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### Guest Editorial

J. Eng. Mater. Technol. 2009;131(4):040301-040301-1. doi:10.1115/1.3187830.
FREE TO VIEW

The papers represented in this edition of JEMT were birthed from a workshop entitled “Predictive Science and Technology in Mechanics and Materials” hosted by the Center of Advanced Vehicular Systems (CAVS) on the campus of Mississippi State University on June 18–20, 2008. The purpose of the workshop was focused on technologies that could drive engineering research such that predictive tools could be realized. Several software companies and corporations that employed state-of-the-art finite element analysis to solve structural problems were present. Although some of the presenters did not submit a review article, the ones who did added more references than usual for a more thorough review on the topic at hand in this particular edition of JEMT. The topics discussed in this context are the following: J. D. Clayton and D. J. Bammann, “Finite Deformations and Internal Forces in Elastic-Plastic Crystals: Interpretations From Nonlinear Elasticity and Anharmonic Lattice Statics.” G. Z. Voyiadjis and B. Deliktas, “Theoretical and Experimental Characterization for the Inelastic Behavior of the Micro/Nanostructured Thin Films Using Strain Gradient Plasticity With Interface Energy.” N. R. Overman, C. T. Overman, H. M. Zbib, and D. F. Bahr, “Yield and Deformation in Biaxially Stressed Multilayer Metallic Thin Films.” D. E. Spearot and D. L. McDowell, “Atomistic Modeling of Grain Boundaries and Dislocation Processes in Metallic Polycrystalline Materials.” K. S. Choi, W. N. Liu, X. Sun, and M. A. Khaleel, “Influence of Manufacturing Processes and Microstructures on the Performance and Manufacturability of Advanced High Strength Steels (AHSS).” J. L. Bouvard, D. K. Ward, D. Hossain, S. Nouranian, E. B. Marin, and M. F. Horstemeyer, “Review of Hierarchical Multiscale Modeling to Describe the Mechanical Behavior of Amorphous Polymers.” T. M. Hatem and M. A. Zikry, “Modeling of Lath Martensitic Microstructures and Failure Evolution in Steel Alloys.” R. Agrawal and H. D. Espinosa, “Multiscale Experiments—State of the Art and Remaining Challenges.” S. Groh and H. M. Zbib, “Advances in Discrete Dislocations Dynamics and Multiscale Modeling.” S.-G. Kim, M. F. Horstemeyer, M. I. Baskes, M. Rais-Rohani, S. Kim, B. Jelinek, J. Houze, A. Moitra, and L. Liyanage, “Semi-Empirical Potential Methods for Atomistic Simulations of Metals and Their Construction Procedures.” D. S. Li, H. Garmestani, S. Ahzi, M. Khaleel, and D. Ruch, “Microstructure Design to Improve Wear Resistance in Bioimplant UHMWPE Materials.”

Commentary by Dr. Valentin Fuster

### Research Papers

J. Eng. Mater. Technol. 2009;131(4):041001-041001-6. doi:10.1115/1.3120409.

Uniaxial tension and compression stress-strain curves are simultaneously evaluated from load and surface strain data measured during a bending test. The required calculations for the uniaxial results are expressed as integral equations and solved in that form using inverse methods. This approach is taken to reduce the extreme numerical sensitivity of calculations based on equations expressed in differential form. The inverse solution method presented addresses the numerical sensitivity issue by using Tikhonov regularization. The use of a priori information is explored as a means of further stabilizing the stress-strain curve evaluation. The characteristics of the inverse solution are investigated using experimental data from bending and uniaxial tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041002-041002-5. doi:10.1115/1.3184028.

A graded binary titanium-nickel alloy with a compositional gradient, from elemental Ti to $Ti–23.2 at. %$ Ni, has been successfully deposited using laser rapid forming. A metallurgical study of the phase evolution along the compositional gradient showed that a series of phase evolutions $α→α+β→α+β+Ti2Ni→β/B2+Ti2Ni$ have occurred. Phase formation and microstructure evolution along the compositional gradient was analyzed by Scheil–Gulliver and eutectic growth models. In particular, the morphology of the $β/B2+Ti2Ni$ anomalous and coupled eutectic is discussed in detail.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041003-041003-7. doi:10.1115/1.3184034.

