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IN THIS ISSUE

### Research Papers

J. Eng. Mater. Technol. 2015;137(2):021001-021001-9. doi:10.1115/1.4029041.

This paper presents a double-slip/double-twin polycrystal plasticity model using finite element solution to investigate the kinetics of deformation twinning of medium manganese (Mn) twinning-induced plasticity (TWIP) steels. Empirical equations are employed to estimate the stacking fault energy (SFE) of TWIP steels and the critical resolved shear stress (CRSS) for dislocation slip and deformation twinning, respectively. The results suggest that the evolution of twinning in Fe–xMn–1.4Al–0.6 C (x = 11.5, 13.5, 15.5, 17.5, and 19.5 mass%) TWIP steels, and its relation to the Mn content, can explain the effect of Mn on mechanical properties. By comparing the double-slip/double-twin model to a double-slip model, the predicted results essentially reveal that the interaction behavior between dislocation slip and deformation twinning can lead to an additional work hardening. Also, numerical simulations are carried out to study the influence of boundary conditions on deformation behavior and twin formation. The nucleation and growth of twinning are found to depend on internal properties (e.g., mismatch orientation of grains and stress redistribution) as well as on external constraints (e.g., the applied boundary conditions) of the material.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021002-021002-9. doi:10.1115/1.4029032.

Conventional fusion joining methods, such as resistance spot welding (RSW), have been demonstrated to be ineffective for magnesium alloys. However, self-pierce riveting (SPR) has recently been shown as an attractive joining technique for lightweight metals, including magnesium alloys. While the SPR joining process has been experimentally established on magnesium alloys through trial and error, this joining process is not fully developed. As such, in this work, we explore simulation techniques for modeling the SPR process that could be used to optimize this joining method for magnesium alloys. Due to the process conditions needed to rivet the magnesium sheets, high strain rates and adiabatic heat generation are developed that require a robust material model. Thus, we employ an internal state variable (ISV) plasticity material model that captures strain-rate and temperature dependent deformation. In addition, we explore various damage modeling techniques needed to capture the piercing process observed in the joining of magnesium alloys. The simulations were performed using a two-dimensional axisymmetric model with various element deletion criterions resulting in good agreement with experimental data. The simulations results of this study show that the ISV material model is ideally suited for capturing the complex physics of the plasticity and damage observed in the SPR of magnesium alloys.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021003-021003-7. doi:10.1115/1.4029112.

This work is concerned with development of sol–gel method for preparation of nanoscaled TiO2 using organometallic precursor—titanium tetraisopropoxide (TTIP) and determination of the present crystalline phases depending on the temperature of further thermal treatment. The characteristic processes and transformations during the thermal treatment were determined by means of thermal gravimetric analysis and/or differential thermal analysis (TGA/DTA) method. The crystalline structure and size of the TiO2 crystallites were analyzed by means of Raman spectroscopy and X-ray powder diffraction (XRPD) method. At 250 °C, cryptocrystalline structure was detected, where amorphous TiO2 is accompanied with crystalline anatase. The anatase crystallite phase is stable up to 650 °C, whereas at higher temperature rutile transformation begins. It was observed that at 800 °C, almost the whole TiO2 is transformed to rutile phase. According to XRPD analysis, the increase of the temperature influences on the increase of the size of the crystalline particles ranging from 6 nm at 250 °C to less than 100 nm at 800 °C. The size and shape of the TiO2 crystalline particles were observed by transmission electron microscopy (TEM). The shape of the studied samples changes from nanospheres (250, 380, and 550 °C) to nanorods (650 and 800 °C). Morphology of the formed TiO2 aggregates was observed by scanning electron microscopy (SEM).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021004-021004-9. doi:10.1115/1.4029196.

In the present work, multiwalled carbon nanotubes (MWNTs) were synthesized by electric arc discharge method in open air atmosphere. The synthesized nanotubes were subjected to multistep purification followed by characterization using Raman spectroscopy and transmission electron microscopy (TEM). These carbon nanotubes (CNTs) have inner and outer diameters of the order of 3.5 nm and 16 nm with an aspect ratio of 63. AA 4032 nanocomposites reinforced with MWNTs were produced by high energy ball milling using elemental powder mixtures. X-ray diffraction (XRD) and scanning electron microscope (SEM) studies showed different phases of composite with and without CNTs. The crystallite size and lattice strain were calculated using an anisotropic model of Williamson–Hall peak broadening analysis, which showed in decreased crystallite size with increasing milling time. TEM studies reveal that the MWNTs were uniformly distributed in the matrix. Thermal stability of the nanocrystalline powders was studied using a differential thermal analyzer (DTA). The mechanically alloyed powders were consolidated using a novel method called equal channel angular pressing (ECAP) at room temperature. The consolidated samples were sintered at 480 °C in argon atmosphere for 90 min. ECAP method was investigated as an alternative to conventionally sintered powder composites. CNT addition has shown significant improvement in the hardness of the system, even though the observed density is relatively low compared with a base alloy. Thus, the results show that ECAP enables sufficient shear deformation results in good metallurgical bonds between particles at lower compaction pressures. Hence, it is proven that ECAP can be effectively used as one of the consolidation technique especially for powders that are difficult to consolidate by other means.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021005-021005-10. doi:10.1115/1.4029291.

