bluuebunny/crossref_metadata_2025_split huggingface dataset:
An Elastoplastic Finite-Element Investigation of Crack Initiation Under Mixed-Mode Static and Dynamic Loading
Authors: Ahmad J , Barnes CR , Kanninen MF 
Published: March 2009

The fracture mechanics techniques needed for materials that fracture in a highly ductile manner must give explicit attention to the extensive plastic deformation surrounding the crack tip. Large-scale crack-tip plasticity has two effects. First, because of crack-tip blunting, crack growth initiation tends to occur under conditions for which conventional linear elastic fracture mechanics analyses are invalid. Second, significant amounts of stable crack growth can occur prior to fracture instability. To cope with these essentially inelastic processes, elastic-plastic fracture mechanics techniques are now being widely pursued [1].2 The research reported in this paper builds on these developments to provide the basis for fracture mechanics treatments under the more general conditions involved in dynamic mixed-mode loading.
Current work in elastic-plastic fracture mechanics is largely confined to quasi-static loading conditions and to crack growth in the opening mode (Mode I). Yet, in impact-loaded structures, crack initiation could occur under dynamic conditions with a combination of both the opening mode and the sliding mode (Mode II). Thus, it is necessary to obtain a quantitative understanding of crack growth initiation, propagation, and arrest under initial combined-mode dynamic loading conditions. The research described here is aimed at developing this understanding through a program of integrated experimental and finite-element analysis work.
Elastic Field Surrounding a Rapidly Tearing Crack
Authors: Kobayashi AS , Lee OS 
Published: March 2009

The transient elastic stress field surrounding a rapidly tearing crack in a single-edged notch (SEN) specimen is analyzed by dynamic photoelasticity using a 16-spark gap Cranz-Schardin camera system. The 1.6-mm-thick annealed polycarbonate SEN specimens produced rapidly tearing cracks with approximately 11-mm-long necked regions which were modeled as Dugdale strip yield zones. In addition, the wake of the residual compressive stress along the rapidly tearing crack was modeled statically by a single force parallel to the crack and acting close to the physical crack tip. Theoretical isochromatics generated by this residual-stress-corrected, propagating Dugdale model of a rapidly tearing crack agreed reasonably well with those obtained experimentally by dynamic photoelasticity.
The experimental analysis showed that the length of Dugdale strip yield zone remained essentially constant through much of the rapid tearing. The crakc-tip opening angle also remained constant, while the crack-mouth opening displacement increased during the terminal stage of crack propagation.
Elastic-Plastic Analysis of a Nozzle Corner Crack by the Finite-Element Method
Authors: Brocks W , Erbe H-H , Noack H-D , Veith H 
Published: March 2009

The paper presents the results of an elastic-plastic three-dimensional finite-element analysis for a nozzle corner crack in a pressurized reactor test vessel. The calculations were performed by the finite-element program ADINA using the von Mises yield condition and isotropic hardening. The crack plane was taken parallel to the axis of the vessel and the crack front straight and perpendicular to the symmetry line of the nozzle corner in order to obtain the worst position for a nozzle corner crack. The calculations were performed up to that pressure level where general yield of the ligament in the nozzle corner section takes place.
For the chosen crack length, which is relatively small in comparison with the ligament, the stress normal to the ligament plane and the crack-opening displacement have a maximum in the center of the crack. The differences between the maximum and minimum values along the crack front increase rapidly in the plastic range when the pressure load is raised above half of the general yield pressure. The results of the finite-element analysis are compared with figures obtained from analytical procedures of elastic-plastic fracture mechanics.
Elastic-Plastic Finite-Element Analysis for Two-Dimensional Crack Problems
Authors: Hilton PD , Gifford LN 
Published: March 2009

