bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Chemo-Mechanical Polishing (CMP) of Si3N4 Bearing Balls With Water Based CeO2 Slurry
Authors: Jiang Ming , Wood Nelson O. , Komanduri R.
Published: February 2008

Among various abrasives investigated for the chemo-mechanical polishing (CMP) of Si3N4 balls (Jiang, 1998), cerium oxide (CeO2) was found to be the most effective polishing medium (even superior to Cr203, Bhagavatula and Komanduri, 1996), yielding an extremely smooth and damage-free surface with a finish Ra of ≈4 nm and Rt of ≈40 nm. In this investigation, the underlying reasons for the superior finish with CeO2 were investigated. Various chemical reactions involved in CMP of Si3N4 balls with CeO2 were investigated (Gibbs free energy minimization) and a mechanism for the CMP is proposed. The two important functions that CeO2 performs in the CMP of Si3N4 are: 1. It participates directly in the chemical reaction (oxidization-reduction reaction) with Si3N4 workmaterial leading to the formation of a thin SiO2 layer, 2. The hardness of CeO2 is closer to that of the thin SiO2 layer formed on Si3N4 but significantly lower than Si3N4 workmaterial (≈1/3). It can thus remove the brittle SiO2 reaction product effectively without damaging the Si3N4 substrate as no abrasion can take place by CeO2 on Si3N4. The kinetic action, which involves the removal of the reaction products from the interface by subsequent mechanical action of flowing water and CeO2 is critical to CMP. The chemical reaction could proceed on a continuing basis so long as the passivation layers are removed by the mechanical action at the same time. CeO2 is found to be very effective in a water environment (hydrolysis) leading to the formation of additional SiO2 by reacting with Si3N4 thereby enhancing the CMP of Si3N4. Several similarities between polishing of Si3N4 and glass (SiO2) (Cook, 1990), including the polishing environment (CeO2 plus the magnetic fluid, pH value ≈6) and the mechanism of polishing were observed. Also, after investigating various reaction species in the CMP of Si3N4 with CeO2 and Cr203, the former is found to be much safer from an environmental point of view (Reddy and Komanduri, 1998).
Modeling of GMA Weld Pools With Consideration of Droplet Impact
Authors: Cao Zhen Ning , Dong Pingsha
Published: February 2008

A three-dimensional weld pool model has been developed to study the fluid flow and heat transfer process during gas metal arc (GMA) welding. Both droplet heat content and impact force were considered in analyzing the effect of droplets on the formation of weld pool. The fluid flow in the weld pool was induced by the presence of surface tension, electromagnetic and buoyancy force. The surface deformation of weld pool was calculated by considering arc pressure and droplet impact force. Computational results under partial and full penetration welding conditions were obtained. The effect of heat flow and fluid flow characteristics on weld pool geometry was discussed, particularly with respect to the presence of droplet heat input and impact force.
Rate Dependent Deformation of Semi-Crystalline Polypropylene Near Room Temperature
Authors: Arruda E. M. , Ahzi S. , Li Y. , Ganesan A.
Published: February 2008

We examine the strain rate dependent, large plastic deformation in isotropic semi-crystalline polypropylene at room temperature. Constant strain rate uniaxial compression tests on cylindrical polypropylene specimens show very little true strain softening under quasi-static conditions. At high strain rates very large amounts (38 percent) of apparent strain softening accompanied by temperature rises are recorded. We examine the capability of a recently proposed constitutive model of plastic deformation in semi-crystalline polymers to predict this behavior. We neglect the contribution of the amorphous phase to the plastic deformation response and include the effects of adiabatic heating at high strain rates. Attention is focused on the ability to predict rate dependent yielding, strain softening, strain hardening, and adiabatic temperature rises with this approach. Comparison of simulations and experimental results show good agreement and provide insight into the merits of using a polycrystalline modeling assumption versus incorporating the amorphous contribution. Discrepancies between experiments and model predictions are explained in terms of expectations associated with neglecting the amorphous deformation.
Influence of Crosslinking on the Plastic Behavior of Amorphous Polymers at Large Strains
Authors: G’Sell Christian , Souahi Abdelhamid
Published: February 2008

The mechanical properties of three grades of PMMA, one linear and two crosslinked, are investigated in tension below and above the glass transition temperature. It is found that strain-hardening decreases gradually with temperature in the glassy state, but keeps a significant value in the rubbery state. The results are analyzed in terms of an entropy-based statistical model (van der Giessen and Wu, 1992) which describes the stress-strain response of a chain network. The junctions of the network are not only associated with the chemical crosslinks, but additionally with physical entanglements, which are more numerous in the glassy state.
Large Deformations of Semicrystalline Polymer Modeled Using the Necking Network Concept
Authors: Sweeney J. , Collins T. L. D. , Coates P. D. , Unwin A. P. , Ward I. M.
Published: February 2008

