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Research Papers

J. Eng. Mater. Technol. 2016;139(1):011001-011001-11. doi:10.1115/1.4034328.
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The objective of this work is to develop a multiscale modeling tool of copolymers with long chains. We propose an enhanced coarse-graining method of thermoplastic polyurethane (TPU) with three beads. The proposed coarse-graining provides an accurate molecular modeling tool to keep the molecular interaction together with computational efficiency. The coarse-grained model with three beads is further improved with pressure-correction of the force-field. The improved coarse-grained model holds similar properties of a bulk model of TPU—varying density with temperature, a close density value of TPU at 1 atm, and the phase separation. Equating potential energy densities of the coarse-grained model to the strain energy functions of the continuum model at volumetric and isochoric deformation modes, bulk and shear moduli of TPU are directly obtained and used to estimate Young's modulus and Poisson's ratio. The molecular simulation with the coarse-grained model of TPU demonstrates its much greater bulk modulus than the shear modulus, which is typically observed in elastomers. Modifying the coarse-grained model of TPU with hard and soft segments, we successfully demonstrated the material design of bulk modulus and Poisson's ratio by varying hard and soft segments at the molecular level. The proposed coarse-graining tool will pave a new way to explore the multiscale modeling of copolymers with long chains and can be directly applied to the multiscale modeling of other thermoplastic elastomers (TPE).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011002-011002-8. doi:10.1115/1.4034692.

In the present study, in situ reaction technique has been employed to prepare AA5052 matrix composites reinforced with different vol. % of ZrB2 particles (i.e., 0, 4.5, and 9 vol. %). Composites have been characterized by X-ray diffraction (XRD) to confirm the in situ formation of ZrB2 particles in the matrix. Optical Microscopy (OM) studies reveal the refinement of aluminum-rich phase due to the presence of ZrB2 particles. Scanning electron microscopy (SEM) studies reveal size and distribution of ZrB2 particles while transmission electron microscopy (TEM) reveals the presence of dislocations in the matrix around ZrB2 particles. Hardness and tensile testing of composites have been carried out at room temperature to evaluate the mechanical properties. The results reveal the improvement in hardness and strength with increased amount of ZrB2 particles. Strength of AA5052/ZrB2 in situ composites has been analyzed by various strengthening mechanism models. The analysis revealed that Orowan and Solid solution strengthening mechanisms are the predominant mechanism for high strength composites. Theoretical yield strength is about 6–10% higher than the experimental values due to clustering tendency of ZrB2 particles.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011003-011003-8. doi:10.1115/1.4034509.

One of the main challenges encountered in modeling the behavior of metal matrix composites (MMCs) during machining is the availability of a suitable constitutive equation. Currently, the Johnson–Cook (J–C) constitutive equation is being used, even though it was developed for homogeneous materials. In such a case, an equivalent set of homogeneous parameters is used, which is only suitable for a particular combination of particle size and volume fraction. The current work presents a modified form of the J–C constitutive equation that suits MMCs, and explicitly accounts for the effects of particle size and volume fraction, as controlled parameters. Also, an energy-based force model is presented, which considers particle cracking and debonding based on the principles of fracture mechanics. In order to validate the new approach, cutting forces were predicted and compared to experimental results, where a good agreement was found. In addition, the predicted forces were compared to other analytical models available in the literature.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011004-011004-9. doi:10.1115/1.4034752.

The differential scheme is extended to predict the effective properties of multiphase magnetoelectroelastic composite materials. The prediction of effective properties is done gradually by adding a series of incremental additions of a small volume of particulate phase materials to an initial material (matrix phase). The construction process is compatible with high volume concentration of inclusion. A system of coupled differential equations is formulated and its numerical solution leads to effective properties of reinforced magnetoelectroelastic composites. For the numerical results, two-phase and three-phase magnetoelectroelastic composites are considered. The effective properties are presented as function of volume fractions and shapes of inclusions and compared with predictions based on the Mori–Tanaka and incremental self-consistent models.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011005-011005-7. doi:10.1115/1.4034820.

We develop a framework for wave tailoring by altering the lattice network topology of a granular crystal consisting of spherical granules in contact. The lattice topology can alternate between two stable configurations, with the spherical granules of the lattice held in stable equilibrium in each configuration by gravity. Under impact, the first configuration results in a wave with rapidly decaying amplitude as it propagates along a primary chain, while the second configuration results in a solitary wave propagating along the primary chain with no decay. The mechanism to achieve such tunability is by having energy diverted to the granules adjacent to the primary chain in the first case but not the second. The tunable design of the proposed network is validated using both numerical simulations and experiments. In terms of potential applications, the proposed bistable lattice network can be viewed either as a wave attenuator or as a device that allows higher amplitude wave propagation in one direction than in the opposite direction. The lattice is analogous to a crystal phase transformation due to the change in atomic configurations, leading to the change in properties at the macroscale.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011006-011006-9. doi:10.1115/1.4034924.

A sizeable number of structures, as key load-bearing components, are currently being made using both high-strength and medium-strength alloys of aluminum. During their service life, these alloys are often exposed to environments spanning a range of aggressiveness. In this study, the corrosion behavior of a high-strength aluminum alloy in both static and flowing saline solution was conducted using both experimental and numerical analysis. The damage resulting from environment-induced degradation, or corrosion, of the test specimens upon exposure to flowing saline solution was noticeably severe in comparison with the damage caused by exposure to static saline solution. Subsequent to flow-induced degradation, an analysis of dispersion of the corrosion products over the surface revealed it to be in the direction of flowing saline solution. The higher the flow rate of saline solution over the sample surface, the more severe and visibly evident was the severity of damage due to environment-induced degradation. Microscopic observations of the corrosion morphology for the three different flow rates revealed a greater degree of damage to the surface with an increase in flow rate of the saline solution. This can be quantified by both an increase in area of the sample that is degraded and depth of the corrosion-induced pits. Using cellular automata algorithm in conjunction with matlab software, the damage caused by flowing saline solution for three different flow rates predicted fairly accurately the severity of the environment-induced damage due to corrosion and resultant morphology of the corrosion-related debris.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011007-011007-7. doi:10.1115/1.4034959.