A compressive split Hopkinson pressure bar (SHPB) was used to investigate the dynamic mechanical behavior of graphene (GR) reinforced polyurethane (PU) composites (GR/PU) at high strain rates ranging from approximately 1500 s−1 to 5000 s−1. Four types of GR/PU composites with different GR contents: 0.25% GR, 0.5% GR, 0.75% GR, and 1% GR were prepared by the solution mixing method and divided into two groups of unheated and postheated specimens. Experimental results show that the GR/PU composite is a strong strain rate dependent material, especially in the high strain rate regime of 3000 s−1–5000 s−1. The dynamic mechanical properties of GR/PU composite in terms of plateau stress, peak stress, and peak load carrying capacity are better than that of pristine PU at most of the applied strain rates. Among the four different GR concentrations used, the 0.5 wt.%-GR specimen shows the highest peak stress, and the 1 wt.% GR specimen has the highest plateau stress; while no significant change in peak strain with changing GR weight fraction was observed. Compared to unheated specimens, the plateau stress, peak stress, and peak strain of postheated specimens are significantly higher.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021006-021006-10. doi:10.1115/1.4029410.

The present work deals with the high temperature flow behavior and the microstructure of the Al-Cu/Mg2Si metal matrix composite. Toward this end, a set of hot compression tests was performed in a wide range of temperature (573–773 K) and strain rate (0.001–0.1 s−1). The results indicated that the temperature and strain rate have a significant effect on the flow softening and hardening behavior of the material. The work hardening rate may be offset due to the occurrence of the restoration processes, the dynamic coarsening, and spheroidization of the second phase particles. In this regard, two phenomenological constitutive models, Johnson–Cook (JC) and Arrhenius-type equations, were employed to describe the high temperature deformation behavior of the composite. The JC equation diverged from experimental curves mainly in conditions which are far from its reference temperature and reference strain rate. This was justified considering the fact that JC model considers thermal softening, strain rate hardening, and strain hardening as three independent phenomena. In contrast, the Arrhenius-type model was more accurate in modeling of the flow behavior in wide range of temperature and strain rate. The minor deviation at some specified conditions was attributed to the negative strain rate sensitivity of the alloys at low temperature deformation regime.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021007-021007-7. doi:10.1115/1.4029530.

This study investigated the hydrogen embrittlement (HE) cracking behavior produced by local contact loading of high-strength steel. When a spherical impression was applied to a hydrogen-absorbed high-strength steel, HE induces contact fracture, where radial cracks are initiated and propagated from the indentation impression. The length of the radial crack was found to be dependent on the hydrogen content in the steel as well as the applied contact force. A combined experimental/computational investigation was conducted in order to clarify the mechanism of hydrogen-induced contact fracture. In the computation, crack propagation was simulated using a cohesive zone model (CZM) in finite element method (FEM), in order to elucidate stress criterion of the present HE crack. It was found that the normal tensile stress was developed around impression, and it initiated and propagated the HE crack. It was also revealed that the hydrogen content enhanced contact fracture damage, especially the resistance of crack propagation (i.e., threshold stress intensity factor, Kth). The findings may be useful for countermeasure of contact fracture coupled with hydrogen in high-strength steel. Such phenomenon is potentially experienced in various contact components in hydrogen environment.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021008-021008-7. doi:10.1115/1.4029532.

