bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of Creep in a Si-SiC C-Ring by Finite Element Method
Authors: Chuang Tze-jer , Wang Zheng-Dong , Wu Dongdi
Published: April 2008

The long-term creep deformation of a siliconized silicon carbide ceramic C-ring subjected to a compression load at 1300°C in air is studied by the finite element method. Based on asymmetric creep responses observed under uniaxial creep tests, multidirectional constitutive equations in power-law form are derived using the parameters of effective stress/strain. The elastic solutions are first solved and used as a guide to reach the final mesh design and as initial conditions. Solving the initial value problem in which equilibrium and constitutive equations are satisfied at all times, this model gives time-history of stresses and displacements. Fair agreements were obtained in load-point displacement rate, damage zone, neutral axis locations, stress relaxation and redistribution when compared with simple curved beam theory and experimental data. As common with other nonlinear problems, the convergence of the finite element solutions strongly depends on the time step as well as the finess of the element size, particularly at the regions where principal stresses are close to the threshold stress for creep damage.
Cyclic Deformation and Anisotropic Constitutive Relations of Al-6061-T6 Under Biaxial Loading
Authors: Lin Hong , Nayeb-Hashemi Hamid , Pelloux R. M. N. , Berg C. A.
Published: April 2008

Anisotropic cyclic stress-strain curves (CSSCs) were obtained from fully reversed cyclic tests in controlled strain on two orientations of specimens made from an Al-6061-T6 plate (along the rolling direction and perpendicular to the rolling direction). The experiments were conducted at room temperature under three loading conditions: tension/compression, pure torsion and combined tension/torsion in-phase. Based on the CSSCs data, the anisotropic constitutive relations of the material were obtained using Hill’s anisotropic plasticity theory. The coefficients in the yield function were evaluated with a 0.2 percent effective plastic strain criterion. Yield loci and flow behavior were determined for tension/torsion biaxial loading and compared to theoretical predictions. Two anisotropic effective stress-effective strain criteria obtained from the anisotropic constitutive relations were evaluated. A Von Mises type criterion correlated very well with the test data of all specimens under all loadings. The Tresca type criterion did not correlate with the test data.
A Visco-Plastic Constitutive Model for 60/40 Tin-Lead Solder Used in IC Package Joints
Authors: Busso E. P. , Kitano M. , Kumazawa T.
Published: April 2008

A new unified visco-plastic constitutive model for the 60 Sn-40 Pb alloy used in solder joints of surface-mount IC packages and semiconductor devices is proposed. The model accounts for the measured stress-dependence of the activation energy and for the strong Bauschinger effect exhibited by the solder. The latter is represented by a back stress state variable which, in turn, evolves according to a hardening-recovery equation. Based on the observed hardening behavior, it is assumed that the isotropic resistance to plastic flow does not evolve within the deformation range covered in this study (ε< 3 percent). The deformation phenomena associated with the solder’s monotonic and steady-state cyclic responses are accurately predicted for −55°C≦T≦150°C and 8 x 10−2 s−1 ≦ ε ≦ 8 x 10−5 s−1. The model also predicts well the overall trend of steady-state creep behavior. The constitutive model is formulated within a continuum mechanics framework and is therefore well suited for implementation into finite element or other structural codes.
A Phenomenologically Based Constitutive Model for Rene´ 95
Authors: Sherwood J. A. , Stouffer D. C.
Published: April 2008

A unified constitutive model incorporating internal state variables based upon the deformation phenomena that are observed to occur at the microstructural level has been developed and applied to Rene´ 95. Material hardening is modeled using dragstress and back-stress state variables, while the reduction in the material’s load-carrying capability is described by using a damage-accumulation state variable. Application of the model to the tensile, cyclic, and creep loadings of Rene´ 95 at 650°C demonstrated that the model is capable of capturing cyclic hardening, damage accumulation, and tertiary creep by using one inelastic flow equation in concert with the state-variable-evolution equations.
Variation of Yield Stress With Strain Rate for Three Carbon Steels
Authors: Yongning Liu , Jinhua Zhu , Huijiu Zhou , Follansbee P. S.
Published: April 2008

This paper presents an experimental study of the strain-rate and temperature dependence of the yield stress for three low carbon steels (percent c = 0.2, 0.3, and 0.4). Tension test results using notched specimens are reported for temperatures of −20°C, 0°C, and 25°C and strain rates from 10−4 s−1 to 200 s−1. Two regions with different kinetics have been observed. The yield stress shows a low sensitivity to strain-rate at higher temperatures and lower strain rates (region I), but the rate sensitivity is high at lower temperatures and higher strain rates (region II). An approximately linear dependence of yield stress on log strain rate is displayed in both regions. The carbon content of the steel strongly affects strengthening in region I, but weakening appears as the strain rate increases in region II. Deformation in the two regions is analyzed according to standard formalisms. When the yield stress is separated into athermal and thermally activated components, the carbon concentration dependence is shown to isolate to the athermal stress component, which becomes rate controlling at high temperatures and low strain rates.
Nonisothermal Constitutive Model for the Small Strain Behavior of 9Cr-1 Mo-V-Nb Pressure Vessel Steel
Authors: Moosbrugger J. C.
Published: April 2008

