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Research Papers

J. Eng. Mater. Technol. 2018;140(3):031001-031001-9. doi:10.1115/1.4039108.

Magnesium alloys with rare earth (RE) elements addition, modified with zirconium are used for cast, light-weight solutions for components applied at temperature up to 250–300 °C. Computational methods are often used by the foundries to decrease the cost of a new product start-up. The commercially available magmasoft software is widely used to simulate a casting process. Nevertheless, its database does not contain complete data for modern alloys. We present the results of our investigations on the thermo-physical properties of a EV31A magnesium alloy and the simulation of a sand casting process with applied data. We compared the simulation and technological trials, recognized problems occurring during the simulation, and applied corrections. Our final simulations gave acceptable results. The differences between the technological tests and the simulation were caused by factors that are difficult to model in the simulation, such as the presence of nonmetallic inclusions and the degree of modification of the liquid alloy.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(3):031002-031002-13. doi:10.1115/1.4038801.

This paper presents a combined experimental and theoretical analysis focusing on the individual roles of microdeformation mechanisms that are simultaneously active during the deformation of twinning-induced plasticity (TWIP) steels in the presence of hydrogen. Deformation responses of hydrogen-free and hydrogen-charged TWIP steels were examined with the aid of thorough electron microscopy. Specifically, hydrogen charging promoted twinning over slip–twin interactions and reduced ductility. Based on the experimental findings, a mechanism-based microscale fracture model was proposed, and incorporated into a visco-plastic self-consistent (VPSC) model to account for the stress–strain response in the presence of hydrogen. In addition, slip-twin and slip–grain boundary interactions in TWIP steels were also incorporated into VPSC, in order to capture the deformation response of the material in the presence of hydrogen. The simulation results not only verify the success of the proposed hydrogen embrittlement (HE) mechanism for TWIP steels, but also open a venue for the utility of these superior materials in the presence of hydrogen.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(3):031003-031003-14. doi:10.1115/1.4038672.

Hybrid micro-architected materials with unique combinations of high stiffness, high damping, and low density are presented. We demonstrate a scalable manufacturing process to fabricate hollow microlattices with a sandwich wall architecture comprising an elastomeric core and metallic skins. In this configuration, the metallic skins provide stiffness and strength, whereas the elastomeric core provides constrained-layer damping. This damping mechanism is effective under any strain amplitude, and at any relative density, in stark contrast with the structural damping mechanism exhibited by ultralight metallic or ceramic architected materials, which requires large strain and densities lower than a fraction of a percent. We present an analytical model for stiffness and constrained-layer damping of hybrid hollow microlattices, and verify it with finite elements simulations and experimental measurements. Subsequently, this model is adopted in optimal design studies to identify hybrid microlattice geometries which provide ideal combinations of high stiffness and damping and low density. Finally, a previously derived analytical model for structural damping of ultralight metallic microlattices is extended to hybrid lattices and used to show that ultralight hybrid designs are more efficient than purely metallic ones.

Topics: Damping , Stiffness , Polymers
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(3):031004-031004-13. doi:10.1115/1.4039290.

The focus in this work is toward an investigation of the fracture response of brittle materials with different specimen size loaded in diametral compression using different boundary conditions. The compacted zone underneath the loading points is assumed to be limited and only responsible for the load transition to the rest of the material. Therefore, the theory of elasticity is used to define the stress state within a circular specimen. A tensile failure criterion is used, and the final load capacity is related to the formation of a subsurface crack initiated in a probabilistic manner in a region in the vicinity of the loaded diameter of the specimen. This process is described by Weibull theory, and it is assumed here that the growth of the subsurface crack occurs in an unstable manner. Therefore, the assumption in Weibull theory that the final failure occurs as soon as a macroscopic fracture initiates from a microcrack is fulfilled. The concept of disk effective volume used in Weibull size effect is presented in a convenient way that facilitates the application of the model to transfer the tensile strength obtained from different methods such as three point bending and Brazilian test. The experimental results for Brazilian test on a selected hard rock are taken from the literature and a fairly close agreement is obtained with the model predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(3):031005-031005-10. doi:10.1115/1.4039291.

