• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.9
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• pp.859-869
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• 2009

The sharp indenters such as Berkovich and conical indenters have a geometrical self-similarity in theory, but different materials have the same load-depth curve in case of single indentation. In this study, we analyze the load-depth curves of conical indenter with angles of indenter via finite element method. From FE analyses of dual-conical indentation test, we investigate the relationships between indentation parameters and load-deflection curves. With numerical regressions of obtained data, we finally propose indentation formulae for material properties evaluation. The proposed approach provides stress-strain curve and the values of elastic modulus, yield strength and strain-hardening exponent with an average error of less than 2%. It is also discussed that the method is valid for any elastically deforming indenters made of tungsten carbide and diamond for instance. The proposed indentation approach provides a substantial enhancement in accuracy compared with the prior methods.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.5
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• pp.657-664
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• 2013

By using shear mixing and ultrasonication, we fabricated specimens of well-dispersed multi-walled carbon nanotube composites. To confirm the proper dispersion of the filler, we used scanning electron microscopy images for quantitative evaluation and a tensile test for qualitative assessment. Furthermore, the coefficients of thermal expansion of several specimens having different filler contents were calculated from the measured thermal strains and temperatures of the specimens. Based on the microscopy images of the well-dispersed fillers and the small deviations in the measurements of the tensile strength and stiffness, we confirmed the proper dispersion of nanotubes in the epoxy. As the filler contents were increased, the values of tensile strength increased from 58.33 to 68.81 MPa, and those of stiffness increased from 2.93 to 3.27 GPa. At the same time, the coefficients of thermal expansion decreased. This implies better thermal stability of the specimen.

• Tunnel and Underground Space
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• v.25 no.1
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• pp.24-36
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• 2015

The Geochang granite widely used in construction works is one of the most popular dimension stones in Korea. In order to evaluate the physical properties of rock, a lot of laboratory tests for the Geochang granite were conducted to find unit weight, absorption ratio, P wave velocity, S wave velocity, uniaxial compressive strength, Young's modulus, Poisson's ratio, tensile strength, cohesion, friction angle and point load strength index. The uniaxial compressive strength of the Geochang granite was 19.5 times tensile strength and also 8.6 times cohesion, besides P wave velocity was 1.5 times S wave velocity. Correlation analyses were also conducted to find the correlation among 11 different physical properties, where the uniaxial compressive strength showed Pearson correlation coefficient of more than 0.8 with Poisson's ratio, point load strength index and Young's modulus, respectively. Regression analyses were finally conducted by means of both linear and multiple analysis and the brief results including coefficient of determination of more than 0.7 were presented.

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.5
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• pp.390-396
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• 2000

The dynamic elastic constants of the simulated weld HAZ (heat-affected zone) of SA 508 Class 3 reactor pressure vessel (RPV) steel were investigated by resonant ultrasound spectroscopy (RUS). The resonance frequencies of rectangular parallelepiped samples woe calculated from the initial estimates of elastic stiffness $c_{11},\;c_{12}\;and\;c_{44}$ with an assumption of isotropic property, dimension and density. Through the comparison of calculated resonant frequencies with the measured resonant frequencies by RUS, very accurate elastic constants of SA 508 Class 3 steel were determined by iteration and convergence processes. Clear differences of Youngs modulus and shear modulus were shown from samples with different thermal cycles and microstructures. Youngs modulus and shear modulus of samples with fine-grained bainite were higher than those with coarse-grained tempered martensite. This tendency was confirmed from other results such as micro-hardness test.

• Composites Research
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• v.33 no.4
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• pp.185-190
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• 2020

Titanium is widely used as an implant material due to its excellent biocompatibility, but has a problem due to high cost and high Young's modulus compared to bone. Magnesium alloy is attracting attention as a material to replace it. Magnesium alloy, like titanium, has excellent biocompatibility and has a Young's modulus similar to that of bone. However, there are corrosion resistance problems due to corrosion, and various surface treatment methods are being studied to solve them. In this study, the ceramic coating layer was grown on the surface of the AZ31 magnesium alloy in an electrolytic solution containing EDTA, and the properties of the formed coating were analyzed through SEM and XRD to analyze the microstructure and shape, and measured the micro hardness of the coating layer. Corrosion properties in the body were evaluated through a corrosion test in SBF solution, a component similar to blood plasma.

• Composites Research
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• v.33 no.4
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• pp.198-204
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• 2020

In this study, we employ the full atomistic molecular dynamics simulation and finite element homogenization method to predict the thermo-mechanical properties of nanocomposites including carbon nanotube bundle. As the number of carbon nanotubes within the single bundle increases, the effective in-plane Young's modulus and in-plane shear modulus decrease, and in-plane thermal expansion coefficient increases, despite the same volume fraction of carbon nanotubes. To investigate the thickness of interphase zone, we employ the radial density distribution. It is investigated that the interphase thickness is almost independent on the number of carbon nanotubes within the single bundle. It is assumed that the matrix and interphase are isotropic materials. According to the predicted thermo-mechanical properties of interphase zone, the Young's modulus and shear modulus of interphase zone clearly decrease, and the thermal expansion coefficient increases. Based on the thermo-mechanical interphase behavior, we developed the multiscale homogenization model to predict the thermo-mechanical properties of PLA nanocomposites that include the carbon nanotube bundle.