The specialized finite-element program, PAPST (plastic axisymmetric and planar structures), has been developed by the Navy for analysis of elastic-plastic crack problems. After a number of years of PAPST development, internal usage [1,2]3 and refinement, preliminary documentation [3] has now been issued for public release. PAPST includes a finite strain treatment [4] (with a small strain option) for incremental (J2) plasticity. Both power-hardening and multilinear models for uniaxial response are available. The singular core element [5] can be employed to enforce the Hutchinson-Rice-Rosengren crack-tip singularity for power-hardening materials [6, 7] while the enriched element [8] may be used to model crack-tip response for the multilinear material model. For crack problems, both path values for the J-integral and values based on the amplitude of the singular solution can be automatically computed.
PAPST has been used within the Navy research and development community to evaluate response of specimens of varying size, geometries, and material properties. Results for planar and axisymmetric configurations are presented to demonstrate the applications of PAPST to elastic-plastic crack problems.
Elastic-Plastic and Fully Plastic Analysis of Crack Initiation, Stable Growth, and Instability in Flawed Cylinders
Authors: Kumar V , German MD , Shih CF 
Published: March 2009

In this paper a procedure has been developed for examining crack initiation, stable growth, and instability in flawed cylinders. Elastic and fully plastic solutions are presented for cylinders containing axial and circumferential cracks located at the inner surface. In the case of an axial crack the loading is due to internal pressure and a plane-strain analysis is carried out. The circumferentially cracked cylinder is loaded by remote uniform tension and is treated as an axisymmetric problem. The solutions are based on the deformation theory of plasticity and employ a special incompressible finite-element method. Solutions cover a broad range of values for the wall thickness to radius ratio, crack length to wall thickness ratio a/b, and strain-hardening exponent n. The relevant crack parameters such as the J-integral, crack mouth opening displacement, and load-point displacement are suitably normalized and tabulated. Accuracy of the elastic solutions is assessed by a comparison with available literature solutions and some other consistency checks.
By appropriately combining the elastic and fully plastic solutions developed, simple estimation formulas are derived through which the relevant crack parameters can be readily computed in the entire range of elastic-plastic deformation. Corresponding results for some illustrative problems are compared with detailed finite-element calculations based on both flow and deformation theories of plasticity. Example J-integral crack driving force diagrams are presented for materials like A533B and 304 stainless steel. Since equilibrium of crack growth requires that applied J equal the material resistance, a superposition of the material JR-curve on such diagrams yields complete information regarding crack initiation, amount of stable growth, instability, and the overall deformation behavior.
Finally, the instability of crack growth under J-controlled growth conditions is examined along the lines of the tearing modulus, TJ, approach. J versus TJ stability diagrams are presented for both axial and circumferential cracks covering a broad range of practical problems of interest. Instability develops when the applied tearing modulus exceeds the material resistance to crack growth. Effects of material properties, flaw size, and cylinder dimensions, including system compliance, on the crack growth and instability phenomenon are investigated in some detail.
Consistency Checks for Power-Law Calibration Functions
Authors: Parks DM , Kumar V , Shih CF 
Published: March 2009

Recently, considerable numerical effort has been devoted to obtaining solutions to crack problems in incompressible J2 deformation theory materials of pure power-law type (ε ∞ σn). Following Il'yushin's observation of the scalability of such solutions, many investigators have conducted broad parametric studies on crack length and material exponent, and have tabulated normalized J and deformation magnitudes. Due to numerical difficulties such as slow convergence, incompressibility, or inadequate finite-element mesh refinement, the solutions do not always agree.
We outline the basis for a very stringent general test of the “quality” of proposed solutions by applying the well-known equivalence of J and energy difference rate with respect to crack length.
The test is applied to data available in the literature, and conclusions are drawn as to their accuracy.
Ductile Growth of Part-Through Surface Cracks: Experiment and Analysis
Authors: White CS , Ritchie RO , Parks DM 
Published: March 2009

