0

IN THIS ISSUE

### Research Papers

J. Eng. Mater. Technol. 2018;141(1):011001-011001-5. doi:10.1115/1.4040554.

This paper outlines a microstructure-based model relating gamma prime microstructure and grain size of Ni-base alloys to their creep behavior. The ability of the model to explain creep of multiple superalloys with a single equation and parameter set is demonstrated. The only parameters that are changed from alloy to alloy are related to the gamma prime characteristics and grain size. This model also allows prediction of creep performance as a function of heat treatment and explains some apparently contradictory data from the literature.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011002-011002-7. doi:10.1115/1.4040556.

Carbon nanotubes (CNTs) are well known as perfect reinforcement for high strength and lightweight composites due to their high specific strength, thermal, electrical, and mechanical characteristics. One of the important challenges is to obtain a homogeneous dispersion of CNTs in metal matrix, so development technologies for producing metal matrix composites (MMCs) is of great interest. Melting followed by solidification, may be successfully utilized for synthesizing CNT-reinforced aluminum-based MMCs. In this study, Al/CNT composites have been produced by direct injection of CNTs in pure aluminum using high-pressure die casting (HPDC) method. The as-produced billets were subjected to cyclic extrusion (CE) to refine CNT agglomerates and to increase CNT dispersion in aluminum. Current investigation demonstrates that more than 50% efficiency of combined HPDC-CE production method has been achieved. The resulting composites demonstrated improved mechanical properties.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011003-011003-12. doi:10.1115/1.4040222.

Arising from long-term high temperature service, the microstructure of nickel-base (Ni-base) superalloy components undergoes thermally and deformation-induced aging characterized by isotropic coarsening and directional coarsening (rafting) of the $γ′$ precipitates. The net result of the morphological evolutions of the $γ′$ particles is a deviation of the mechanical behavior from that of the as-heat treated properties. To capture the influence of a rafted and isotropic aged microstructure states on the long-term constitutive behavior of a Ni-base superalloy undergoing thermomechanical fatigue (TMF), a temperature-dependent crystal viscoplasticity (CVP) constitutive model is extended to include the effects of aging. The influence of aging in the CVP framework is captured through the addition of internal state variables that measure the widening of the γ channels and in-turn update the material parameters of the CVP model. Through the coupling with analytical derived kinetic equations to the CVP model, the enhanced CVP model is shown to be in good agreement when compared to experimental behavior in describing the long-term aging effects on the cyclic response of a directionally solidified (DS) Ni-base superalloy used in hot section components of industrial gas turbines.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011004-011004-8. doi:10.1115/1.4040553.

Mechanical properties of additive manufactured metal components can be affected by the orientation of the layer deposition. In this investigation, Ti–6Al–4V cylindrical specimens were fabricated by electron beam melting (EBM) at four different build angles (0 deg, 30 deg, 60 deg, and 90 deg) and tested as per ASTM E8 Standard Test Methods for Tension Testing of Metallic Materials. With the layer-by-layer fabrication suggesting granting anisotropic properties to the builds, strain fields were recorded by digital image correlation (DIC) in the search for shear effects under uniaxial loads. For the validation of this measuring method, axial strains were measured with a clip extensometer and a virtual extensometer, simultaneously. Failure analysis of the specimens at different orientations was conducted to evidence the recording of shear strain fields. The failure analysis included fractography, optical micrographs of the microstructure distribution, and failure profiles displaying different failure features associated with the layering orientation. Additionally, an experimental study case of how the failure mode of components can potentially be designed from the fabrication process is presented. At the end, remarks about the shear effects found, and an insight of the possibility of designing components by failure for safer structures are discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011005-011005-11. doi:10.1115/1.4040555.

Cellular architectures are promising in a variety of engineering applications due to attractive material properties. Additive manufacturing has reduced the difficulty in the fabrication of three-dimensional (3D) cellular materials. In this paper, the numerical homogenization method for 3D cellular materials is provided based on a short, self-contained matlab code. It is an educational description that shows how the homogenized constitutive matrix is computed by a voxel model with one material to be void and another material to be solid. A voxel generation algorithm is proposed to generate the voxel model easily by the wireframe scripts of unit cell topologies. The format of the wireframe script is defined so that the topology can be customized. The homogenization code is then extended to multimaterial cellular structures and thermal conductivity problems. The result of the numerical homogenization shows that different topologies exhibit anisotropic elastic properties to a different extent. It is also found that the anisotropy of cellular materials can be controlled by adjusting the combination of materials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011006-011006-10. doi:10.1115/1.4040593.

Important from exploitation point of view mechanical properties of single-layer, double-layer, and mixed alumina and ceria films and their stainless steel (SS) substrate were investigated by means of nanoindentation experiments. As a result, we obtained the experimental load–displacement curves and calculated the indentation hardness (HIT) and indentation modulus (EIT), by means of Oliver and Pharr approximation method. Numerical simulations of the process of nanoindentation by means of finite element method were performed as well, in order to obtain more information about the plastic properties of the investigated films. The obtained results show that the mixed Al2O3+Ce2O3 film, obtained at dominant concentration of cerium ions in the working electrolyte, has the highest indentation hardness and modulus, followed by the single Ce2O3-CeO2 film, the mixed Al2O3+Ce2O3 film, obtained at dominant concentration of aluminum ions in the working electrolyte, the double Ce2O3-CeO2/Al2O3 layer, and single Al2O3 layer.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011007-011007-10. doi:10.1115/1.4040591.

