• Korean Journal of Metals and Materials
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• v.49 no.10
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• pp.760-765
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• 2011

This paper presents a study of the tensile behavior of carbon and glass fiber reinforced epoxy hybrid laminates manufactured by vacuum assisted resin transfer molding (VARTM). The objective of this study was to develop and characterize carbon fiber reinforced plastic hybrid composite material that is low cost and light-weight and that possesses adequate strength and stiffness. The effect of position and content of the glass fabric layer on the tensile properties of the hybrid laminates was examined. The strength and stiffness of the hybrid laminates showed a steady decrease with an increase of the glass fabric content this decrease was almost linear. Fracture strain of these laminates showed a slight increasing trend when glass fabric content was increased up to 3 layers, but at a glass fabric content > 3 layers the strain was almost constant. When glass fabric layers were at both outer surfaces, the hybrid laminate exhibited a slightly higher tensile strength and elastic modulus due to the small amount of glass yarn pull-out.

• Journal of the Korean Geotechnical Society
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• v.16 no.4
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• pp.95-105
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• 2000

Anisotropy of stiffiness, from extremely small strains to post-failure strains, of isotropically consolidated air-pulviated sands in plane strain compression was studied by using the newly developed instrumentation for small strain measurements. Seven types of sand of the world-wide origins were tested, which have been extensively used for research purposes. Stress-strain relationships for a wide range of strain from about 0.0001% to 10% were obtained with measuring axial and lateral strains locally free from the effects of bedding and membrane penetration errors at the specimen boundaries. It was found that the maximum shear modulus Gmax was irrespective of the angle $\delta$of the $\sigma$1 direction relative to the bedding plane. However, the normalized Gmax was varied with the types of sand. Furthermore, the dependency of the strain and stress level on the stiffness increased as decreased.

• Proceedings of the Korean Geotechical Society Conference
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• 2008.03a
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• pp.71-77
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• 2008

Microstructural changes may arise due to the particle vanishing, fluid diffusion, heating, etc. This study focuses on the changes in small-strain shear stiffness in k0 loading produced by local straining in particular system made of sand-salt mixtures. Local strains were induced by dissolution of salt particles. Experiments were carried out in a conventional oedometer cell equipped with bender elements. Axial displacement and shear wave signals are recorded at each loading stage and during saturation process. Experimental data showed that microstructural changes due to particle vanishing were clearly captured by using shear wave measurement. Saturation of sand-salt mixture at a larger axial stress did not always create a more condense soil at the end of loading stage. Sand-salt mixture is useful for laboratory test on controlled artificial specimen.

• Geomechanics and Engineering
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• v.36 no.3
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• pp.277-293
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• 2024

Shored Mechanically Stabilized Earth (SMSE) walls are types of soil retaining structures that increase soil stability under static and dynamic loads. The damage caused by an earthquake can be determined by evaluating the probabilistic seismic response of SMSE walls. This study aimed to assess the seismic performance of SMSE walls and provide fragility curves for evaluating failure levels. The generated fragility curves can help to improve the seismic performance of these walls through assessing and controlling variables like backfill surface settlement, lateral deformation of facing, and permanent relocation of the wall. A parametric study was performed based on a non-linear elastoplastic constitutive model known as the hardening soil model with small-strain stiffness, HSsmall. The analyses were conducted using PLAXIS 2D, a Finite Element Method (FEM) program, under plane-strain conditions to study the effect of the number of geogrid layers and the axial stiffness of geogrids on the performance of SMSE walls. In this study, three areas of damage (minor, moderate, and severe) were observed and, in all cases, the wall has not completely entered the stage of destruction. For the base model (Model A), at the highest ground acceleration coefficient (1 g), in the moderate damage state, the fragility probability was 76%. These values were 62%, and 54%, respectively, by increasing the number of geogrids (Model B) and increasing the geogrid stiffness (Model C). Meanwhile, the fragility values were 99%, 98%, and 97%, respectively in the case of minor damage. Notably, the probability of complete destruction was zero percent in all models.

• Structural Engineering and Mechanics
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• v.5 no.6
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• pp.715-728
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• 1997

The aim of this paper is to investigate the secondary buckling behaviour and mode-coupling of spherical caps under uniformly external pressure. The analysis makes use of a rotational finite shell element on the basis of strain-displacement relations according to Koiter's shell theory (Small Finite Deflections). The post-buckling behaviours after a bifurcation point are analyzed precisely by considering multi-mode coupling between several higher order harmonic wave numbers: and on the way of post-buckling path the positive definiteness of incremental stiffness matrix of uncoupled modes is examined step by step. The secondary buckling point that has zero eigen-value of incremental stiffness matrix and the corresponding secondary mode are obtained, moreover, the secondary post-buckling path is traced.