It is an essentially open question as to how well characterizations of through-thickness crack growth based on parameters such as J, crack-tip opening displacement, and tearing modulus describe ductile cracking in important structural configurations such as the part-through surface-cracked plate.
In this work we compare experimental observations and predictions of ductile crack extension in surface-cracked plates subject to bending. Predictions are made by performing finite-element analysis of this three-dimensional geometry using the line-spring model, in conjunction with experimental plane-strain crack growth data from R-curves obtained from through-thickness-cracked compact-tension specimens.
Analysis of the surface flaw is based upon the incremental plasticity theory line-spring element, which has been incorporated into the ABAQUS© general-purpose finite-element program.
The material under study is a low-alloy bainitic pressure vessel steel; namely, fossil energy ASTM A542, Class 3.
Results show that for the crack configuration considered here the amount of stable tearing is quite uniform along the crack front as is the local value of the J-integral. Crack growth initiation occurs at similar J's for both through-crack and surface-crack specimens, but the surface-crack specimens tested exhibit a higher apparent tearing modulus.
J-Integral Analysis of Surface Cracks in Pipeline Steel Plates
Authors: King RB , Cheng Y-w , Read DT , McHenry HI 
Published: March 2009

A capability for direct experimental evaluation of the J-integral in surface-cracked members under elastic-plastic deformation is useful for providing understanding of the driving force for fracture. In addition, such a capability makes it possible to evaluate analytical and numerical predictions of J. Experimental procedures have been developed for direct evaluation of J at the root of surface cracks from quantities measured at the specimen surfaces. These procedures rely on the assumption that the line J-integral around the root of a surface flaw is sufficiently path-independent for practical purposes. Arguments in favor of this assumption are made.
Experimentally measured J-integral results are presented for specimens containing surface cracks. Several specimens of pipeline steel 16 mm thick and 75 to 100 mm wide, containing surface cracks varying in length from 31 to 48 mm and in depth from 5.6 to 10.5 mm, were loaded in tension, and the J-integral was evaluated. Plots of measured J as a function of applied stress show a parabolic dependence followed by a nonlinear region and a vertical asymptote at the stress corresponding to net section yield. The experimental results are used to evaluate a simplified line-spring model for predicting J, and the model results compare favorably with the experimental data.
Use of J-Integral Estimation Techniques to Determine Critical Fracture Toughness in Ductile Steels
Authors: Green G , Miles L 
Published: March 2009

The use of J-integral estimation techniques is discussed with reference to the evaluation of fracture toughness values for ductile materials. Two applications are considered, tensile plates containing elliptical surface cracks and two specimens designed to produce Mode II sliding shear.
In all cases good agreement was obtained between critical fracture toughness values obtained using J-integral estimation techniques and those obtained in established specimen geometries or by using fractographic techniques. Caution is advised in ensuring that the specimen geometry utilized for J-integral analysis is sufficiently constrained to produce a crack-tip stress-strain field uniquely characterized by the J-integral.
Effects of Creep Recovery and Hardening on the Stress and Strain-Rate Fields Near a Crack Tip in Creeping Materials
Authors: Kubo S 
Published: March 2009

It is thought that creep recovery may be one of the main causes of acceleration of crack growth rates under variable loads. To explore this possibility, the effects of creep recovery and hardening on the near-tip fields are examined in this paper. Robinson's constitutive relation, which is based on the Bailey-Orowan model, is used to take these effects into account. In this model, the strain rate is proportional to the nth power of an effective stress, which is the difference between applied stress and internal stress. The rate of the internal stress is given by the difference between a hardening term and a recovery term, which are proportional to the strain rate divided by the βth power of the internal stress and the (n − β)th power of the internal stress, respectively.
It is found that the Hutchinson-Rice-Rosengren-type singular fields prevail near a creep crack tip irrespective of the combination of the material parameters n and β. When β &lt; n − 1, the “steady-state creep conditions” prevail near the crack tip; when β &gt; n − 1, the “initial-state creep conditions” prevail, under which the internal stress is small compared with the applied stress.
The intensities of the singular stress and strain-rate fields are obtained rigorously for the case of β = n − 1. For the case of β &lt; n − 1 and β &gt; n − 1, the intensities are estimated by using a hypothesized path independence of the J-integral or the C*-integral. For any combination of n and β, the intensity of strain rate is very high immediately after load increase. Then the strain rate decreases and approaches its steady-state value. On the other hand, for load decrease the strain rate is low immediately after load decrease, and increases until it attains the steady-state value.
From a survey of the values of n and β, the case of β &lt; n − 1 seems to be prevailing for metallic creep. In this case C* can be a characterizing parameter of the near-tip fields. The observed acceleration and deceleration of crack growth rates under variable loads are found to be explained by the present analysis.
Estimates of the C* Parameter for Crack Growth in Creeping Materials
Authors: Smith DJ , Webster GA 
Published: March 2009