Previously, a physically-based elastic model has been used in the modeling of high temperature polymer deformation. While this has been successful in producing realistic shapes and forces in necking polymers, the elastic nature of the theory causes some discrepancies between the predictions and the experimental observations. The theory does not describe the observed rate dependence of drawing forces, and at some stages of deformation the predicted inhomogeneity is greater than is observed. The latter effect is the result of the elastic theory’s ability to change instantaneously from a homogeneous to an inhomogeneous state of strain, corresponding to the development of a neck. In this paper, we model the development of necks in tensile specimens using an extension of the model in which rate dependence has been incorporated in a simple way. The incorporation of rate dependence results in a model of neck development in which the changes in strain are less abrupt and the predicted shapes more realistic. To evaluate the model, specimens of polypropylene were stretched at 150°C and their images captured and analysed digitally. Predictions of the shapes were generated by incorporating the model into the finite element package ABAQUS.
Recovering Nonisothermal Physical Aging Shift Factors Via Continuous Test Data: Theory and Experimental Results
Authors: Bradshaw R. D. , Brinson L. C.
Published: February 2008

For isothermal physical aging, a few simple tests to characterize the aging shift factors allow reasonable prediction of the mechanical response. In this paper, a new technique is developed to extract aging shift factors from creep data during a nonisothermal history. Previous methods have generated discrete experimental shift factors by a series of short-term creep tests, in which the load portion alone is used for evaluation; this is particularly time consuming for nonisothermal histories, since many data points (requiring several tests) may be needed for an adequate characterization of the response. This paper presents a new continuous shift factor (CSF) method, based on the validity of effective time theory, which generates a continuous experimental shift factor curve from a single test. Results are presented for this method when applied to a polyimide/carbon fiber composite material tested in shear under temperature jump conditions; this nonisothermal aging data for a polymer matrix composite is shown to exhibit similar response to that of homogeneous polymers. The new CSF technique will be useful in the development of models to predict the shift factor due to coupled aging and thermal history.
Effect of Cure State on Stress Relaxation in 3501-6 Epoxy Resin
Authors: White S. R. , Hartman A. B.
Published: February 2008

Little experimental work has been done to characterize how the viscoelastic properties of composite material matrix resins develop during cure. In this paper, the results of a series of creep tests carried out on 3501–6 epoxy resin, a common epoxy matrix material for graphite/epoxy composites, at several different cure states is reported. Beam specimens were isothermally cured at increasing cure temperatures to obtain a range of degrees of cure from 0.66 to 0.99. These specimens were then tested in three-point bending to obtain creep compliance over a wide temperature range. The master curves and shift functions for each degree of cure case were obtained by time-temperature superposition. A numerical technique and direct inversion were used to calculate the stress relaxation modulus master curves from the creep compliance master curves. Direct inversion was shown to be adequate for fully cured specimens, however it underpredicts the relaxation modulus and the transition for partially cured specimens. Correlations with experimental stress relaxation data from Kim and White (1996) showed that reasonably accurate results can be obtained by creep testing followed by numerical conversion using the Hopkins-Hamming method.
Free Edge Stresses in Elastic and Viscoelastic Composites Under Uniaxial Extension, Bending, and Twisting Loadings
Authors: Yi Sung , Hilton H. H.
Published: February 2008

Time-dependent interlaminar stresses in elastic and viscoelastic laminated composites subjected to arbitrary combinations of axial extension, bending and/or twisting loads are obtained based on integral constitutive relations and Pipes and Pagano’s displacement field for laminates under a generalized plane deformation state. Numerical results obtained from the present formulation are compared against experimental data and excellent agreement within two percent was obtained between these results. Time-dependent interlaminar stresses for cross-ply and angle-ply laminates subjected to uniaxial extension, bending and twisting are also presented. Appreciable stress relaxation occurred during the loading period resulting in decreased magnitudes of residual stresses. It is seen that the rate of interlaminar shear stress relaxation is greater than the normal one, since the relaxation of shear moduli is larger than that of the normal moduli.
Constitutive Modeling and Material Characterization of Polymeric Foams
Authors: Zhang J. , Lin Z. , Wong A. , Kikuchi N. , Li V. C. , Yee A. F. , Nusholtz G. S.
Published: February 2008

In this study, mechanical properties of three types of polymeric foams (polypropylene (PP), polystyrene (PS), and polyurethane (PU) foams) are investigated. Focus has been placed on the strain rate and temperature effects on these foams under large deformations. Selected experimental results from uniaxial compression, hydrostatic compression, and simple shear tests are presented. A phenomenological hydrodynamic elastoplastic constitutive law is developed to model these polymeric foams. Numerical implementation and validation of the constitutive model are also described.
Visco-Hyperelastic Model for Filled Rubbers Used in Vibration Isolation
Authors: Papoulia K.-D. , Kelly J. M.
Published: February 2008

