• Geomechanics and Engineering
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• 제8권1호
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• pp.33-52
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• 2015

Based on the Mohr-Coulomb failure criterion, a gradient-dependent plastic model that considers the strain-softening behavior is presented in this study. Both triaxial shear tests on conventional specimen and precut-specimen, which were obtained from an ancient landslide, are performed to plot the post-peak stress-strain entire-process curves. According to the test results of the soil strength, which reduces from peak to residual strength, the Mohr-Coulomb criterion that considers strain-softening under gradient plastic theory is deduced, where strength reduction depends on the hardening parameter and the Laplacian thereof. The validity of the model is evaluated by the simulation of the results of triaxial shear test, and the computed and measured curves are consistent and independent of the adopted mesh. Finally, a progressive failure of the ancient landslide, which was triggered by slide of the toe, is simulated using this model, and the effects of the strain-softening process on the landslide stability are discussed.

• 대한기계학회논문집
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• 제7권3호
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• pp.270-277
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• 1983

Plastic plane stress solutions are given for a center cracked strip, characterized by the Ramberg-Osgood plastic index, under bi-axial tension. Using a power law hardening stress-strain relation, an incremental plasticity finite element formulation is developed, and simple formulation is given for computing J-integral with nodal displacements. The near tip angular distribution of von Mises effective stress doesn't differ significantly in magnitude according to the change of loading stress and bi-axial load combination factor. But, for smaller plastic index, the location of its maximum value moves vertically at a head of crack. J-integral value, in the plastic zone near crack tip, decreases with load combination factor for large and small plastic index.

• 산업기술연구
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• 제26권A호
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• pp.69-73
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• 2006

Mach peening treatment is one of the various kinds of techniques to improve the fatigue properties. The mach peening process gives high-level work hardening and compressive residual stress near the surface layer, improving the fatigue strength. In addition, this treatment reduces slip bands that initiate the fatigue cracks near the surface. During impingement, a plastic indentation surrounded by a plastic zone is formed. Mach peening treatment characteristic is less energy consumption and is an environmental friendly processing methods that is not accompanied by pollution. It is machining process that can prevent fatigue fracture beforehand in structure using already as well as process of production. The test results showed that fatigue crack propagation delay appeared by drilling type 43%, mach peening type 110%.

• 한국해양공학회지
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• 제14권1호
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• pp.44-51
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• 2000

Safety and integrity are required for reactor pressure vessels because they are operated in high temperature. There are single specimen method multiple specimen method and load ratio analysis method which used as evaluation of safety and integrity for reactor pressure vessels. In this study the fracture resistance curve(J-R curve) elastic-plastic fracture toughness($J_{IC}$) and material tearing modulus ($T_{mat}$) of SA 508 class 3 alloy steel used as reactor pressure vessel steel are measured and evaluated at room temperature 20$0^{\circ}C$ and 30$0^{\circ}C$ according to unloading compliance method and load ration analysis method. And then the comparison with experimental $J_{IC}$ and theoretical$J_{IC}$ by local fracture strain is managed.

• International Journal of Naval Architecture and Ocean Engineering
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• 제8권4호
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• pp.360-374
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• 2016

The present paper studies the ultimate torsional strength of stiffened box girders with large deck opening due to the influence of cracks. Three types of hull girders with different spans are provided for comparison. Potential parameters which may have effects on the torsional strength including the mesh refinement, initial deflection, material strain hardening, geometric properties of crack and stiffener are discussed. Two new concepts that play an significant role in the ultimate strength research of damaged box girders are introduced, one of which is the effective residual section (ERS), the other is the initial damage of the failure zone (IDFZ) for intact structures. New simple formulas for predicting the residual ultimate torsional strength of cracked stiffened box girders are derived on the basis of the two new concepts.

• International Journal of Naval Architecture and Ocean Engineering
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• 제9권1호
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• pp.66-76
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• 2017

This paper is concerned with investigating the plastic material properties of steel plate formed by line heating method, and is aimed at implementing more rational design considering the accidental limit states such as collision or grounding. For the present study, line heating test for marine grade steel plate has been carried out with varying plate thickness and heating speed, and then microscopic examination and tensile test have been carried out. From the microscopic, it is found that the grain refined zones like ferrite and pearlite are formed all around the heat affected zone. From the tensile test results, it is seen that yield strength, tensile strength, fracture strain, hardening exponent and strength coefficient vary with plate thickness and heat input quantity. The formulae relating the material properties and heat input parameter should be, therefore, derived for the design purpose considering the accidental impact loading. This paper ends with describing the extension of the present study.