This paper discusses methods of determining the creep equivalent of the J-contour integral termed C* for a cracked body subjected to creep deformation. It is shown that C* can be expressed in terms of a creep displacement rate, which can be measured either at the loading points, or locally across the crack plane and an appropriate nondimensional factor. It is found that this factor can be estimated from numerical calculations for a material undergoing power-law hardening plastic deformation or from approximate limit analysis techniques. With either approach the nondimensional factor is found to be dependent on specimen geometry and crack length but to be relatively insensitive to the creep stress index n.
The form of expression developed for C* is particularly suitable when experimental measurements of displacement rate can be made since it enables C* to be determined with little knowledge of the creep properties of the material. Comparisons with experimental creep crack growth data have indicated that the approximate limit analysis estimates of C* are satisfactory for the most part.
Influence of Time-Dependent Plasticity on Elastic-Plastic Fracture Toughness
Authors: Ingham T , Morland E 
Published: March 2009

The paper describes the results of preliminary experiments to examine the effect of time-dependent plasticity on ductile crack growth, resistance to ductile tearing, and instability in laboratory fracture toughness tests. Experiments on a carbon-manganese (C-Mn) steel and two specimens of A533B-1 steel have shown that unstable ductile fractures can be induced at ambient temperature when 20- and 40-mm-thick compact specimens are held at constant loads between the load approximating that for crack initiation and the maximum load attained in a conventional displacement-controlled fracture toughness test; the time to failure after holding at constant load decreases with increasing constant load level and increasing strain rate prior to load-holding. The data suggest that final failure occurs under conditions paralleling “tertiary creep” behavior. Specimens were loaded at typical rates used in “static” fracture toughness testing and at a slower rate where the maximum load condition was reached in approximately 24 h. Crack extension under constant load has been related to J-resistance curves derived from multispecimen displacement-controlled tests.
The J-resistance curve for C-Mn steel specimens is dependent upon strain rate at rates of loading which are conventionally taken to be representative of “static” conditions. This effect was not observed in tests on A533B-1 steel.
Comparison of Ashing Techniques for Determination of the Inorganic Content of Peats
Authors: Andrejko MJ , Fiene F , Cohen AD 
Published: March 2009

In recent years there has been an increased interest in peat both as a precursor of coal and as an alternative fossil fuel. One of the most important tests used to evaluate or describe peat is measurement of its inorganic (or ash) content. Numerous methods for determining ash content have been proposed by various researchers. The purpose of this research, therefore, was to compare ash yields produced from the same samples of peat by several of these methods. These include the present ASTM standards for peat (D 2974) and coal (D 3174), along with more recent developments such as electronic Low Temperature Ashing (LTA).
For this study a series of large bulk samples were utilized from several presently or potentially exploitable peat deposits. These samples represented a wide range of depositional environments, botanical compositions, and inorganic and mineral suites. In some samples the mean difference in ash weight between methods was as high as 26% on a dry weight basis, while in other samples there was little if any significant distinction. The overall results indicate that there is a direct relation between the mineral composition of the inorganic fraction and the tendency for distinct statistical differences in the ash results from the different methods utilized.
The conclusions from this study indicate that a revised standard for the determination of ash content in peat should be considered that would incorporate parts of ASTM D 2974 (peat) and D 3174 (coal) with new procedures utilizing LTA technology.
Geotechnical Classification of Peats and Organic Soils
Authors: Landva AO , Korpijaakko EO , Pheeney PE 
Published: March 2009

