• Journal of the Korea Concrete Institute
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• v.13 no.3
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• pp.228-236
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• 2001

Even though numerous researches have been performed for the prediction of thermal stresses in mass concrete structures by both analytical and experimental means, the limitations exist for both approaches. In analytical approach, the fundamental limitation is derived from the difficulty of predicting concrete properties such as modulus of elasticity, coefficient of thermal expansion, etc.. In experimental approach, there are many uncertainties related to in-situ conditions, because a majority of researches have focused on measuring thermal stresses in actual and simulated structures. In this research, an experimental device measuring thermal stresses directly in a laboratory setting is developed. The equipment is located in a temperature chamber that follows the temperature history previously obtained from temperature distribution analysis. Thermal strains are measured continuously by a strain gauge in the device and the corresponding thermal stresses are calculated simply by force equilibrium condition. For the verification of the developed device, a traditional experiment measuring thermal strains from embedded strain gauges is performed simultaneously. The results show that the thermal strain values measured by the newly developed device agree well with the results from the benchmark experiment.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.31 no.2
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• pp.97-104
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• 2018

In this study, a structure satisfying the fatigue constraint is designed by applying the topology optimization based on the phase field design method. In order to predict life based on the stress value, high cycle fatigue failure theory in which stress acts within the range of elastic limit is discussed and three fatigue theories of modified-Goodman, Smith-Watson-Topper and Gerber theory are applied. To calculate the global maximum stress, a modified P-norm stress correction method is used. As a result, it is possible to obtain topology optimization results that minimize the volume while satisfying the fatigue constraints. By applying the phase field design method, a simple shape with a minimized gray scale was obtained, and the maximum stress value acting on the optimization result became very close to the allowable stress value due to the modified P-norm stress method. While previous studies does not consider the stress correction factor, this study proposes the determination method regarding the stress correction factor considering loading effects related to axial stress components.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.8
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• pp.829-837
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• 2012

Direct numerical simulations were carried out for turbulent channel flows with $Re_{\tau}$ = 180, 395 and 590 to investigate the turbulent flow structure related to the Reynolds shear stress. By examining the probability density function, the second quadrant (Q2) events with the largest contribution to the mean Reynolds shear stress were identified. The change in the inclination angle of Q2 events varies with wall units in $y^+<50$ and with the channel half height in y/h > 0.5. Conditionally averaged flow fields for the Q2 event show that the flow structures associated with Reynolds shear stress are a quasi-streamwise vortex in the buffer layer and a hairpin-shaped vortex in the outer layer. Three-dimensional visualization of the distribution of high Reynolds shear stress reveals that the organization of hairpin vortices in the outer layer having a size of 1.5~3 h is associated with large-scale motions with high Reynolds shear stress in the outer layer.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.12 no.1
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• pp.75-82
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• 1999

일반 해양 구조물이나 해저면에 설치되는 해저 관로는 외력에 의한 변형이 발생된다. 구조물 형상이 복잡하거나, 구성 요소의 개수가 많을 경우 응력해석 시 많은 초기값이 필요하고 해석 시간 또는 장 시간 소요된다. 해양 구조물에 작용하는 대표적인 외력은 파도, 조류, 바람이고 이런 외력은 구조물의 사용 기간(operation life)동안 계속적으로 작용하기 때문에 구조물의 변형율은 항상 허용치 안에서 발생되도록 설계되어야 한다. 허용 변형은 탄성범위 내에 존재해야 하며, 비교적 큰 변형을 일으키는 구조물이나 해저파이프라인의 응력해석을 수치적으로 접근하는 방법을 고찰하였다. 평행상태의 하중 벡터값만 직각 좌표계에서 인트린직(intrinsic) 좌료로 변환시킬 때 변형이 발생함으로, 본 논문에서 소개하는 이차 요소(quadratic element)방법을 사용할 경우 수치해석 시 많은 장점이 있다는 것을 보여준다. 본 방법을 도입함으로써 비교적 큰 변형이 발생되는 구조물 해석 시 일반 수치해석 방법과 그 결과는 같으나 해석 시간을 단축시킬 수 있다는 장점이 있다. 응력 해석 시 국부 강도 행열(element stiffness matrix)은 방향과 무관하며 이차요소 방법을 사용하여 각 요소 벡터를 발생시켰다. 해저관로 일점 상승 시 관로에 작용하는 변형과 상승력에 따른 휨 모멘트를 산출하여 일반적으로 사용되는 선형이론과 비교하였다.

• Journal of the Korean Society for Nondestructive Testing
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• v.26 no.5
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• pp.336-342
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• 2006

Uniaxial stress in ail axisymmetric body is the simplest example of ultrasonic stress measurement. However, the birefringence theory cannot be applied for axisymmetric solids because the axisymmetric stress field in the body does not make shy velocity difference in SH waves propagating in the axisymmetric direction. Conventional ultrasonic technique using the time-of-flight method also needs ultrasonic lengths of the unstressed and stressed body, which is very impractical. In this paper, the birefringence effect in axisymmetric solids under uniaxial stress is formulated to evaluate the axial stress inside the solid without measuring tile ultrasonic length. Theoretical derivation for the birefringence characteristics in the axisymmetric solids is made using the longitudinal and shear waves instead of two horizontally polarized shear waves. Tension test is conducted for carbon-steel specimen to measure the birefringence coefficient and investigate the validity of the theory. It is observed from experimental results that the velocity difference in two differently polarized acoustic waves is proportional to the uniaxial stress in the axisymmetric solid with a good agreement with the theoretical value.

