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Research Papers

J. Eng. Mater. Technol. 2018;141(2):021001-021001-12. doi:10.1115/1.4040831.

In this paper, a protocol for interpretation of static creep tests on closed-cell polyurethane foams is defined, considering the influence of a finite loading duration when identifying creep compliance parameters. Experiments were conducted at isothermal conditions with temperatures ranging from 20 to 120 °C. The experimental results indicate Lomnitz, i.e., logarithmic-type creep behavior. We discuss uniqueness of the backcalculated parameters. Furthermore, the viscoelastic material parameters obtained were verified in independent experiments: elastic compliance by ultrasonic wave velocity measurements, viscous material parameters by relaxation tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(2):021002-021002-9. doi:10.1115/1.4041388.

The structure–property relationships of a vintage ASTM A7 steel is quantified in terms of stress state, temperature, and strain rate dependence. The microstructural stereology revealed primary phases to be 15.8% ± 2.6% pearlitic and 84.2% ± 2.6 ferritic with grain sizes of 13.3 μm ± 3.1 μm and 36.5 μm ± 7.0 μm, respectively. Manganese particle volume fractions represented 0.38–1.53% of the bulk material. Mechanical testing revealed a stress state dependence that showed a maximum strength increase of 85% from torsion to tension and a strain rate dependence that showed a maximum strength increase of 38% from 10−1 to 103 s−1at 20% strain. In tension, a negative strain rate sensitivity (nSRS) was observed in the quasi-static rate regime yet was positive when traversing from the quasi-static rates to high strain rates. Also, the A7 steel exhibited a significant ductility reduction as the temperature increased from ambient to 573 K (300 °C), which is uncommon for metals. The literature argues that dynamic strain aging (DSA) can induce the negative strain rate sensitivity and ductility reduction upon a temperature increase. Finally, a tension/compression stress asymmetry arises in this A7 steel, which can play a significant role since bending is prevalent in this ubiquitous structural material. Torsional softening was also observed for this A7 steel.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(2):021003-021003-7. doi:10.1115/1.4041389.

Ultra-high-molecular-weight-polyethylene (UHMWPE) has the greatest impact strength of any thermoplastic and has a variety of both industrial and biomedical applications. Equal channel angular processing (ECAP) is a fabrication method for UHMWPE that introduces shear into the polymer matrix by deforming the polymer through an angular channel, with the goal of enhancing mechanical properties. Both nanographite (NG) and carbon black (CB) attract interest as potential carbon additives for use in creating UHMWPE conductive polymer composites (CPC), but they have not yet been extensively tested in conjunction with ECAP. This study presents a systematic evaluation of the mechanical and electrical properties of 1.0 wt % CB/UHMWPE and NG/UHMWPE composites created using ECAP. These samples are compared against pure UHMWPE ECAP controls as well as compression molded (CM) composite samples. Results indicate that both NG and CB carbon additives successfully create CPCs with a corresponding decrease in mechanical properties. ECAP results in comparatively high mechanical and conductive properties when compared with compression molding. Electrical conductivity is shown to be inversely correlated with tensile strain in a repeatable manner, and microstructural theory is discussed. This work suggests a method to produce flexible, conductive UHMWPE composites that vary consistently and predictably with applied strain, which could have a variety of biomedical and industrial applications.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(2):021004-021004-8. doi:10.1115/1.4041853.

Nanofibers can be used in fields/applications such as medical care, environment protection, apparel, and agriculture. In addition, we believe that this field would continue to show substantial growth in the future. In this study, we focused on its application to oil adsorption. Oil adsorbing performances achieved polymeric nanofiber mass production by a melt-blowing method. We first tested the oil adsorption performance of fiber experimentally under different bulk densities and thicknesses. We also conducted the suction experiment with different bulk densities. Based on experimental result, we considered contact angle, capillarity, and surface tension to be the causes of oil adsorption. We also proposed a three-direction physical model for oil adsorption and used it to calculate the theoretical oil adsorption rate by different free volumes. As a result, we confirmed that the proposed three-direction model could accurately estimate the oil adsorption rate. Moreover, nanofiber has exceptional oil adsorption performance. Further, the fiber with average diameter of 1500 nm exceeds 60 times its self-weight. Therefore, we believe that the proposed nanofiber nonwoven fabric oil adsorption pad could adequately be used as oil adsorption material.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(2):021005-021005-7. doi:10.1115/1.4041854.