A viscoplastic constitutive model characterization is presented for the small strain, nonisothermal behavior of a 9Cr-1Mo-V-Nb pressure vessel steel between 25°C and 600°C. A framework based on previously proposed viscoplastic models is reviewed. The technique for including deformation softening is discussed and the procedure used to determine model constants and parameters is presented. Results of model integration are compared with various isothermal and nonisothermal, uniaxial test results.
Significance of the Relative Orientation of the Mean and Alternating Principal Stress Axes in Biaxial Fatigue
Authors: Munday E. G.
Published: April 2008

In certain biaxial fatigue problems, the stress components can be represented as a superposition of mean components, and proportionally applied alternating components. Such representation is the first step in applying some of the more popular methods for long-life fatigue analysis. It is commonly assumed, perhaps unwittingly, that the relative orientation of the mean and alternating principal stress axes does not influence fatigue behavior. It is explained how this assumption is inherent in several popular methods. It is pointed out that the validity of the assumption cannot be demonstrated by existing data.
Models for Inelastic Deformation of Particles Associated With Hot Pressing of Metal Matrix Composites
Authors: Tszeng T. C. , Ohriner E. K. , Sikka V. K.
Published: April 2008

During fabrication of fiber reinforced metal-matrix composites by hot pressing, fiber breakage due to particles impingement during consolidation of fiber/particle system is very common. In studying the fiber breakage, one of the main issues is the interactions between fibers and particles during consolidation. In this study, we proposed to investigate the problem of fiber/particle interactions by examining a unit problem consisting of a deformable particle and a cylinder. A rather engineering model for axisymmetric deformation of a particle induced by a rigid sphere was developed first and then extended to the interactions between a deformable particle and a rigid cylinder. The calculations were compared with experiments on lead balls, good agreement was observed. The model was then applied to determining the maximum bending stress in fibers using the simple beam theory. A safe criterion for preventing fibers from breaking was found.
Relationship Between Alloy Composition and Tool-Life of High-Speed Steel Twist Drills
Authors: Henderer W. E.
Published: April 2008

Tool-life tests are reported which show the relationship between the alloy composition of high-speed steel twist drills and performance. Tool-life is shown to depend primarily on the composition of the matrix consisting of tempered martensite and precipitated secondary carbides. The longest tool-life was obtained from alloys with high vanadium content and low tungsten or molybdenum content. This observation is consistent with the dispersion characteristics of vanadium carbide which precipitate during tempering.
Quantitative Comparison Between Precision Closed-Die Forging-Force Data and Computer Simulations
Authors: Majerus J. N. , Jen K. P. , Gong H.
Published: April 2008

This paper presents a study of precision closed-die, isothermal, forgings via both experiments and computer simulation. The closed-die cross-section was an “H” shape and Tin/Lead eutectic solder was used for the billet material. Extensive statistical analysis of the axial force versus displacement history was conducted using replicated forging experiments. The purpose of the experiment was to obtain statistically significant data so that accuracy tests could be conducted on different FEM computer models, e.g., ALPID, EPIC2D, NIKE2D, and DYNA2D. Overall, the forging history exhibited complex behavior consisting of five distinct regions. The experimental results yield a 5.2 percent COV in the required forging force for a specific top-die displacement. A 6.5 percent COV in the “stiffness” of the first region (elastic behavior) of the forging history was also obtained. One set of simulations with one FEM computer model, ALPID Version 2.1 for rigid-thermoviscoplastic behavior, was conducted. The occurrence of all four viscoplasticflow regions was qualitatively predicted by the simulations. Quantitatively, the simulations are within the experimental bounds for the early viscoplastic regions, but out of bounds for the later regions. It appears that, for the eutectic tin/lead billet material, there is no combination of “power-law” material parameters that yield good agreement with the later stages of the forging force history.
Parameter Evaluation for a Unified Constitutive Model
Authors: Senseny P. E. , Brodsky N. S. , DeVries K. L.
Published: April 2008

Parameters for the unified constitutive model MATMOD [1] were evaluated for rock salt (NaCl) by using nonlinear least squares to fit the model to isothermal laboratory data. MATMOD incorporates two internal variables that represent the effects of both kinematic and isotropic hardening. The constitutive equations contain nine parameters that must be evaluated to model isothermal deformation. Laboratory data from stress relaxation, constant strain rate, and long-term creep tests were used. The latter two test types included staged tests in which the strain rate or stress was changed step-wise during the test. The test conditions were precisely controlled by a computer and the constitutive equations were integrated to simulate the laboratory conditions closely. The MATMOD parameters were then evaluated by fitting the integrated equations to the laboratory data using nonlinear least squares. The model fits the data well, but the fit may be improved by changing the evolutionary equation for the internal variable that accounts for isotropic hardening.
Effect of Bulk Deformation on Viscoplastic Adhering Process—A Numerical Study of Solid State Pressure Welding
Authors: Takahashi Y. , Koguchi T. , Nishiguchi K.
Published: April 2008