• Proceedings of the KSR Conference
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• 2006.11b
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• pp.31-34
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• 2006

A new continuum damage theory (CDT) has been proposed by Lee et al. (1996) based on the SEEP. The CDT has the apparent advantage over the other related theories because the complete constitutive law can be readily derived by simply replacing the virgin elastic stiffness with the effective orthotropic elastic stiffness obtained by using the proposed continuum damage theory. In this paper, the CDT is evaluated by the numerical experiment comparing the mode shapes and natural frequencies of a square plate containing a small line-through crack with those of the same plate with a damaged site replaced with the effective orthotropic elastic stiffness computed by using the CDT.

• Structural Engineering and Mechanics
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• v.51 no.4
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• pp.601-625
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• 2014

Based on continuum mechanics and the principle of virtual displacements, incremental total Lagrangian formulation (T.L.) and incremental updated Lagrangian formulation (U.L.) were presented. Both T.L. and U.L. considered the large displacement stiffness matrix, which was modified to be symmetrical matrix. According to the incremental updated Lagrangian formulation, small strain, large displacement, finite rotation of three dimensional Timoshenko fiber beam element tangent stiffness matrix was developed. Considering large displacement and finite rotation, a new type of tangent stiffness matrix of the beam element was developed. According to the basic assumption of plane section, the displacement field of an arbitrary fiber was presented in terms of nodal displacement of centroid of cross-area. In addition, shear deformation effect was taken account. Furthermore, a nonlinear finite element method program has been developed and several examples were tested to demonstrate the accuracy and generality of the three dimensional beam element.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.148-151
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• 2004

Mechanical properties such as Young's modulus and hardness of thin film in coated steel are difficult to determine by nano-indentation from the conventional analysis using the load-displacement curve. Therefore, an analysis of the nano-indentation loading curve was used to determine the Young's modulus, hardness and strain hardening exponent. A new method is recently being developed for plasticity properties of materials from nano-indentation. Elastic modulus of the thin films shows relatively small influence whereas yield strength and strain hardening are found to have significant effect on measured data. The load-displacement behavior of material tested with a Berkovich indenter and nano-indentation continuous stiffness method is used to measure the modulus and hardness through thin films.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1991.10a
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• pp.133-138
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• 1991

The aim of the present paper is to develop a computer program predicting ultimate fracture strength of initially cracked structure under monotonically increasing external loads. For this purpose, two kinds of 3-D isoparametric solid elements, one 6-node wedge element and another 8-node brick element are formulated along the small deformation theory. Plasticity in the element is checked using von Mises' yield criterion. Elasto-plastic stiffness matrix of the element is calculated taking account of strain hardening effect. If the principal strain at crack tip which is one nodal point exceeds the critical strain dependin on the material property, crack tip is supposed to be opened and the crack tip node which was previously constrained in the direction perpendicular to the crack line is released. After that, the crack lay be propagated to the adjacent node. Once a crack tip node is fractured, the energy of the newly fractured node should be released which is to be absorbed by the remaining part. The accumulated reaction force which was carried by the newly fractured node so far is then applied in the opposite direction. During the action of crack tip relief force, since unloading may be occured in the plastic element, unloading check should be made. If a plastic element unloads, elastic stress-strain equation is used in the calculation of the stiffness matrix of the element, while for a loading element, elasto-plastic stress-strain equation is continuously used. Verification of the computer program is made comparing with the experimental results for center cracked panel subjected to uniform tensile load. Also some factors affecting ultimate fracture strength of initially cracked plate are investigated. It is concluded that the computer program developed here gives an accurate solution and becomes useful tool for predicting ultimate fracture load of initially cracked structural system under monotonically increasing external loads.

• Journal of the Earthquake Engineering Society of Korea
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• v.6 no.4
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• pp.39-49
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• 2002

In this paper, the characteristics of RB(rubber bearing) were studied by various prototype tests on RB with low hardness rubber. The characteristics of RB were tested on displacements, repeated cycles, frequencies, vertical pressures, temperature, vertical stiffness and the capability of shear deformation. The prototype test showed that the displacement and vertical pressures were the most governing factors influencing on characteristics of RB. The effective stiffness and equivalent damping of RB showed small increment in high frequency range. After the repeated cyclic test with 50's cycles, the effective stiffness and equivalent damping of RB were almost constant compared with those of the 1st cycles due to low hysteretic damping. The shear modulus of RB was reduced after large deformation, and this value of RB was partly recovered after 40 days. Finally, the shear failure test of RB was conducted, the prototype was failed over 490% of shear strain, and real size RB was failed over 430% of shear strain.