• Journal of Welding and Joining
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• 제30권2호
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• pp.47-53
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• 2012

This paper aimed to study and understand the mechanical phenomena of thermal elasto-plastic behavior on the dissimilar butt joint (Al 6061-T6 and STS304) by TIG assisted Friction Stir Welding. Heat conduction and residual stress analysis is carried out using in-house solver. Two-dimensional results of the heat distribution and residual stresses in dissimilar joint for particular tool geometry and material properties are presented. The predicted stress along longitudinal direction in Al 6061-T6 and STS304 are approximately between 12-15% of their respective yield strengths. A comparison is made between experimentally measured and numerically predicted equivalent residual stress values.

• Steel and Composite Structures
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• 제29권1호
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• pp.23-37
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• 2018

In this paper, seismic provisions related to built-up special concentrically braced frames (BSCBFs) are investigated under cyclic loading using non-linear finite element analysis of a single-bay single-story frame. These braces, which contain double angle and double channel brace sections, are considered in two types of single diagonal and X-braced frames. The results of this study show that current seismic provisions such as observing the 0.4 ratio for slenderness ratio of individual elements between stitch connectors are conservative in BSCBFs, and can be increased according to the type of braces. Furthermore, such increments will lead to decreasing or remaining the current middle protected zone requirements of each BSCBFs. Failure results of BSCBFs, which are related to the plastic equivalent strain growth of members and ductility capacity of the models, show that the behaviors of double channel back-to-back diagonal braces are more desirable than those of similar face-to-face ones. Also, for double angle diagonal braces, results show that the failure of back-to-back BSCBFs occurs faster in comparison with face-to-face similar braces. In X-braced frames, cyclic and failure behaviors of built-up face-to-face models are more desirable than similar back-to-back braces in general.

• Geomechanics and Engineering
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• 제8권5호
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• pp.707-726
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• 2015

This paper investigates the time dependent behaviour of Haarajoki test embankment on soft structured clay deposit. Half of the embankment is constructed on an area improved with prefabricated vertical drains, while the other half is constructed on the natural deposit without any ground improvement. To analyse the PVD-improved subsoil, axisymmetric vertical drains were converted into equivalent plane strain conditions using three different approaches. The construction and consolidation of the embankment are analysed with the finite element method using a recently developed anisotropic model for time-dependent behaviour of soft clays. The constitutive model, namely ACM-S accounts for combined effects of plastic anisotropy, interparticle bonding and degradation of bonds and creep. For comparison, the problem is also analysed with isotropic Soft Soil Creep and Modified Cam Clay models. The results of the numerical analyses are compared with the field measurements. The results show that neglecting effects of anisotropy, destructuration and creep may lead to inaccurate predictions of soft clay response. Additionally, the numerical results show that the matching methods accurately predict the consolidation behaviour of the embankment on PVD improved soft clays and provide a useful tool for engineering practice.

• Steel and Composite Structures
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• 제1권4호
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• pp.411-426
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• 2001

This paper treats the failure analysis of prestressing steel wires with different kinds of localised damage in the form of a surface defect (crack or notch) or as a mechanical action (transverse loads). From the microscopical point of view, the micromechanisms of fracture are shear dimples (associated with localised plasticity) in the case of the transverse loads and cleavage-like (related to a weakest-link fracture micromechanism) in the case of cracked wires. In the notched geometries the microscopic modes of fracture range from the ductile micro-void coalescence to the brittle cleavage, depending on the stress triaxiality in the vicinity of the notch tip. From the macroscopical point of view, fracture criteria are proposed as design criteria in damage tolerance analyses. The transverse load situation is solved by using an upper bound theorem of limit analysis in plasticity. The case of the cracked wire may be treated using fracture criteria in the framework of linear elastic fracture mechanics on the basis of a previous finite element computation of the stress intensity factor in the cracked cylinder. Notched geometries require the use of elastic-plastic fracture mechanics and numerical analysis of the stress-strain state at the failure situation. A fracture criterion is formulated on the basis of the critical value of the effective or equivalent stress in the Von Mises sense.

