• Journal of the Microelectronics and Packaging Society
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• v.21 no.4
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• pp.125-131
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• 2014

In order to develop stretchable substrates for wearable packaging applications, the variation behavior of elastic modulus was evaluated for transparent PDMS Sylgard 184 and black PDMS Sylgard 170 as a function of the base/curing agent mixing ratio. Both for Sylgard 184 and Sylgard 170, the true elastic modulus evaluated on a true stress-true strain curve was higher more than two times compared to the engineering elastic modulus obtained from an engineering stres-sengineering strain curve, and their difference became larger with increasing the stiffness of the PDMS. Sylgard 184 exhibited a maximum engineering elastic modulus of 1.74 MPa and a maximum true elastic modulus of 3.57 MPa at the base/curing agent mixing ratio of 10. A maximum engineering elastic modulus of 1.51 MPa and a maximum true elastic modulus of 3.64 MPa were obtained for Sylgard 170 at the base/curing agent mixing ratio of 2.

• Journal of the Korean Society for Nondestructive Testing
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• v.33 no.2
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• pp.147-153
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• 2013

Recently the primary water stress corrosion cracking(PWSCC) has occurred in the dissimilar metal weld region between pressurizer nozzle and safe-end in nuclear power plants(NPPs). As material of the pressurizer nozzle, SA508 Gr. 3 low alloy steel was used. F316L stainless steel and Alloy 82/182 were used as safe-end and weld metal, respectively. Although mechanical properties are needed for evaluation of the structural integrity against flaw in the material, material specification and standard don't supply those properties. Therefore, the present study conducted tensile and fracture toughness tests on SA508 Gr.3 and F316L stainless steel at ambient temperature and operating temperature of NPPs and reported the tested results.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.12
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• pp.1529-1534
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• 2012

In this study, tensile tests for 12-${\mu}m$-thick copper thin foils were performed by using the DIC method. The DIC method provided precise stress-strain curves for thin film materials, and a commercial inkjet printer can be simply and effectively used for printing speckle patterns on the specimen of Cu thin films whose surface contrast is too low to apply the DIC method. The mechanical properties of Cu thin foils obtained in this study are as follows: elastic modulus E = 89.2 GPa, 0.2% offset yield stress $S_{0.2%}$= 232.8 MPa, tensile strength $S_u$= 319.2 MPa, elongation at fracture ${\varepsilon}_f$=16.8 %, and Poisson's ratio ${\nu}$= 0.34.

• Journal of the Korean Geotechnical Society
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• v.24 no.4
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• pp.67-78
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• 2008

In this study, artificial cemented sand made of a few portland cement and Nak-Dong river sand was researched closely to investigate cementing effect quantitatively through unconfined tests and triaxial tests. The peak strength and elastic modulus increased and dilation of cemented sand was restricted by the cementation, but after breakage of the cementation, dilation and negative excess pore water pressure increased. In stress-strain curve, strain-softening behavior appeared in drained condition but strain-hardening behavior was appeared in undrained condition as a result of the increase of effective stress. The test was quantitatively analyzed by multiple regression models, correlating each response variable with input variable. The equations are valid only over the range investigated. Its adjusted coefficient of determination was $0.81{\sim}0.91$, and dry density is important factor for estimating strength of cemented sand.

• Journal of the Korea Concrete Institute
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• v.15 no.1
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• pp.110-116
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• 2003

Recently, as concrete structure becomes high rise and large scaled tendency, demands for high performance concrete such as high strength, high fluidity and high durability has been increased. Even though high performance concrete performs high strength, workability and durability, compared to with those of normal concrete, it is more brittle than normal concrete. Accordingly, this paper is intended to improve toughness and compressive strength through investigating the mechanical properties of the high performance concrete confined with metal lath, glass fiber and carbon fiber laterally in the case of 30% and 40% of W/B. According to the results, the compressive strength increases in order of metal lath, carbon fiber and glass fiber. Considering strain-stress curve with the kinds of material for lateral confinement, while brittleness failure occurs in plain concrete just after maximum load, it is improved in some degree in confined concrete due to increase of the strain by increase of toughness. Elastic modulus increases slightly in case of confined concrete, like the compressing strength.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.20 no.5
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• pp.641-652
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• 2007