• 한국전산구조공학회논문집
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• 제12권4호
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• pp.537-549
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• 1999

선형탄성 파괴해석은 균열을 갖는 변형률 경화재료의 파괴거동을 예측하는데 불충분하기 때문에 최근에는 균열 선단 부에서 대규모 소성 역을 갖는 균열 체에 적용할 수 있는 많은 파괴역학개념이 제안되고 있다. 따라서, 본 연구에서는 대규모항복 조건하의 연성파괴를 보이는 평판을 정확하게 해석할 수 있는 새로운 유한요소모델을 제시하고자 한다. 균열 선단 부의 응력 장을 정의하는데 가장 지배적인 파괴매개변수인 J-적분 값과 소성 역의 크기 및 형상을 J-적분법과 등가영역적분법을 통해 파괴거동을 설명할 수 있도록 증분소성이론에 기초를 둔 p-version 유한요소해석이 채택되었다. 제안된 유한요소모델에 의한 수치해석결과는 이론 해와 h-version 유한요소해석과 비교되었다.

• 한국재료학회지
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• 제19권9호
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• pp.457-461
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• 2009

Recently, metal molding has become essential not only for automobile parts, but also mass production, and has greatly influenced production costs as well as the quality of products. Its surface has been treated by carburizing, nitriding and induction hardening, but these existing treatments cause considerable deformation and increase the expense of postprocessing after treatment; furthermore, these treatments cannot be easily applied to parts that requiring the hardening of only a certain section. This is because the treatment cannot heat the material homogeneously, nor can it heat all of it. Laser surface treatment was developed to overcome these disadvantages, and, when the laser beam is irradiated on the surface and laser speed is appropriate, the laser focal position is rapidly heated and the thermal energy of surface penetrates the material after irradiation, finally imbuing it with a new mechanical characteristic by the process of self-quenching. This research estimates the material characteristic after efficient and functional surface treatment using HPDL, which is more efficient than the existing CW Nd:YAG laser heat source. To estimate this, microstructural changes and hardness characteristics of three parts (the surface treatment part, heat affect zone, and parental material) are observed with the change of laser beam speed and surface temperature. Moreover, the depth of the hardened area is observed with the change of the laser beam speed and temperature.

• Journal of Welding and Joining
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• 제33권4호
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• pp.30-36
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• 2015

Austenite stainless steel(SA213-TP347H) has widely been used for the superheater & reheater tube in USC(ultra-supercritica) coal boiler because of its high creep rupture strength and anti-oxidation. But recently, the short-term failures have happened frequently in heat affected zone for only 4,000~15,000hours of service. Many investigations have been conducted to understand the failure mechanism. The root cause of failure was comfirmed to "strain induce participation hardening crack" or "reheat cracking". This mechanism often occurred due to weld residual stress and precipitation of the Cr, Nb carbides in the stabilized stainless steel such as TP347H. This paper presents an analysis of failure tube and effect of the sample tubes that conducting stabilizing heat treatment in site after 11,380hours & 16,961hours of service. Visual inspection was performed. In addition, microscopic characteristics was identified by O.M, SEM, and hardness test was carried out to find out the heat treatment effects. Failures seem to happen because of being not conducted stabilizing heat treatment in site. And another cause is inadequate weld parameter such as pass, ampere, voltage, inter-pass temperature. Thus, this paper has the purpose to describe that how to prevent similar failures in those weld-joints.

• 콘크리트학회논문집
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• 제13권5호
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• pp.491-498
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• 2001

본 연구에서는 강우와 같은 열 충격으로 보수체의 손상(표면 균열, 경계면 파괴)을 유발하는 표면 인장 응력과 기층과 보수층 경계면의 연직 인장 응력 및 진단 응력이 해석적으로 조사되었다. 응력 계산 시에는 사용 재료의 비선형 응력-변형률 곡선이 사용되었고, 특히 변형률 경화, 변형률 연화 특성이 고려되었다. 응력 계산은 보수층의 두께와 보수 재료를 변수로 하여 강우 강도별로 조사되었다. 강우 초기의 보수체의 온도는 하절기에 55$^{\circ}C$까지 가열됐다고 가정하였고, 강우 온도는 1$0^{\circ}C$로 결정하였다. 강우 빈도 1년 1회, 강우 지속 시간 10분, 60분의 강우 하중으로 응력을 계산한 결과, 보수체 표면에서는 실제 균열은 발생치 않고, 단지 변형률 연화 단계의 가상 균열만 형성되었다. 한편 경계면에서는 부착 전단 파괴는 발생치 않았으며, 연직 인장 파괴를 유발하는 음력이 일정한 강우 강도 및 보수 재료에서 보수층의 두께(do)가 클수록 증가하는 것으로 나타나, 경계면의 연직 인장 응력이 보수체의 안전성에 주요한 변수임을 알 수 있었다. 따라서 강우시 경계면의 파괴 예방을 위해 보수 재료의 성질 및 보수층의 두께를 결정할 수 있는 연직 인장 음력의 예측식을 제안하였다.