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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.
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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

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