The short time and cyclic behavior of filled rubbers used in vibration isolation in the frequency range 10−2 to 102 rad/s is examined. A form of the free-energy function consistent with the assumption of an additive stress decomposition is employed. A constitutive law for the inelastic part of the stress is provided, in the form of an integro-differential equation, which involves the fractional order derivative of the internal variable. It is assumed that the volumetric response of the material is elastic. The elasticity of rubber is modeled following classical models (e.g., Rivlin, Ogden), extended to include compressibility. Step-by-step integration of the constitutive law is performed. Simple shear experiments are used to assess the capability of the model to capture essential response characteristics, such as stiffness reduction under cyclic loading of increasing amplitude and the variation of dissipated energy with amplitude and frequency.
Thermomechanical Effects Associated With Crystallization of Rubber Under Stretch and During Slow Extension
Authors: Negahban Mehrdad
Published: February 2008

A natural rubber sample which crystallizes after stretching normally shows stress relaxation associated with this crystallization and normally ends up at a stress lower than that of the fully amorphous rubber before crystallization. On the other hand, a natural rubber sample which crystallizes during stretching becomes more rigid as a result of the crystallization and the stress required to extend it to a given stretch increases substantially above the stress needed to extend the fully amorphous rubber to the same elongation. Even though the former effect has been modeled and studied by the likes of Flory (1947), the latter effect has not yet been properly modeled or studied. The difference between crystallization during or after stretching will be studied in this article based on a thermodynamic model developed by the author to capture the thermomechanical effects of crystallization in natural rubber. The two limit cases of very rapid and very slow extension to a given stretch are singled out for comparison of the equilibrium stress.
Numerical Analysis of Deep Drawing Process for Thermoplastic Composite Laminates
Authors: Hsiao Shih-Wei , Kikuchi Noboru
Published: February 2008

A numerical analysis including flow, heat transfer and residual stress is developed to simulate the deep drawing process of composite laminates with woven fabric microstructures. The governing equations and material properties of thermoplastic composites at the forming temperature are obtained by the homogenization method based on the assumption of instantaneously rigid solid fibers suspended in a viscous non-Newtonian polymer melt. The processing rheology of the composites is characterized by a power-law constitutive model for this anisotropic, non-isothermal and shear thinning fluid. To simulate the thermoforming and cooling stages of the entire forming process, the three-dimensional finite element method incorporating a fiber orientation model of woven-fabric microstructures is developed. This global-local numerical methodology is capable of predicting macroscopic and microscopic deformation mechanics during the thermoforming process. As an illustration, a comparison between the fiber orientation prediction and experimental data for a deep drawn cup is presented.
The Role of Rate Effects and of Thermomechanical Coupling in Shear Localization
Authors: Estrin Y. , Molinari A. , Mercier S.
Published: February 2008

A dislocation density related constitutive model that accounts for the strain-rate sensitivity of the flow stress and, notable, of the strain-hardening coefficient was applied to describe adiabatic shear banding in face centered cubic metals. The parameter values of a prototype material, for which numerical simulations were carried out, are close to those of copper. The effect of the material parameters, especially of those reflecting the two rate sensitivities, on the occurrence of strain localization in a thin-walled tube under torsion containing a geometrical defect was investigated systematically. The results obtained provide some guidance with respect to design of materials with high resistance to strain localization and high mechanical energy absorption. Another outcome of this study is the recognition that for the problem in question linear stability analysis cannot provide a reliable criterion even for the onset of strain localization, and that numerical simulations have to be invoked.
A Predictive Tool for Delaying Wrinkling and Tearing Failures in Sheet Metal Forming
Authors: Cao J. , Boyce M. C.
Published: February 2008

In the sheet metal forming industry, there is increasing demand to lower manufacturing costs while also providing a decrease in product development turnaround period as well as lighter weight products. These demands have put increasing pressure on the development and use of predictive numerical simulations and in the design and optimization of new forming technologies. In this paper, two of the primary in-process failure modes of sheet metal, wrinkling and tearing, are examined followed by construction of an advanced forming technology—Variable Binder Force—using numerical tools. Specifically, a methodology of capturing the onset of wrinkling and postbuckling behavior proposed in Cao and Boyce (1997) is used to predict wrinkling failure in conical and square cup forming. The results obtained from simulations and experiments demonstrate that the proposed method is not only accurate, but also robust. A tearing criterion based on Forming Limit Diagrams of non-proportional loading paths is then developed and again shows excellent predictability. Finally, a Variable Binder Force (VBF) trajectory for conical cup forming is designed using simulations which incorporate feedback control to the binder based on the predictions of wrinkling and tearing of the sheet. Experiments using this predefined VBF trajectory show a 16 percent increase informing height over the best conventional forming method, that is, constant binder force. The uniqueness of this paper is that numerical simulation is no longer utilized only as a verification tool, but as a design tool for, advanced manufacturing process with the help of the predictive tools incorporated directly into the numerical model.
Spall Study in Mild Steel
Authors: Wang Ze-Ping
Published: February 2008

