• Korean Chemical Engineering Research
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• v.46 no.1
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• pp.124-130
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• 2008

In order to fabricate a modified poly (lactic acid) (PLA) for applications requiring high melt strength, a PLA has been irradiated in the presence of functional monomer of glycidyl methacrylate (GMA). Samples were prepared with various contents of GMA and irradiation dosages, and were characterized by observing their thermal and melt viscoelastic properties and gel faction. The complex viscosity and storage modulus of the modified PLA without GMA were lower than those of the original PLA. Those of the modified PLA decreased with increasing dosage. In the case of the modified PLA containing 0.1 phr or 0.3 phr of GMA, their changing tendency with dosage was similar to the irradiated PLA without GMA. However, the 5 kGy irradiated PLA containing 0.5 phr of GMA showed the greatly enhanced complex viscosity and storage modulus, which were about 3 and 10 times higher than those of the original PLA at a frequency of 0.1 rad/s, respectively.

• Elastomers and Composites
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• v.37 no.2
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• pp.86-98
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• 2002

The application of silica/carbon black dual phase fillers to natural rubber(NR) compound was investigated. When the amounts of filler content were restricted to 60phr, the optimum ratio of dual phase fillers were 25phr/35phr of silica/carbon black. It was found that these new fillers give better overall performance in comparison with carbon black in tear strength, crack resistance, and cutting resistance. Also the thermal degradation resistance of NR vulcanizates which were filled with dual phase fillers was better than that of the carbon black. Dual phase fillers filled NR vulcanizates showed better viscoelastic properties, like tan${\delta}$, for the wet skid resistance and rolling resistance of motor vehicle tires.

• Elastomers and Composites
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• v.34 no.1
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• pp.11-19
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• 1999

Rebound, storage and loss modulus, and tan ${\delta}$ were investigated on cured rubbers with various ratios of sulfur to accelerator and the volume fraction of carbon black in the cured rubbers. The rebound was increased as the sulfur to accelerator ratio and the volume fraction of carbon black decreased. The storage modulus decreased as the loading of carbon black and the strain increased regardless of the cure systems. The network structure formed by filler-filler interaction was destroyed above 6% strain regardless of the loading of carbon black, because constant storage modulus was shown at the higher strain than 3% for $40{\sim}50phr$ loading of carbon black and at the higher strain than 6% for 60 phr and above loading of carbon black. Little effect on loss modulus was found at the low loading of carbon black, but the peak of loss modulus was shown at 1% strain as the loading of carbon black was increased. Tan ${\delta}$ increased as the loading of carbon black and the strain were increased regardless of the cure system, and maximum tan ${\delta}$ was shown at 2% strain regardless of the loading of carbon black.

• Journal of the Earthquake Engineering Society of Korea
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• v.19 no.1
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• pp.37-43
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• 2015

This paper presents a detailed procedure for a nonlinear soil-structure interaction of a seismically isolated NPP(Nuclear Power Plant) structure using the boundary reaction method (BRM). The BRM offers a two-step method as follows: (1) the calculation of boundary reaction forces in the frequency domain on an interface of linear and nonlinear regions, (2) solving the wave radiation problem subjected to the boundary reaction forces in the time domain. For the purpose of calculating the boundary reaction forces at the base of the isolator, the KIESSI-3D program is employed in this study to solve soil-foundation interaction problem subjected to vertically incident seismic waves. Wave radiation analysis is also employed, in which the nonlinear structure and the linear soil region are modeled by finite elements and energy absorbing elements on the outer model boundary using a general purpose nonlinear FE program. In this study, the MIDAS/Civil program is employed for modeling the wave radiation problem. In order to absorb the outgoing elastic waves to the unbounded soil region, spring and viscous-damper elements are used at the outer FE boundary. The BRM technique utilizing KIESSI-3D and MIDAS/Civil programs is verified using a linear soil-structure analysis problem. Finally the method is applied to nonlinear seismic analysis of a base-isolated NPP structure. The results show that BRM can effectively be applied to nonlinear soil-structure interaction problems.

• Polymer(Korea)
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• v.36 no.2
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• pp.235-244
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• 2012

A thiazole type accelerator MBT (2-mercapto benzothiazole) was added into silica filled natural rubber (NR) compound with various concentrations (0, 1, 2, 3, 4 phr). The effects of MBT on the cure rate, mechanical property, degree of rubber-filler interaction (${\alpha}_F$), crosslinking density, and viscoelastic property ($tan{\delta}$) were investigated. As accelerator concentration increased, the $t_{s2}$ and $t_{90}$ decreased and the crosslinking density and modulus at 300% elongation increased. The tensile strength and elongation increased up to 3 phr and no further increased at 4 phr. The $tan{\delta}$ value measured at room temperature was higher than that of the $70^{\circ}C$. The ${\alpha}_F$ value was not affected by the addition of MBT. The mechanisms for the vulcanization rate were reviewed.

