Authors: Rashid MM , Roy R
Published: March 2009

The Exclusion Region theory is a new theoretical construct that attempts to address both the difficulty of solving the boundary-value problem in the presence of a crack, and the subsequent difficulty of extracting from the solution a physically relevant fracture criterion. The theory starts with the presumption that, within a small radius of the separation front, the stress field as determined from the local constitutive model does not provide a meaningful representation of the internal force distribution for purposes of imposing equilibrium. Equilibrium (or momentum conservation) therefore cannot be applied pointwise within this exclusion region to solve for the displacement field. Instead, the material displacement field within the exclusion region is given by an assumed form that accommodates the separation of a material surface into a pair of new free surfaces. The enriched kinematics of the exclusion region may therefore be regarded as simply leading to a broadened constitutive model, which only applies to a small material neighborhood that is suffering surface separation. Further, and in contrast to conventional fracture mechanics, the form of the crack advance criterion does not restrict the material model that may be applied in the bulk continuum. The Exclusion Region theory has been implemented in a two-dimensional finite element code. The code uses a novel numerical procedure in which the moving separation front lies at the center of a translating, disc-shaped patch of elements. Compatibility at the interface between the mesh patch and the fixed background mesh is enforced weakly. Numerical studies have demonstrated complete correspondence between the new theory and linear elastic fracture mechanics. Preliminary results involving large deformations and extensive plastic deformation are also presented.
On the Gurson Micro-Mechanical Parameters
Authors: Zhang ZL , Hauge M
Published: March 2009

The Gurson model based approach has in recent years gained surprising popularity in describing ductile tearing phenomena. A lot of pioneer work has demonstrated the predicting power of the Gurson model. An important aspect in the application of the approach lies in how to characterize the parameters involved in the Gurson model. There is a non-uniqueness problem in determining the Gurson parameters, and how to link the Gurson parameters to material properties is generally open. In this study, the parameters involved in the approach are classified into gradient-sensitive parameter which is active when there is a strong strain gradient, and gradient-insensitive parameters, which include hardening parameters, constitutive parameters, initial micro- mechanical parameters and critical micro-mechanical parameters. It is suggested that the gradient-insensitive parameters are to be determined from smooth and notched tension tests where the strain gradient is not severe, while the gradient-sensitive parameter is fitted from fracture toughness tests. Regarding the fact that there is no built in coalescence criterion in the Gurson model, a physical mechanism based failure criterion for void coalescence is implemented, resulting in a complete Gurson model. It is pointed out that the initial parameters which describe the material's void nucleation law are the intrinsic parameters of a material, while the critical void volume fraction at void coalescence is a material specific response, which in general is dependent on stress state. A method based on the failure criterion is proposed for calibrating the initial parameters. According to the method, the number of unknowns in the approach can be reduced and the non-uniqueness problem of the micro-mechanical parameters for some materials can be solved.
Conditions Causing Intergranular Cracking in High Strength Nickel-Copper Alloys
Authors: Natishan ME , Wagenhofer M
Published: March 2009

Nickel-Copper Alloy K-500 products have exhibited rate-dependent, intergranular failure at temperatures as low as room temperature under ambient (low humidity), static load conditions. Failure probability has been correlated to the amount of grain boundary carbon precipitates existing in this alloy [1]. As part of an ongoing investigation to bound the service conditions of intergranular failure in nickel-copper alloy K-500 containing grain boundary precipitates of carbon, previously tested stress-rupture specimens were examined to document and characterize the deformation associated with grain boundary failures. Of particular interest was defining the role of the grain boundary carbon precipitates in the rate-dependent intergranular fracture process under ambient conditions. The failed creep/stress rupture specimens were sectioned perpendicular to the fracture surface and prepared for optical and electron microscopy of the microstructure adjacent to the primary fracture. High resolution microscopy revealed voids opened along the grain boundaries at carbon precipitate/nickel grain boundary interfaces. A diffusional-creep failure mechanism is postulated.
New Perspectives on the Fracture of Nicalon Fibers
Authors: Taylor ST , Zhu YT , Blumenthal WR , Stout MG , Butt DP , Lowex TC
Published: March 2009

Experimental studies have been conducted to examine the strength and fracture behavior of monofilament Nicalon™ SiC fibers ranging in diameter from 8 to 22 μm. The effects of varying fiber diameter, flaw location and flaw population on the tensile behavior of individual fibers were investigated using fractography. Results indicate that variations in fiber diameter influence the apparent fiber fracture toughness (K1c), with higher K1c values observed for decreasing fiber diameters. Observations also suggest that the location of the critical flaw may play a role in the fracture of Nicalon fibers. In addition to surface flaws, three distinct internal flaw populations are seen to cause fracture in Nicalon fibers.
Stress Ratio Effects on Crack Opening Loads and Crack Growth Rates in Aluminum Alloy 2024
Authors: Riddell WT , Piascik RS
Published: March 2009

The effects of stress ratio (R) and crack-opening behavior on fatigue crack growth rates (da/dN) for Aluminum Alloy (AA) 2024-T3 were investigated using constant-ΔK testing, closure measurements, and fractography. Fatigue crack growth rates were obtained for a range of ΔK and stress ratios. Results show that constant ΔK fatigue crack growth for R ranging from near 0 to near 1 is divided into three regions. In Region I, at low R, da/dN increases with increasing R. In Region II, at intermediate R, fatigue crack growth rates are relatively independent of R. In Region III, at high R, further increases in da/dN are observed with increasing R.