• Corrosion Science and Technology
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• 제20권6호
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• pp.466-474
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• 2021

A wet type scrubber for merchant vessel uses super austenitic stainless steels with pitting resistance equivalent number (PREN) of 40 or higher for operation in a harsh corrosive environment. However, it is expensive due to a high nickel content. Thus, electrochemical behavior and cavitation erosion characteristics of UNS S32750 as an alternative material were investigated. Microstructure analysis revealed fractions of ferritic and austenitic phases of 48% and 52%, respectively, confirming the existence of ferritic matrix and austenitic island. Potentiodynamic polarization test revealed damage at the interface of the two phases because of galvanic corrosion due to different chemical compositions of ferritic and austenitic phases. After a cavitation test, a compressive residual stress was formed on the material surface due to impact pressure of cavity. Surface hardness was improved by water cavitation peening effect. Hardness value was the highest at 30 ㎛ amplitude. Scanning electron microscopy revealed wave patterns due to plastic deformation caused by impact pressure of the cavity. The depth of surface damage increased with amplitude. Cavitation test revealed larger damage caused by erosion in the ferritic phase due to brittle fracture derived from different strain rate sensitivity index of FCC and BCC structures.

• 한국강구조학회 논문집
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• 제14권4호
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• pp.499-508
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• 2002

리브로 보강된 철골 모멘트 내진접합부에 대한 실용적이고 간단한 설계법이 등가스트럿 모델을 바탕으로 최근에 제안된 바가 있다. 본 논문에서는 제안된 설계법을 검증하고 응력집중에 의한 리브 단부의 균열방지 방안을 모색하기 위한 실험결과를 기술하였다. 제안된 설계법에 의해 설계된 모든 실물대 시험체는 접합부의 소성회전각이 0.04(radian)에 이르는 뛰어난 내진성능을 발휘하였다. 접합부를 리브로 보강함과 동시에 보의 플래지 일부를 잘라내어 보의 소성힌지와 국부좌굴의 발생위치를 리브의 끝단으로부터 보 안쪽으로 약간 이동시킴으로서, 리브단부의 균열발생을 효과적으로 제어할 수 있음을 실험적으로 확인하였다. 또한 리브의 스트럿 작용과 이로 인한 보 웨브에서의 전단역전을 스테레인 게이지 계측을 통하여 실험적으로 입증하였다.

• 콘크리트학회지
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• 제5권2호
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• pp.181-188
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• 1993

본 논문에서는 철근콘크리트 쉘 구조물의 크리프와 건조수축에 의한 시간의존성 효과를 포함하여 해석할 수 있는 기법을 개발하였다. Degenerate 쉘 요소를 해석에 사용하였으며 층 분할 기법을 이용하였다. 콘크리트의 압축 거동 모델은 탄-소성 모델 혹은 변형율 경화 모델을 사용할 수 있도록 하였고, 인장 영역에서는 균열 발생시 까지 선형 탄성으로 가정하였다. 철근은 등가의 두께를 가지는 철근 층으로 근사되었으며 각 철근 층은 철근의 배치 방향으로만 저항하는 일축거동을 하며 응력-변형율 곡선을 두 개의 직선으로 이상화 하였다. 비선형 해석을 위해 하중 증분 기법과 반복계산 기법을 사용하였으며 시간 의존성 효과를 고려하기 위해 시간영역을 같은 간격이 아닐 수도 있는 여러 개의 구간으로 나누어 해석하였다. 몇 개의 계산 예를 제시하고 다른 연구자들의 결과와 비교하여 본 연구의 타당성을 검토하였다.