The influence of inertial, thermal, and rate-sensitive effects on the void growth at high strain rate in a thermal-viscoplastic solid is investigated by means of a theoretical model proposed in the present paper. Numerical analysis of the model suggests that inertial, thermal, and rate-sensitive effects are three major factors which greatly influence the behavior of the void growth in porous ductile materials in high strain rate case. One and two-dimensional plate-impact tests of mild steel are performed. Microscopic observations of the softly recovered specimens reveal the mechanism of micro-damage. As an application of the theoretical model, the processes of one and two dimensional spallation in mild steel are successfully simulated by a finite-difference Lagrangian dynamic code in which the mathematical model presented in this paper is incorporated.
Fatigue Crack Growth of β-21S Titanium Alloy Under Constant Amplitude and miniTWIST Flight Spectra at 25°C and 175°C
Authors: Stephens R. R. , Stephens R. I. , Veit A. L. , Albertson T. P.
Published: February 2008

β-21S titanium alloy sheet fatigue crack growth behavior was investigated at 25°C and 175°C under constant amplitude (R = 0.1 and 0.5) and miniTWIST flight spectra. Based upon nominal ΔK values, constant amplitude fatigue crack growth behavior at 175°C was either similar to (R = 0.1), or slightly better than (R = 0.5) 25°C. With crack closure taken into account, the fatigue crack growth curves at 175°C, plotted as a function of Keff, were shifted to the left of the fatigue crack growth curves at 25°C at near threshold values. Under flight spectra conditions, fatigue crack growth life at 175°C was 40 to 90 percent longer than at 25°C. Flight spectra life calculations using NASA/FLAGRO based upon constant amplitude fatigue crack growth data, were primarily conservative but in good agreement with experimental data. Fatigue crack growth was transgranular with crystalline facets and striations that were evident at higher constant amplitude fatigue crack growth rates and with the miniTWIST spectra. Striations were observed to a limited extent at threshold and near threshold conditions at 25°C, but not at 175°C. Based upon desirable constant and variable amplitude fatigue crack growth and fatigue/fracture crack morphology, this β-21S sheet alloy appears to be an acceptable material for damage tolerant aerospace situations between 25°C and 175°C.
Creep-Induced Cleavage Fracture in WIPP Salt Under Indirect Tension
Authors: Chan K. S. , Brodsky N. S. , Fossum A. F. , Munson D. E. , Bodner S. R.
Published: February 2008

The phenomenon of cleavage fracture initiation in rock salt undergoing concurrent creep was studied experimentally using the Brazilian indirect tension test technique. The tensile creep and cleavage fracture behaviors were characterized for rock salt from the Waste Isolation Pilot Plant (WIPP) site. The Brazilian test consists of a compressive line load applied diametrically on a disk specimen to produce a region of tensile stress in the center of the disk. The damage processes were documented using video photography. The experimental results were analyzed in terms of a wing-crack fracture model and an independently developed, coupled time-dependent, mechanism-based constitutive model whose parameters were obtained from triaxial compression creep tests. Analytical results indicate that coupling between creep and cleavage fracture in WIPP salt results in a fracture behavior that exhibits time-dependent characteristics and obeys a failure criterion involving a combination of stress difference and tensile stress. Implications of creep-induced cleavage fracture to the integrity of WIPP structures are discussed.
Modeling Damage Evolution in a Hybrid Ceramic Matrix Composite Under Static Tensile Load
Authors: Bonora N. , Newaz G.
Published: February 2008

In this investigation, damage evolution in a unidirectional hybrid ceramic composite made from Nicalon and SiC fibers in a Lithium Aluminosilicate (LAS) glass matrix was studied. The static stress-strain response of the composite exhibited a linear response followed by load drop in a progressive manner. Careful experiments were conducted stopping the tests at various strain levels and using replication technique, scanning and optical microscopy to monitor the evolution of damage in these composites. It was observed that the constituents of the composite failed in a sequential manner at increasing strain levels. The matrix cracks were followed by SiC fiber failures near ultimate tensile stress. After that, the load drop was associated with progressive failure of the Nicalon fibers. Identification of these failure modes were critical to the development of a concentric cylinder model representing all three constituent phases to predict the constitutive response of the CMC computationally. The strain-to-failure of the matrix and fibers were used to progressively fail the constituents in the model and the overall experimental constitutive response of the CMC was recovered. A strain based analytical representation was developed relating stiffness loss to applied strain. Based on this formulation, damage evolution and its consequence on tensile stress-strain response was predicted for room temperature behavior of hybrid CMCs. The contribution of the current work is that the proposed strain-damage phenomenological model can capture the damage evolution and the corresponding material response for continuous fiber-reinforced CMCs. The modeling approach shows much promise for the complex damage processes observed in hybrid CMCs.
Theoretical and Experimental Evaluation of the Bond Strength Under Peeling Loads
Authors: Nayeb-Hashemi Hamid , Jawad Oussama Cherkaoui
Published: February 2008

