• Steel and Composite Structures
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• 제48권3호
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• pp.293-303
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• 2023

In this paper, geometrically nonlinear free vibration analysis of Mindlin viscoelastic plates with various geometrical and material properties is studied based on the Von-Karman assumptions. A novel solution is proposed in which the nonlinear frequencies of time-dependent plates are predicted according to the nonlinear frequencies of plates not dependent on time. This method greatly reduces the cost of calculations. The viscoelastic properties obey the Boltzmann integral law with constant bulk modulus. The SHPC meshfree method is employed for spatial discretization. The Laplace transformation is used to convert equations from the time domain to the Laplace domain and vice versa. Solving the nonlinear complex eigenvalue problem in the Laplace-Carson domain numerically, the nonlinear frequencies, the nonlinear viscous damping frequencies, and the nonlinear damping ratios are verified and calculated for rectangular, skew, trapezoidal and circular plates with different boundary conditions and different material properties.

• 자원환경지질
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• 제56권1호
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• pp.13-21
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• 2023

합성 제올라이트 X의 고온 고압 하에서 압력 전달 매개체에 따른 체적탄성계수와 상전이 특성을 이해하기 위해 X-선 분말 회절 연구를 진행하였다. 제올라이트 X에 물과 탄산 용액을 압력전달매개체로 사용하여 상온 상압에서 최대 250 ℃, 5.18 GPa까지 가열 및 가압하는 과정에서 나타나는 단위포 부피 변화와 상전이를 방사광 X-선 회절을 통해 관찰하였다. 르바일의 전체 분말 패턴 분해법이 적용된 GSAS2 프로그램을 사용하여 각 압력 단계에서 제올라이트 X와 그멜리나이트, 나트로라이트, 스멕타이트의 격자상수와 단위포 부피를 도출하였다. 버치-머내한 2차 방정식이 적용된 EosFIt 프로그램을 사용하여 각 제올라이트 X와 스멕타이트의 체적탄성계수를 구하였다. 물을 사용한 실험에서 제올라이트 X의 체적탄성계수는 89(3) GPa, 탄산 수용액을 사용한 실험에서 제올라이트 X의 체적탄성계수는 92(3) GPa이다. 두 실험 모두 최초 가압 시 제올라이트 X 내부로 압력 전달 매개체의 유입으로 인한 부피 증가 현상이 발생하였다. 물을 사용한 실험에서 제올라이트 X는 그멜리나이트, 나트로라이트, 스멕타이트로 상전이 하였으며, 탄산 수용액을 사용한 실험에서 제올라이트 X는 스멕타이트로 상전이 하였다. 물을 사용한 실험에서 나타난 제올라이트 간 변화는 탄산 수용액을 사용한 실험에서는 발생하지 않았으며, 이는 특정 제올라이트 생성 조건이 압력 전달 매개체 pH와 연관이 있는 것으로 판단된다.

• Structural Engineering and Mechanics
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• 제12권6호
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• pp.657-668
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• 2001

Fiber reinforced cementitious composites are nowadays widely applied in civil engineering. The postcracking performance of this material depends on the interaction between a steel fiber, which is obliquely across a crack, and its surrounding matrix. While the partly debonded steel fiber is subjected to pulling out from the matrix and simultaneously subjected to transverse force, it may be modelled as a Bernoulli-Euler beam partly supported on an elastic foundation with non-linearly varying modulus. The fiber bridging the crack may be cut into two parts to simplify the problem (Leung and Li 1992). To obtain the transverse displacement at the cut end of the fiber (Fig. 1), it is convenient to directly solve the corresponding differential equation. At the first glance, it is a classical beam on foundation problem. However, the differential equation is not analytically solvable due to the non-linear distribution of the foundation stiffness. Moreover, since the second order deformation effect is included, the boundary conditions become complex and hence conventional numerical tools such as the spline or difference methods may not be sufficient. In this study, moment equilibrium is the basis for formulation of the fundamental differential equation for the beam (Timoshenko 1956). For the cantilever part of the beam, direct integration is performed. For the non-linearly supported part, a transformation is carried out to reduce the higher order differential equation into one order simultaneous equations. The Runge-Kutta technique is employed for the solution within the boundary domain. Finally, multi-dimensional optimization approaches are carefully tested and applied to find the boundary values that are of interest. The numerical solution procedure is demonstrated to be stable and convergent.

• 한국세라믹학회지
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• 제21권1호
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• pp.27-32
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• 1984

During the last decade there have been many studies on the new ceramics especially engineering ceramics. Sintered silicon carbide is one of the main materials in engineering ceramics. This study shows the effects of surface treatment and microstructure especially the abnormal grain growth on the strength of sintered SiC. Surface of sintered SiC and treated with 400, 800 and 1200 grit diamond wheel. Grain growth is introduced by increasing the sintering times at 205$0^{\circ}C$. The $\beta$longrightarrow$\alpha$ transformation occurs during the sintering of $\beta$-starting materials and is often accompanied by abnormal grain growth. The overall strength distribution are estimated using the Weibull statistics. The results show that the strength of sintered SiC is limited by extrinsic surface flaws in normal-sintered specimens. And it is sound that the finer the surface finishing and the grain size the higher the strength results. But the strength of abnormal sintering specimens is limited by the abnormally-grown large tabular grains. The Weibull modulus increases with the decreasing grain size and the decreasing grit size of grinding.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2006년도 춘계 학술발표회 논문집
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• pp.888-895
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• 2006