Viscoplastic intimate contact process of uneven surfaces is numerically studied by using the finite element model proposed in our previous paper. The model treats only the case that the interfacial contact is the rate determining step of the solid state bonding process. The distribution of the equivalent strain rate around the void surface is strongly influenced by the bulk constraint conditions, i.e., the interfacial deformation is greatly affected by the bulk deformation. The strain rate at the void tip is strikingly increased by the bulk deformation, which accelerates the void shrinkage on the bond interface. If the bulk is deformed, the contacting process is also affected by the asperity angle α0 due to surface waviness. When α0 < 30 deg, the bonded area growth is mainly produced by the folding phenomena of the faying surfaces.
Delamination in Thickness Tapered Composite Laminates
Authors: Trethewey B. R. , Gillespie J. W. , Wilkins D. J.
Published: April 2008

The structural performance of thickness-tapered laminates has been investigated using an energy-based damage tolerance methodology. The geometry studied is a thin laminate with discontinuous internal plies and a through-width delamination embedded at the interface between continuous and discontinuous sublaminates. An analytic model, based on shear deformation plate theory and linear-elastic fracture mechanics is employed to determine the Mode I and Mode II components of strain energy release rate. A two-dimensional plane strain finite element analysis is conducted to confirm the accuracy of the analytic predictions. The resulting pure mode strain energy release rates are combined with a mixed-mode growth criterion to predict the axial load required to induce delamination growth. Finally, the analytic and numerical model were used to predict failure in a delamination critical test specimen. Reasonable agreement of the actual and predicted failure loads was observed.
An Experimental Study on the Effect of Prior Plastic Straining on Creep Behavior of 304 Stainless Steel
Authors: Xia Z. , Ellyin F.
Published: April 2008

Constant strain-rate plastic straining followed by creep tests were conducted to investigate the effect of prior plastic straining on the subsequent creep behavior of 304 stainless steel at room temperature. The effects of plastic strain and plastic strain-rate were delineated by a specially designed test procedure, and it is found that both factors have a strong influence on the subsequent creep deformation. A creep model combining the two factors is then developed. The predictions of the model are in good agreement with the test results.
Methodology to Predict Delayed Failure Due to Slow Crack Growth in Ceramic Tubular Components Using Data From Simple Specimens
Authors: Jadaan O. M. , Tressler R. E.
Published: April 2008

The methodology to predict the liftime of sintered α-silicon carbide (SASC) tubes subjected to slow crack growth (SCG) conditions involved the experimental determination of the SCG parameters of that material and the scaling analysis to project the stress rupture data from small specimens to large components. Dynamic fatigue testing, taking into account the effect of threshold stress intensity factor, of O-ring and compressed C-ring specimens was used to obtain the SCG parameters. These SCG parameters were in excellent agreement with those published in the literature and extracted from stress rupture tests of tensile and bend specimens. Two methods were used to predict the lifetimes of internally heated and pressurized SASC tubes. The first is a fracture mechanics approach that is well known in the literature. The second method used a scaling analysis in which the stress rupture distribution (lifetime) of any specimen configuration can be predicted from stress rupture data of another.
Ultrasonic Assessment of Interfacial Oxidation Damage in Ceramic Matrix Composites
Authors: Chu Y. C. , Rokhlin S. I. , Baaklini G. Y.
Published: April 2008

A new approach to characterizing oxidation damage in ceramic matrix composites using ultrasonic techniques is proposed. In this approach the elastic constants of the composite are determined nondestructively by measuring the angular dependence of both longitudinal and transverse wave velocities. A micromechanical model for composites with anisotropic constituents is used to find the anisotropic properties of an effective fiber which is a combination of the fiber and the interface. Interfacial properties are extracted from the properties of this effective fiber by analyzing the difference between effective and actual fiber properties. Unidirectional [0]28 SiC/Si3N4 composites with 30 percent fiber volume fraction and 30 percent matrix porosity are used. The samples are exposed in a flowing oxygen environment at elevated temperatures, up to 1400°C, for 100 hours and then measured by ultrasonic methods at room temperature. The Young’s modulus in the fiber direction of the sample oxidized at 600° C decreased significantly but it was unchanged for samples oxidized at temperatures above 1200° C. The transverse moduli obtained from ultrasonic measurements decrease continuously up to 1200°C. The shear stiffnesses show behavior similar to the transverse moduli. The effective elastic moduli of the interfacial carbon coating are determined from the experimental data and their change due to thermal oxidation is discussed.
High Temperature Fracture Toughness in Silicon Nitride and Sialon
Authors: Mutoh Y. , Miyahara N. , Yamaishi K. , Oikawa T.
Published: April 2008