Reliable applications of adhesively bonded joints require understanding of the stress distribution along the bond-line and the stresses that are responsible for the joint failure. To properly evaluate factors affecting peel strength, effects of defects such as voids on the stress distribution in the overlap region must be understood. In this work, the peel stress distribution in a single lap joint is derived using a strength of materials approach. The bonded joint is modeled as Euler-Bernoulli beams, bonded together with an adhesive, which is modeled as an elastic foundation which can resist both peel and shear stresses. It is found that for certain adhesive and adherend geometries and properties, a central void with the size up to 50 percent of the overlap length has negligible effect on the peak peel and shear stresses. To verify the solutions obtained from the model, the problem is solved again by using the finite element method and by treating the adherends and the adhesive as elastic materials. It is found that the model used in the analysis not only predicts the correct trend for the peel stress distribution but also gives rather surprisingly close results to that of the finite element analysis. It is also found that both shear and peel stresses can be responsible for the joint performance and when a void is introduced, both of these stresses can contribute to the joint failure as the void size increases. Acoustic emission activities of aluminum-adhesive-aluminum specimens with different void sizes were monitored. The AE ringdown counts and energy were very sensitive and decreased significantly with the void size. It was observed that the AE events were shifting towards the edge of the overlap where the maximum peeling and shearing stresses were occurring as the void size increased.
Fatigue Behavior of a Cross-Ply Metal Matrix Composite at Elevated Temperature Under the Strain Controlled Mode
Authors: Sanders B. P. , Mall S. , Dennis L. B.
Published: February 2008

A study was conducted to investigate the fatigue behavior of a cross-ply metal matrix composite subjected to fully-reversed, strain-controlled fatigue cycling at elevated temperature. The stress-strain response, maximum and minimum stresses, and modulus during cycling were analyzed to characterize the macro-mechanical behavior. Additionally, microscopy and fractography were conducted to identify damage mechanisms. Damage always initiated in the 90 deg plies, but the governing factor in the fatigue life was damage in the 0 deg plies. The dominant failure mode was fracturing of fibers in the 0 deg plies when the maximum strain was greater than 0.55 percent, but the dominant failure mode was matrix cracking when the maximum strain was less than 0.55 percent. Combining the fatigue life data with the macro-mechanical and microscopic observations, a fatigue life diagram was developed and partitioned into three regions. These regions showed relationships between the maximum applied strain and the dominant damage mechanisms. Also, on a strain range basis, the fatigue lives of the specimens tested under the strain-controlled mode in this study were compared with its counterpart under the load-controlled mode of the previous study. It was found that the fatigue lives for these two conditions were the same within the experimental scatter.
Investigation of Localized Deformation in NiAl Single Crystals
Authors: Kumar Ashok V. , Yang Chulho , Seelam Vijay B. R.
Published: February 2008

Deformation of NiAl single crystals was studied using finite element analysis to investigate the modes of localized deformation. Constitutive parameters and hardening characteristics of the active slip systems were estimated by comparing numerical simulation results with experimental results. Deformation of tensile specimens of NiAl single crystal was simulated when loaded along different crystal orientations to understand the deformation mechanism that results in various localized modes of deformation. In particular, the formation of shear bands and kink bands was studied and the material and geometric characteristics that influence the formation of such localization were investigated.
Strain Space Formulation of the Armstrong-Frederick Family of Plasticity Models
Authors: Wang Haiyang , Barkey M. E.
Published: February 2008

Strain space based plasticity models have certain advantages in theoretical development and numerical implementation. Previous efforts have been made to formulate cyclic plasticity models in strain space using the idea of multiple-yield surface theory. Recently, however, Armstrong-Frederick type plasticity models have received increasingly more attention because of their enhanced performance in predicting ratchetting behavior. In this paper, the strain space formulation of the Armstrong-Frederick family of cyclic plasticity models is established, and several representative strain controlled loading paths are used to compare the results from the proposed formulation and previous experimental data. The excellent agreement suggests the proposed strain space formulation is very promising in strain controlled cyclic plasticity such as finite element analysis, strain gage rosette applications, and multiaxial notch analysis using pseudo-stress or pseudo-strain approaches.
Elastic Constants of Single Crystal Hastelloy X at Elevated Temperatures
Authors: Canistraro Howard A. , Jordan Eric H. , Shixiang Shi , Favrow Leroy H. , Reed Francis A.
Published: February 2008

An acoustic time of flight technique is described in detail for measuring the elastic constants of cubic single crystals that allows for the constants to be determined at elevated temperature. Although the overall technique is not new, various aspects of the present work may prove extremely useful to othersinterested in finding these values, especially for aerospace materials applications. Elastic constants were determined for the nickel based alloy, Hastelloy X from room temperature to 1000°C. Accurate elastic constants were needed as part of an effort to predict both polycrystal mechanical properties and the nature of grain induced heterogeneous mechanical response. The increased accuracy of the acoustically determined constants resulted in up to a 15 percent change in the predicted stresses in individual grains. These results indicate that the use of elastic single crystal constants of pure nickel as an approximation for the constants of gas turbine single crystal alloys, which is often done today, is inaccurate.
Tensile and Bending Behavior of Sintered Alloys: Experimental Results and Modeling
Authors: Bertini L. , Fontanari V. , Straffelini G.
Published: February 2008

