• Korean Journal of Materials Research
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• v.27 no.12
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• pp.688-694
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• 2017

Porous materials such as polymeric foam are widely adopted in engineering and biomedical fields. Porous materials often exhibit complex nonlinear behaviors and are sensitive to material and environmental factors including cell size and shape, amount of porosity, and temperature, which are influenced by the type of base materials, reinforcements, method of fabrication, etc. Hence, the material characteristics of porous materials such as compressive stress-strain behavior and void volume fraction according to aforementioned factors should be precisely identified. In this study, unconfined uniaxial compressive test for two types of closed-cell structure polyurethane foam, namely, 0.16 and $0.32g/cm^3$ of densities were carried out. In addition, the void volume fraction of three different domains, namely, center, surface and buckling regions under various compressive strains (10 %, 30 %, 50 % and 70 %) were quantitatively observed using Micro 3D Computed Tomography(micro-CT) scanning system. Based on the experimental results, the relationship between compressive strain and void volume fraction with respect to cell size, density and boundary condition were investigated.

• Steel and Composite Structures
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• v.38 no.2
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• pp.177-187
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• 2021

The effect of nanoparticle volume fraction on the elastic properties of a polymer-based nanocomposite was experimentally investigated and the obtained results were compared with various existing theoretical models. The nanocomposite was consisted of high density polyethylene (HDPE) as polymeric matrix and 0, 0.5, 1 and 1.5 wt.% multi walled carbon nanotubes (MWCNTs) prepared using twin screw extruder and injection molding technique. Nanocomposite samples were molded in injection apparatus according to ASTM-D638 standard. Therefore, in addition to morphological investigations of the samples, tensile tests at ambient temperature were performed on each sample and stress-strain plots, elastic moduli, Poisson's ratios, and strain energies of volume units were extracted from primary strain test results. Tensile test results demonstrated that 1 wt.% nanoparticles presented the best reinforcement behavior in HDPE-MWCNT nanocomposites. Due to the agglomeration of nanoparticles at above 1 wt.%, Young's modulus, yielding stress, fracture stress, and fracture energy were decreased and Poisson's ratio and failure strain were increased.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.9
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• pp.1337-1343
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• 2004

Shape optimization was performed to obtain a precise shape of cutouts including the internal shape of cutouts in a laminated composite plate by three dimensional modeling using solid element. Volume control of the growth-strain method was implemented and the distributed parameter chosen as Tsai-Hill fracture index for shape optimization. It makes Tsai-Hill failure index at each element uniform in laminated composites under the predetermined volume a designer requires. Shapes optimized by Tsai-Hill failure index were compared with those of the initial shapes for the various load conditions and cutouts. The following conclusions were obtained in this study; (1) It was found that growth-strain method was applied efficiently to shape optimization of three dimensional cutouts in a laminate composite, (2) The optimal shapes of the various load conditions and cutouts were obtained, (3) The maximum Tsai-Hill failure indices of the optimal shapes were remarkably reduced comparing with those of the initial shapes.

• Computers and Concrete
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• v.26 no.1
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• pp.31-52
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• 2020

This paper investigates the size dependent effect on the vibration analysis of a porous nanocomposite viscoelastic plate reinforced by functionally graded-single walled carbon nanotubes (FG-SWCNTs) by considering nonlocal strain gradient theory. Therefore, using energy method and Hamilton's principle, the equations of motion are derived. In this article, the effects of nonlocal parameter, aspect ratio, strain gradient parameter, volume fraction of carbon nanotubes (CNTs), damping coefficient, porosity coefficient, and temperature change on the natural frequency are perused. The innovation of this paper is to compare the effectiveness of each mentioned parameters individually on the free vibrations of this plate and to represent the appropriate value for each parameter to achieve an ideal nanocomposite plate that minimizes vibration. The results are verified with those referenced in the paper. The results illustrate that the effect of damping coefficient on the increase of natural frequency is significantly higher than the other parameters effect, and the effects of the strain gradient parameter and nonlocal parameter on the natural frequency increase are less than damping coefficient effect, respectively. Furthermore, the results indicate that the natural frequency decreases with a rise in the nonlocal parameter, aspect ratio and temperature change. Also, the natural frequency increases with a rise in the strain gradient parameter and CNTs volume fraction. This study can be used for optimizing the industrial and medical designs, such as automotive industry, aerospace engineering and water purification system, by considering ideal properties for the nanocomposite plate.

• Transactions of Materials Processing
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• v.9 no.1
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• pp.72-79
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• 2000

Dynamic recrystallization (DRX), which may occur during hot deformation, is important for the microsturctural evolution of 304 stainless steel. Especially, the current interest in modelling hot rolling demands quantitative relationships among the thermomechanical process variables, such as strain, temperature, strain rate, and etc. Thus, this paper individually presents the relationships for flow stress and volume fraction of DRX as a function of processing variables using torsion tests. The hot torsion tests of 304 stainless steel were performed at the temperature range of 900~110$0^{\circ}C$ and the strain rate range of 5x10-2~5s-1 to study the high temperature softening behavior. For the exact prediction of flow stress, the equation was divided into two regions, the work hardening (WH) and dynamic recovery (DRV) region and the DRX region. Especially, The flow stress of DRX region could be expressed by using the volume fraction of DRX (XDRX). Since XDRX was consisted of the critical strain($\varepsilon$c) for initiation of dynamic recrystallization (DRX) and the strain for maximum softening rate ($\varepsilon$*), that were related with the evolution of microstructure. The calculated results predicted the flow stress and the microstructure of the alloy at any deformation conditions well.