Fracture Toughness of HIP-sintered silicon nitride decreased with increasing temperature up to 1200°C. The brittle-to-ductile transition was observed in the temperature range from 1200°C to 1275°C: the fracture toughness rapidly increased in the transition region. Above the transition temperature, the fracture toughness decreased with increasing temperature. Fracture toughness of sialon increased with increasing temperature. Transition of fracture mechanism was observed in sialon around 1300°C. The differences of temperature dependence of fracture toughness between two materials are interpreted in terms of the effects of grain-boundary glass phase on fracture.
High Strain Rate Deformation and Damage in Ceramic Materials
Authors: Raiser G. , Clifton R. J.
Published: April 2008

The objective of this investigation is to use a plate impact recovery experiment to identify the dominant failure mechanisms in conventional α-Al2O3 ceramics and thereby gain insight into the most promising, failure-resistant microstructures. A “soft-recovery” configuration is used wherein a star-shaped flyer impacts a square specimen. The impedances, shapes, thicknesses and orientation of all plates are designed to ensure a known history of longitudinal, planar stress waves throughout a central octagonal region of the specimen. The plane waves generated from this experiment are monitored by a laser interferometer system that allows data to be collected at four separate locations. The validity of the approach is demonstrated by a shot in which all plates were stressed within their elastic range. Subsequently, several experiments were conducted at nearly the same stress level with commercially sintered aluminas having different grain size and different glass content. These experiments, taken as a whole, demonstrate that improvement in alumina’s dynamic compressive properties is obtained by reducing the grain size. In compression, a reduction in grain size lowers average residual stresses at triple junctions and grain boundaries and makes the material less susceptible to inelastic deformation and sliding at triple junctions and grain boundaries. A reduction in the weight percent of pre-processing impurities (and therefore the amount of intergranular glassy phase) yields strong improvements in the dynamic tensile strength of the ceramic. A decrease in the amount of glassy phase tends to make tensile damage less likely by improving grain boundary strength. These trends were tested by conducting recovery experiments on a high-purity, small-grain alumina, processed in-house through hot pressing. Both the compressive resistance and, especially, the tensile resistance were superior to those found for all other tested specimens. The overall results suggest that the best failure resistance will be obtained for new, high-purity, ultrafine-grain ceramics that are prepared by hot pressing of nanometer scale powders.
Compressive Failure of Fiber Composites: The Roles of Multiaxial Loading and Creep
Authors: Slaughter W. S. , Fleck N. A. , Budiansky B.
Published: April 2008

The roles of multiaxial loading and creep in compressive failure of aligned fiber composites are considered. Analytical models are developed based on the model given by Budiansky and Fleck (1992). The critical microbuckling stress in multiaxial loading is calculated for a rigid-perfectly plastic solid and an elastic-plastic strain hardening solid. The rigid-perfectly plastic results predict a plane compressive failure surface in stress space. The rigid-perfectly plastic results are sufficiently accurate, when compared to the strain hardening results, so long as the remote shear stress and stress normal to the fiber direction are not too large relative to the remote stress in the fiber direction. The model given for creep microbuckling is suitable for power law viscous composite behavior. Deformation within a localized kink band is computed as a function of time. A creep life is predicted, based on a critical strain failure criterion.
ASME 1992 Nadai Lecture—Micromechanics of Inelastic Composite Materials: Theory and Experiment
Authors: Dvorak George J.
Published: April 2008

Some recent theoretical and experimental results on modeling of the inelastic behavior of composite materials are reviewed. The transformation field analysis method (G. J. Dvorak, Proc. R. Soc. London, Series A437, 1992, pp. 311–327) is a general procedure for evaluation of local fields and overall response in representative volumes of multiphase materials subjected to external thermomechanical loads and transformations in the phases. Applications are presented for systems with elastic-plastic and viscoelastic constituents. The Kroner-Budiansky-Wu and the Hill self-consistent models are corrected to conform with the generalized Levin formula. Recent experimental measurements of yield surfaces and plastic strains on thin-walled boron-aluminum composite tubes are interpreted with several micromechanical models. The comparisons show that unit cell models can provide reasonably accurate predictions of the observed plastic strains, while models relying on normality of the plastic strain increment vector to a single overall yield surface may not capture the essential features of the inelastic deformation process.
Strain Hardening of Annealed 304 Stainless Steel by Creep
Authors: Wu Han-Chin , Ho Chin-Cheng
Published: April 2008

Combined axial-torsional experiments have been conducted at room temperature on thin-walled tubes to investigate the strain hardening behavior of annealed 304 stainless steel due to creep. The constant strain-rate dynamic loading (or SCISR) surfaces representing the state of material before and after creep have benn determined. It has been found that transient creep essentially causes the loading surface to undergo kinematic hardening with insignificant amount of isotropic hardening for this material. A conclusion is drawn that the loading surface hardened by transient creep is the same as that hardened by plastic deformation. This is true both for specimens with pure tension and pure torsion loading paths. The results confirm assumptions of the overstress theory of viscoplasticity.
Constitutive Equations for the Thermomechanical Response of Rene´ 80: Part 1—Development From Isothermal Data
Authors: Bhattachar V. S. , Stouffer D. C.
Published: April 2008

