bluuebunny/crossref_metadata_2025 huggingface dataset:
Discussion of a New Experimental Method in Measuring Fracture Toughness Initiation at High Loading Rates by Stress Waves
Authors: Klepaczko J. R.
Published: October 2009

An experimental method is described for measuring the fracture initiation properties of metals and alloys over a wide range of loading rates, which can cover over six orders of magnitude in K˙I (1 MPam s−1 ≤ K˙I ≤ 106 MPam s−1). With some modification of the standard compact tension specimen, a large series of screening tests can be performed in the high loading region at a relatively low cost. At the lower loading rates a standard closed loop testing machine can be used. To evaluate fracture initiation at a very high loading rate, a special arrangement of the split Hopkinson pressure bar has been proposed. Specimens of the same geometry as those used in quasi-static tests are placed between the Hopkinson bars. Since the wedge is attached to the incident bar, and the specimen is backed by the transmitter bar (Fig. 2), the course of specimen loading and fracturing can be exactly monitored by recording the incident, reflected and transmitted longitudinal waves. Using this technique, fracture initiation of the prefatigued specimen has been achieved within ∼ 20 μs after the beginning of specimen loading. The effects of inertia acting on the specimen and an error introduced by friction are both considered. Experiments performed on some aluminum alloys as well as on medium carbon steel revealed a complicated pattern of the fracture toughness behavior. Generally, for the strain rate sensitive materials a substantial decrease in fracture toughness was observed under high loading rates.
Statistical Evaluation of Some Hot Rolling Theories
Authors: Murthy A. , Lenard J. G.
Published: October 2009

The accuracy and precision of four mathematial models of varying complexity are evaluated by comparing their predictions to experimental data generated in carefully controlled laboratory experiments and to production logs obtained from the finishing trains of several Canadian, American, and European hot strip mills. The materials rolled are low carbon and HSLA steels; the models used are Orowan’s formulation with Shida’s flow strength and Ford and Alexander’s formulation with Shida’s flow strength; then both these formulations are combined with Ekelund’s flow strength equation. It is concluded that Orowan’s formulation with Shida’s flow strength relation is the most consistently accurate technique of analysis. Further, the behavior of HSLA steels is not well described by either Shida’s or Ekelund’s equations.
Relationship Between Some Physico-Mechanical Properties and Numerical Indexes of Graphite Shape in Compacted-Vermicular Graphite Cast Irons
Authors: Tanaka Yuichi , Saito Hakaru , Tokura Ikuo , Ikawa Katsuya
Published: October 2009

The main purpose of this paper is to present some data on the mechanical and physical properties of compacted-vermicular graphite cast iron and to give a reasonable interpretation to the characteristics by using numerical indexes indicating the shape of graphite flakes in the structure. After describing the preparation of the material and a new method for measuring thermal diffusivity of the iron, the influence of kind and amount of alloy added for treatment and of cooling rate upon the graphite shape is discussed by using the indexes of the structure. The thermal diffusivities and mechanical characteristics such as tensile strength and hardness are shown as functions of the indexes to clarify the relationship between them. Furthermore, the present indexes are compared with those proposed earlier to find which is most suitable for the cast iron treated in this work. Authors propose a definition of compacted-vermicular graphite cast iron, which is reasonable from the physico-mechanical point of view, and also show some typical mechanical properties and measures required to produce such cast iron with desirable features.
Tensile and Torsional Fatigue of Fiber-Reinforced Composites at Cryogenic Temperatures
Authors: Wang S. S. , Chim E. S.-M. , Socie D. F. , Gauchel J. V. , Olinger J. L.
Published: October 2009

A study of the fundamental fatigue behavior of G-10 grade, glass fiber-reinforced composite laminate in a liquid nitrogen environment is presented. Both uniaxial tensile and pure torsional fatigue failure of the fiber composite are investigated. Cryogenic fatigue degradation of the composite in terms of stiffness reduction, energy dissipation, and cyclic fracture is examined in detail. Influences of loading mode, fiber orientation, surface condition, and geometric variables are studied also. Fatigue damage mechanisms at cryogenic temperatures under various loading conditions are discussed.
Acoustic Emission From Simple Upsetting of Solid Cylinders
Authors: Dornfeld David A. , Diei Edward
Published: October 2009

Acoustic emission (AE) generated during simple upsetting (forging) of solid cylinders contains information that could potentially be used to separate the upsetting process into a range of zones of plastic deformation and a zone of both plastic deformation and cracking. This investigation monitored the AE signals during the upsetting of cylindrical specimens of 7075-T6 Aluminum from the start of plastic deformation through eventual cracking. The count rate (N˙) and cumulative count (N) as a function of effective strain were determined. The count rate data are characterized by three distinct regions, an initial peak during yielding of the material, a period of gradual increase during the progression of plastic deformation and the accompanying changes in specimen geometry, and finally a region of rapid increase in N˙ as cracking begins. The cumulative count and rms data follow similar patterns. An analysis of the amplitude distribution of the cumulative count data over a range of strain was made, and in the region of higher amplitude emissions, changes in the distribution of up to two orders of magnitude are observed for data obtained during plastic deformation and surface cracking compared to that from the region of pure deformation only. If plotted as log count (N) versus signal amplitude, the resulting data can be fit with a line using least squares methods yielding a power law relationship sensitive to the degree of deformation.
Deformation Characteristics of Nickel Alloys During Machining
Authors: Wright P. K. , Chow J. G.
Published: October 2009