In this paper, a half-scaled substructure test was performed to evaluate the buckling and structural safety of an existing transmission tower subjected to wind load. A loading scheme was devised to reproduce the dead and wind loads of a prototype transmission tower, which uses a triangular jig that is mounted on the reduced model to which the similarity law of a half length was applied. As a result of the preliminary numerical analysis carried out to evaluate the stability of a specimen for the design load, is was confirmed that the calculated axial forces of tower leg members were distributed to $80{\sim}90%$ of an admissible buckling load. When the substructured transmission tower was loaded by 270% of its maximum admissible buckling load, it was failed due to the local buckling that is occurred in joints with weak constraints for out-of-plane behavior of leg members. By inspection of load-displacement curves, displacements and strains of members, it is considered that this local buckling was due to additional eccentric force by unbalanced deformation because the time that is reached to yielding stress due to the bending moment is different at each point of a same section.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.35 no.4
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• pp.769-777
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• 2015

In this study, the rheological models were introduced and developed to reflect rate dependent tensile behaviour of concrete. In general, mechanical properties(e.g. strength, elasticity, and fracture energy) of concrete are increased under high loading rates. The strength of concrete shows high rate dependency among its mechanical properties, and the tensile strength has higher rate dependency than the compressional strength. To simulate the rate dependency of concrete, original spring set of RBSN(Rigid-Body- Spring-Network) model was adjusted with viscous and friction units(e.g. dashpot and Coulomb friction component). Three types of models( 1) visco-elastic, 2) visco-plastic, and 3) visco-elasto- plastic damage models) are considered, and the constitutive relationships for the models are derived. For validation purpose, direct tensile test were simulated, and characteristics of the three different rheological models were compared with experimental stress-strain responses. Simulation result of the developed visco-elasto-plastic damage(VEPD) model demonstrated well describing and fitting with experimental results.

• Magazine of the Korea Concrete Institute
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• v.5 no.2
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• pp.181-188
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• 1993

In this study, a numerical method for the material nonlinear analysis of reinforced concrete shell structures including the time dependent effects due to creep and shrinkage is developed. Degenerate shell elements with the layered approach are used. The perfect or strain hardening plasticity model in compression and the linearly elastic model in tension until cracking for concrete are employed. The reinforcing bars are considered as a steel layer of equivalent thickness. Each :steel layer has an uniaxial behaviour resisting only the axial force in the bar direction. A bilinear idealization is adopted to model elasto-plastic stress-strain relationships. For the nonlinear anaysis, incremental load method combined with unbalanced load iterations for each load increment is used. To include time dependent effects of concrete, time domain is divided into several time steps which may have different length. Some numerical examples are presented to study the validity and applicability of the present method. The results are compared with experimental and numerical results obtained by other investigator.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.10 no.3
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• pp.99-106
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• 1990

This study was aimed to see the effect of chemical grout on fresh granite joint shear strength. The grouting chemical used in this study was composed of 25% water glass. Direct shear tests were performed on the chemical filled joints, which had been made artificially with granite. The test results show that chemical grouted rock jonts have markedly reduced shear strength comparing with the ungrouted fresh joints and they sheared within chemical grout before the rock to rock contact had been established, while the ungrouted joint sheared between rock surfaces from the beginning of shear deformation. With chemical grouted joints the shear stress slowly reached its maximum without showing distinct peak shear strength. Therefore the shear stiffness of joints were decreased with increasing thickness of grout. but the shear strain at failure was increased with it.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.9 s.180
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• pp.2183-2190
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• 2000

A thermal aging is observed in a primary reactor cooling system(RCS) made of a casting stainless steel when the RCS is exposed for long period at the reactor operating temperature, 290~3300C An investigation of effects of thermal aging on a low cycle fatigue characteristics included a stress variations caused by a reactor operation and trip, is required. The purpose of the present investigation is to find an effect of a thermal aging of the CF8M on a low cycle fatigue life. The specimen of CF8M are prepared by an artificially accelerated aging technique holding 300 and 1800hr at 4300C respectively. The low cycle fatigue tests for the virgin and two aged specimens are performed at the room temperature for various strain amplitudes($\varepsilon$ta), 0.3, 0.5, 0.8, 1.0, 1.2 and 1.5% strain. Through the experiment, it is found that the fatigue life is rapidly reduced with an creasing of the aging time. The experimental fatigue life estimation formulas between the virgin and two aged specimen are obtained and are proposed to a analysis purpose.