This work describes the development of a unified nonisothermal constitutive model to predict the thermomechanical fatigue (TMF) response of a Nickel base superalloy, Rene´ 80. Nonisothermal deformation mechanisms are modeled using state variables. The flow equation of the Ramaswamy-Stouffer model was rewritten in the form of an Arrhenius equation with explicit temperature dependence. The isothermal predictions were correlated with the test data at four test temperatures between 538°C and 982°C. Material parameters were verified using nonisothermal tensile calculations. This verification showed that modeling the transition between planar slip and dislocation climb accurately is crucial for obtaining reliable TMF predictions. The revised constitutive model could successfully predict Rene´ 80 response from several TMF tests between 760°C and 982°C.
The Use of Classification Zones for Fatigue Behavior in Steels—2
Authors: Weiss M. P.
Published: April 2008

This is the second and updated version of a paper with a similar name, that was published in this journal in [1], 1977. The basic idea of classification of the Fatigue Regime is implemented again, using updated fatigue studies, and the result is a new and updated diagram. Numerous studies on Fatigue Damage and Fatigue Crack Propagation dealt with these problems either by the cumulative damage approach, or by using crack propagation equations based on linear elastic fracture mechanics. Although these two approaches are not compatible, each is useful for predicting fatigue behavior with acceptable scatter, but only within defined limits. This study updates the “Fatigue Diagram,” which classifies different combinations of stresses and crack lengths in a given specimen, as zones in which different fatigue evaluation criteria govern. The whole fatigue regime is divided into six zones, that fit to known and to new prediction methods. For the more problematic zone, where the stress amplitude is higher than the fatigue limit, and the stress intensity factor range is higher than the threshold, a superposition of two prediction procedures is shown to match closely to test results in AISI 4XXX types of steel specimens. The fatigue diagram is shown to be a useful tool for dealing with real life, fatigue design problems. The “Threshold Crack Length” is reintroduced, and directions for additional studies are indicated.
Application and Verification of Fatigue Life Calculation Methods for AZ91E-T6 Cast Magnesium Alloy Under Variable Amplitude Loading
Authors: Perov S. N. , Ogarevic V. V. , Stephens R. I.
Published: April 2008

The goal of this research was to determine the applicability of commonly used fatigue life calculation methods for variable amplitude loading with sand cast AZ91E-T6 magnesium alloy using notched “keyhole” specimens. Commercially available fatigue life calculation software packages were used to calculate the fatigue crack “initiation” life (1 mm crack) and fatigue crack growth life corresponding to the experiments. The resulting correlation between experimental and calculated fatigue lives ranged from good to poor, and from conservative to nonconservative, depending on the analysis method used. It was shown that these fatigue life calculation methods can be used with this sand cast magnesium alloy but with certain apprehension.
A General Fatigue Theory and Its Application to Out-of-Phase Cyclic Loading
Authors: Ellyin F. , Xia Z.
Published: April 2008

Results of an experimental investigation of a constant-amplitude, fully reversed biaxial tension-compression out-of-phase cyclic loading are presented first. An energy-based fatigue theory is extended to include nonproportional (out-of-phase) cyclic loading. Finally, a constitutive model is presented to predict the cyclic multiaxial stress-strain response, and to calculate the energy based damage parameter. Thus, a general theory, applicable to uniaxial, proportional, and nonproportional multiaxial cyclic fatigue is presented. The predicted results are in fairly good agreement with the constant amplitude test data. A few uniaxial fatigue tests and an out-of-phase cyclic test are required to specify material constants of the theory.
General-Form Solution of Shock Fitting Equation Including Die Wall Friction for the Multishock Powder Compaction
Authors: Sano Y. , Tokushima K. , Yamashita M.
Published: April 2008

In this paper, shock fitting equations including wall friction force for predicting the one-dimensional compaction process of a powder medium caused by punch impact are first derived. The medium is assumed to be discontinuously compressed only at a shock wave front both when the front propagates toward an assumed rigid plug and when it propagates back to an assumed rigid punch. The equations suggest that the effect of the friction force on the process becomes large as the front propagates toward the plug. This friction effect suggests that a continuous compression will occur in the medium between the impacted surface and the front if the effect is large. Next, the general-form solution of the shock fitting equations is obtained. This solution is compared with the solution by the pseudo-viscosity method without using the assumption that the medium is compressed only at the front. Both the solutions agree well for the compaction with a short initial medium length where the effect is not remarkable. For the compaction with a long initial medium length where the effect is remarkable, however, the solutions predict different types of the process, especially in its earlier stage. Explicitly, the former predicts the discontinuous compression only at the front, as is clear from the assumption made, while the latter predicts not only the discontinuous compaction at the front but also the continuous compression between the impacted surface and the front due to the remarkable friction effect. In its later stage, they predict the compression only at the front. Thus, the general-form solution is valid for the compaction with short initial medium lengths, but results in errors in the earlier stage for long initial medium lengths.
A Spectral Element Simulation of Gravitational Flow During Plastic Pipe Extrusion
Authors: Githuku D. N. , Giacomin A. J.
Published: April 2008