Machining experiments have been carried out on iron base and nickel base alloys in order to investigate the substantial differences in machinability between these materials. Calculations of the dynamic material strength show that the nickel base alloys have a much higher strength in both the primary and secondary shear zones. In addition, the nickel alloys are shown to “clean” the initial part of the chip-tool contact length more readily than do the iron base materials. The combination of the high primary shear zone strength and the high secondary shear stress close to the cutting edge means that, when machining the nickel alloys, the edge temperature is very high. By contrast, when machining ferrous materials the temperatures further back along the rake face are considerably greater than the cutting edge temperatures. The high strength of the nickel materials also causes high stresses to act normal to the cutting edge. The stress-temperature states at the cutting edge for the two kinds of workmaterials have been summarized in a deformation map for machining. The deformation map summarizes the tool material properties versus temperature, and the normal stress acting on the cutting edge versus temperature. Such maps lead to a prediction of cutting edge collapse and a ranking of materials in terms of their machinability.
Evaluation of Creep Constitutive Equations for Type 304 Stainless Steel Under Repeated Multiaxial Loading
Authors: Ohashi Y. , Ohno N. , Kawai M.
Published: October 2009

Four kinds of creep constitutive models, i.e., strain-hardening, modified strain-hardening, kinematic-hardening, and mixed-hardening theory, are evaluated on the basis of creep-test results on type 304 stainless steel at 650°C under repeated multiaxial loading. The predictions of the four models are compared with the experimental results. It is shown that substantial differences appear among these predictions under large rotations of the principal axes of the deviatoric stress tensor, and that none of them can describe with sufficient accuracy the transient increase of strain-rate and the noncollinearity between the deviatoric stress and creep strain-rate vectors which are observed just after the stress-rotations.
2 1/4Cr-1Mo Steels for Coal Conversion Pressure Vessels
Authors: Viswanathan R. , Jaffee R. I.
Published: October 2009

Pressure vessels for coal conversion processes are likely to be made of a 2 1/4 Cr-1Mo steel, covered by ASTM specification A387, class 2, grade 22. Under the projected operating conditions, degradation of the mechanical properties of the steel by temper embrittlement and hydrogen embrittlement is a major concern. The significance of these degradation mechanisms is reviewed here in the light of experimental results recently generated by EPRI projects and other results published in the literature.
Lack of Hardening in Drawn Bars of Copper and Aluminum in Tension Tests
Authors: Thomsen E. G. , Holthuis J. T. , Thomsen H. H.
Published: October 2009

Oxygen-free copper, 6061 aluminum, and 1100 aluminum bars were drawn from various initial diameters in the annealed state to bars of 11.8 mm (0.464 in.) at maximum reductions in area up to 79.5 percent. It was found that an apparent lack of hardening existed at the higher strains up to the maximum investigated as was determined in tension tests of the drawn bars. The lack of hardening observed was thought to be due, at least in part, to the development of microcracks at the larger strains, a theory proposed by Shaw. It was also found that the oxygen-free copper of the two different heats had appreciable variations in ductility at fracture in tension tests. The reason for this behavior was not determined.
Hydrogen-Induced Crack Propagation in a QT-Steel Weldment
Authors: Andersson B. A. B.
Published: October 2009

A hydrogen-induced fracture process in the HAZ of a QT-steel weldment is studied numerically. Two-dimensional models are used to estimate the transient distributions of stress and hydrogen during the cooling and during the subsequent crack propagation. The distributions of hydrogen in front of a blunted crack are studied for different void volumes in the fracture zone corresponding to brittle and ductile fracture mechanisms. It is tentatively concluded that the incubation time at slow crack growth is not controlled by the hydrostatic stress. A ductile fracture mechanism leading to a relative void volume of 0.02-0.05 in the fracture zone is found to imply long incubation times resulting in a low average crack propagation speed.
Effect of Stress Frequency on Fatigue Crack Propagation in Titanium
Authors: Takezono S. , Satoh M.
Published: October 2009

