Evaluation of compressibility and small strain stiffness characteristics of sand reinforced with discrete synthetic fibers

Choo, H.,Yoon, B.,Lee, W.,Lee, C. Elsevier Applied Science Publishers, Ltd ; Elsevie 2017 Geotextiles and geomembranes Vol.45 No.4

<P>This experimental investigation evaluates the compressibility and small strain stiffness of sand reinforced with discrete synthetic fibers. Varying fiber contents (FC), fiber aspect ratios (AR), and void ratios were selected as testing variables in this study, and the modified oedometer tests were conducted to measure the compression index (C-c) and maximum shear modulus (Gmax) of fiber-reinforced sand. The results of this study demonstrate that the Cc of the tested fiber-reinforced sand increases with an increase in FC because the packing of sand grains in the fiber-reinforced sand is very loose due to a disruption of direct contact between the sand grains due to the presence of long discrete fibers. Additionally, this disruption of direct contact between sand grains due to the fibers results in a reduction of interparticle contact and coordination number between sand grains. Therefore, the Gmax of tested fiber-reinforced sand decreases with an increase in FC. Most notably, the Gmax of the tested fiber-reinforced sand with varying FC and AR can be expressed as a single function of the void ratio at a given applied stress, which implies that the inclusion of fibers just alters the packing state of sand grains, and the interparticle contact stiffness is mainly determined by the contacts between sand grains. (C) 2017 Elsevier Ltd. All rights reserved.</P>

Surrogate-based Pareto optimization of annealing parameters for severely deformed steel

Ghiabakloo, H.,Lee, K.,Kazeminezhad, M.,Kang, B.S. Elsevier Ltd 2016 Materials & Design Vol.92 No.-

<P>Severe plastic deformation (SPD) is a metalworking technique that is used for the enhancement of the strength and hardness of metallic materials. As SPD causes ductility deterioration, materials typically necessitate annealing for ductility increase; however, annealing may conversely affect strength and hardness. Thus, to optimally balance strength, hardness, and ductility, this study determined annealing conditions with a severely deformed low carbon steel sheet by adjusting annealing time and temperature. For the facilitation of the annealing process optimization, measurements of strength, hardness, and ductility under various annealing conditions were represented by regression Kriging. Then, because of the conflicting nature of the desired metal properties, a set of optimal annealing conditions was identified by Pareto multi-objective optimization. Finally, the best combination on the Pareto front was selected with TOPSIS. The results of Pareto optimization with regression Kriging showed that the best candidates for annealing conditions can be determined at a significantly reduced experimental cost. (C) 2015 Published by Elsevier Ltd.</P>

XFEM investigation on Knoop indentation cracking: Fracture toughness and aspect-ratio of radial-median cracks

Rickhey, F.,Lee, J.H.,Lee, H. Elsevier Ltd 2016 Materials & Design Vol.107 No.-

<P>In sharp indentation of brittle materials, cracks form below the impression or at its corners and propagate further during unloading. The dimensions of radial-median cracks can be exploited to derive the fracture toughness. Knoop indentation has the merit that only one large crack is produced. This and the shallow plastic zone make it the preferred method for crack growth experiments, but which require information on the initial crack shape. For investigation of the complex indentation stress field, which is moreover perturbed by cracking, the extended finite element method (XFEM) is an ideal tool. Results show that the point load assumption holds for sufficient loading, which means that Knoop indentation can be used for fracture toughness evaluation. During loading the plastic zone evolves linearly with depth h, while crack depth c(z) is found to evolve according to h proportional to c(z)(3/4). For well-developed cracks, the crack aspect-ratio rho equivalent to c(z)/c (c is the length of the crack on the surface) is load independent. The XFE model is validated through comparison with experimental results,from the literature. Based on parameter studies, we establish functions that allow determination of fracture toughness and crack aspect-ratio. It is demonstrated that the mapping functions work well. (C) 2016 Elsevier Ltd. All rights reserved.</P>

New results on passivity-based H_~ control for networked cascade control systems with application to power plant boiler-turbine system

Mathiyalagan, K.,Park, J.H.,Sakthivel, R. Elsevier Ltd 2015 NONLINEAR ANALYSIS HYBRID SYSTEMS Vol.17 No.-