This paper reports the results of theoretical and experimental work leading to the construction of a dedicated finite element method (FEM) system allowing the computer simulation of physical phenomena accompanying the steel sample testing at temperatures that are characteristic for integrated casting and rolling of steel processes, which was equipped with graphical, database oriented pre- and postprocessing. The kernel of the system is a numerical FEM solver based on a coupled thermomechanical model with changing density and mass conservation condition given in analytical form. The system was also equipped with an inverse analysis module having crucial significance for interpretation of results of compression tests at temperatures close to the solidus level. One of the advantages of the solution is the negligible volume loss of the deformation zone due to the analytical form of mass conservation conditions. This prevents FEM variational solution from unintentional specimen volume loss caused by numerical errors, which is inevitable in cases where the condition is written in its numerical form. It is very important for the computer simulation of deformation processes to be running at temperatures characteristic of the last stage of solidification. The still existing density change in mushy steel causes volume changes comparable to those caused by numerical errors. This paper reports work concerning the adaptation of the model to simulation of plastic behavior of axial-symmetrical steel samples subjected to compression at temperature levels higher than $1400°C$. The emphasis is placed on the computer aided testing procedure leading to the determination of mechanical properties of steels at temperatures that are very close to the solidus line. Example results of computer simulation using the developed system are presented as well.

Commentary by Dr. Valentin Fuster

### PREDICTIVE SCIENCE AND TECHNOLOGY IN MECHANICS AND MATERIALS

J. Eng. Mater. Technol. 2009;131(4):041201-041201-15. doi:10.1115/1.3183773.

Large deformation kinematics and internal forces arising from defects in crystalline solids are addressed by a nonlinear kinematic description and multiscale averaging concepts. An element of crystalline material with spatially uniform properties and containing defects such as dislocation lines and loops is considered. The average deformation gradient for this element is decomposed multiplicatively into terms accounting for effects of dislocation flux, recoverable elastic stretch and rotation, and residual elastic deformation associated with self-equilibrating internal forces induced by defects. Two methods are considered for quantifying average residual elastic deformation: continuum elasticity and discrete lattice statics. Average residual elastic strains and corresponding average residual elastic volume changes are negligible in the context of linear elasticity or harmonic force potentials but are not necessarily inconsequential in the more general case of nonlinear elasticity or anharmonic interactions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041202-041202-15. doi:10.1115/1.3183774.

Thin film technology is pervasive in many applications, including microelectronics, optics, magnetic, hard and corrosion resistant coatings, micromechanics, etc. Therefore, basic research activities will be necessary in the future to increase knowledge and understanding and to develop predictive capabilities for relating fundamental physical and chemical properties to the microstructure and performance of thin films in various applications. In basic research, special model systems are needed for quantitative investigation of the relevant and fundamental processes in thin film material science. Because of the diversity of the subject and the sheer volume of the publications, a complete a review of the area of the current study is focused particularly on the experimental and theoretical investigations for the inelastic behavior of the micro-/nanostructured thin films.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041203-041203-6. doi:10.1115/1.3183775.

Multilayer thin films of CuNb, CuNi, and CuNbNi with 20 nm individual layer thicknesses were fabricated by magnetron sputtering on oxide coated silicon wafers. The mechanical properties of the films were measured using bulge testing and nanoindentation. Elastic and plastic properties were determined for freestanding films in both square and rectangular window geometries. A finite element model of the window was used to determine the yielding behavior to account for stress concentrations in the membranes. The initial yield in a pressurized membrane, on the order of 400–500 MPa, is approximately one-half of the flow stress inferred from nanoindentation hardness results between 1.9 GPa and 3.4 GPa, and is a result of microscale yielding prior to uniform deformation in these high strength systems.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041204-041204-9. doi:10.1115/1.3183776.

The objective of this review article is to provide a concise discussion of atomistic modeling efforts aimed at understanding the nanoscale behavior and the role of grain boundaries in plasticity of metallic polycrystalline materials. Atomistic simulations of grain boundary behavior during plastic deformation have focused mainly on three distinct configurations: (i) bicrystal models, (ii) columnar nanocrystalline models, and (iii) 3D nanocrystalline models. Bicrystal models facilitate the isolation of specific mechanisms that occur at the grain boundary during plastic deformation, whereas columnar and 3D nanocrystalline models allow for an evaluation of triple junctions and complex stress states characteristic of polycrystalline microstructures. Ultimately, both sets of calculations have merits and are necessary to determine the role of grain boundary structure on material properties. Future directions in grain boundary modeling are discussed, including studies focused on the role of grain boundary impurities and issues related to linking grain boundary mechanisms observed via atomistic simulation with continuum models of grain boundary plasticity.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041205-041205-9. doi:10.1115/1.3183778.

