• 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.

• 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%.

• 한국안전학회지
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• 제12권1호
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• pp.19-28
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• 1997

This study was made of the effect of cold working on the stress corrosion cracking(SCC) of austenitlc 304 stainless steel in boiling 42% $MgCl_2$ solution. For this experiment, specimens cold-worked of 0%, 10%, 20%, 30%, 40% were fabricated respectively, and then experiments of mechanical properties and stress corrosion cracking(SCC) of these specimens were carried out. The results of these experiments indicate that the maximum resistance to SCC showed at 20% of cold working degree and that the SCC susceptibility depended on the volume fraction of deformation-induced martensite by cold working and the work hardening of matrix. On the other hand, the fracture mode was changed. This phenomenon was considered that deformation-induced martensite was grown from transgranular fracture mode to intergranular fracture mode and caused by increased of dislocation density along the slip planes.

• 대한금속재료학회지
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• 제48권4호
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• pp.305-314
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• 2010

The development of textures and microstructures during plastic deformation in a hot-extruded AZ 31 Mg alloy was investigated using a compression test with such parameters as deformation temperature, strain rate. It was observed from true stress-strain curves that twinning involves changes of the flow stresses. In the early stages of deformation at temperatures lower than $200^{\circ}C$, the occurrence of twins resulted in a decrease of the work-hardening rate, which increased drastically at a true strain of -0.05. The evolution of the deformation textures were assessed with the aid of EBSD analyses in terms of the competition between twinning and slip activity.

• Steel and Composite Structures
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• 제9권4호
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• pp.381-395
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• 2009

This paper presents the details of an advanced Finite Element (FE) analysis of a plane steel portal frame with semi-rigid beam-to-column connections subjected cyclic loading. In spite of several component models on cyclic behaviour of connections presented in the literature, works on numerical investigations on cyclic behaviour of full scale frames are rather scarce. This paper presents the evolution of an FE model which deals comprehensively with the issues related to cyclic behaviour of full scale steel frames using ABAQUS software. In the material modeling, combined kinematic/isotropic hardening model and isotropic hardening model along with Von Mises criteria are used. Connection non-linearity is also considered in the analysis. The bolt slip which happens in friction grip connection is modeled. The bolt load variation during loading, which is a pivotal issue in reality, has been taken care in the present model. This aspect, according to the knowledge of the authors, has been first time reported in the literature. The numerically predicted results using the methodology evolved in the present study, for the cyclic behaviour of a cantilever beam and a rigid frame, are validated with experimental results available in the literature. The moment-rotation and deflection responses of the evolved model, match well with experimental results. This proves that the methodology for evolving the steel frame and connection model presented in this paper is closer to real frame behaviour as evident from the good comparison and hence paves the way for further parametric studies on cyclic behaviour of flexibly connected frames.

• 한국재료학회지
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• 제10권2호
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• pp.155-159
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• 2000

고주기 피로조건에서 응력진폭은 항복점이하의 응력이므로 변형은 일반적으로 탄성적이다. 만약 변형이 완전히 탄성적이라면 피로는 생겨나지 않을 것이다. 그러나 이는 항복점의 개념과 항복점 아래에서의 순수탄성변형의 가정을 과도하게 단순화한 것이다. 인장실험 시 시편 전체가 파괴 절차를 따르는 반면, 고주기 피로실험에서는 국부적 영구 슬립띠가 파괴절차를 따른다. 그러나 두 경우에서 파괴 전변형영역의 단위체적 당 변형의 축적은 두 재료가 동일하기 때문에 국부적으로 동일하다. 미소 소성변형이나 Luders band, 탄성영역에서의 인장실험곡선의 기울기변화는 재료속에 포함된 경도가 높은 침입형 또는 침탄형 원자의 구름에 기인한다. 이들이 구름운동(Rolling movement)을 일으켜 다음 격자로 이동하면 소성변형이 발생되는 반면, 완전히 구르지 못하고 제자리로 되돌아오는 운동을 반복하는 경우가 바로 피로한계인다.