<P>This paper is concerned with the problem of passivity-based H-infinity controller design for a class of networked cascade control systems (NCCSs) with random packet dropouts. The NCCS under consideration is modeled by using state feedback controllers and the network-induced imperfections like packet dropouts and time-varying delays. The model is defined with a stochastic packet-dropout case by using the Bernoulli distributed white sequence with time-varying probability measures. The probability-dependent conditions for stabilization of NCCSs are established to guarantee the resulting closed-loop system to be stochastically stable and achieve a prescribed mixed H-infinity and passivity performance. The Lyapunov stability theory and linear matrix inequality (LMI) approach are used to derive criteria for the existence of the state feedback controllers. The proposed probability-dependent gain scheduled controller can be designed by solving the convex optimization problem by means of a set of LMIs, which can be easily solved by using some standard numerical packages. Finally, a practical application is presented to illustrate the effectiveness and potential of the proposed results. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Mechanical and high temperature wear properties of extruded Al composite reinforced with Al₁₃Fe₄ CMA nanoparticles

Nemati, N.,Emamy, M.,Penkov, O.V.,Kim, J.,Kim, D.E. Elsevier Ltd 2016 Materials & Design Vol.90 No.-

<P>The mechanical and tribological properties of extruded aluminum matrix composites reinforced with various weight percentages (1, 3, 5, 7, 10 wt.%) of Al13Fe4 complex metallic alloys (CMAs) were investigated. The nano-composites were produced using conventional powder metallurgy and a hot extrusion process. The tribological behavior of the composites was investigated under normal loads in the range of 20-80 N using a reciprocating high-temperature tribo-tester over a temperature range of 25-350 degrees C. At an optimized reinforcing agent concentration of 5 wt.%, the composite showed a significant enhancement in Young's modulus (similar to 108 MPa) and hardness (similar to 1.85 GPa). The lowest coefficient of friction of 0.1 was attained at a temperature of 250 degrees C with a reinforcing agent concentration of 5 wt.%. Also, the wear rate was reduced by a factor of similar to 25 compared to the unreinforced aluminum specimen. The significant improvement in the tribological properties of the nanocomposite was attributed to the enhanced mechanical properties due to severe plastic deformation incurred during the extrusion process and incorporation of well distributed CMA nanoparticles in the matrix which provided oobstacles for dislocation motion. Detailed microstructural analyses revealed that incorporation of the second phase to the Al matrix led to microstructure refinement and increased the hardness up to similar to 2 GPa. Furthermore, the nanoparticles aided in the formation of hard and temperature-resistant tribo-layers which reduced the wear rate of the composite (Al-5 wt.% Al13Fe4) down to 1.5 x 10(-4) at 250 degrees C. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Surface energy and wettability control in bio-inspired PEG like thin films

Javid, A.,Kumar, M.,Wen, L.,Yoon, S.,Jin, S.B.,Lee, J.H.,Han, J.G. Elsevier Ltd 2016 Materials & Design Vol.92 No.-

<P>Tailoring of chemical functionalities in polymer films can induce interesting biocompatibility, however the sequential process of polymerization followed by functionalization imposes surface-interface complexities and inhomogeneity of functional groups across the thickness. Here, a single-step plasma process, enabling the simultaneous polymerization-functionalization, is demonstrated to control the surface energy and wettability of polyethylene glycol-like thin films. Chemical studies, carried out by Fourier transform infra-red spectroscopy and X-ray photoelectron spectroscopy, confirm the evolution and enhancement in amide functionalities, owing to the increase in the electronic transitions related to nitrogen based ions/radicals (independently confirmed by optical emission spectroscopy). In present case, the evolution and control over amide functionalities lead to the enhancement in wettability and surface energy tailoring in 60.5-67.5 mJ/m(2) range. Excellent growth of L-929 fibroblast cells is obtained by the synergic contribution of plasma power and N-2 flow rate via enriching the amide functionalities in these films. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Microstructural and kinetic investigation on the suppression of grain growth in nanocrystalline copper by the dispersion of silicon carbide nanoparticles

Akbarpour, M.R.,Farvizi, M.,Kim, H.S. Elsevier Ltd 2017 Materials & Design Vol.119 No.-

<P>In this paper, the thermal stability and grain growth kinetics of nanocrystalline Cu, reinforced with SiC nanoparticles and obtained using a mechanical milling process, were investigated during isothermal annealing. The presence of the nanoparticles in the nanocrystalline copper matrix resulted in a significant decrease in grain growth, the formation of partially textured microstructure and twin boundaries at higher temperatures, and an increase in the volume fraction of recrystallized grains, as estimated by grain orientation spread, in comparison to unreinforced Cu during annealing. The lower volume fraction of recrystallized grains at higher temperatures was attributed to dynamic recovery. Normal grain growth was observed in the annealing range of 400-600 degrees C, and significant abnormal grain growth was observed at higher temperatures. An analysis of the grain growth kinetics in the temperature range of 400-600 degrees C revealed a time exponent of n approximate to 3.6 and activation energy of approximate to 34 kJ mol(-1), based on the parabolic equation. The calculated activation energy for grain growth in the SiC dispersion strengthened Cu was found to be less than that of nanocrystalline Cu. The low activation energy and high thermal stability were attributed to high lattice strain and the retarding effect of nanoparticles by the Zener mechanism. (C) 2017 Elsevier Ltd. All rights reserved.</P>