• Journal of Biomedical Engineering Research
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• v.19 no.5
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• pp.495-502
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• 1998

A mathematical model of viscoelasticity on the material property of human pharyngeal tissue utilizing Y.C. Fung's Quasi-linear viscoelastic theory is proposed based on cyclic load, stress relaxation, incremental load, and uniaxial tensile load tests. The material properties are characterized and compared with other biological materials' results. The mathematical model is proposed by combining two characteristic functions determined from the stress relaxation and uniaxial tensile load tests. The reduced stress relaxation function G(t) and elastic response function S(t) are obtained from stress relaxation test and uniaxial tensile load test results respectively. Then the model describing stress-time history of the tissue is implemented utilizing two functions. The proposed model is evaluated and validated by comparing the model's cyclic behaviour with experimental results. The model data could be utilized as an important information for constructing 3-dimensional biomechanical model of human pharynx using FEM(Finite Element Method).

• Transactions of the Korean Society of Mechanical Engineers A
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• v.40 no.12
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• pp.1021-1026
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• 2016

In this study, crack resistance is evaluated by the crack tip opening angle (CTOA) using a wedge splitting test (WST) on a viscoelastic particulate reinforced composite based on an HTPB binder. Generally, CTOA, as a function of crack extension, is used in order to determinate fracture resistance and has a steady state relative angle. Digital image correlation (DIC) is used to measure the crack tip opening displacement (CTOD) and crack extension for the critical crack tip opening angle (CTOAc). In these results, the CTOAc value of a particulate reinforced composite tends to approach a constant angle after a small amount of crack extension. The CTOAc value increases with decreasing temperature, from $50^{\circ}C$ to $-40^{\circ}C$. These CTOAc values may be used to measure fracture mechanics parameters for particulate reinforced composite.

• Polymer(Korea)
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• v.28 no.3
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• pp.273-280
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• 2004

Acrylic resins (ethyl methacrylate-2-hydroxypropyl methacrylate-n-butyl acrylate-acrylic acid EHBCs) containing 80％ of solid were synthesized. Then, high-solid coatings (ethyl methacrylate-2-hydroxypropyl methacrylate-n-butyl acrylate-acrylic acid/hexamethylene diisocyanate-biuret : EHBCNs) were prepared by curing of the acrylic resin with curing agent hexamethylene diisocyanate-biuret at room temperature. The cure time of prepared coatings EHBCN-4 (EHBC-4 : $T_{g}$ = $0^{\circ}C$) and EHBCN-7 (EHBC-7 : $T_{g}$ = 3$0^{\circ}C$), measured by rigid-body pendulum method, was recorded 6.2 hours and 4.5 hours, respectively. Dynamic viscoelastic experiment revealed the glass transition temperature of EHBCN-4 and EHBCN-7 to be $14^{\circ}C$ and $39^{\circ}C$, respectively. It was found that the adhesion and flexural properties among various properties of coatings were enhanced by the incorporation of caprolactone acrylate monomer into the acrylic resins.

• Macromolecular Research
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• v.16 no.6
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• pp.517-523
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• 2008

A RGD (Arg-Gly-Asp) conjugated chitosan hydrogel was used as a cell-supporting scaffold for articular cartilage regeneration. Thermosensitive chitosan-Pluronic (CP) has potential biomedical applications on account of its biocompatibility and injectability. A RGD-conjugated CP (RGD-CP) copolymer was prepared by coupling the carboxyl group in the peptide with the residual amine group in the CP copolymer. The chemical structure of RGD-CP was characterized by $^1H$ NMR and FT IR. The concentration of conjugated RGD was quantified by amino acid analysis (AAA) and rheology of the RGD-CP hydrogel was investigated. The amount of bound RGD was $0.135{\mu}g$ per 1 mg of CP copolymer. The viscoelastic parameters of RGD-CP hydrogel showed thermo-sensitivity and suitable mechanical strength at body temperature for cell scaffolds (a> 100 kPa storage modulus). The viability of the bovine chondrocyte and the amount of synthesized glycosaminoglycans (GAGs) on the RGD-CP hydrogels were evaluated together with the alginate hydrogels as a control over a 14 day period. Both results showed that the RGD-CP hydrogel was superior to the alginate hydrogel. These results show that conjugating RGD to CP hydro gels improves cell viability and proliferation, including extra cellular matrix (ECM) expression. Therefore, RGD conjugated CP hydrogels are quite suitable for a chondrocyte culture and have potential applications to the tissue engineering of articular cartilage tissue.

• Journal of the Korean Society for Nondestructive Testing
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• v.32 no.5
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• pp.469-483
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• 2012

The propagation characteristics of a mechanical wave in human soft tissue depend on its elastic properties. Investigation of these propagation characteristics is of paramount importance because it may enable us to diagnose cancer or tumor from the vibration response of the tissue. This paper investigates and compares displacement patterns generated in soft tissue due to several forms of low-frequency vibration sources placed on a surface. Among vibration sources considered are a normal load, tangential load, and antiplane shear load. We derive analytical expressions for displacements in viscoelastic single layers, and calculate displacement patterns in half space and infinite plate type tissue. Also, we simulate the vibration response of a finite-sized tissue using finite element method. The effects of the type of stress, the size and frequency of vibration sources, and medium boundaries on displacement patterns are discussed.