Advanced high strength steels (AHSS) are performance-based steel grades and their global material properties can be achieved with various steel chemistries and manufacturing processes, leading to various microstructures. In this paper, we investigate the influence of the manufacturing process and the resulting microstructure difference on the overall mechanical properties, as well as the local formability behaviors of AHSS. For this purpose, we first examined the basic material properties and the transformation kinetics of three different commercial transformation induced plasticity (TRIP) 800 steels under different testing temperatures. The experimental results show that the mechanical and microstructural properties of the TRIP 800 steels significantly depend on the thermomechanical processing parameters employed in making these steels. Next, we examined the local formability of two commercial dual phase (DP) 980 steels which exhibit noticeably different formability during the stamping process. Microstructure-based finite element analyses are carried out to simulate the localized deformation process with the two DP 980 microstructures, and the results suggest that the possible reason for the difference in formability lies in the morphology of the hard martensite phase in the DP microstructure. The results of this study suggest that a set of updated material acceptance and screening criteria is needed to better quantify and ensure the manufacturability of AHSS.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041206-041206-15. doi:10.1115/1.3183779.

Modern computational methods have proved invaluable for the design and analysis of structural components using lightweight materials. The challenge of optimizing lightweight materials in the design of industrial components relates to incorporating structure-property relationships within the computational strategy to incur robust designs. One effective methodology of incorporating structure-property relationships within a simulation-based design framework is to employ a hierarchical multiscale modeling strategy. This paper reviews techniques of multiscale modeling to predict the mechanical behavior of amorphous polymers. Hierarchical multiscale methods bridge nanoscale mechanisms to the macroscale/continuum by introducing a set of structure-property relationships. This review discusses the current state of the art and challenges for three distinct scales: quantum, atomistic/coarse graining, and continuum mechanics. For each scale, we review the modeling techniques and tools, as well as discuss important recent contributions. To help focus the review, we have mainly considered research devoted to amorphous polymers.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041207-041207-10. doi:10.1115/1.3183780.

A multiple-slip dislocation-density-based crystalline formulation, specialized finite-element formulations, and Voronoi tessellations adapted to martensitic orientations were used to investigate dislocation-density activities and crack tip blunting in high strength martensitic steels. The formulation is based on accounting for variant morphologies and orientations, retained austenite, and initial dislocations densities that are uniquely inherent to martensitic microstructures. The effects of variant distributions and arrangements are investigated for different crack and void interaction distributions and arrangements. The analysis indicates that for certain orientations related to specific variant block arrangements, which correspond to random low angle orientations, cracks can be blunted by dislocation-density activities along transgranular planes. For other variant block arrangements, which correspond to random high angle orientations, sharp crack growth can occur due to dislocation activities along intergranular planes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041208-041208-15. doi:10.1115/1.3183782.

In this article we review recent advances in experimental techniques for the mechanical characterization of materials and structures at various length scales with an emphasis in the submicron- and nanoregime. Advantages and disadvantages of various approaches are discussed to highlight the need for carefully designed experiments and rigorous analysis of experimentally obtained data to yield unambiguous findings. By examining in depth a few case studies we demonstrate that the development of robust and innovative experimentation is crucial for the advancement of theoretical frameworks, assessment of model predictive capabilities, and discovery of new physical phenomena.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041209-041209-10. doi:10.1115/1.3183783.

Discrete dislocation dynamics is a numerical tool developed to model the plasticity of crystalline materials at an intermediate length scale, between the atomistic modeling and the crystal plasticity theory. In this review we show, using examples from the literature, how a discrete dislocation model can be used either in a hierarchical or a concurrent multiscale framework. In the last section of this review, we show through the uniaxial compression of microcrystal application, how a concurrent multiscale model involving a discrete dislocation framework can be used for predictive purposes.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041210-041210-9. doi:10.1115/1.3183784.