When plastic pipe is extruded, it emerges from an annular die and passes through a sizing sleeve to set its outer diameter. The pipe is then solidified in a cooling tank by spraying the outer surface with cold water. For thick walled pipe, material on the inside stays molten for a long time, and flows under its own weight. This gravitational flow (or sag) therefore governs the final pipe wall thickness distribution. Thus the solidification of extruded plastic pipe involves heat transfer with phase-change coupled with gravitational flow. The inside thermal boundary condition is arguably adiabatic. For the outside boundary, one can use either Newton’s law of cooling or an isothermal condition. In this paper, a commercially available spectral element code for simulating unsteady incompressible fluid flow with heat transfer, has been used to simulate sag flow. The model predictions of the solid pipe thickness distribution compared well with process data. Also, the effects of different heat transfer parameters on the thickness profile are evaluated. The extrusion temperature is found to have the greatest effect on the pipe wall profile.
Bending Analysis of Laminated Composite Box Beams
Authors: Tripathy A. K. , Patel H. J. , Pang S. S.
Published: April 2008

Box beams are widely used in weight reduction structures such as aircraft wings. The use of composite box beams further reduces the weight factor for such structures with the same deflection and stress as that of isotropic box beams. The difference in the behavior of composite box beams with different fiber orientation, number of plies, and number of stringers also provides a wide range of designing parameters to achieve the required performance for a given problem. A bending analysis has been carried out for the study of deflections and stresses for box beams of different material (isotropic and laminated composites), size, and number of stringers subjected to different kinds of loading conditions. A finite element model has been developed based on the strain energy principle, and the results are compared with an available commercial code “COSMOS/M.” Experiments using aluminum and scotchply composite laminates were conducted to verify the results. An optimal design for size and number of stiffeners for a given loading condition has been achieved. Investigations have also been carried out to find the effect of transverse shear on the span-wise normal stress.
Influence of Environment and Creep on Fatigue Crack Growth in a High Temperature Aluminum Alloy 8009
Authors: Jata K. V. , Maxwell D. , Nicholas T.
Published: April 2008

Frequency effects on fatigue crack growth rates are examined in aluminum alloy 8009 in sheet and extruded product forms. The investigations show that frequency effects on the fatigue crack growth rates are pronounced in the sheet but minimal in the extrusion. The influence of creep cracking on fatigue crack growth rate is studied through tests with a 60 s hold-time at maximum load at several stress intensity ranges. A 60 s hold-time at maximum load at 315°C tends to retard fatigue crack growth in both the sheet and the extrusion. The mechanism by which this retardation occurs is attributed to stress relaxation at the crack tip. At 204°C a 60 s hold at Pmax accelerates crack growth rate in the sheet but not in the extrusion. Vacuum and laboratory air tests show that fatigue crack growth rates in vacuum are lower than in air by about a factor of four. A 60 s hold-time at minimum load has only a minor effect on the fatigue crack growth rates at 315°C and no effect at 204°C, confirming the absence of any strong environmental contribution to crack growth rate. Fracture modes in fatigue, creep crack growth and hold-time at Pmax are significantly different. The fractographic results are discussed in relation to the mechanical property data.
Analysis of Plastic Behavior to Cyclically Uniaxial Tests Using an Endochronic Approach
Authors: Yeh Wei-Ching , Cheng Jung-Yuan , Her Rong-Song
Published: April 2008

This paper is devoted to the modeling of metallic materials behaving path dependent strain/loading hardening using endochronic theory. Characteristics of the model include the description of the strain path dependent rate of kinematic hardening and of the strain path dependent distortion of the yield surface. Comparisons between theoretical and experimental results are presented to proof the theory. It is shown that the present model can reasonably predict stress response to uniaxially cyclic loading under a variety of test conditions. In particular, the celebrated Bauschinger effect, which is well-known to be inadequately described by the classical plasticity theories, can be quantitatively predicted well by the model.
A Finite-Element Work-Hardening Plasticity Model of the Uniaxial Compression and Subsequent Failure of Porous Cylinders Including Effects of Void Nucleation and Growth—Part I: Plastic Flow and Damage
Authors: Lee J. H. , Zhang Y.
Published: April 2008

Gurson’s mixed hardening plasticity model (which takes into account the progressive damage due to void nucleation and growth of an initially dense material), with strain and stress-controlled nucleations, was used in a large deformation finite element program to study the plastic flow and damage in the uniaxial compression of cylinders under sticking friction. Effects of strain hardening, nucleation models, yield surface curvature, and geometry on the distributions and evolutions of stresses, strains, mean stress, void fractions, and coalescence are studied in detail. Using Gurson’s isotropic hardening model, positive mean and axial stresses developed at the bulge of the cylinder with growth of voids at latter stages of deformation. Due low stress triaxiality (Σm/σe<0.6) at the bulge, the process is nucleation rather than growth dominated for the majority of the cases studied. At failure, the maximum void fraction at the bulge among all cases studied is 0.085 and is far less than the critical void fraction (≈0.15) for coalescence.
Boundary Element Implementation of a Rough Crack Constitutive Model
Authors: Divakar M. P. , Fafitis A.
Published: April 2008

