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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

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