In this study, anisotropically consolidated undrained shear(CAU) test was performed to examine the variation of the shear strength according to the stress history. The specimen having 30% relative density was homogeniously prepared, and 200KPa of back pressure was applied to increase the B value more than 0.95. To make NC specimens, the vertical stress was applied on the specimen by 100KPa, 200KPa and 400KPa, and to make OC specimens, the vertical stress was applied upto 400KPa and was reduced to 200KPa and 100KPa resulting in OCR 2 and 4 respectively. The test result indicates the shear strength for the OC specimens are slightly higher then that of the NC specimens at the same confining pressure. The elastic modulus varies according to the confining stress and considerably affected by preconsolidation stress.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2006년도 춘계 학술발표회 논문집
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• pp.808-817
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• 2006

There were huge damages of human beings and their properties in many areas of the basin of the Nak-Dong river by the unusual weather and the localized downpour recently. In this research against disasters, we want to know the special quality of strength of the cemented sand that is mixed with cement and poor-graded sand which is the materials of riverbed in the basin of the Nak-Dong river as levee's material. For that, we want to provide the fundamental data which need in the examination of adaptation of levee's material, design and analysis by investigating compressive strength by curing period and cement ratio, elastic modulus and stress by transformation from compaction test, CBR test, unconfined compression test, triaxial compression test as changing ratio of sand and cement from 2% to 8% at two points in the basin of the Nak-Dong river.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2005년도 춘계 학술발표회 논문집
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• pp.916-923
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• 2005

In this study, to observe aging effect of undrained shear behavior for Nak-Dong River sand, undrained static and cyclic triaxial tests were performed with changing relative density ($D_r$), consolidation stress ratio($K_c$) and consolidation time. As a result of the test, the modulus of elasticity to all samples estimated within elastic zone by the micro strain of about 0.05% in case of static shear behavior increased with the lapse of consolidation time significantly, so aging effect was shown largely. Also strength of phase transformation point(S_{PT}$) and strength of critical stress ratio point($S_{CSR}$) increased with the lapse of consolidation time. Undrained cyclic shear strength($R_f$) obtained from the failure strain 5% increased in proportion to relative density($D_r$) and initial static shear stress($q_{st}$), $R_f$ of consolidated sample for 1,000 minutes increased about 10.6% compared to that for 10 minutes at the loose sand, and $R_f$ increased about 7.0% at the medium sand. In situ application range of $R_f$ to the magnitude of earthquake for Nak-Dong River sand was proposed by using a increasing rate of $R_f$ as being aging effect shown from this test result.

• Composites Research
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• 제15권6호
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• pp.30-37
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• 2002

꼰 섬유(twisted yarn)로 이루어진 복합재료의 탄성계수를 예측하기 위한 강성모델을 제시하였다. 이 모델은 단위 셀 구조를 바탕으로 하여 좌표변환과 구성 재료의 연성 상수를 체적 평균함으로써 복합재료의 탄성계수를 예측하는 방법이다. 해석적인 결과와 실험치와의 비교를 위하여 두 종류의 꼬임 각을 가지는 섬유로 구성된 복합재료에 대한 시험을 하였으며 인장, 압축, 전단 강도와 탄성계수를 구하였다. 이 시험편은 유리섬유 및 에폭시 수지를 사용한 RTM 성형법으로 제조하였다. 탄성계수에 대한 비교 곁과 예측치와 실험치는 비교적 잘 일치하였다. 이 모델에 의해 구해진 3차원적인 탄성계수 값은, 꼬인 섬유로 이루어진 직조형 복합재료 구조물의 구조해석을 위한 기본 물성치 입력값으로 사용될 수 있다.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2003년도 춘계학술발표대회 논문집
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• pp.18-21
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• 2003

The effect of interface bonding strength on the recovery force of SMA wire reinforced polymer matrix composites was investigated by pullout test. Firstly, the recovery forces and transformation temperatures of various prestrained SMA wires were measured and 5% prestrained SMA wires were prepared for the reinforcements of composites. EPDM incorporated with 20vol% silicon carbide particles(SiCp) of 6, 12, $60{mutextrm{m}}$ size were used as matrix. Pullout test results showed that the interface bonding strength increased when the SiCp size decreased due to the increase of elastic modulus of matrix. Cyclic test of composites was performed through control of DC current at the constant displacement mode. The abrupt decrease of recovery force during cycle test at high current was occurred by thermal degradation of matrix. This was in good agreement with temperature related in the thermal degradation of matrix. The hysteresis of recovery force with respect to the temperature was compared between wire and composite and the hysterisis of composites was smaller than the wire due to less thermal conduction.

• 한국자동차공학회논문집
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• 제18권2호
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• pp.61-66
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• 2010

A virtual manufacturing system that is composed of JMatPro, a material modeler and $DEFORM^{TM}$-HT, a finite element package is applied to the hot press forming process: high temperature material properties for each phase such as flow stress, elastic modulus, Poisson's ratio, thermal expansion coefficient, in addition to TTT curve are predicted by JMatPro and taken into $DEFORM^{TM}$-HT to predict the material behavior considering phase transformation and heat transfer simultaneously. In order to verify the accuracy of computation, the residual stress and the springback were compared with the experimental measurements. Both the predicted and measured principal residual stresses and amount of springback were in good agreement. It was also found that the residual stresses generated from hot press forming are not negligible as it has been generally assumed, although the springback deformation is quite small.