Design principle of super resolution near-field structure using thermally responsive optical phase change materials for nanolithography applications

Park, G.,Lee, J.,Kang, S.,Kim, M.,Kang, S.,Choi, W. Elsevier Ltd 2016 Materials & Design Vol.102 No.-

<P>The super resolution near-field structure (Super-RENS) which is composed of a thin layer of a thermally responsive optical phase change material (PCM) between two dielectric layers, can be a means of resolving the limited resolution of the laser beam for direct laser lithography. In Super-RENS, incident laser irradiation induces the direct, reversible opening and closing of a nanoaperture in the PCM layer, and a nanoscale pattern is realized in the lithography system. Here, we first introduce the complete modeling procedures and optimization methodology for Super-RENS in nanolithography based on a rigorous analysis of near-field structure, thermal analysis in the finite-element method, and analysis of the corresponding feature size on the photoresist (PR) layer. Multiple combinations of the PCM layer and the two dielectric layers with varying dimensions are considered as design parameters to achieve the required resolution in the nanolithography system. The feasible line profiles are investigated at the general operating conditions of the pulsed laser beam, based on varying dimensions of the PCM layer (5-30 nm) and the two dielectric layers (10-200 nm). This work will provide a detailed methodology for the design and optimization of the Super-RENS for applications in the nanolithography system. (C) 2016 Elsevier Ltd. All rights reserved.</P>

Understanding the relationship between microstructure and mechanical properties of Al-Cu-Si ultrafine eutectic composites

Kim, J.T.,Lee, S.W.,Hong, S.H.,Park, H.J.,Park, J.Y.,Lee, N.,Seo, Y.,Wang, W.M.,Park, J.M.,Kim, K.B. Elsevier Ltd 2016 Materials & Design Vol.92 No.-

<P>Systematic investigations for the influence of the microstructural change derived from compositional tuning in Al-rich corner of Al-Cu-Si system on mechanical properties demonstrate that mechanical characteristic of ultra fine eutectic composites strongly depend on the crystallinity, length scale and volume fraction of constituent phases. Ultrafine eutectic composites can be divided into two categories: 1) whether primary phases exist or not and 2) the types of matrix phases, i.e., single eutectic or bimodal eutectic. The features of primary phases play a principal role on the macroscopic property. Ductile alpha-Al phase is very effective to improve the plasticity while brittle Al2Cu intermetallic phase deteriorates obviously mechanical performance. In addition, bimodal eutectic matrix alloys composed of eutectics with different length scale and morphology show superior mechanical properties than single eutectic matrix alloys. Based on the microstructural studies, it is believed that high strength is originated from ultrafine scale microstructure and enhanced plasticity is supported by strain gradient plasticity. These results reveal that mechanical properties of the ultrafine eutectic composites can be properly optimized via control of chemical and topological governing factors. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Joining of metal-ceramic using reactive air brazing for oxygen transport membrane applications

Raju, K.,Muksin,Kim, S.,Song, K.s.,Yu, J.H.,Yoon, D.H. Elsevier Ltd 2016 Materials & Design Vol.109 No.-

<P>This study examined the joining of dense Ce0.9Gd0.1O2 (-) (delta)-La0.6Sr0.4Co0.2Fe0.8O3 (-) (delta) (GDC-LSCF) ceramics to high temperature metal alloys for the fabrication of multilayered oxygen transport membrane (OTM) stacks. Reactive air brazing using a silver-based paste was performed at 1050 degrees C for 30 min in air to join GDC-LSCF/high temperature alloys, such as AISI 310S, Inconel 600 and Crofer 22 APU. The effects of the various filler materials, including CuO, GDC, LSCF, and GDC-LSCF mixture, in the Ag paste were also examined. The Ag-10 wt% CuO braze filler ensured in a reliable and compact joining without the formation of cracks and voids at the joining interface, while the addition of other ceramic fillers resulted in incomplete joining. Although none of the GDC-LSCF/metal alloy joints showed gas leakage at room temperature, the GDC-LSCF/Crofer joint only maintained the gas-tightness up to 800 degrees C under pressurized air up to 7 bars, which was explained by the microstructural rigidness of the oxide layer formed on the filler/alloy interface at high temperatures. This was supported by the minimal decrease in shear strength of the GDC-LSCF/Crofer joint, which was 91.1 and 88.3 MPa for the as-brazed and isothermally aged joint at 800 degrees C for 24 h in air, respectively. (C) 2016 Published by Elsevier Ltd.</P>