The effect of stress frequency on the fatigue crack propagation rate in 99.5 percent pure titanium, which has a strong strain rate dependency in the plastic region, was studied. Fatigue crack propagation tests were carried out under three stress frequencies (0.02Hz, 0.2Hz, 20Hz). An elasto/visco-plastic analysis of fatigue crack propagation was performed by the finite element method (FEM), and an analysis of the dependency of crack propagation rate on frequency and strain behavior at the crack tip. The results obtained in this study are summarized as follows: (1) It was found from the experiments that the crack propagation rate, dl/dN, was approximately in inverse proportion to fn (n>0), where f is the frequency. (2) A parameter closely related to the fatigue crack growth rate is the strain range, Δεy, or visco-plastic strain range, Δεyνp at the crack tip. (3) The effect of stress frequency on the fatigue crack propagation rate may be explained by variations of Δεy or Δεyνp based on the strain rate dependency of the material.
Why Ductile Fracture Mechanics?
Authors: Ritchie R. O.
Published: October 2009

Until recently, the engineering application of fracture mechanics has been specific to a description of macroscopic fracture behavior in components and structural parts which remain nominally elastic under loading. While this approach, termed linear elastic fracture mechanics, has been found to be invaluable for the continuum analysis of crack growth in brittle and high strength materials, it is clearly inappropriate for characterizing failure in lower strength ductile alloys where extensive inelastic deformation precedes and accompanies crack initiation and subsequent propagation. Accordingly, much effort has been devoted in recent years toward the development of nonlinear or ductile fracture mechanics methodology to characterize fracture behavior under elastic/plastic conditions; an effort which has been principally motivated by problems in nuclear industry. In this paper, the concepts of ductile (elastic/plastic) fracture mechanics are introduced and applied to the problem of both stationary and nonstationary cracks. Specifically, the limitations inherent in this approach are defined, together with a description of the microstructural considerations and applications relevant to the failure of ductile materials by fracture, fatigue, and creep.
Microstructural Development and Cracking Behavior of AISI 304 Stainless Steel Tested in Time Dependent Fatigue Modes
Authors: Ermi A. M. , Moteff J.
Published: October 2009

Transmission and scanning electron microscopy were used to study the substructural development, crack initiation and crack propagation of AISI 304 stainless steel tested in low-cycle fatigue with various hold times. Tests at 593°C, a strain rate of 4 E-03 s−1 and strain ranges of 0.5, 1.0 and 2.0 percent were interrupted at various fractions of the fatigue life, both during rapid hardening and during saturation. Cells developed during the first few percent of the fatigue life, depending on a threshold stress value, below which cells were not observed. The cell size had a different dependence on the relaxed tensile stress during rapid hardening than during saturation. A work-hardening model relating the peak saturation stress to strain and cell size also applied during the late stages of rapid hardening. The number of cycles to initiate a crack of one grain diameter and to propagate the crack to failure were proportionally reduced for one minute tension holds. For a 60 minute tension hold, crack propagation occupied a much smaller fraction of the fatigue life.
Response of Various Metals to Large Torsional Strains Over a Large Range of Strain Rates—Part 1: Ductile Metals
Authors: Johnson G. R. , Hoegfeldt J. M. , Lindholm U. S. , Nagy A.
Published: October 2009

This paper presents torsional test results for six ductile metals subjected to large shear strains and high strain rates. Included are OFHC copper, Cartridge brass, Nickel 200, Armco IF iron, Carpenter electrical iron, and 1006 steel. Torsional shear strains as high as 700 percent are achieved and strain rates vary from quasi-static to over 300 s−1. At the lower strain rates all of the materials exhibit positive strain hardening and strain rate hardening under essentially isothermal conditions. At the higher strain rates there is significant adiabatic thermal softening and strong evidence for shear instabilities and localizations. Constitutive relationships are derived from the test data and finite element computations of the tests are performed.
The Effect of Superplastic Deformation on Subsequent Service Properties of Fine Grained 7475 Al
Authors: Bampton C. C. , Edington J. W.
Published: October 2009

Intergranular cavitation may be a limiting factor in the otherwise highly promising commercial exploitation of the recently developed superplastic forming process for the high strength aluminum alloy, 7475. A preliminary investigation has been carried out to quantify the effects of superplastic deformation on the major ambient temperature service properties. It is shown that the properties most degraded by the development of cavitation after large superplastic strains are those that depend on the local strain at tensile fracture. This affects, mainly, ductility and fatigue life. However, the effects are not generally significant below effective superplastic strains of about 0.8 to 0.9 (120–150 percent elongation) with the optimum forming conditions (T=516°C, ε˙=2×10−4 s−1). Furthermore, since the fine grain processing has an initial beneficial effect on the ductility-related properties, compared to conventionally processed sheet, most superplastically formed components would suffer very litle, if any, design penalties.
Behavior of Soft Spheres During Impact by High-Speed Photography
Authors: Tatara Yoichi
Published: October 2009