Sintered materials show a different stress-strain behavior when subjected to tensile or compressive loading, the response to compression being characterized by a higher elastic modulus, yield stress, and strain hardening rate. These differences tend to make the bending behavior somewhat more complex to analyze, particularly in the elasto-plastic field, as compared to conventional materials, having equal mechanical properties under tension and compression. As a consequence, the use of widely applied test techniques, such as the Three Point Bending (TPB), becomes more difficult for sintered materials, due to the lack of reliable analytical models capable of evaluating elasto-plastic stress-strain distribution as a function of applied load and deflection. In the present investigation, the results of uniaxial tensile-compressive and bending tests conducted on sintered ferrous alloys characterized by different microstructures and porosity are reported and briefly discussed. Then an analytical model, specifically aimed to analyze the elasto-plastic monotonic behavior of a TPB specimen made with a material having different tensile and compressive properties, is presented. Its predictions as regards load-deflection curves and elasto-plastic stress-strain distributions are compared with the results of TPB tests and of numerical (Finite Element) analysis, showing a fairly good agreement.
Atomistic Simulations for Multiscale Modeling in bcc Metals
Authors: Moriarty John A. , Xu Wei , So¨derlind Per , Belak James , Yang Lin H. , Zhu Jing
Published: February 2008

Quantum-based atomistic simulations are being used to study fundamental deformation and defect properties relevant to the multiscale modeling of plasticity in bcc metals at both ambient and extreme conditions. Ab initio electronic-structure calculations on the elastic and ideal-strength properties of Ta and Mo help constrain and validate many-body interatomic potentials used to study grain boundaries and dislocations. The predicted Σ5 (310) [100] grain boundary structure for Mo has recently been confirmed in HREM measurements. The core structure, γ surfaces, Peierls stress, and kink-pair formation energies associated with the motion of a/2〈111〉 screw dislocations in Ta and Mo have also been calculated. Dislocation mobility and dislocation junction formation and breaking are currently under investigation.
Elastic Interaction of Defects on Crystal Surfaces
Authors: Kouris Demitris , Peralta Alonso , Sieradzki Karl
Published: February 2008

Surface defects corresponding to adatoms, vacancies and steps interact, affecting and often dominating kinetic processes associated with thin-film growth. A discrete harmonic model for the evaluation of the interaction energy between surface defects is presented. It is based on the concept of eigenstrains and allows for the accurate evaluation of the elastic field, both at the immediate vicinity of the defects, as well as in the far field. Results for the interaction energy suggest conditions for which a body-centered-cubic crystal surface will grow in a stable, two-dimensional, step-flow mode. In order to verify the accuracy of the discrete elastic model, we present results of atomic simulations that incorporate Embedded Atom Method (EAM) potentials. The discrete elastic model results compare favorably with results from our atomic EAM simulations and agree with the far-field predictions of continuum elastic theory.
Dislocation Dynamics Simulations in the Presence of Interacting Cracks
Authors: Demir I. , Gulluoglu A. N.
Published: February 2008

For an understanding failure behavior of crystalline solids, considerable interest is given to investigating interaction effects between the main crack and microcracks in the presence of mobile dislocations. Accurate analysis of these types of interaction problems may lead to accurate models for failure prevention and the history of plastic zone development. High stress concentration areas such as crack tips are the places where dislocations are subjected to higher forces. Therefore, a computer simulation technique based on dislocation dynamics has been developed to investigate the movement of dislocations in the presence of multiple cracks. Dislocation structures, dislocation distribution and strain rate results are presented as functions of applied stresses for different microcrack positions and orientations. Simulation results give a reasonable description of dislocation pattern development during deformation around the cracks and explain the shape and development of the plastic zone.
Experimental Investigation of Size Effects of FCC Polycrystal by Shear Banding
Authors: Watanabe Osamu , Kurata Takayuki
Published: February 2008

Several plasticity phenomena display a size effect where the smaller the size is the stronger its response. This effect relates to the plastic gradients, appearing in plastically inhomogeneous material. The present paper describes results of an experimental meso-scale study using the specimens having rectangular cross section made of FCC polycrystal of pure Aluminum and OFHC Copper under the tensile or compressive loading. Experimental measurements are carried out to investigate thickness effect and grain size effect in connection with size effect, and the internal mechanism of plastic flow in the specimens is also discussed.
The Effect of Tungsten on the Mechanical Properties of Tantalum
Authors: Briant C. L. , Lassila D. H.
Published: February 2008

This paper reports a study of the mechanical behavior of Ta-W alloys. The results show that tungsten additions increase the yield strength and the rate of work hardening of tantalum. These additions also cause a change in the deformed microstructure from one that is primarily cellular to one that consists mostly of dislocation tangles. It is proposed that the increase in yield strength arises from solid solution hardening and that the increase in the work hardening can be correlated with an increase in the density and arrangement of dislocations present in the material.
Orientation Imaging Microscopy Investigation of the Compression Deformation of a [011] Ta Single Crystal
Authors: Schwartz A. J. , King W. E. , Campbell G. H. , Sto¨lken J. S. , Lassila D. H. , Sun S. , Adams B. L.
Published: February 2008

