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Research Papers

J. Eng. Mater. Technol. 2015;138(1):011001-011001-7. doi:10.1115/1.4031426.

This paper discusses an approach to incorporate density and temperature terms in the well-known stretched exponential (SE) model for predicting the stress relaxation behavior of polymer foams. We have developed this approach for closed-cell polyurethane foams (PUFs) and verified using experimental data for accuracy. The SE model was first examined using short-term experimental data to predict long-term stress relaxation behavior of PU solid (PUS). The corresponding model parameters were then extracted for PUS and two PUFs with different densities (PU404 and PU415) at three different test temperatures. Finally, an expression was developed in conjunction with the modified Gibson–Ashby relationship and the Arrhenius equation and validated for other foam density (PU420) and test temperatures. The predictions were found to be reasonably good with more than 90% accuracy.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;138(1):011002-011002-9. doi:10.1115/1.4031427.

High damping rubber (HDR) is used in the manufacturing of elastomeric bearings for seismic isolation of building and structures. In practical situations, rubber bearings are subjected to a permanent vertical load which may change at the occurrence of the earthquake, and concurrent shear deformation, due to either service movements of the structure or earthquake-induced ground motion. The study presents an experimental procedure for the assessment of HDR specimens under combined compression and shear, reproducing the same typical load regimes which rubber isolators experience in service. Five commercial HDRs were tested according to the procedure. The results point to the importance of considering the influence of the compression stress for a correct understanding of the behavior of HDRs under cyclic shear.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;138(1):011003-011003-12. doi:10.1115/1.4031615.

In this work, the static and dynamic compaction response of a six-material mixture, containing both brittle and ductile constituents, is compared. Quasi-static and dynamic compaction experiments were conducted on samples and the results compared to simulations. Optical analyses of compacted samples indicate that dynamically compacting samples to near 300 m/s is not sufficient for complete compaction or localized grain melt. Simulations indicate that a wide distribution of temperature and stress states are achieved in the dynamically compacted samples; compaction speeds should be increased to near 800 m/s at which point copper grains achieve melt temperatures on their surfaces. The experimental data is used to fit a bulk P-α equation of state (EOS) that can be used for simulating large-scale dynamic compaction for industrial applications.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;138(1):011004-011004-10. doi:10.1115/1.4031830.

This paper is intended to quantify the relationship between the peen forming effectiveness and various involved parameters through a realistic numerical study. For this purpose, a new finite element (FE) model is proposed with full geometry representation, random shots generation, and rate-dependent material law of kinematic strain-hardening. The mesh sensitivity and effects of boundary conditions are carefully examined. The FE model is validated by comparing the results with the experimental measurements. The proposed model is then used to investigate the effects of the peening intensity (represented as the shot velocity) and the strip thickness on the peen-formed deflection and the residual stress distribution for strips made of Ti-6Al-4V. Our results indicate the existence of a maximum convex deflection for different strip thicknesses. In addition, a reversed deflection (i.e., concaved curvature) is observed for severe peening conditions (i.e., thin strip under high peening intensity). Our simulations verify the previous proposition that a concaved curvature can be generated only when the whole cross section is plastically deformed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;138(1):011005-011005-9. doi:10.1115/1.4031960.

In real physical experiments, three typical deformation stages including elastic deformation stage, symmetric deformation stage, and asymmetric deformation stage appear step by step when the stainless steel hemispherical shell structure is under axial compression loading. During the asymmetric deformation stage, the rolling-plastic-hinge-radius which characterizes the size of the deformation area evolves along the circumferential direction with the compressive displacement. For the hemispherical shell structures with apparent asymmetric deformation stage, the double-buckling phenomenon of the structures in experiments can be clearly detected. The traditional theoretical analysis based on the assumption with circumferentially constant rolling-plastic-hinge-radius is not suitable to predict this phenomenon. For these hemispherical shell structures, load capacity and absorbed energy predicted by the traditional analysis are usually higher than experimental results in the asymmetric deformation stage. In this paper, a new description based on experimental observation for the evolution of rolling-plastic-hinge-radius has been proposed. Minimum energy principle was employed to obtain the postbuckling behavior. The energy evolution of different buckling stages during compression loading is investigated to evaluate the structure load capacity. Stainless steel hemispherical specimens with different sizes are tested under axial compression between two rigid plates to verify the theoretical modification. Good agreement is achieved between proposed model and experimental results. The theoretical model proposed in this paper can be used in prediction of postbuckling behavior for different deformation patterns in the asymmetric deformation stage. It also provides higher flexibility and efficiency for the postbuckling behavior prediction of hemispherical shell structures.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Eng. Mater. Technol. 2015;138(1):014501-014501-3. doi:10.1115/1.4031665.

This technical brief considers bending stress minimization as a basis for obtaining the optimal Poisson's ratio of simply supported equilateral triangular plates under (a) bending loads at the plate boundary, (b) uniform load throughout the entire plate, and (c) concentrated load at the plate center. Results suggest that the use of auxetic materials is not appropriate for triangular plates under applied bending at the boundary, while mildly auxetic and highly auxetic materials are appropriate for triangular plates under uniform and central point loads, respectively. In addition, obtained results show that the optimal Poisson's ratios for circular and square auxetic plates are not necessarily applicable for triangular plates. The use of auxetic materials offers an additional choice for decreasing bending stresses under specific boundary conditions and loading patterns.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2015;138(1):014502-014502-3. doi:10.1115/1.4031736.

Sharp indentation problems are examined based on finite element methods (FEMs) and self-similarity considerations. The analysis concerns classical elastic–plastic materials with low, or no, strain-hardening and especially the details of the behavior of the size of the plastic zone are at issue. The results are correlated using a single parameter, comprising both geometrical and mechanical properties, and compared with previously presented semi-analytical findings. The numerical analysis is restricted to cone indentation of elastic-ideally plastic materials.

Commentary by Dr. Valentin Fuster

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