Different Al-SiC metal matrix composites (MMCs) with a different matrix, reinforcement sizes, and volume fractions were fabricated using ball milling (BM) and powder metallurgy (PM) techniques. Al and Al-SiC composites with different volume fractions were milled for 120 h. Then, the Al and Al-SiC composites were pressed under 125 MPa and finally sintered at 450 °C. Moreover, microsize and combination between micro and nano sizes Al-SiC samples were prepared by the same way. The effect of the Al matrix, SiC reinforcement sizes and the SiC volume fraction on the microstructure evolution, physical and mechanical properties of the produced composites was investigated. The BM and powder metallurgy techniques followed by sintering produce fully dense Al-SiC composite samples with different matrix and reinforcement sizes. The SiC particle size was observed to have a higher effect on the thermal conductivity, electrical resistivity, and microhardness of the produced composites than that of the SiC volume fraction. The decreasing of the Al and SiC particle sizes and increasing of the SiC volume fraction deteriorate the physical properties. On the other hand, the microhardness was enhanced with the decreasing of the Al, SiC particle sizes and the increasing of the SiC volume fraction.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011008-011008-9. doi:10.1115/1.4034925.
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This work reports the elastic modulus and four-point flexural strength of a gelcast ceramic, cerium dioxide (ceria), with a microporosity of nominally 20% and a grain size of 11 μm from 23 to 1500 °C. The data augment the sparse data published for ceria and extend previous results by 150 °C. The ceria tested is representative of that constituting the ligaments of a reticulated porous ceramic. The elastic modulus decreases from 90 GPa at 23 °C to 16 GPa at 1500 °C. The flexural strength is 78 MPa below 900 °C and then decreases rapidly to 5 MPa at 1500 °C. These trends are consistent with data reported for other ceramics. Comparing the measured elastic modulus to prior data obtained for lower porosity shows the minimum solid area (MSA) model can be used to extend the modulus data to other porosities. Similarly, the flexural strength data agree with prior data when the effects of specimen size, porosity, and grain size are taken into account.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011009-011009-10. doi:10.1115/1.4034944.

The paper presents the derived equations for calculations of the initial wall thickness g0 of a tube bent to elbow. The expressions for calculating g0 are presented in a suitable measure of the “great active actual radius Rj” in the bending zone for an exact-generalized solution (continuous fields) and for three formal simplifications (discontinuous fields) of the first-, second-, and third-orders. The expressions to calculate the components of deformation for a generalized solution (continuous fields) are obtained on the basis of kinematically admissible fields of plastic deformations. In any case, a value of initial tube thickness depends on the radius and on the angle of bending αb on the external diameter of the tube, on the displacement of the neutral axis, and on the allowable (required) elbow thickness according to European, American, or other national technical standard or regulations. The initial thickness also depends on the coordinates of the point where the allowable thickness was determined and on the technological–material coefficient of the bending zone range k (defined during the tests). The obtained calculation results are presented in the form of graphs and in table.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011010-011010-10. doi:10.1115/1.4034987.

In this paper, the authors present an internal state variable (ISV) cap plasticity model to provide a physical representation of inelastic mechanical behaviors of granular materials under pressure and shear conditions. The formulation is dependent on several factors: nonlinear elasticity, yield limit, stress invariants, plastic flow, and ISV hardening laws to represent various mechanical states. Constitutive equations are established based on a modified Drucker–Prager cap plasticity model to describe the mechanical densification process. To avoid potential numerical difficulties, a transition yield surface function is introduced to smooth the intersection between the failure and cap surfaces for different shapes and octahedral profiles of the shear failure yield surface. The ISV model for the test case of a linear-shaped shear failure surface with Mises octahedral profile is implemented into a finite element code. Numerical simulations using a steel metal powder are presented to demonstrate the capabilities of the ISV cap plasticity model to represent densification of a steel powder during compaction. The formulation is general enough to also apply to other powder metals and geomaterials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2016;139(1):011011-011011-10. doi:10.1115/1.4035098.

A model to predict the elastic material properties of reticulated porous ceramics (RPCs) based on the microstructural geometry is presented. The RPC is represented by a repeating unit structure of truncated octahedrons (tetrakaidecahedrons) with the ligaments represented by the cell edges. The deformations of the ligaments in the cellular structure under applied loads are used to determine the effective moduli and Poisson's ratio of the bulk material. The ligament cross section is represented as having a Plateau border exterior surface with a cusp half-angle that is varied between 0 and 90 deg, and a Plateau border interior void with a cusp half-angle of zero, representative of the ranges seen in RPCs. The ligament cross-sectional area is permitted to vary along its length and the distance between internal and external cusps is assumed constant. The relative density of the foam, corresponding to the length, cusp distances, and external-cusp half-angle of the ligaments, is determined using solid geometry. The relative density has the dominant effect on the moduli, while normalized ligament length varies the moduli by 11–49% at a specified relative density. The impact of the external shape of a ligament on the relative moduli is insignificant. The model is validated through comparisons with the measured elastic properties of RPCs in the literature and new data. The model is the first to consider the effect of the microstructural features of ligaments of RPCs on the elastic moduli of the bulk material.

Commentary by Dr. Valentin Fuster

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