High-purity tantalum single crystal cylinders oriented with [011] parallel to the cylinder axis were deformed 10, 20, and 30 percent in compression. The samples took on an ellipsoidal shape during testing, elongated along the [100] direction with almost no dimensional change along [011]. Two orthogonal sectioning planes were selected for characterization using Orientation Imaging Microscopy (OIM): one in the plane containing [100] and [011] (longitudinal) and the other in the plane containing [011] and [011] (transverse). OIM revealed patterns of alternating crystal rotations that develop as a function of strain and exhibit evolving length scales. The spacing and magnitude of these alternating misorientations increase in number density and decrease in spacing with increasing strain.
Multiscale Polycrystal Plasticity
Authors: McGinty Robert D. , McDowell David L.
Published: February 2008

Polycrystal plasticity models are commonly developed with a narrow focus on the grain as the fundamental unit of crystallographic orientation and anisotropic behavior. However, deformation and strengthening mechanisms occur simultaneously at multiple length scales and may lead to bulk deformation behavior of metals that is substantially different from that predicted by simple forms of polycrystal plasticity. The development of dislocation substructure occurs at subgrain scales while, at the same time, geometrically necessary dislocation boundaries (GNBs) are generated that extend over several grain diameters. A framework is presented here for the efficient treatment of multiple, simultaneously evolving strengthening mechanisms. The theory focuses on a macroscale hardening surface representation of the strengthening due to GNB formation. Crystallographic shear flow resistance is determined via a mapping procedure of the macroscale hardening surface to the length scale of grains. Predicted stress-strain curves based on the hardening surface formulation are compared to experimental data and polycrystal plasticity predictions for OFHC Cu. It is demonstrated that the hardening surface model of GNB strengthening mechanisms can provide improved predictive capability of nonproportional loading behavior of Cu compared to conventional slip system hardening laws commonly used in polycrystal plasticity applications.
Constitutive Modeling of Inelastic Solids for Plastic Flow Processes Under Cyclic Dynamic Loadings
Authors: Dornowski W. , Perzyna P.
Published: February 2008

The main objective of the paper is the description of the behavior and fatigue damage of inelastic solids in plastic flow processes under dynamic cyclic loadings. Experimental motivations and physical foundations are given. Recent experimental observations for cycle fatigue damage mechanics at high temperature of metals suggest that the intrinsic microdamage process does very much depend on the strain rate effects as well as on the wave shape effects. The microdamage process has been treated as a sequence of nucleation, growth and coalescence of microcracks. The microdamage kinetics interacts with thermal and load changes to make failure of solids a highly rate, temperature and history dependent, nonlinear process. A general constitutive model of elasto-viscoplastic damaged polycrystalline solids is developed within the thermodynamic framework of the rate type covariance structure with finite set of the internal state variables. A set of the internal state variables is assumed and interpreted such that the theory developed takes account of the effects as follows: (i) plastic non-normality; (ii) plastic strain induced anisotropy (kinematic hardening); (iii) softening generated by microdamage mechanisms; (iv) thermomechanical coupling (thermal plastic softening and thermal expansion); (v) rate sensitivity. To describe suitably the time and temperature dependent effects observed experimentally and the accumulation of the plastic deformation and damage during dynamic cyclic loading process the kinetics of microdamage and the kinematic hardening law have been modified. The relaxation time is used as a regularization parameter. By assuming that the relaxation time tends to zero, the rate independent elastic-plastic response can be obtained. The viscoplastic regularization procedure assures the stable integration algorithm by using the finite difference method. Particular attention is focused on the well-posedness of the evolution problem (the initial-boundary value problem) as well as on its numerical solutions. The Lax-Richtmyer equivalence theorem is formulated and conditions under which this theory is valid are examined. Utilizing the finite difference method for regularized elasto-viscoplastic model, the numerical investigation of the three-dimensional dynamic adiabatic deformation in a particular body under cyclic loading condition is presented. Particular examples have been considered, namely, a dynamic, adiabatic and isothermal, cyclic loading processes for a thin steel plate with small rectangular hole located in the centre. Small two regions which undergo significant deformations and temperature rise have been determined. Their evolution until occurrence of final fracture has been simulated. The accumulation of damage and equivalent plastic deformation on each considered cycle has been obtained. It has been found that this accumulation distinctly depends on the wave shape of the assumed loading cycle.
Coalescence-Controlled Anisotropic Ductile Fracture
Authors: Benzerga A. A. , Besson J. , Pineau A.
Published: February 2008