Previously, it has been verified experimentally for durations of impact that the Hertz theory (the quasi-statical theory) holds during impact of spheres without any exception. However, no measurement of duration of impact has been presented for spheres of materials other than metal. This study presents exceptional cases of impacts of spheres during which the Hertz model does not directly hold. By the use of a high-speed camera running at a speed of 5000 frames/s, durations of impact are measured directly for impacts of two solid rubber spheres of the same size and content and impacts of a soft ball (Japanese type-soft tennis ball) on a rigid foundation. As a result, the measured durations of impact in the two impacting cases are found to be decreased as the impact velocity is increased, similar in tendency to durations of impact of elastic metal spheres during which the Hertz theory holds. However, the measured durations of impact are found to be clearly shorter than results calculated according to the Hertz theory, approximately half in the former impacts at high impact velocities, and about 70 percent of the Hertzian results in the latter impacts at almost all impact velocities. Deformation process of the ball impacting on the foundation is also presented to indicate both durations in the compressive process and the restitution one to be shorter than those expected by the Hertz theory. The other results observed on the films are noted to investigate the origin of the great discrepancies between the measured and Hertzian durations (that is, the impacting mechanism of the rubber spheres or the rubber ball packed with air treated here).
Effects of Temperature on Deep Drawing of Sheet Metals
Authors: Kumpulainen J. O. , Ranta-Eskola A. J. , Rintamaa R. H. O.
Published: October 2009

The influence of temperature on the flow stress and the uniform strain of steel, brass, aluminium and AISI 304 and 316 type stainless steels is determined by using uniaxial tensile tests. Sliding and stretching friction coefficients at several surface temperatures are measured for various sheets and lubricants using a bending under tension type strip drawing test. The validity of the results is verified by deep drawing experiments. The influence of temperature difference between the punch nose region and the flange on the drawing and fracture loads and on the limiting drawing ratio is determined.
Effects of Type of Lubricant in Cold Extrusion of Sintered Preforms
Authors: Tabata T. , Masaki S. , Hosokawa K.
Published: October 2009

Four types of lubricant, i.e., dry film, liquid, powdered solid and liquid with powdered solid lubricants, were evaluated for cold extrusion of sintered copper powder preforms. Effects of the lubricants on extrusion pressure, densification and crack formation were examined with respect to variations of density of the preforms, extrusion ratio and die angle. It was found that liquid and liquid with powdered solid lubricants were not suitable for extrusion of sintered preforms.
Effect of Materials on the Mechanism of Electric Discharge Machining (E.D.M.)
Authors: Erden A.
Published: October 2009

Electric Discharge Machining process is investigated both theoretically and experimentally to determine the effects of electrode materials on the machining performance. For this purpose a single and isolated spark is physically and mathematically modelled, and its three phases; viz., Breakdown, Discharge and Erosion are investigated. Resolidified electrode materials as suspended particles in the dielectric liquid are found to be the most significant factor in the breakdown phase. Mathematical expressions relating the time lags to particle concentration are given which can be used to determine the effects of particle concentration on the machining performance. Discharge properties are shown to be dependent on the discharge medium which includes vapours of the electrode materials. The polarity effect has been studied both theoretically and experimentally. Some qualitative explanation is given for the erosion phase. Importance of electrical forces is discussed and a simple mathematical expression is given for the erosion phase. It is concluded that optimum machining conditions can only be obtained by proper selection of the tool material, workpiece material and discharge medium since they affect the initiation and development of the discharge and erosion of electrode materials.
A Numerical Assessment of the Tolerance of Manual Pressure Vessel Cladding Repairs
Authors: Alberry P. J. , Nicholson R. D.
Published: October 2009

A numerical method for the prediction of the distribution of heat affected zone (HAZ) microstructures for dissimilar metal combinations involving austenitic and nickel base electrodes is described. The method is extended to consider possible variations in HAZ structure after local repairs to pressure vessel cladding. The calculated tolerance of the ASME III half-bead technique is compared with the tolerance of a two-layer refinement technique which does not involve hot grinding.
Effect of Strain Wave Shape on Thermal-Mechanical Fatigue Crack Propagation in a Cast Low-Alloy Steel
Authors: Okazaki M. , Koizumi T.
Published: October 2009

The effect of strain wave shape on low cycle thermal-mechanical fatigue crack propagation in a cast low-alloy steel was investigated under in-phase loading condition with a temperature range of 300–550°C. It was found that the rate of crack propagation increased with the asymmetry of strain wave shape, the effect being greatest in the high strain range. This behavior was discussed on the basis of observations in electron fractography and the change of inelastic tensile creep strain range with the repeated strain cycles. It was shown that the accumulation of inelastic tensile creep strain, which was not recovered during the compressive strain period contributed to this behavior. Further, the parameter called the range of total J-integral was introduced by applying the strain range partitioning approach and the rate of crack propagation was correlated with this.
Fatigue Crack Retardation of High Strength Steel in Saltwater
Authors: Tokaji K. , Ando Z. , Imai T. , Kojima T.
Published: October 2009