• 대한조선학회논문집
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• 제34권1호
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• pp.77-86
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• 1997

최근들어 안전하고 합리적인 구조를 설계하기 위하여 구조물의 내충돌 또는 내충격에 대한 요구와 관심이 높아지고 있는데, 이러한 문제들은 아주 짧은 시간동안에 대변형이 일어나는 비선형문제라는 특징이 있다. 구조재료는 변형속도가 빨라짐에 따라 정적인 범주에서 보이는 거동과는 달리 변형률 의존적인 거동을 보인다. 따라서 대변형 소성문제인 충돌해석 등에는 종래 사용하여 온 변형률 비의존 재료구성방정식으로는 한계가 있다. 이 논문에서는 이러한 점을 개선하기 위하여 연강의 소성거동을 잘 나타낼 수 있는 소성슬립모델을 채용하고, 비선형경화를 도입하여 변형도 적용범위를 확장한 대변형 탄소성 변형률의존 재료구성방정식을 제시하였다. 본 구성방정식의 특징으로 항복조건과 하중조건이 필요없기 때문에 계산이 간편하며, 전위밀도와 속도로써 소성을 표현하기 때문에 보다 물리적인 의미를 가지고 금속재료의 소성현상을 나타낼 수 있다.

• 한국재료학회지
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• 제21권11호
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• pp.617-622
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• 2011

The effect of oxygen on the shape memory characteristics in Ti-18Nb-6Zr-XO (X = 0-1.5 at%) biomedical alloys was investigated by tensile tests. The alloys were fabricated by an arc melting method at Ar atmosphere. The ingots were cold-rolled to 0.45 mm with a reduction up to 95% in thickness. After severe cold-rolling, the plate was solution-treated at 1173 K for 1.8 ks. The fracture stress of the solution-treated specimens increased from 450 Mpa to 880 MPa with an increasing oxygen content up to 1.5%. The fracture stress increased by 287MPa with 1 at% increase of oxygen content. The critical stress for slip increased from 430 MPa to 695 MPa with an increasing oxygen content up to 1.5 at%. The maximum recovery strain of 4.1% was obtained in the Ti-18Nb-6Zr-0.5O (at%) alloy. The martensitic transformation temperature decreased by 140 K with a 1.0 at% increase in O content, which is lower than that of Ti-22Nb-(0-2.0)O (at%) by 20 K. This may have been caused by the effect of the addition of Zr. This study confirmed that addition of oxygen to the Ti-Nb-Zr alloy increases the critical stress for slip due to solid solution hardening without being detrimental to the maximum recovery strain.

• Geomechanics and Engineering
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• 제34권4호
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• pp.409-424
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• 2023

Response of the pipeline crossing fault is considered as the large strain problem. Proper estimation of the pipeline response plays important role in mitigation studies. In this study, an advanced continuum modeling including material non-linearity in large strain deformations, hardening/softening soil behavior and soil-pipeline interaction is applied. Through the application of a fully nonlinear analysis based on an explicit finite difference method, the mechanics of the pipeline behavior and its interaction with soil under large strains is presented in more detail. To make the results useful in oil and gas engineering works, a continuous pipeline of two steel grades buried in two clayey soil types with four different crossing angles of 30°, 45°, 70° and 90° with respect to the pipeline axis have been considered. The results are presented as the fault movement corresponding to different damage limit states. It was seen that the maximum affected pipeline length is about 20 meters for the studied conditions. Also, the affected length around the fault cutting plane is asymmetric with about 35% and 65% at the fault moving and stationary block, respectively. Local buckling is the dominant damage state for greater crossing angle of 90° with the fault displacement varying from 0.4 m to 0.55 m. While the tensile strain limit is the main damage state at the crossing angles of 70° and 45°, the cross-sectional flattening limit becomes the main damage state at the smaller 30° crossing angles. Compared to the stiff clayey soil, the fault movement resulting 3% tensile strain limit reach up to 40% in soft clayey soil. Also, it was seen that the effect of the pipeline internal pressure reaches up to about 40% compared to non-pressurized condition for some cases.

• 콘크리트학회지
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• 제3권4호
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• pp.97-106
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• 1991

본 연구의 목적은 휨거동을 하는 철근콘크리트 부재의 변위를 해석적으로 정확하게 구하기 위해 평균 휨균열간격 및 휨모멘트-등가곡률 관계(M-$\Phi_eg$)의 해석법을 제안한 것이다. 제안식은 비균열 구간에서의 철근과 콘크리트 간의 부착특성 및 재료의 소성영역을 고려하여 정확한 곡률분포를 계산함으로써 구할 수 있다. 제안된 해석법의 타당성을 검증하기 위해 34개의 철근콘크리트 보 부재를 제작, 휨재하 실험을 실시하였으며 해석치와 비교검토하였다. 그 결과 실험치와 해석치는 매우 잘 일치하여 본 해석법의 실용성 및 정확성이 입증되었다.