The boundary element application of a micromechanics based constitutive model for rough cracks previously developed by the authors is presented. This constitutive model considers the off-diagonal terms in the crack stiffness matrix, and it relates the normal and shear stresses on the rough crack to the corresponding displacement discontinuities. A brief introduction to the boundary element method is presented to illustrate the application of the constitutive model. Formulations are then developed for modeling discontinuities such as frictional interfaces present in continuous media. The constitutive model is used as a boundary condition to couple the subregions on either side of the discontinuity. Examples of boundary element modeling of frictional interfaces are then presented and discussed with respect to test results. It is shown that the constitutive model developed by the authors can be implemented in the boundary element method without any difficulty.
A Model for Friction in Metal Forming
Authors: Carter William T.
Published: April 2008

The role of hydrostatic stress on the plastic deformation of asperities is examined in the context of the adhesion theory of friction. A lower bound analysis gives the frictional shear strength to be proportional to the normal component of the deviatoric stress, rather than the total stress as is currently held in the Coulomb friction model. When applied to specific regimes, the new model is shown to agree qualitatively with various existing models for friction, including the Coulomb and Prandtl rules.
A Yield Surface Approach to the Estimation of Notch Strains for Proportional and Nonproportional Cyclic Loading
Authors: Barkey M. E. , Socie D. F. , Hsia K. J.
Published: April 2008

An analytical method is developed to estimate notch root strains in a notched bar of elastic-plastic, isotropic material subjected to proportional and nonproportional multiaxial nominal loading. The method uses anisotropic plasticity theory to define a structural yield surface in nominal stress space that incorporates both the isotropic material properties and the anisotropic geometry factors of the notch. Notch root plastic strain increments and anisotropic work-hardening effects are then related to this yield surface using standard methods of plasticity. Comparisons of the proposed method with previously published strain estimates using the finite element method for a notched shaft under proportional nominal bending and torsion, and with strain gage measurements of a circumferentially notched solid steel shaft subjected to a series of box-shaped nonproportional loading paths in tension-torsion nominal stress space are presented. The strain calculations agree well both qualitatively and quantitatively using an appropriate nominal load-notch plastic strain relationship, and are suitable for strain-life fatigue calculations.
Fatigue Growth Analysis of an Inclined Crack Under Uniaxial Cyclic Loading in Materials With Different Yield Strengths in Tension and Compression
Authors: Yan Xiangqiao , Lei Weisheng
Published: April 2008

In the present paper, an improved strain-energy-density criterion presented recently for the commonly used fracture criterion, the minimum strain-energy-density criterion, is extended to the case of cyclic loading to predict mixed-mode fatigue crack growth in materials with different yield strengths in tension and compression. The analysis of the mixed-mode fatigue crack growth process is very complex. For the purpose of more precisely predicting the mixed mode fatigue crack growth process, we developed a numerical scheme in which the improved fatigue crack growth criterion is combined with the displacement discontinuity method, a boundary element method. In the fatigue crack growth analysis of an inclined crack under uniaxial cyclic loading, the stress intensity factors for each increment of the crack growth are calculated by means of the displacement discontinuity method. Fatigue growth analysis of an inclined crack under uniaxial cyclic loading in materials with different yield strengths in tension and compression is carried out.
Influence of Weld Discontinuities on Strain Controlled Fatigue Behavior of 308 Stainless Steel Weld Metal
Authors: Bhanu Sankara Rao K. , Valsan M. , Sandhya R. , Mannan S. L. , Rodriguez P.
Published: April 2008

Detailed investigations have been performed for assessing the importance of weld discontinuities in strain controlled low cycle fatigue (LCF) behavior of 308 stainless steel (SS) welds. The LCF behavior of 308 SS welds containing defects was compared with that of type 304 SS base material and 308 SS sound weld metal. Weld pads were prepared by shielded metal arc welding process. Porosity and slag inclusions were introduced deliberately into the weld metal by grossly exaggerating the conditions normally causing such defects. Total axial strain controlled LCF tests have been conducted in air at 823 K on type 304 SS base and 308 SS sound weld metal employing strain amplitudes in the range from ±0.25 to ±0.8 percent. A single strain amplitude of ±0.25 percent was used for all the tests conducted on weld samples containing defects. The results indicated that the base material undergoes cyclic hardening whereas sound and defective welds experience cyclic softening. Base metal showed higher fatigue life than sound weld metal at all strain amplitudes. The presence of porosity and slag inclusions in the weld metal led to significant reduction in life. Porosity on the specimen surface has been found to be particularly harmful and caused a reduction in life by a factor of seven relative to sound weld metal. Defect combination of porosity and slag inclusions was found to be more deleterious than the case when either the slag inclusions or porosity was present alone. Discontinuties acted as crack initiation sites and also enhanced crack propagation. The LCF properties of weld samples containing discontinuities have been correlated with the damage and fracture behavior.
A Finite Element Analysis of Electromagnetic Forming for Tube Expansion
Authors: Lee Sung Ho , Lee Dong Nyung
Published: April 2008

