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Research Papers

J. Eng. Mater. Technol. 2017;140(1):011001-011001-6. doi:10.1115/1.4037275.

Superalloys are high temperature materials which are indispensable in many high temperature applications such as the gas turbines. IN738LC is a nickel-based superalloy that is extensively used in the hot sections of the gas turbines. The strengthening in this alloy is provided mainly by the γ′ precipitates. In this research, precipitate size and morphology of a serviced IN738LC polycrystalline turbine blade is investigated. Specimens from the trailing edge, middle, and leading edge positions of the tip, middle, and root sections on their hot (exterior) and cooled (interior) surfaces are analyzed for the precipitate size and morphology. The size and morphology are then linked to the temperature and stress/strain distribution in the blade. In general, the hot surfaces have larger precipitates that indicate a higher temperature exposure. In particular, the precipitate size is larger in the tip and middle sections than the root section, implying that the latter has a lower temperature. As the precipitates transforms to rafts at high temperature and stress/strain, the middle positions of the tip and middle sections, the trailing edge of the tip section, and the leading edge of the middle section are predicted to have high temperature–stress/strain coupling.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011002-011002-5. doi:10.1115/1.4037169.

Exfoliated graphite (EG) was prepared from commercially available natural graphite flakes (NGF), through strong acid treatment followed by thermal shock at 950 °C. The EG sheets were characterized with respect to their thermal stability via thermogravimetric analysis (TGA) and Raman spectra. Their morphology and particle size were evaluated using scanning electron microscope (SEM) and particle size analyzer. The potential of EG as reinforcement on the mechanical and thermal properties of the dynamically vulcanized polystyrene/styrene butadiene rubber (PS/SBR) composites was evaluated. The influence of EG on the electrical properties of the composites was measured as well.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011003-011003-10. doi:10.1115/1.4037023.

A scale-dependent numerical approach is developed through combining the finite element (FE)-based averaging process with the Monte Carlo method to determine the desired size of a characteristic volume element (CVE) for a random magnetoactive composite (MAC) under applied magnetic field and large deformations. Spatially random distribution of identically magnetic inclusions inside a soft homogeneous matrix is considered to find the appropriate size of the characteristic volume element. Monte Carlo method is used to generate ensembles of a randomly distributed magnetoactive composite to be applied in the homogenization study. The ensemble is utilized as a statistical volume element (SVE) in a scale-dependent numerical algorithm to search the desired characteristic volume element size. Results of this study can be used to investigate effective behavior and multiscale modeling of randomly particulate magnetoactive composites.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011004-011004-8. doi:10.1115/1.4037272.

Several experimental studies have revealed that the size-dependent deformation in polymers at nano- to micro-meter length scales is significantly associated with elastic deformation. Such size-dependent deformation in polymers is expected to affect the in-plane macroscopic elastic properties of cellular polymers with micrometer-sized cells. A finite element (FE) formulation of a higher-order elasticity theory is applied to evaluate the in-plane macroscopic elastic properties of different polymer cellular geometries by varying the cell size from the macroscopic to micron length scale. For a given relative density of the cellular solid, a reduction in the cell size from the macroscopic to micron length scale resulted in geometry-specific variations in the in-plane macroscopic elastic moduli and Poisson's ratios. Furthermore, an increase in the relative density for a given cell size revealed variations in the size dependence of the elastic properties. The size dependence of elastic properties is interpreted based on the influence of rotation gradients with varying cell size of the cellular solid. Also, the evaluated size-dependent elastic properties are compared with the available analytical solutions from the literature.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011005-011005-8. doi:10.1115/1.4037273.

Hybrid titanium composite laminates (HTCLs) combine the benefits of thin titanium sheets and fiber-reinforced polymer (FRP) composite laminates to design high performance light-weight materials with optimized impact resistance, fracture toughness, durability, and/or thermal performance. This paper starts with a detailed review of typical failure modes observed in HTCLs. The critical manufacturing process of thin grade II titanium sheets combined with HexPly G947/M18 carbon fiber-reinforced polymer laminates is described in detail. This includes the evaluation of titanium surface preparation techniques, which guarantee good adhesive bonding. A systematic experimental study of different HTCL configurations under tensile loading confirms that the major failure modes are debonding between the titanium sheet and the FRP laminate, matrix cracking in the 90 deg plies of the FRP laminate and interlaminar delamination. The results show that HTCLs made from woven carbon FRP plies show higher ultimate strengths and strain at breaks than HTCLs containing a cross-ply composite core made from unidirectional (UD) prepreg.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011006-011006-10. doi:10.1115/1.4037170.

Understanding the degradation of material properties and stress–strain behavior of rubberlike materials that have been exposed to elevated temperature is essential for rubber components design and life time prediction. The complexity of the relationship between hyperelastic materials, crosslinking density (CLD), and chemical composition presents a difficult problem for the accurate prediction of mechanical properties under thermal aging. In this paper, a new and relatively simple mathematical formulation is presented to expresses the change in material properties of hyperelastic materials under thermal aging. The proposed formulation has been applied to a natural rubber (NR). Testing was performed on more than 130 specimens that were thermally aged then subjected uniaxial tension and hardness tests. The aging temperatures ranged from 76.7 °C to 115.5 °C, and the aging times ranged from 0 to 600 h. Based on the recorded experimental data, the NR mechanical properties under thermal aging showed a similar behavior to the rate of change of the CLD with aging time and temperature. Three mechanical properties have been chosen to be studied in this paper: the ultimate tensile strength, the fracture stretch value, and the secant modulus at 11.0% strain. The proposed mathematical formulation is a phenomenological equation that relates the material properties with the change in CLD based on a form of Arrhenius equation. The proposed equation showed promising results compared to the experimental data with an acceptable error margin of less than 10% in most of the cases studied.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011007-011007-11. doi:10.1115/1.4037024.