A previously developed energy based high cycle fatigue (HCF) life assessment framework is modified to predict the low cycle fatigue (LCF) life of aluminum 6061-T6. The fatigue life assessment model of this modified framework is formulated in a closed form expression by incorporating the Ramberg–Osgood constitutive relationship. The modified framework is composed of the following entities: (1) assessment of the average strain energy density and the average plastic strain range developed in aluminum 6061-T6 during a fatigue test conducting at the ideal frequency for optimum energy calculation, and (2) determination of the Ramberg–Osgood cyclic parameters for aluminum 6061-T6 from the average strain energy density and the average plastic strain range. By this framework, the applied stress range is related to the fatigue life by a power law whose parameters are functions of the fatigue toughness and the cyclic parameters. The predicted fatigue lives are found to be in a good agreement with the experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021009-021009-8. doi:10.1115/1.4029571.

Steel and Al were friction stir lap welded using two different W-25% Re-4% HfC pin tools, having two different pin diameters and pin lengths. The effects of plunge depth, bonding area, and top sheet positions on the microstructure and mechanical properties were investigated. Morphology of the joint interface showed severe steel flash on the retreating side, which controlled the joint strength when the top sheet was placed on the retreating side. A joint efficiency of 58% was achieved when right-handed lap welds were made using the pin tool with longer pin length.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021010-021010-11. doi:10.1115/1.4029570.

In this study, a crystal plasticity finite element model (CPFEM) has been revisited to study the microstructure effects on macroscopic mechanical behavior of ultrafine-grained (UFG) nickels processed by severe plastic deformation (SPD). The microstructure characteristics such as grain size and dislocation density show a strong influence on the mechanical behavior of SPD-processed materials. We used a modified Hall–Petch relationship at grain level to study both grain size and dislocation density dependences of mechanical behavior of SPD-processed nickel materials. Within the framework of small strain hypothesis, it is quite well shown that the CPFEM predicts the mechanical behavior of unimodal nickels processed by SPD methods. Moreover, a comparison between the proposed model and the self-consistent approach will be shown and discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):021011-021011-9. doi:10.1115/1.4029660.

The aim of this paper is to contribute to the prediction of the failure of materials (ductile and brittle) with a single criterion (rule) not violating the assumptions of continuum mechanics. In this work, the failure behavior of isotropic materials is connected with the ability of a material to store elastic strain energy from the very start of loading until its fracture. This elastic strain energy is known that is separated in a distortional and a dilatational part. So, when one of these quantities takes a critical value, then the material fails either by slip or by cleavage. The behavior of a material is described with regard to the secant elastic moduli depending on both unit volume expansion Θ and equivalent strain $ɛeq$. This dependence enlightens, in physical terms, the different reaction of materials in normal and shear stresses. T-criterion is applied for the prediction of failure in a series of experiments that took place to an aluminum alloy (Al-5083) and to PMMA (Plexiglas). A single criterion was used for two totally different materials and the predictions are quite satisfactory. This work is a step toward the direction of using one criterion in order to explain and predict failure in materials independently of the plastic strain that developed before fracture.

Commentary by Dr. Valentin Fuster

### Technical Brief

J. Eng. Mater. Technol. 2015;137(2):024501-024501-5. doi:10.1115/1.4029249.

In geomechanics and civil engineering, the distinct element method (DEM) is employed in a top-down manner to simulate problems involving mechanics of granular media. Because this particle-based method is well adapted to discontinuities, we propose here to adapt DEM at the mesoscale in order to simulate the mechanics of nanocrystalline structures. The modeling concept is based on the representation of crystalline nanograins as mesoscopic distinct elements. The elasticity, plasticity, and fracture processes occurring at the interfaces are captured with contact models of interaction between elements. Simulations that rely on the fitting of the peak stress, strain, and failure mode on the experimental testing of Au and CdS hollow nanocrystalline particles illustrate the promising potential of mesoscopic DEM for bridging the atomistic-scale simulations with experimental testing data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;137(2):024502-024502-3. doi:10.1115/1.4029531.

This short brief develops a model for the velocity of longitudinal wave propagation in auxetic rods. Due to the large density change in auxetic solids and significant lateral deformation for Poisson's ratio between −1 and −0.5, this note takes into consideration density correction and lateral inertia. Results show that deviation from the elementary wave propagation model becomes more significant the more the Poisson's ratio of the rod material deviates from 1/4, in which the deviation of wave velocity is insignificant for Poisson's ratio in the positive range, but significant in the negative range. Specifically, the tensile and compressive wave velocity increases and decreases, respectively, for Poisson's ratio less than 1/4, but this trend reverses for Poisson's ratio greater than 1/4. In addition to showing that the elementary wave propagation model is invalid for describing the longitudinal wave velocity in auxetic rods, the results also suggest that auxetic materials are useful for applications that require slowing down and speeding up of compressive and tensile wave propagations, respectively.

Commentary by Dr. Valentin Fuster