The electromagnetic field and dynamic deformation analyses for tube expansion by electromagnetic forming were performed by the finite element method. A realistic pressure distribution was calculated by taking into account both coil and workpiece. The calculated values of displacement along the tube axis and with time were in very good agreement with the measured ones.
Nonlocal Model Based on Crack Interactions: A Localization Study
Authors: Bazˇant Z. P. , Jira´sek M.
Published: April 2008

A micromechanically based enrichment of the nonlocal operator by a term taking into account the directional dependence of crack interactions (Bazˇant, 1992) can be expected to improve the performance of the nonlocal model. The aim of this paper is to examine this new model in the context of a simple localization problem reducible to a one-dimensional description. Strain localization in an infinite layer under plain stress is studied using both the old and the new nonlocal formulations. The importance of renormalization of the averaging function in the proximity of a boundary is demonstrated and the differences between the localization sensitivity of the old and new model are pointed out.
Microscale Arrangement Effects on the Thermomechanical Behavior of Advanced Two-Phase Materials
Authors: Bo¨hm H. J. , Rammerstorfer F. G. , Fischer F. D. , Siegmund T.
Published: April 2008

Finite Element based micromechanical methods are used to study the influence of microscale phase arrangements on the overall and microscale thermomechanical properties of two advanced two-phase materials, duplex steels and unidirectional continuously reinforced metal matrix composites (MMCs). Both inclusion-matrix topologies and interwoven microgeometries are investigated for duplex steels, and the predicted macroscopic transverse elastoplastic responses are correlated with quantitative stereological descriptions of the microgeometries. For the MMCs, the emphasis is put on the influence of the fiber arrangement on the microscale residual stress states of the as-produced material and on their effects on damage mechanisms.
The Interface Ring Dislocation in Fiber-Matrix Composites: Approximate Analytical Solution
Authors: Demir I. , Zbib H. M.
Published: April 2008

An analytical solution for the extended stress field of the fundamental problem of interfacial Somigliana ring dislocation in fiber-matrix composites is obtained. The result of this work provides a powerful tool which can be used to address a class of three-dimensional defect problems encountered in heterogeneous materials, such as fiber-matrix debonding, fiber pull-out, broken fibers and others, as well as interaction among defects.
Micromechanics and Effective Elastoplastic Behavior of Two-Phase Metal Matrix Composites
Authors: Ju J. W. , Chen Tsung-Muh
Published: April 2008

A micromechanical framework is presented to predict effective (overall) elasto-(visco-)plastic behavior of two-phase particle-reinforced metal matrix composites (PRMMC). In particular, the inclusion phase (particle) is assumed to be elastic and the matrix material is elasto-(visco-)plastic. Emanating from Ju and Chen’s (1994a,b) work on effective elastic properties of composites containing many randomly dispersed inhomogeneities, effective elastoplastic deformations and responses of PRMMC are estimated by means of the “effective yield criterion” derived micromechanically by considering effects due to elastic particles embedded in the elastoplastic matrix. The matrix material is elastic or plastic, depending on local stress and deformation, and obeys general plastic flow rule and hardening law. Arbitrary (general) loadings and unloadings are permitted in our framework through the elastic predictor-plastic corrector two-step operator splitting methodology. The proposed combined micromechanical and computational approach allows us to estimate overall elastoplastic responses of PRMMCs by accounting for the microstructural information (such as the spatial distribution and micro-geometry of particles), elastic properties of constituent phases, and the plastic behavior of the matrix-only materials. Comparison between our theoretical predictions and experimental data on uniaxial elastoplastic tests for PRMMCs is also presented to illustrate the capability of the proposed framework. A straightforward extension to accommodate viscoplastic matrix material is also presented to further enhance the applicability of the proposed method.
Micromechanics of Active Composites With SMA Fibers
Authors: Lagoudas D. C. , Boyd J. G. , Bo Z.
Published: April 2008

The study of the effective thermomechanical response of active fibrous composites with shape memory alloy (SMA) fibers is the subject of this work. A 3-D constitutive response for the SMA fibers is formulated first. To model thermomechanical loading path dependence, an incremental approach is used assuming that within each stress and temperature increment the volume fraction of the martensitic phase remains constant in the SMA fibers. The Mori-Tanaka averaging scheme is then used to give an estimate of the instantaneous effective thermomechanical properties in terms of the thermomechanical properties of the two phases and martensitic volume fraction. A unit cell model for a periodic active composite with cubic and hexagonal arrangement of fibers is also developed to study the effective properties using finite element analysis. It is found that since the fibers and not the matrix undergo the martensitic phase transformation that induces eigenstrains, the Mori-Tanaka averaging scheme accurately models the thermomechanical response of the composite, relative to the finite element analysis, for different loading paths. Specific results are reported for the composite pseudoelastic and shape memory effect for an elastomeric matrix continuous SMA fiber composite.