A high strength steel was studied in 3 percent saltwater to investigate the effects of a corrosive environment and sheet thickness on fatigue crack propagation behavior following the application of a single tensile overload. Experiments were carried out under sinusoidally varying loads at a load ratio of 0 and frequency of 10 Hz. A single tensile overload was found to cause delayed retardation, and the crack propagation rate at first increased, followed by fairly rapid decrease to a minimum value and then increased gradually to its steady-state value, just as it did in air. The overload affected zone size and the retardation cycles increased with decreasing sheet thickness, just as they did in air. However, the zone size and the cycles were larger in 3 percent saltwater than in air. Since the crack propagation rates through the overload affected zone were not affected by the test environment, the longer retardation cycles in 3 percent saltwater were attributed to an enlargement of the overload affected zone size. The crack propagation behavior following the application of a single tensile overload in 3 percent saltwater was well explained by the crack closure concept.
A Time-Temperature Parameter Study of Stress-Rupture of 316SS Under Increasing Stress
Authors: Rowe G. H.
Published: October 2009

Experimental results for eight linearly increasing stress-to rupture tests at 649°C, 732°C, and 816°C agree with analytical rupture time predictions using the Larson-Miller time-temperature parameter stress-rupture curve for the temperature range, 649°C to 899°C. An anomaly in 649°C stress-rupture behavior is disclosed, whereby the increasing stress-to-rupture time predictions at 649°C do not agree with the stress-rupture curve constructed from the actual 649°C stress-rupture data.
Study of Friction in Cold Strip Rolling
Authors: Lai-Seng Lim , Lenard J. G.
Published: October 2009

Experiments were conducted to measure the effects of roll pressure and roll rpm on the magnitude and variation of the coefficient of friction in the roll gap in cold strip rolling. Two aluminum alloys (1100-T0 and 5052-H34) were used in the experiments. Roll pressures were found not to affect the frictional coefficients in a significant manner. Speed of rolling was identified as the most important parameter as far as the values of μ are concerned. Increased speeds appeared to lower the values of the average coefficient of friction.
Surface Integrity in Machining Solution-Treated and Aged 2024-Aluminum Alloy, Using Natural and Controlled Contact Length Tools. Part I—Unlubricated Conditions
Authors: Elkhabeery M. M. , Bailey John A.
Published: October 2009

The effects of cutting speed, chip-tool contact length and tool surface finish on the surface integrity of a solution-treated and aged 2024-Aluminum alloy, that is machined under dry, orthogonal conditions are investigated. The surface region of machined workpieces is examined using a wide variety of diagnostic techniques. In addition, tool forces are measured and both the cutting geometry and cutting temperatures are calculated. The results of the investigation show that a wide variety of geometrical surface features are generated that contribute to the natural surface roughness. The results also show that a plastically deformed surface region is generated that contains variations in hardness, microstructure and residual strain. The results of the investigation are interpreted in terms of the type of chip produced, formation of the built-up edge, the mechanics and thermodynamics of the cutting process and the interaction between the tool nose region and freshly machined workpiece surface.
Gross Crack Initiation and Propagation in Brittle Thin Solid and Annular Disks Subjected to Impact Loading
Authors: Johnson W. , Bai Y. L. , Ghosh S. K.
Published: October 2009

This paper derives from a study of grinding wheel break-up behavior due to impact. The impact fracture characteristics of circular disks of plaster of Paris with a concentric central hole were studied experimentally [1] for three types of loading: (a) when the disks were suspended freely and loaded intensely at one point on their circumference by an explosive detonator; (b) when the disks were allowed to fall under gravity from a certain height on to a rigid base; and (c) when a disk, resting on a rigid base, was struck by a flat ended rigid body which was dropped on to it from a certain height. Quasi-static flattening tests on the disks were also carried out. The paper describes a theoretical investigation into the stress analysis of disks under impact, classifies the relevant damage sustained by them and attempts to unify the “gross” impact fracture patterns which arise in different modes of dynamic loading. The extent of local flattening of the quasi-statically loaded disks before fracture, is also reported. Good correlation between the theory and experimental results is obtained, especially for rings of diameter ratio (Di/D0) of around 0.5.
The Tensile Fracture Characteristics of Nickel, Monel, and Selected Superalloys Broken in Liquid Mercury
Authors: Price C. E. , Good J. K.
Published: October 2009

An investigation was performed, using the slow strain-rate tensile test technique, to determine whether nickel and typical nickel base alloys were susceptible to embrittlement by liquid mercury at room temperature. The alloys Monel 400, Monel R405, Monel K500, Inconel 625, Inconel 718, and Inconel X750 displayed intergranular embrittlement to different degrees. Nickel 200 and Inconel 600 had transgranular breaks. The alloys Incoloy 800 and Incoloy 825 were not embrittled under these test conditions, giving cup and cone fractures, but some surface cracking did arise in the necked region. The fracture mode was governed by the strain at which cracking initiated with lower values favoring integranular separations. Some alloys showed a progression from intergranular to transgranular to microvoid coalescence fractures across the cross section, the latter failures occurring at higher strain levels. The relative embrittlement of the alloys did not correlate with the mechanical properties or the stacking fault energies, but is perhaps related to composition, for the alloys higher in iron content were affected less.
Prediction of Fatigue Threshold of Notched Components
Authors: Tanaka K. , Nakai Y.
Published: October 2009