• YAKHAK HOEJI
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• v.45 no.5
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• pp.491-493
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• 2001

Coagulase-negative Staphylococcus sp #39, isolated from raw-milk showed reduced susceptibility to vancomycin. The minimun inhibitory concentration for strain #39 was at 8$\mu\textrm{g}$ of vancomycin per ml. Transmitting electron microscopy displayed that this strain had a 2.5-3.5 times thicker cell wall than a vancomucin sensitive strain of Staphylococcus sp. The strain #39 also had an increased cell volume. These data indicate that the reduced susceptibility may be due to the thickness of the cell wall of the test strain.

• Advances in concrete construction
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• v.17 no.4
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• pp.233-243
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• 2024

The behavior of confined steel fiber-reinforced concrete (including confinement models) with compressive strengths ranging from normal to high strength is still rarely studied. This paper presents the results of an investigation of fifteen confined concrete cylinders containing steel fiber. The design parameters evaluated in the experiment included concrete compressive strength (covers normal to high strength), volume fraction of steel fiber and hoop spacing. The main objective of this study was to evaluate the behavior of confined steel fiber concrete by reviewing several design parameters, such as concrete strength (normal to high strength). It is then developed to be an analytical stress-strain expression for confined steel fiber concrete. The experimental program was carried out by making cylindrical specimens with a diameter of 100 mm and a height of 200 mm. The cylindrical test object is compressed in a monotonic uniaxial loading. Experimental results have shown steel fiber in concrete has an important role in increasing the compressive strength and strain of cylindrical concrete without steel fiber. In addition, the value of strength enhancement of confined concrete (K) along with increasing fiber fraction volume; which applies to normal to high-strength concrete. The value of K also increases if the compressive strength of the concrete tends to decrease and the spacing of the hoops is closer. The comparison of stress-strain behavior between the confined steel fiber concrete proposed by other researchers and the experimental results in general significantly different in post-peak response. The statistical analysis indicates that the value of Coefficient of Variation for the confinement model by Campione is the closest compared to other existing confinement models in predicting the values of K and Toughness Index. Furthermore, the analytic stress-strain expression of confined steel fiber concrete was developed by adopting and modifying several equations from the present models. The proposed analytical expression is then verified with the experimental results. The results of the verification show that the stress-strain behavior of confined steel fiber concrete is relatively close.

• KSBB Journal
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• v.17 no.3
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• pp.295-301
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• 2002

The effects of nonionic surfactant (SPAN 20) on the desulfurization process by Rhodococcus sp. strain IGTS8 have been investigated at various oil/water ratios, pHs and concentrations of surfactant. The hexadecane containing DBT was employed as model oil. The presence of surfactant in the oil/water mixture stabilized the oil/water interface, thus enhanced the efficiency of desulfurization. The volume percentages of oil in the oil/water mixture were 30, 50 and 70%. The concentrations of surfactant were varied from 0 to 0.33 wt% relative to water phase. In general, the biodesulfurization efficiencies were decreased as the concentration of SPAN 20 and the volume percentage of oil phase increased.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.12
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• pp.1933-1943
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• 2001

A micromechanical model is presented for superplasticity in which heterogeneous microstructures are coupled with deformation behavior. The effects of initial distributions of grain size, and their evolutions on the mechanical properties can be predicted by the model. Alternative stress rate models such as Jaumann rate and rotation incremental rate have been employed to analyze uniaxial loading and simple shear problems and the appropriate modeling was studied on the basis of hypoelasticity and elasto-viscoplasticity. The model has been implemented into finite element software so that full process simulation can be carried out. Tests have been conducted on Ti-6Al-4V alloy and the microstructural features such as grain size, distributions of grain size, and volume fraction of each phase were examined for the materials that were tested at different strain rates. The experimentally observed stress-strain behavior on a range of initial grain size distributions has been shown to be correctly predicted. In addition, the effect of volume fraction of the phases and concurrent grain growth were analyzed. The dependence of failure strain on strain rate has been explained in terms of the change in mechanism of grain growth that occurs with changing strain rate.

• Journal of the Korean Society for Heat Treatment
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• v.3 no.2
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• pp.20-31
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• 1990

This investigation has been carried out to make clear the effect of deformation temperature, strain rate and grain size on the tensile properties of 304L stainless steel. Tensile properties of the metastable austenitic 304L steel remarkably influenced by deformation temperature. Tensile strength increased with decreasing deformation temperature and the elongation showed maximum value near $40^{\circ}C$. In order to obtain the high elongation, a large amount of deformation is available in austenite before martensitic transformation and the martensite has to be induced gradually. Tensile strength and elongation increased with decreasing grain size. The temperature representing the maximum elongation shifted to low temperature and the peak width of elongation became broaden with decreasing austenite grain size. The volume fraction of strain induced martensite decreased with decreasing austenite grain size. As the strain rate increase, the temperature representing the maximum elongation value shifted to high temperature and volume fraction of strain induced martensite decreased.