The prediction of the structural performance of additive manufacturing (AM) parts has become one of the main challenges to boost the use of AM in industry. The structural properties of AM are very important in order to design and fabricate parts not only of any geometrical shape but also with variable or customized mechanical properties. While AM experimental studies are common in the literature, a limited number of investigations have focused on the analysis and prediction of the mechanical properties of AM parts using theoretical and numerical approaches, such as the finite element method (FEM); however, their results have been not accurate yet. Thus, more research work is needed in order to develop reliable prediction models able to estimate the mechanical performance of AM parts before fabrication. In this paper, the analysis and numerical simulation of the structural performance of fused deposition modeling (FDM) samples with variable infill values is presented. The aim is to predict the mechanical performance of FDM components using numerical models. Thus, several standard tensile test specimens were fabricated in an FDM system using different infill values, a constant layer thickness, one shell perimeter, and polylactic acid (PLA) material. These samples were measured and modeled in a computer-aided design (CAD) system before performing the experimental tensile tests. Numerical models and simulations based on the FEM method were then developed and carried out in order to predict the structural performance of the specimens. Finally, the experimental and numerical results were compared and conclusions drawn.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(2):021006-021006-7. doi:10.1115/1.4041715.

This paper presents an experimental investigation on the effect of interference-fit on the bearing strength and fatigue life of pin-loaded plain-woven and cross ply carbon fiber-reinforced plastic laminate (CFRP). Stainless steel pins are installed to five different sized holes on the CFRP specimens to achieve transition-fit and four interference-fits (0.2%, 0.4%, 0.6%, and 1.0%). The quasi-static and fatigue (R = 0.1) properties of the pin-loaded CFRP are then compared to each other. From the experimental results, it is demonstrated that the interference-fit can improve the joint stiffness per unit bearing area, or the joint stiffness, under both the static and dynamic bearing load conditions. The ultimate bearing strength, fatigue life, and joint stiffness of interference-fit samples are higher than those of the transition-fit samples and they are maximized at an interference-fit percentage of 0.4%. Regardless of interference-fit percentage, the fatigue life of a pin-loaded CFRP specimen tends to be proportional to its joint stiffness in the beginning of a fatigue test. During fatigue testing, the joint stiffness of pin-loaded CFRP gradually decreases to the range of 18.8 GPa/mm to 18.6 GPa/mm when bearing failure occurs. The increased joint stiffness by interference-fit delays CFRP hole damage growth by reducing pin displacement under fatigue cycles.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(2):021007-021007-6. doi:10.1115/1.4041865.

In this study, we used powder metallurgy process to develop gradient concentrated single-phase fine magnesium–zinc alloy particles. Fine magnesium particles were initially dry coated with nanometer size zinc particles in homogeneous manner and cold compacted to cylindrical billet. Zinc atoms were diffused in to the magnesium particles during high-temperature sintering process and produced the single-phase gradient solid solution. The gradient concentration of zinc induced gradual grain refinement in the magnesium particles. The powder metallurgy processed gradient concentrated alloy particles showed an excellent level of hardness, strength, ductility, and fracture toughness in their bulk form, which was even much higher when compared with unalloyed magnesium. Despite having gradient solid solution structure, the developed alloy particles showed homogeneous properties in their bulk form.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;141(2):021008-021008-10. doi:10.1115/1.4041869.

Direct numerical simulations (DNS) of knitted textile mechanical behavior are for the first time conducted on high performance computing (HPC) using both the explicit and implicit finite element analysis (FEA) to directly assess effective ways to model the behavior of such complex material systems. Yarn-level models including interyarn interactions are used as a benchmark computational problem to enable direct comparison in terms of computational efficiency between explicit and implicit methods. The need for such comparison stems from both a significant increase in the degrees-of-freedom (DOFs) with increasing size of the computational models considered as well as from memory and numerical stability issues due to the highly complex three-dimensional (3D) mechanical behavior of such 3D architectured materials. Mesh and size dependency, as well as parallelization in an HPC environment are investigated. The results demonstrate a satisfying accuracy combined with higher computational efficiency and much less memory requirements for the explicit method, which could be leveraged in modeling and design of such novel materials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(2):021009-021009-12. doi:10.1115/1.4041874.