A consistent method is proposed to predict significant properties concerning the notch fatigue threshold from three experimental data: the fatigue limit of smooth specimens, the threshold stress intensity range for a long crack, and the effective stress intensity range at the threshold for a long crack. Based on the model of the intrinsic crack combined with crack closure, the fatigue limits for crack initiation and fracture from a notch, the length of a nonpropagating crack, and the notch tip radius at the branch point are all derived in a consistent way. The predicted results are compared with the experimental data of a low-carbon steel, and good agreement is obtained.
On the Thickness Effect of Axisymmetric Deformation of Sheet Metals
Authors: Samanta Shyam K. , Grab G.
Published: October 2009

In the present investigation, we have studied the influence of sheet thickness on the limiting strain, i.e., forming limit, of a mild steel by punch stretching experiments. Our results show that for a two-fold increase in thickness the limiting strain of a material does not substantially increase with increasing thickness of the sheet. These experimental observations were well supported by the results of theoretical analysis using Finite Element modelling of the process.
A Physical Theory of Plasticity and Creep
Authors: Lin T. H.
Published: October 2009

In 1971, Lin showed a physical theory of plasticity which seems to represent the actual mechanism of plasticity better than other theories. Since then, much work has been done to improve this theory and to extend this theory to creep. This improved and extended theory is reviewed and given here. Some differences between the present theory and the self consistent theory are discussed. Present theory satisfies the conditions of equilibrium, continuity of displacement and the dependency of slip on the resolved shear stress throughout the polycrystal. The calculated results agree well with most experiments.
Improved Constitutive Equations for Modeling Strain Softening—Part II: Predictions for Aluminum
Authors: Lowe T. C. , Miller A. K.
Published: October 2009

The capabilities of a new deformation model, MATMOD-4V, are demonstrated by comparison of computer predictions against independent data for high purity aluminum. In particular, model predictions of strain softening are presented and discussed. The predictive abilities of MATMOD-4V represent a substantial improvement over earlier versions of the MATMOD constitutive equations. The strength of the approach is illustrated by MATMOD-4V’s ability to predict deformation behaviors not explicitly built into the model.
Power-Formula Viscoplasticity, Its Modification and Some Applications
Authors: Chen Yu
Published: October 2009

A power viscoplastic constitutive equation was first proposed by Bodner and Partom in 1972. This specific formula has its origin in the physical phenomenon of dislocation dynamics and due to the simplicity of its mathematical form, it is a useful constitutive equation in solving boundary value problems. In this paper a brief review of the basic relation is given, followed by discussion of some relatively less known features of this formula. The body of the paper deals with the modification of the equation to explore its potential in several directions. The first modification consists of making the “threshold stress” strain rate dependent. The second modification aims of modeling strain rate history dependency. Due to space the formulation of the boundary value problems of uniaxial testing and its implementation through finite element programs will not be reported here. Results demonstrating the effect of strain rate and strain-rate history are presented. They are in good qualitative agreement with available experimental data.
A Large Deformation Plasticity Model With Rate Sensitivity and Thermal Softening
Authors: Nicholson D. W. , Kiddy K. C.
Published: October 2009

In this paper, a previously published small deformation constitutive model with rate sensitive plasticity and thermal softening is extended to large deformation. The extended model appears suitable for describing a deleterious thermoplastic process manifested by adiabatic shear banding in materials such as titanium under severe dynamic loads. The nature of the instability admitted by the model is described. Also, calculations are reported on the rapid extension of a titanium strip. For applied stresses several times the yield stress, a deleterious temperature is attained in times of the order of 10−2 s.
Creep Rupture of Metals—An Analytical Model
Authors: Krajcinovic Dusan , Selvaraj Sebastine
Published: October 2009

Introducing a special vectorial internal variable quantifying the density, distribution and orientation of planar microcracks and assuming the existence of a potential in the space of affinities developed is a continuum creep theory which accounts for gradual deterioration of material. The reported numerical results demonstrate the utility of the theory in modelling the tertiary creep phase and predicting the onset of the creep rupture.
Solid Phase Sheet Forming of Thermoplastics—Part I: Mechanical Behavior of Thermoplastics to Yield
Authors: Stokes V. K. , Nied H. F.
Published: October 2009

The detailed mechanical behavior to yield of three thermoplastics—polycarbonate, polybutylene terephthalate, and polyetherimide—subjected to simulated forming histories, is examined in order to gain an insight into the sheet forming process for thermoplastics. The phenomenology of yield is shown to be quite different for semicrystalline polybutylene terephthalate when compared with amorphous polycarbonate and polyetherimide. The dependence of the mechanical properties of thermoplastics on temperature, deformation rate and hold-time periods are shown to be important for understanding and controlling the solid phase sheet forming process.
Transient Small-Scale Yield Behavior of Textured Copper Tubing Under Biaxial Loading
Authors: Garmestani Hamid , Adams Brent L.
Published: October 2009