Hybrid nanocomposites with multiple fillers like carbon nanotubes (CNT) and graphene nanoplatelets (GNP) are known to exhibit improved electrical and electromechanical performance when compared to monofiller composites. We developed a two-dimensional Monte Carlo percolation network model for hybrid nanocomposite with CNT and GNP fillers and utilized it to study the electrical conductivity and piezoresistivity as a function of nanocomposite microstructural variations. The filler intersections are modeled considering electron tunneling as the mechanism for electrical percolation. Network modification after elastic deformation is utilized to model the nanocomposite piezoresistive behavior. Systematic improvement in electrical conductivity and piezoresistivity was observed in the hybrid nanocomposites, compared to monofiller CNT nanocomposites. Parametric studies have been performed to show the effect of GNP content, size, aspect ratio, and alignment on the percolation threshold, the conductivity, and piezoresistivity of hybrid CNT–GNP polymer composites.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011008-011008-5. doi:10.1115/1.4037274.

Grain boundary (GB) embrittlement by sulfur in fcc CuΣ5(012)[100] symmetrical tilt grain boundary (STGB) is simulated by first-principles calculations. The surface and grain boundary segregation energies are estimated by progressively placing solute atoms in the potential segregation sites in the boundaries. Based on the calculated segregation energies, the cohesive energy of the grain boundary is evaluated as a function of the sulfur atoms concentration. It is found that, when a two atomic layers’ concentration is attained, the cohesive energy is reduced by one order of magnitude compared to its value for the clean grain boundary.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011009-011009-7. doi:10.1115/1.4037393.

This paper presents a study of hydrogen diffusion for a spiral weld pipe considering the effect of weld residual stress. The results show that the hydrogen mainly gathers at heat-affected zone (HAZ). HAZ is the weakest zone where hydrogen-induced cracking (HIC) occurs. The effect of helix angle on the hydrogen diffusion is also discussed. It shows that different helix angles generate different hydrogen concentrations. As the helix angle increases, both the hydrogen concentration and residual stresses decrease. As the helix angle increases from 40 deg to 50 deg, the equivalent pressure stresses reduce a little, resulting in the change of hydrogen concentration being small. The smaller the helix angle is, the larger the diffusion rate is. The most suitable helix angle should be optimized at 40–50 deg.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011010-011010-11. doi:10.1115/1.4037656.

In this paper, an elastoplastic-damage constitutive model is presented. The formulation is cast within the framework of continuum damage mechanics (CDM) by means of the internal variable theory of thermodynamics. The damage is assumed as a tensor type variable and its evolution is developed based on the energy equivalence hypothesis. In order to discriminate the plastic and damage deformation, two surfaces named as plastic and damage are introduced. The damage surface has been developed so that it can model the nonlinear variation of damage. The details of the model besides its implicit integration algorithm are presented. The model is implemented as a user-defined subroutine user-defined material (UMAT) in the abaqus/standard finite element program for numerical simulation purposes. In the regard of investigating the capability of model, the shear and tensile tests are experimentally conducted, and corresponding results are compared with those predicted numerically. These comparisons are also accomplished for several experiments available in the literature. Satisfactory agreement between experiments and numerical predictions provided by the model implies the capability of the model to predict the plastic deformation as well as damage evolution in the materials.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011011-011011-10. doi:10.1115/1.4037657.

The mechanical behavior of closed-cell aluminum foam composites under different compressive loadings has been investigated. Closed-cell aluminum foam composites made using the liquid metallurgy route were reinforced with multiwalled carbon nanotubes (CNTs) with different concentrations, namely, 1%, 2%, and 3% by weight. The reinforced foams were experimentally tested under dynamic compression using the split Hopkinson pressure bar (SHPB) system over a range of strain rates (up to 2200 s−1). For comparison, aluminum foams were also tested under quasi-static compression. It was observed that closed-cell aluminum foam composites are strain rate sensitive. The mechanical properties of CNT reinforced Al-foams, namely, yield stress, plateau stress, and energy absorption capacity are significantly higher than that of monolithic Al-foam under both low and high strain rates.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011012-011012-8. doi:10.1115/1.4037524.

Warm hydromechanical deep drawing (WHDD) has increasingly been implemented by automotive industry due to its various benefits including mass reduction opportunities in auto body-in-white components and improved formability for lightweight alloys. In the first part of the current study, WHDD of AA 5754-O was studied. In order to obtain the highest formability, an optimization study was performed for AA 5754-O WHDD process parameters (tool temperature, hydraulic pressure (HP), and blank holder force (BHF) loading profiles) through finite element analysis (FEA) + experimentation approach. Results showed that the optimal temperature for punch is 25 °C and 300 °C for die and blank holder. In addition, HP was found to be more effective on formability when compared to BHF. Both fast increasing HP and blank holder loading profiles contributes to higher formability.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(1):011013-011013-7. doi:10.1115/1.4037525.

Shot peening is well known as a surface deformation process which can induce compressive residual stresses into the subsurface of materials in order to improve the fatigue life. In this paper, the effects of the process conditions for both single and double shot peening on the fatigue life of AISI 4340 low alloy steel is investigated. The fatigue tests revealed that the shot peening process could significantly enhance the fatigue life of the treated components. However, a side effect of the process was an increase in surface roughness which was more prevalent under higher peening pressures and led to a reduction in the fatigue life. Therefore, to maximize the performance of the process, the peening parameters need to be carefully selected. Microstructure analysis of the shot peened parts indicated that the nucleation cracks or initiation cracks occurred in the subsurface at depths of 10–20 μm in the case of as-received samples but moved up to the free surface for the shot peened parts.

Commentary by Dr. Valentin Fuster

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