The anisotropic ductile fracture of rolled plates containing elongated inclusions is promoted by both the dilational growth of voids and the coalescence process. In the present article, the emphasis is laid on the latter process. The effects of void shape and mainly of inter-particle spacings are investigated. Two types of coalescence models are compared: a localization-based model and plastic limit-load models. The capabilities of both approaches to incorporate shape change and spacing effects are discussed. These models are used to predict the fracture properties of two low alloy steels containing mainly manganese sulfide inclusions. Both materials are characterized in different loading directions. Microstructural data inferred from quantitative metallography are used to derive theoretical values of critical void volume fractions at incipient coalescence. These values are used in FE-calculations of axisymmetrically notched specimens with different notch radii and loading directions.
Equivalence Between Higher-Order Stress Power and Heat Flux in Energy Equation Based on Lattice Dynamics
Authors: Yasui Y. , Shizawa K. , Takahashi K.
Published: February 2008

The essence of macroscopic quantities in solid mechanics can be grasped by expressing these quantities in terms of kinematic and mechanical quantities of atoms. In this paper, a method is proposed for obtaining the microscopic definitions of internal forces of continua such as stress, higher-order stresses and heat flux. Moreover, the relation between higher-order stress power and heat flux is discussed expressing the first law of thermodynamics with microscopic quantities in the mesodomain. Comparing heat flux with higher-order stress power, it is clarified that the divergence of heat flux is equivalent to the total of each order power due to higher-order stresses.
A Thermodynamical Theory of Plastic Spin and Internal Stress With Dislocation Density Tensor
Authors: Shizawa K. , Zbib H. M.
Published: February 2008

A thermodynamical theory of elastoplasticity including kinematic hardening and dislocation density tensor is developed. The theory is self-consistent and is based on two fundamental principles of thermodynamics, i.e., the principle of increase of entropy and maximal entropy production rate. The thermodynamically consistent governing equations of plastic spin and back stress are rigorously derived. An expression for the plastic spin tensor is obtained from the constitutive equation of dislocation drift rate tensor and an expression for the back stress tensor is given as a balance equation expressing an equilibrium between internal stress and microstress conjugate to the dislocation density tensor. Moreover, it is shown that, in order to obtain a thermodynamically consistent theory for kinematic hardening, the free energy density should have the dislocation density tensor as one of its arguments.
Statistical Modeling of Creep Rupture Data
Authors: Davies R. B. , Hales R. , Harman J. C. , Holdsworth S. R.
Published: February 2008

A flexible statistical modeling framework for the analysis of creep rupture data is proposed, which offers an improvement on traditional methods of deriving creep rupture strength values and confidence limits. The paper reviews a family of models that can be used to represent the trend relationship between failure times about the trend line, and examines the reliability of extrapolations. Areas of statistical research which would lead to model improvement are discussed, such as variance heterogeneity, left censoring and allowance for the cluster (cast) structure of the data.
Quantitative Prediction of Impact Forces In Elastomers
Authors: Goyal Suresh , Larson Ronald G. , Aloisio Charles J.
Published: February 2008

We measure the impact forces and deflections resulting from drop tests of a mass with a flat impact surface onto flat pads of various elastomeric materials, and show that the forces can be predicted quantitatively with no adjustable parameters by using a theory whose only inputs are the linear viscoelastic characteristics of the materials, measured in small-amplitude oscillatory deformations. The theory, which models the elastomer as a nonlinear neo-Hookean material, is accurate for several elastomeric solids including polyurethanes, polynorbomene, and poly-vinyl-chlorides (PVCs), over a wide range of impact velocities, masses, temperatures and pad thicknesses. Some steps are taken to extend the model to surfaces which are not flat. The application in mind is the rational design of elastomeric components in impact-tolerant portable electronic equipment.
A Method for Determining the Elastic-Plastic Response Ahead of a Notch Tip
Authors: Wang C. H. , Guo W. , Rose L. R. F.
Published: February 2008

The stress distribution ahead of a notch tip is the prerequisite to calculating the driving force for cracks emanating from notches. This article first examines whether two commonly used engineering methods, which are often employed to determine the response at a notch tip, can be applied to evaluate the elastic-plastic response ahead of a notch tip. It is found that both methods would significantly underestimate the stress-strain distribution ahead of a notch tip. Based deformation theories of plasticity, and analytical method is then developed, taking into account of the effects of stress redistribution induced by notch plasticity and the in-plane and through-thickness constraints near the notch root. Predictions are shown to be in close correlation with finite element results.
Fatigue Crack Growth Rate Characteristics of Laser Shock Peened Ti-6Al-4V
Authors: Ruschau John J. , John Reji , Thompson Steven R. , Nicholas Theodore
Published: February 2008

The fatigue crack growth rate (FCGR) characteristics of Laser Shock Peened (LSP) titanium 6Al-4V were examined and compared to those of unprocessed material. The FCGR resistance of LSP processed material tested at low stress ratios (R) is shown to be significantly greater than for unprocessed, baseline material. The increased damage tolerance can be attributed to the large residual compressive stresses generated by the LSP process. Differences in the growth rate behavior due to LSP can be accounted for by using the closure corrected effective stress intensity range, ΔKeff, which takes into account only the portion of loading above the crack opening load. The rationale of using ΔKeff is also demonstrated through fractographic investigations.