Biaxial microplastic yielding (8 microstrain) of 101 copper tubing was studied at room temperature to assess the transient time-dependent behavior of subsequent yielding following small prestrains (2000 microstrain). The specimens investigated were thin-walled tubes loaded in variable combinations of uniaxial tension/compression and internal pressurization. Prestraining in three different directions introduced a Bauschinger effect as manifested by a translation of the yield surface in the direction of stressing. The yield surface also showed an expansion in size. Subsequent yield surfaces, measured at other time intervals, showed that the Bauschinger effect recovered up to 90 percent after 120 hours, and the final yield locus retained the same shape anisotropy as the initial surface. This implies a shift from kinematic to isotropic hardening. Hart’s phenomenological model was used to predict the experimental data. In this model, the Bauschinger effect and other shape changes of the yield surface are attributed to anelastic phenomena.
On the Mechanics of Grey Cast Iron Under Pure Bending
Authors: Shawki G. S. A. , Naga S. A. R.
Published: October 2009

This paper presents the results of experiments conducted on lamellar graphite grey cast iron of rectangular section subjected to pure bending. Strain measurements confirm the traditional speculation that plane sections remain plane under strain. Owing to the nonlinear relationship between stress and strain, however, the neutral axis of a loaded specimen is shown to shift away from the centroidal axis. This shift is evidently amplified with increased loading. A computer program is herein specially devised for calculating the shift in neutral axis through satisfaction of the conditions of equilibrium together with checking for possible crack initiation at the extension side. While the simple flexural formula holds very nearly true for the compression side, it fails, however, to predict stresses on the extension side, the situation being further aggravated by higher bending moments. The apparent high bending strength of grey cast iron is fully accounted for.
Ultrasonic Fretting Wear of a Plain Carbon Steel
Authors: So¨derberg Staffan , Colvin Todd , Salama Kamel , Vingsbo Olof
Published: October 2009

The fretting wear characteristics of a low-carbon steel (AISI 1018) at a 20 kHz ultrasonic testing frequency have been studied. The tests show that after an initial running-in period there exists a linear relationship between wear volume and number of cycles. Fretting wear is also found to increase exponentially with load and amplitude, the effect of load being the strongest within the studied range. By comparing obtained wear data and observed wear mechanism with results in literature from low-frequency fretting of the same material, it is found that a similar behavior is obtained in spite of the large difference in frequency. These results suggest that ultrasonic fretting can be utilized as an accelerated method for fretting wear testing.
Formulation of Cross-Hardening in Creep and its Effect on the Creep Damage Process of Copper
Authors: Murakami S. , Sanomura Y. , Saitoh K.
Published: October 2009

The present paper is concerned with the modelling of creep and creep damage in polycrystalline metals and the experimental evaluation of the proposed model. By ascribing the reduction of creep rates caused by the principal stress rotation (i.e., cross-hardening) to the intersection mechanism of dislocations on active slip planes in crystal grains, a constitutive equation of creep describing the cross-hardening is first formulated. Then, in view of the metallurgical observations on the nucleation and the growth of grain boundary cavities in the creep damage process, an evolution equation of anisotropic creep damage is expressed as a function of the stress, a second rank damage tensor and the creep rate of the material. Finally, the validity of the proposed theory is discussed by performing systematic creep damage tests of thin-walled copper tubes under nonsteady multiaxial states of stress at 250°C.
Variation of Fatigue Notch Factor With Lifetime, Stress Ratio and Temperature
Authors: Berkovits Avraham
Published: October 2009

An understanding of the influence of various loading parameters on the fatigue notch factor Kf is important to the designer, who often relies exclusively on the limited fatigue data available in handbooks. The significance and limitations of Kf values so obtained are discussed in the light of geometry, fatigue lifetime, stress ratio, and temperature. Ultimately fatigue notch factor data in the literature are very limited, and often there is no recourse but to make use of the theoretical stress concentration factor Kt. The results of this procedure are usually very conservative. Handbook fatigue failure data were reexamined in order to determine the variation of Kf for a representative range of materials, including aluminum, steel, titanium and nickel alloys. Results are presented in the form of Kf over a wide range of notch and loading conditions. The results are interpreted and generalized in terms of stress concentration, and material mechanical and microstructural response.
An Engineering Approach to the Prediction of Creep Crack Growth
Authors: Nikbin K. M. , Smith D. J. , Webster G. A.
Published: October 2009

This paper is concerned with assessing the integrity of cracked engineering components which operate at elevated temperatures. Fracture mechanics parameters are discussed for describing creep crack growth. A model is presented for expressing growth rate in terms of creep damage accumulation in a process zone ahead of the crack tip. Correlations are made with a broad range of materials exhibiting a wide spread of creep ductilities. It is found that individual propagation rates can be predicted with reasonable accuracy from a knowledge only of the material uni-axial creep ductility. An engineering creep crack growth assessment diagram is proposed which is independent of material properties but which is sensitive to the state of stress at the crack tip. Approximate bounds are presented for plane stress and plane strain situations and it is shown that crack growth rates about fifty times faster are expected under plane strain conditions than when plane stress prevails.
Determination of Stress Intensity Factors for Partial Penetration Axial Cracks in Thin-Walled Cylinders
Authors: Cheng Weili , Finnie Iain
Published: October 2009