General theory of semi-empirical potential methods including embedded-atom method and modified-embedded-atom method (MEAM) is reviewed. The procedures to construct these potentials are also reviewed. A multi-objective optimization (MOO) procedure has been developed to construct MEAM potentials with minimal manual fitting. This procedure has been applied successfully to develop a new MEAM potential for magnesium. The MOO procedure is designed to optimally reproduce multiple target values that consist of important material properties obtained from experiments and first-principle calculations based on density-functional theory. The optimized target quantities include elastic constants, cohesive energies, surface energies, vacancy-formation energies, and the forces on atoms in a variety of structures. The accuracy of the present potential is assessed by computing several material properties of Mg including their thermal properties. We found that the new MEAM potential shows a significant improvement over previously published potentials, especially for the atomic forces and melting temperature calculations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):041211-041211-7. doi:10.1115/1.3183786.

A microstructure design framework for multiscale modeling of wear resistance in bioimplant materials is presented here. The increase in service lifetime of arthroplasty depends on whether we can predict wear resistance and microstructure evolution of a bioimplant material made from ultra high molecular weight polyethylene during processing. Experimental results show that the anisotropy introduced during deformation increases wear resistance in desired directions. After uniaxial compression, wear resistance along the direction, perpendicular to compression direction, increased 3.3 times. Micromechanical models are used to predict microstructure evolution and the improvement in wear resistance during processing. Predicted results agree well with the experimental data. These models may guide the materials designer to optimize processing to achieve better wear behavior along desired directions.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Eng. Mater. Technol. 2009;131(4):044501-044501-5. doi:10.1115/1.3120384.

Aluminum 6XXX alloys show high strength to weight ratios and are thus promising materials for today’s transport industry lightweight construction efforts. When considering both deformation and final mechanical properties, high ductility is interesting from the conformability point of view. On the other hand, high resistance is necessary in the automotive structural parts (which can be obtained through T6 precipitation heat treatment) but leads to reduced ductility. In order to increase aluminum alloys’ formability, warm forming is commonly applied. In contradiction to this, this article shows how the tensile deformation behavior of the 6082 T6 alloy is not affected by the temperature. In this work, the necessary formability values to obtain the parts are achieved, deforming the material under O annealed condition. But this strategy is focused on the formability perspective; therefore, the final mechanical properties do not achieve the necessary strength requirements. As a solution, the possibility of applying the T6 heat treatment after forming the parts (in annealed condition) is studied. A tensile characterization of the post-heat-treated specimens obtained from the deformed experimental part results in high flow stress levels, and thus, the strategy is validated. Nevertheless, the heat treatment leads to geometrical distortions in the final part, and thus, a last calibration step should be added to the forming process in order to obtain the desired shape.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2009;131(4):044502-044502-5. doi:10.1115/1.3120394.

The twinning-induced plasticity (TWIP) phenomenon is established as the most effective mechanism to enhance the formability of the advanced high-Mn (15–30 wt %) austenitic steels (known as TWIP steels). As the formability is very sensitive to the steel microstructure, the study of their hot deformation characteristics is highly desired. The aim of the present work is to investigate the effects of strain rate on the high temperature flow behavior, dynamic recrystallization (DRX) and the microstructural evolution of a grade of TWIP steels (with 29 wt % Mn) through single hit compression testing. The hot compression tests were carried out at two different temperatures ($850°C$ and $1150°C$) applying a range of strain rates $(0.001–0.1 s−1)$. The results indicated a greater deformation resistance at higher strain rates. The detected broad stress peaks at higher strain rates were related to the occurrence of DRX. The microstructural studies revealed that, in addition to DRX, a geometrical dynamic recrystallization occurred at $850°C$. This results in a microstructure with finer equiaxed grains.

Commentary by Dr. Valentin Fuster

### Errata

J. Eng. Mater. Technol. 2009;131(4):047001-047001-1. doi:10.1115/1.3227670.
FREE TO VIEW

Corrections were made to Table 1 where the number of significant digits therein were increased to twelve, which is necessary to obtain accurate numerical results (tables available for download from http://mae.ucdavis.edu/~mhill/jemt129table.html).

Topics: Stress , Strain gages
Commentary by Dr. Valentin Fuster