• Composites Research
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• v.36 no.5
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• pp.369-376
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• 2023

This study focuses on investigating the influence of epoxy polymer crosslinking density, a crucial aspect in composite material matrices, on the yield surface using molecular dynamics simulations. Our approach involved generating epoxy models with diverse crosslinking densities and subjecting them to both uniaxial and multiaxial deformation simulations, accounting for the elasto-plastic deformation behaviors. Through this, we obtained key mechanical parameters including elastic modulus, yield point, and strain hardening coefficient, all correlated with crosslinking conversion ratios. A particularly noteworthy finding is the rapid expansion of the yield surface in the biaxial compression region with increasing crosslinking ratios, compared to the uniaxial tensile region. This unique behavior led to observable yield surface variations, indicating a significant pressure-dependent relationship of the yield surface considering plastic strain and crosslinking conversion ratio. These results contribute to a deeper understanding of the complex interplay between crosslinking density and plastic mechanical response, especially in the aspect of multiaxial deformation behaviors.

• Korean Journal of Remote Sensing
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• v.35 no.6_1
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• pp.933-944
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• 2019

This study analyzes the tide deformation of land boundary regions on the east (Region A) and west (Region B) sides of the Ross Ice Shelf in Antarctica using Double-Differential Interferometric Synthetic Aperture Radar (DDInSAR). A total of seven Sentinel-1A SAR images acquired in 2015-2016 were used to estimate the accuracy of tide prediction model and Young's modulus of ice shelf. First, we compared the Ross Sea Height-based Tidal Inverse (Ross_Inv) model, which is a representative tide prediction model for the Antarctic Ross Sea, with the tide deformation of the ice shelf extracted from the DDInSAR image. The accuracy was analyzed as 3.86 cm in the east region of Ross Ice Shelf and it was confirmed that the inverse barometric pressure effect must be corrected in the tide model. However, in the east, it is confirmed that the tide model may be inaccurate because a large error occurs even after correction of the atmospheric effect. In addition, the Young's modulus of the ice was calculated on the basis of the one-dimensional elastic beam model showing the correlation between the width of the hinge zone where the tide strain occurs and the ice thickness. For this purpose, the grounding line is defined as the line where the displacement caused by the tide appears in the DDInSAR image, and the hinge line is defined as the line to have the local maximum/minimum deformation, and the hinge zone as the area between the two lines. According to the one-dimensional elastic beam model assuming a semi-infinite plane, the width of the hinge region is directly proportional to the 0.75 power of the ice thickness. The width of the hinge zone was measured in the area where the ground line and the hinge line were close to the straight line shown in DDInSAR. The linear regression analysis with the 0.75 power of BEDMAP2 ice thickness estimated the Young's modulus of 1.77±0.73 GPa in the east and west of the Ross Ice Shelf. In this way, more accurate Young's modulus can be estimated by accumulating Sentinel-1 images in the future.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2009.04a
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• pp.243-246
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• 2009

본 연구에서는 분자동역학 전산모사와 미시역학 모델을 이용하여 나노입자의 크기와 체적분율 변화가 나노복합재의 물성변화에 미치는 영향을 효과적으로 묘사할 수 있는 순차적 브리징 해석기법을 개발하였다. 나노 입자의 크기변화와 체적분율 변화에 따른 영률과 전단계수를 분자동역학 전산모사를 통해 예측한 후, 이를 연속체 모델에서 구현하기 위해 다중입자 모델을 적용하였다. 나노입자의 크기효과를 반영하기 위해 입자와 기지 사이에 유효계면을 추가적인 상으로 도입하였고, 체적분율 효과는 나노복합재를 둘러싸는 무한영역의 물성값을 통해 조절되도록 하였다. 유효계면과 무한영역의 물성을 입자의 반경과 체적분율의 함수로 근사한 후, 다양한 입자의 크기와 체적분율에서 나타나는 나노복합재의 물성변화를 예측하였다. 제안된 해석기법의 적용을 통해 분자동역학 전산모사 결과와 잘 일치하는 예측해를 효과적으로 얻을 수 있었다.

• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.144-144
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• 2003

Fe-Co-Ge 자기변형 합금 복합체는 낮은 자기장에서도 높은 자기 변형 민감도와 고분자 바인더에 의한 절연으로 인하여 저항이 크고, 와전류 손실이 작아 고주파영역 에서도 우수한 자기변형을 가지는 것으로 보고되고 있다. 그러나 희토류계 자기변형 복합체에 비하여 그 성능은 열세이지만, 제조 단가가 저렴하여 높은 성능/가격 대비 효과를 가지므로 초음파 발진소자와 같은 대량의 상업적 응용분야에 적용 가능한 소재로 있다. [1]. 한편 이와 같은 자기 변형 복합체는 고분자 바인더와 결합되어 있어 그 기계적 특성의 향상과 사용주파수 대역의 증가가 요구되어진다. 이에 이들 문제점을 보안하기 위하여 670 GPa의 큰 영률을 갖는 WC 분말의 첨가에 의한 합금복합체의 기계적 및 자기변형 (정적 및 동적) 특성에 미치는 영향에 관하여 연구하고자 하였다.