An approach based on the use of rotation and displacement solutions for a cracked element in plane strain is used to obtain the stress intensity factor for a long axial crack in a thin-walled cylinder. The hoop stress distribution in the cylinder prior to introduction of the crack is arbitrary. Results obtained with this approach are in good agreement with numerical solutions for several hoop stress distributions.
Fatigue Crack Growth Under General-Yielding Cyclic-Loading
Authors: Minzhong Zheng , Liu H. W.
Published: October 2009

In low cycle fatigue, cracks are initiated and propagated under general-yielding cyclic-loading. For general-yielding cyclic-loading, Dowling and Begley have shown that fatigue crack growth rate correlates well with the measured ΔJ. The correlation of da/dN with ΔJ has also been studied by a number of other investigators. However, none of these studies has correlated da/dN with ΔJ calculated specifically for the test specimens. Solomon measured fatigue crack growth in specimens in general-yielding cyclic-loading. The crack tip fields for Solomon’s specimens are calculated, using the finite element method, and the J-values of Solomon’s tests are evaluated. The measured crack growth rate in Solomon’s specimens correlates very well with the calculated ΔJ.
Nondestructive Evaluation of Adhesively Bonded Joints
Authors: Kline R. A. , Hsiao C. P. , Fidaali M. A.
Published: October 2009

In this experiment, the method of phase spectral analysis was used to monitor ultrasonic moduli changes in a two part epoxy adhesive bonded between steel adherends during cure as well as during cyclic loading. Ultrasonic signals were generated using a piezoelectric transducer operated in the pulse-echo mode. These signals were digitized using a high-speed transient digitizer (100 MHz sampling rate) and stored for post-test analysis in the memory of a minicomputer. Based on this information, ultrasonic attenuation and phase velocity measurements (as a function of frequency) were obtained for both longitudinal and shear waves throughout the cure cycle. This technique was also used to monitor the cure of adhesives with different mix ratios in order to evaluate the utility of this approach for quality control purposes. The potential use of the method to study fatigue damage development was also considered.
Determination of Tensile Properties by a Three-Point Method
Authors: Thomsen E. G. , Negroni F. D.
Published: October 2009

It is shown that it is possible to use a three-point method for the evaluation of tensile data for some ductile metals. The experimental data show also that average tensile stresses and axial tensile stresses along the axis of the specimens during the necking portion of a test remain linear functions of the effective strain. Prediction made by the use of an electronic calculation sheet and the use of a computer program yield results which are in substantial agreement with the experimental results. The three-point method would seem to reduce the experimental work necessary to obtain σ versus ε curves and apparently would permit higher testing speeds than those of ε ≈ 0.03 used in this investigation.
Assessment of Work-Hardening Characteristics and Limit Strains of Anisotropic Aluminum Sheets in Biaxial Stretching
Authors: Ragab A. R. , Abbas A. T.
Published: October 2009

In this work, commercially pure aluminum sheets in both the as-received and annealed conditions are tested in uniaxial and biaxial tension. Biaxial stretching is performed in dies giving different degrees of stress biaxiality. Resulting effective stresses and plastic strains are estimated according to the original Hill’s theory in the two situations where planar anisotropy is either neglected or taken into consideration. In both situations discrepancies between biaxial and uniaxial flow curves are observed. By analyzing the above uniaxial and biaxial test results according to the flow rule associated with a yield function recently proposed by Hill, a new material index describing the anisotropic behavior has been evaluated. This new material behavior description realized a satisfactory agreement between work-hardening characteristics of the tested aluminum sheets under various biaxial stress systems. The same tested aluminum sheet materials have been then tested in order to determine their forming limit curves. Correlation between these curves and the theoretical predictions of limit strains according to various instability analyses, is sought through the use of the above description of material work-hardening.
Constitutive Models Suitable for Thermal Loading
Authors: Slavik D. , Sehitoglu Huseyin
Published: October 2009

A non-unified creep plasticity constitutive model and a unified creep-plasticity model have been considered. In the non-unified model, creep and plastic strains were added separately; in the unified model, they were treated in a unified manner. These models were used to predict cyclic hardening and mean stress relaxation for isothermal loading conditions. The results indicate that certain instabilities occured in unified creep-plasticity simulations at low temperatures. Material behavior for thermal loading was studied using the two-bar structure. Both constitutive models were modified to handle material behavior changes with temperature. The thermal loading response was predicted satisfactorily with both models for most cases. However, certain model limitations were encountered in the unified model. The capabilities of both models are outlined and discussed.