Near-crack-tip surface closure measurements, using a new noncontact displacement method, show that Region I transient crack growth rates are a result of crack closure. Increasing fatigue surface microvoid density, coinciding with increasing da/dN for Region III fatigue crack growth, suggests that a Kmax-dependent damage mechanism is responsible for the transient fatigue crack growth rates characteristic of Region III. For AA 2024-T3, the onset of Region III correlates with a critical value of Kmax (13 MPa m1/2). Region II fatigue crack growth is unaffected by both closure and Kmax effects. In this intrinsic region, ΔK = ΔKeff Rates from this region are used to test the validity of closure measurements for Region I fatigue crack growth. Poor correlation of da/dN and ΔKeff occurs when ΔKeff are calculated from the fully open load obtained by the reduced strain technique. Good correlation of da/dN and ΔKeff are found when ΔKeff is calculated from local opening loads approximately 0.1 mm behind the crack-tip.
Reflecting on the Mechanical Driving Force of Fatigue Crack Propagation
Authors: Lang M , Marci G
Published: March 2009

The goal of the work to be presented is a fracture mechanics methodology for the description of fatigue crack propagation (FCP) in which the mechanical driving force has a unique relation to FCP. A review of FCP literature leads to two basic premises for FCP, namely that a) ΔK indirectly describes the stress and strain excursions in the plastic zone, and b) tensile stresses and strains in the near crack front region are a necessary condition for FCP. It is shown that closure as defined by Elber can only act like some externally applied tensile load. Therefore, closure cannot be the mechanism that transfers the external cyclic loading condition to the mechanical driving force. A model is proposed which includes the closure mechanism, but the transfer from external cyclic loading to the mechanical driving force is solely due to the stresses and strains in the plastic zone ahead of the crack front, as originally proposed by Schijve. The stress intensity factor at which the stresses and strains in the near crack front region switch from compression to tension during the loading part of a cycle has been experimentally determined for various cyclic loading conditions. This level is denoted as KPR, the crack propagation stress intensity factor, and allows the determination of the mechanical driving force, ΔKeff.
An Analytical Model for Studying Roughness-Induced Crack Closure
Authors: Chen N , Lawrence FV
Published: March 2009

A 2-D analytical model of the combined effects of plasticity-induced crack closure (PICC) and roughness-induced crack closure (RICC) has been developed. A zigzag crack growth path was employed to simulate surface roughness. The two opposing fatigue crack surfaces were translated by the mixed-mode displacement occurring near the deflected crack tip. This translation caused a mechanical interference of the opposing crack surfaces as the crack closed, that is, caused RICC. The gradual transition from RICC- to PICC-dominated crack closure was handled naturally by this model. The influence of the geometrical features of the surface roughness, R-ratio, and the cyclic load range on RICC were examined using the PICC-RICC model. The effect of microstructure on the RICC was examined. Despite the simple, 2-D nature of the model, it predicted the trends of experimental data on near-threshold crack growth behavior reported in the literature.
Fatigue Durability Enhancement by Controlled Overloading
Authors: Tipton SM , Sorem JR
Published: March 2009

Controlled overloading techniques, such as the proof loading of lifting components, the overspeed testing of turbomachinery, and the overpressurization of high-pressure components, are routinely employed to affirm the structural integrity of engineering components and systems. This usually occurs following the manufacturing process but prior to service, and typically without regard to its influence on fatigue behavior under service loading. Proof loading often induces localized yielding and subsequent residual compressive stress at regions of stress concentration, which can lead to an increase in fatigue durability for some types of service loading. The nature of residual stresses imposed by overloads is described and their interaction with fatigue mechanisms overviewed. A technique using notch strain analysis and the “reverse plasticity criterion” is described for predicting optimal proof load levels and its domain of applicability is discussed. Through example applications, it is demonstrated how the approach can enhance fatigue lives by orders of magnitude.
Residual Stress Effects in Railroad Rail Fatigue
Authors: Fry GT , Jones HL , Jones SL
Published: March 2009

Subsurface, and thus invisible, rail-head fatigue defects are sometimes the first link in a chain of events leading to train derailment. This paper presents critical-plane fatigue analyses of the head region of railroad rail. Particular attention is given to assessing the role of residual stresses in the fatigue crack nucleation process. The results of the analyses suggest that nucleation of rail-head defects occurs preferentially in a material region below the running surface of the rail: a fact borne out by field observations. Analytically, subsurface nucleation can be explained by the presence of subsurface tensile residual stresses which counterbalance surficial compressive residual stresses. Based on the results from the fatigue study, an analysis of rail grinding is discussed in the context of preventing the formation of subsurface rail-head fatigue defects.
The Use of Almost Complete Contacts for Fretting Fatigue Tests
Authors: Ciavarella M , Demelio G , Hills DA
Published: March 2009

In this paper we analyse the frequently used bridge-type fretting fatigue tests, by idealizing the specimens as having almost-flat-ended feet, but with small blend radii. An estimate of the shearing force developed is given, and the relevant partial slip contact problem is solved. We then go on to examine further characteristics of the problem such as the local density of energy expenditure against friction, and the crack tip stress intensity factors found for cracks.