Historically, the approach in material selection was to find the proper material that serves a specific application. Recently, a new approach is implemented such that materials are being architected and topologically tailored to deliver a specific functionality. Periodic cellular materials are increasingly gaining interest due to their tunable structure-related properties. However, the concept of structure–property relationship is not fully employed due to limitations in manufacturing capabilities. Nowadays, additive manufacturing (AM) techniques are facilitating the fabrication of complex structures with high control over the topology. In this work, the mechanical properties of additively manufactured periodic metallic cellular materials are investigated. The presented cellular materials comprise a shell-like topology based on the mathematically known triply periodic minimal surfaces (TPMS). Maraging steel samples with different topologies and relative densities have been fabricated using the powder bed fusion selective laser sintering (SLS) technique, and three-dimensional printing quality was assessed by means of electron microscopy. Samples were tested in compression and the compressive mechanical properties have been deduced. Effects of changing layer thickness and postprocessing such as heat treatment are discussed. Results showed that the diamond TPMS lattice has shown superior mechanical properties among the examined topologies.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(2):021010-021010-9. doi:10.1115/1.4042034.

Nano-scale multilayer composite thin films are potential candidates for coating applications at harsh environments due to their promising mechanical and thermal properties. In this study, a viscoplasticity continuum model based on the plastic flow potential of metal/ceramic nanolayer composites, obtained from molecular dynamics (MD) simulations, is developed to build up a multiscale model bridges atomistic simulation with continuum models for the thin film composites. The model adopts a power law hardening considering confined layer slip (CLS) mechanism and accounts for the evolution of dislocation density based on the statistically stored dislocations and geometrically necessary dislocations. It is then implemented into a finite element code (ls-dyna) to investigate the deformation behavior of nanolayer composites at the macroscale. The deformation behavior of a high strength steel coated with Nb/NbC multilayer is also examined.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(2):021011-021011-10. doi:10.1115/1.4040333.

Fusion welding of nickel-based alloys is often associated with coarse grains and severe segregation, which finally results in the increase of hot cracking susceptibility and poor mechanical properties. Conventional gas tungsten arc welding (GTAW) can aggravate these phenomena, which is mainly due to its high heat input and low cooling rate. In this paper, the cooling rate was enhanced by spraying liquid nitrogen during the welding process. Compared to conventional GTAW, the rapid cooling produced narrower heat affected zone (HAZ) width and more equiaxed grains in the fusion zone, thus higher hardness distribution was also achieved in this condition. In addition, γ′ phase exhibited a dispersed distribution, and segregation has been improved. The results show that the HAZ width is decreased by about 50%, and the fusion zone consisting of the finest equiaxed grains and the lowest segregation was obtained, when the heat sink located on one side 10 mm away from the weld centerline. Also, fine equiaxed grains and the dispersed distribution of γ′ phase could improve the grain boundary strength and reduce the incidence of liquid films along grain boundaries, contributing to prevent nickel-based alloys welding hot cracking from initiating.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(2):021012-021012-12. doi:10.1115/1.4042382.

Johnson–Cook (JC) strength and failure models have been widely used in finite element analysis (FEA) to solve a variety of thermo-mechanical problems. There are many techniques to determine the required JC parameters; however, a best practice to obtain the most reliable JC parameters has not yet been proposed. In this paper, a genetic-algorithm-based optimization strategy is proposed to calibrate the JC strength and failure model parameters of AISI/SAE 1018 steel. Experimental data were obtained from tensile tests performed for different specimen geometries at varying strain rates and temperatures. FEA was performed for each tensile test. A genetic algorithm was used to determine the optimum JC parameters that best fit the experimental force-displacement data. Calibrated JC parameters were implemented in FEA to simulate the impact tests of standard V-notch Charpy bars to verify the damage mechanism in the material. Considering good agreement of the experimental and FEA results, the current strategy is suggested for calibration proposes in other kind of materials in which plastic behavior could be represented by the JC strength and failure models.

Commentary by Dr. Valentin Fuster

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