• 한국전산구조공학회논문집
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• 제27권3호
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• pp.191-198
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• 2014

본 논문에서는 노심용융사고 시 관통노즐이 제거된 원자로용기 하부헤드의 구조 건전성 평가를 수행하였다. 열응력, 노심용융물의 질량 그리고 내압조건의 해석결과를 고려할 때, 하부헤드의 열응력에 의한 영향이 가장 크게 나타났다. 손상 가능성은 파손기준에 따라 평가하였으며, 등가소성변형률이 임계변형률 파손기준보다 낮은 수준으로 평가되었다. 열-구조물 연성해석 결과 하부헤드의 두께 중간층에서 항복강도보다 낮은 응력이 발생한 탄성영역 구간을 확인하였다. 내압이 커지면서 탄성영역 범위가 점차 좁아지면서 탄성영역이 내벽으로 이동하는 결과를 확인하였고, 노심용융사고 시 구조적 건전성을 만족하는 것으로 평가되었다.

• 한국재료학회지
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• 제26권7호
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• pp.370-375
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• 2016

Tensile tests and creep tests were carried out at high temperatures on an Al-$Al_4C_3$ alloy prepared by mechanical alloying technique. The material contains about 2.0% carbon and 0.9% oxygen in mass percent, and the volume fractions of $Al_4C_3$ and $Al_2O_3$ particles are estimated at 7.4 and 1.4%, respectively, from the chemical composition. Minimum creep rate decreased steeply near two critical stresses, ${\sigma}_{cl}$ (the lower critical stress) and ${\sigma}_{cu}$ (the upper critical stress), with decreasing applied stress at temperatures below 723 K. Instantaneous plastic strain was observed in creep tests above a critical stress, ${\sigma}_{ci}$, at each test temperature. ${\sigma}_{cu}$ and ${\sigma}_{ci}$ were fairly close to the 0.2% proof stress obtained by tensile tests at each test temperature. It is thought that ${\sigma}_{cl}$ and ${\sigma}_{cu}$ correspond to the microscopic yield stress and the macroscopic yield stress, respectively. The lower critical stress corresponds to the local yield stress needed for dislocations to move in the soft region within subgrains. The creep strain in the low stress range below 723 K arises mainly from the local deformation of the soft region. The upper critical stress is equivalent to the macroscopic yield stress necessary for dislocations within subgrains or in subboundaries; this stress can extensively move beyond subboundaries under a stress above the critical point to yield a macroscopic deformation. At higher temperatures above 773 K, the influence of the diffusional creep increases and the stress exponent of the creep rate decreases.

• 한국전산구조공학회논문집
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• 제33권2호
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• pp.121-128
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• 2020

본 논문에서는 고속열차-차륜-레일-지반부의 상호작용에 따른 지반부의 변위장과 가속도장의 분포양상을 연구하고자 하였다. 이를 위해 고속열차 주행을 실제 차륜으로 모형하여 차륜-레일간의 연직접촉과 사행동에 의한 횡접촉을 모사하고, 이동질량해석을 근간으로 하였다. 이 때 지반부는 Modified Drucker-Prager 모델을 이용하여 상면 지반부의 비선형 거동을 부여하고 이에 따른 변위, 가속도의 변화를 탄성지반의 거동과 비탄성지반의 거동을 상호 비교하였다. 이를 통해, 실제 지반거동과 가까운 변위와 가속도 범위를 예측하고자 하였다. 이 때 지반부의 von-Mises응력과 등가소성변형도를 검토하고, 각 파괴면에서의 등가소성변형도, 전체 체적변형도 등을 검토하였다. 이동질량을 이용한 차륜-레일 접촉부의 수직응력, 횡압, 종방향 구속압 등의 시간이력에 따른 응력변화도 검토하였다. 비선형 지반모델의 경우 탄성 지반모델에 비해 열차 주행에 따른 변위 차이는 크지 않는 반면에 가속도의 경우는 큰 감소를 발생시키는 것으로 나타났다.