On reviewing the literature on peat classification, it becomes evident that most of the existing classification systems are based on the use of peats and organic soils as a growing medium in horticulture, agriculture, and forestry, or as a horticultural additive (peat moss), or as a fuel. One exception is the Radforth classification system for peat, which does pertain to engineering applications. Since this system is not based on geotechnical properties, however, it is not in itself sufficient for geotechnical classification.
The term peat as used today also includes a vast range of organic soils: from jelly-like, almost liquid muds whose organic content may not even originate from peat, to extremely coarse-fibrous meshes of woody remnants and fibers. The mud is often referred to as an amorphous-granular peat and the woody mesh as a fibrous peat. These are the only two terms that have been generally adopted by geotechnical engineers, with the inevitable results that each term covers a large range of materials and that considerable overlapping occurs.
An attempt is made in this paper to distinguish between various peats and organic soils on the basis of geotechnical engineering considerations. The existing von Post and Radforth systems are examined with respect to geotechnical applications. These systems are supplemented with routine geotechnical tests yielding numerical values for classification purposes.
A Comparative Study of Preparation and Analytical Methods for Peat
Authors: Schelkoph GM , Hassett DJ , Weber BJ 
Published: March 2009

The United States has large deposits of peat, with recoverable fuel value roughly estimated at 538 quads (1015 Btu), which is equivalent to 90 billion barrels of oil. The potential importance of peat as a feedstock for gasification and as a fuel source is currently being evaluated by the U.S. Department of Energy.
Owing to the high moisture content, high volatiles, and fibrous nature of most peats, standard techniques for the preparation and analysis of coal may not always apply to peat. The purpose of this study was to determine whether current preparation and analytical methods used for coal were adequate for characterizing peat and to determine the accuracy of these methods for peat. Peat samples were obtained from four different locations in the United States and were packed under an inert atmosphere. These samples were sent to six laboratories, where ultimate, proximate, heating value, and elemental ash analyses were performed. The results were then statistically correlated, and the various analytical methods were evaluated by the Grand Forks Energy Technology Center as to their validity for peat. These results will be extremely useful in developing ASTM standards and selecting the best techniques for testing and analysis of peat samples.
Repeated Short-Term Consolidation of Peats
Authors: Ng SY , Eischens GR 
Published: March 2009

The behavior of peats under varying loading processes was noted by the authors while working on various geotechnical engineering projects. This paper gives the comparative results of repeated short-term loading tests and continuous one-load consolidation tests on peats. Peat samples were obtained from three separate sites. The test results show that peats undergo significant consolidation when subjected to repeated loadings, comparable to continuous loading. Loading times and frequencies do not significantly affect the rate and amount of compression.
The test results offer a possible explanation for uneven settlements of parking lots and storage areas of lumber yards, coal yards, canneries, etc.
Immunological Aspects of Silica
Authors: Burrell R 
Published: March 2009

This paper reviews the immunological aspects of silica and silicosis. Since crystalline silica by itself is nonantigenic, emphasis is placed on how silica can effect immunological consequences, on what immunological abnormalities are observed in both spontaneous and experimental forms of silicosis, and on how these immunological activities may influence silicosis.
The effect of silica on immunoglobulin levels, interferon induction, lymphocyte function, and macrophages in the afferent limb of the immune response is critically examined. The toxic and adjuvant effects of silica are discussed in relation to how these effects influence humoral and cellular immune responses to extrinsic and intrinsic antigens.
Finally, the relationship between the immunological effects of silica and immune reactions involving connective tissue antigens is explored in relation to its possible role in the pathogenesis of disease.