Characterization of materials undergoing severe plastic deformation requires the careful measurement of instantaneous sample dimensions throughout testing. For compressive testing, it is insufficient to simply estimate sample diameter from an easily measured height and volume. Not all materials exhibit incompressibility, and friction during testing can lead to a barreled sample with diameter that varies with height. Video extensometry has the potential to greatly improve testing by capturing the full profile of a sample, allowing researchers to account for such effects. Common two-dimensional (2D) video extensometry algorithms require thin, planar samples, as they are unable to account for out-of-plane deformation. They are, therefore, inappropriate for standard compressive tests which use cylindrical samples that exhibit large degrees of out-of-plane deformation. In this paper, a new approach to 2D video extensometry is proposed. By using background subtraction, the profile of a cylindrical sample can be isolated and measured. Calibration experiments show that the proposed system has a 3.1% error on calculating true yield stress—similar to ASTM standard methods for compressive testing. The system is tested against Aluminum 2024-T351 in a series of cold upsetting tests. The results of these tests match very closely with similar tests from the literature. A preliminary finite element model constructed using data from these tests successfully reproduced experimental results. Diameter data from the finite element model undershot, but otherwise closely matched experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(3):031006-031006-15. doi:10.1115/1.4039292.

Recent studies on developing dual phase (DP) steels showed that the combination of strength/ductility could be significantly improved when changing the volume fraction and grain size of phases in the microstructure depending on microstructure properties. Consequently, DP steel manufacturers are interested in predicting microstructure properties as well as optimizing microstructure design at different strain rate conditions. In this work, a microstructure-based approach using a multiscale material and structure model was developed. The approach examined the mechanical behavior of DP steels using virtual tensile tests with a full micro-macro multiscale material model to identify specific mechanical properties. Microstructures with varied ferrite grain sizes, martensite volume fractions, and carbon content in DP steels were also studied. The influence of these microscopic parameters at different strain rates on the mechanical properties of DP steels was examined numerically using a full micro-macro multiscale finite element method. An elasto-viscoplastic constitutive model and a response surface methodology (RSM) were used to determine the optimum microstructure parameters for a required combination of strength/ductility at different strain rates. The results from the numerical simulations were compared with experimental results found in the literature. The developed methodology proved to be a powerful tool for studying the effect and interaction of key strain rate sensitivity and microstructure parameters on mechanical behavior and thus can be used to identify optimum microstructural conditions at different strain rates.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(3):031007-031007-11. doi:10.1115/1.4039293.

Carbon nanotube (CNT)-based conductors are the focus of considerable ongoing experimental research, which has demonstrated their potential to offer increased current carrying capacity or higher specific conductance, as compared to conventional copper cabling. Complementary analytical research has been hindered by the high computational cost of large-scale quantum models. The introduction of certain simplifying assumptions, supported by critical comparisons to exact solutions and the published literature, allows for quantum modeling work to assist experiment in composite conductor development. Ballistic conductance calculations may be used to identify structure–property relationships and suggest the most productive avenues for future nanocomposite conductor research.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(3):031008-031008-7. doi:10.1115/1.4039505.

Origami-based sheet metal (OSM) folding is a novel approach regarded as extension of the origami technique to sheet metal. It requires creating numerous features along the bend line, called material discontinuities (MD). Material discontinuities control the material deformation and result in reduced bending force (BF), minimal tooling, and machinery requirements. Despite the promising potential of OSM, there is little understating of the effect of the selected MD shape and geometry on the final workpiece. Specifically, this is of interest when comparing the manufacturing energy and cost allocations for OSM with a well-establish process for sheet metal such as stamping. In this work, wiping die bending of aluminum sheet with different MD shapes and geometries along the bend line is investigated using finite element analysis (FEA) and compared to traditional sheet bending in terms of stress distribution along the bending line, required bending force and springback. The FEA results are validated by comparing it to the available empirical models in terms of bending forces. This study found that OSM technique reduced the required bending force significantly, which has important significance in energy and cost reduction. The study also found each MD resulted with different bending force and localized stress. Hence, MD are ranked in terms of the required force to bend the same sheet metal type and thickness for further future investigation. Springback is decreased due to application of MD. Meanwhile, MD generated localized high stress regions along the bending line, which may affect load-bearing capability of the final part.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(3):031009-031009-11. doi:10.1115/1.4039506.

The strain-hardening behavior of metal during the uniaxial tension can be treated as the competing result of generation and annihilation of statistically stored dislocations (SSDs). Geometrically necessary dislocations (GNDs) are generated to accommodate a lattice mismatch and maintain deformation compatibility in dual-phase (DP) steels because of the heterogeneous deformation of the microstructure. In this study, a dislocation-based strain-hardening model that encompasses GNDs was developed to describe the mechanical properties of dual-phase steel. The GNDs were obtained based on a cell model of uniaxial deformation and the SSDs were calculated using a dynamic recovery model. The strain of each phase is a nonlinear function of the overall material strain obtained by the point-interpolation method (PIM). The proposed strain-hardening model was verified by using commercially produced DP600 steel. The calculated results obtained with GNDs are able to predict more precisely the experimental data than that without. The effects of martensite volume fraction and grain size on the strain-hardening behaviors of individual phases and material were studied.

Commentary by Dr. Valentin Fuster

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