• Tunnel and Underground Space
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• v.14 no.3
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• pp.188-202
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• 2004

If an underground research facility for the validation of disposal concept is constructed in KAERI, it is expected to have a thick weathered zone and varying surface topology. In this study, the influence of different geological conditions, tunnel slope, tunnel size, and sequential excavation is investigated by 3D mechanical analysis using FLAC3D. Around the tunnel, it is not expected to develop any plastic zone and the maximum stress might be as high as 5 ㎫. The maximum compressive stress will be developed at about 20 m to e dead end of the tunnel. There is no difference on stress and displacement distributions between the cases with and without sequential excavation. It is expected to have stress release in the roof and floor after the excavation of the tunnel. There is no significant influence of weathered zone size, tunnel size, and tunnel slope on the stress and displacement distributions. The modeling for the intersection shows the minimum factor of safety is above 3, when the in situ stress ratio K is 3. From the study, it was possible to demonstrate that the small scale disposal research tunnel in KAERI will be mechanically stable.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.24 no.2
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• pp.149-156
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• 2011

In this paper, a methodology is proposed to improve the stress prediction of plates via Saint Venant's principle. According to Saint Venant's principle, the stress resultants can be used to describe linear elastic problems. Many engineering problems have been analyzed by Euler-Bernoulli beam(E-B) and/or Kirchhoff-Love(K-L) plate models. These models are asymptotically correct, and therefore, their accuracy is mathematically guaranteed for thin plates or slender beams. By post-processing their solutions, one can improve the stresses and displacements via Saint Venant's principle. The improved in-plane and out-of-plane displacements are obtained by adding the perturbed deflection and integrating the transverse shear strains. The perturbed deflection is calculated by applying the equivalence of stress resultants before and after post-processing(or Saint Venant's principle). Accuracy and efficiency of the proposed methodology is verified by comparing the solutions obtained with the elasticity solutions for orthotropic beams.

• Journal of the Korea institute for structural maintenance and inspection
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• v.16 no.3
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• pp.41-50
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• 2012

Improvement of steel material quality made fatigue problems more critical than failure of the material itself. In many cases, cracks on the welded parts of steel deck bridges are reported against the failure of steel materials. And the cracks are caused by alternate stress on the welded parts due to live loads on the bridge. The range of alternate stress on the welded part is related to property of the sections which compose othortropic steel deck. Othortropic steel deck is mainly composed of deck plate, ribs and floor beams, wearing surface, etc. In this paper, a methology to estimate the alternate stress for pthortropic steel deck using Pelikan-Esslinger method and signed Von-Mises equivalent stress is proposed first. Parametric study served references for fatigue stresses when designing or repairing othortropic steel deck bridges, by analyzing relationship between alternate stress range and properties of steel deck members.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.4A
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• pp.267-279
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• 2009

A numerical model that can simulate the nonlinear behavior of ultra high strength fiber reinforced concrete (UHSFRC) structures subjected to monotonic loading is introduced. The material properties of UHSFRC, such as compressive and tensile strength or elastic modulus, are different from normal strength reinforced concrete. The uniaxial compressive stress-strain relationship of UHSFRC is designed on the basis of experimental result, and the equivalent uniaxial stress-strain relationship is introduced for proper estimation of UHSFRC structures. The steel is uniformly distributed over the concrete matrix with particular orientation angle. In advance, this paper introduces a numerical model that can simulate the tension-stiffening behavior of tension part of the axial member on the basis of the bond-slip relationship. The reaction of steel fiber is considered for the numerical model after cracks of the concrete matrix with steel fibers are formed. Finally, the introduced numerical model is validated by comparison with test results for idealized UHSFRC beams.

• Proceedings of the International Microelectronics And Packaging Society Conference
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• 2003.11a
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• pp.265-269
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• 2003

Sn(또는 SnAg)/Ni(P)와 Sn/Cu 계의 열사이클동안 형성되는 금속간화합물에 의해 유기되는 응력의 변화를 in-situ로 관찰하였다. Sn(또는 SnAg)/Ni(11.7P) 박막은 계면반응으로 인해 $Ni_3P$와 $Ni_3Sn_4$ 상이 형성되고 이때 인장응력이 발생하였으며, 한편, Sn(또는 SnAg)/Ni(3P) 박막의 계면반응에 의해서는 동일한 $Ni_3P$와 $Ni_3Sn_4$ 상이 형성됨에도 불구하고 압축응력이 발생하였다. SmAg를 사용할 때 형성되는 $Ag_3Sn$이 응력에 미치는 영향은 거의 없었다. Sn/Cu 박막의 경우는 계면반응 초기에는 인장응력이 발생하였고 어느 정도 이상 반응이 진전됨에 따라 압축응력이 발생하였고 최종적으로 $Cu_3Sn$ 상이 형성되었다. 초기의 인장응력은 계면에서 원자들의 intermixing 베 의한 것이고 압축응력은 Sn 방향으로 일방향 성장하는 금속간화합물 형성에 기인한다.