This paper compared the mechanical behavior of 6H SiC under quasi-static and dynamic compression. Rectangle specimens with a dimension of 3 × 3 × 6 mm3 were used for quasi-static compression tests under three different loading rates (i.e., 10−5/s, 10−4/s, and 10−3/s). Stress–strain response showed purely brittle behavior of the material which was further confirmed by scanning electron microscopy (SEM)/transmission electron microscopy (TEM) examinations of fractured fragments. For dynamic compression, split Hopkinson pressure bar (SHPB) tests were carried out for cubic specimens with a dimension of 6 × 6 × 4 mm3. Stress–strain curves confirmed the occurrence of plastic deformation under dynamic compression, and dislocations were identified from TEM studies of fractured pieces. Furthermore, JH2 model was used to simulate SHPB tests, with parameters calibrated against the experimental results. The model was subsequently used to predict strength and plasticity-related damage under various dynamic loading conditions. This study concluded that, under high loading rate, silicon carbide (SiC) can deform plastically as evidenced by the development of nonlinear stress–strain response and also the evolution of dislocations. These findings can be explored to control the brittle behavior of SiC and benefit end users in relevant industries.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011008-011008-8. doi:10.1115/1.4040592.

This work documents the development of a tool to perform automated parameter fitting of constitutive material models. Specific to this work is the fitting of a Swift hardening rule and isotropic linear plasticity model to aluminum 2024-T351, C36000 brass, and C10100 copper. Material characterization was conducted through the use of compressive, cold upsetting tests. A noncontact, optical displacement measurement system was applied to measure the axial and radial deformation of the test specimens. Nonlinear optimization techniques were then applied to tune a finite element model to match experimental results through the optimization of material model parameters as well as frictional coefficient. The result is a system, which can determine constitutive model parameters rapidly and without user interaction. While this tool provided material parameters for each material and model tested, the quality of the fit varied depending on how appropriate the constitutive model was to the material's actual plastic behavior. Aluminum's behavior proved to be an excellent match to the Swift hardening rule while the behavior of brass and copper was described better by the linear plasticity model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011009-011009-10. doi:10.1115/1.4040761.

Accurate estimation of material stress–strain response is essential to many fatigue life analyses. In cases where variable amplitude loading conditions exist, the ability to account for transient material deformation behavior can be particularly important due to the potential for periodic overloads and/or changes in the degree of nonproportional stressing. However, cyclic plasticity models capable of accounting for these complex effects often require the determination of a large number of material constants. Therefore, an Armstrong–Frederick–Chaboche style plasticity model, which was simplified in a previous study, was extended in the current study to account for the effects of both general cyclic and nonproportional hardening using a minimal number of material constants. The model was then evaluated for its ability to predict stress–strain response under complex multiaxial loading conditions by using experimental data generated for 2024-T3 aluminum alloy, including a number of cyclic incremental step tests. The model was found to predict transient material response within a fairly high overall level of accuracy for each loading history investigated.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011010-011010-8. doi:10.1115/1.4040830.

The present paper introduces a simple method to predict the modulus of elasticity and the hardness of polymeric materials that range from soft elastomers to hard plastics. Hertzian elastic impact model is used to define the relationship between the contact time duration and the maximum force of normal contact due to the impact of a hard sphere indenter with the tested polymer sample. It is shown that the adopted model and experimental method can be used as a tool for extracting the magnitude of the complex modulus of elasticity. Moreover, a new impact index is shown to be proportional to the polymer shore hardness. Theoretical and experimental results based on the force–time signals are consistent and show good correlation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(1):011011-011011-11. doi:10.1115/1.4041139.

In addition to processing a troubling agricultural by-product and reducing demands on our landfills, prepared agro-waste composites are suitable for a variety of practical applications. However, enhancing value-added options for these agricultural by-products can necessitate ability to assess their mechanical integrity. This paper accordingly describes the preparation of a cellulosic-manure composite and demonstrates ability to determine stresses in a perforated structure of the material from measured displacement data. Processing digital image correlation (DIC) recorded displacement information with an Airy stress function gives reliable results full-field as well as at the edge of geometric discontinuities without having to differentiate the recorded data. Required constitutive properties are evaluated in situ and results are substantiated independently.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Eng. Mater. Technol. 2018;141(1):014501-014501-5. doi:10.1115/1.4040671.

The transient data obtained during stress relaxation test of polycrystalline materials has broader implications. The test is influenced by the material length scale. Efforts to mathematically bridge data at different length scales is scarce. In the present work, it is attempted to modify a recently proposed stress relaxation model with additional coefficients to accommodate the mechanical behavior at different length scales. The macroscale stress relaxation test was performed using a tensile testing machine, whereas the micro- and nanoscale specimens were tested using indentation technique. Assuming power law rate behavior, a scaling relation is derived initially to correlate the indentation pressure and flow stress.

Commentary by Dr. Valentin Fuster