• 대한기계학회논문집A
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• 제25권12호
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• pp.2056-2061
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• 2001

Non-proportional loading tests of Nylon 66 at room temperature exhibit path dependent behavior and plasticity-relaxation interactions. The uniaxial formulation of the viscoplasticity theory based on overstress (VBO), which has been used to reproduce the nonlinear strain rate sensitivity, relaxation, significant recovery and cyclic softening behaviors of Nylon 66, is extended to three-dimensions to predict the response in strain-controlled, comer-path tests. VBO consists of a flow law that is easily written for either the stress or the strain as the independent variable. The flow law depends on the overstress, the difference between the stress and the equilibrium stress that is a state variable in VBO. The evolution law of the equilibrium stress in turn contains two additional state variables, the kinematic stress and the isotropic stress. The simulations show that the constitutive model is competent at modeling the deformation behavior of Nylon 66 and other solid polymers.

• Structural Engineering and Mechanics
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• 제25권1호
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• pp.107-126
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• 2007

The problem related to the computation of bounds on plastic deformations for structures in plastic shakedown condition (alternating plasticity) is studied. In particular, reference is made to structures discretized by finite elements constituted by elastic perfectly plastic material and subjected to a special combination of fixed and cyclic loads. The load history is known during the steady-state phase, but it is unknown during the previous transient phase; so, as a consequence, it is not possible to know the complete elastic plastic structural response. The interest is therefore focused on the computation of bounds on suitable measures of the plastic strain which characterizes just the first transient phase of the structural response, whatever the real load history is applied. A suitable structural model is introduced, useful to describe the elastic plastic behaviour of the structure in the relevant shakedown conditions. A special bounding theorem based on a perturbation method is proposed and proved. Such theorem allows us to compute bounds on any chosen measure of the relevant plastic deformation occurring at the end of the transient phase for the structure in plastic shakedown; it represents a generalization of analogous bounding theorems related to the elastic shakedown. Some numerical applications devoted to a plane steel structure are effected and discussed.

• Structural Engineering and Mechanics
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• 제26권5호
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• pp.569-589
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• 2007

This paper presents an analytical model for RC beam-column connections that takes into account bond deterioration between reinforcing steel and concrete. The model is based on the Lumped Damage Mechanics (LDM) theory which allows for the characterization of cracking, degradation and yielding, and is extended in this paper by the inclusion of the slip effect as observed in those connections. Slip is assumed to be lumped at inelastic hinges. Thus, the concept of "slip hinge", based on the Coulomb friction plasticity theory, is formulated. The influence of cracking on the slip behavior is taken into account by using two concepts of LDM: the effective moment on an inelastic hinge and the strain equivalence hypothesis. The model is particularly suitable for wide beam-column connections for which bond deterioration dominates the hysteretic response. The model was evaluated by the numerical simulation of five tests reported in the literature. It is found that the model reproduces closely the observed behavior.

• 한국구조물진단유지관리공학회 논문집
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• 제3권1호
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• pp.137-146
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• 1999

Analysis of R/C columns requires modeling of the plastic hinge region, as well as nonlinear material characteristics. This becomes a challenging task in view of the nonlinearity of both steel and concrete. Furthermore, formation and progression of plasticity in the hinge is a difficult phenomenan to simulate, especially under reversed cyclic loading and decaying strength conditions. This research provide one analytical model employed in column analysis, including the analysis procedure for establishing inelastic force-deformation relationships. The analytical results show good correlation with experimental data. The employed procedure with the adopted analytical models can be used to compute inelastic displacements of concrete columns with welded reinforcement grids. The inelastic deformability beyond the peak was similar to those indicated by columns with conventional ties. The superior performance of columns with welded grids may be attributed to the improved confinement characteristics of grids associated with increased rigidity of welded ties.

• 한국방재학회 논문집
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• 제7권5호
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• pp.27-35
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• 2007

본 연구는 지진운동의 영향 하에서 강제 모멘트 골조로 이루어진 post-Northridge 연결부를 갖는 보의 탄성 및 비탄성 거동을 모델하기 위한 부등단면 보 요소를 제시한다. 단면감소 연결부를 갖는 부등단면 보의 탄성강성 행렬은 수치적분이 필요치 많은 수식으로 표현된다. 소성모델은 분포형이며 강체링크로 연결된 일련의 비선형 힌지로 구성 되어있고 경화법칙은 단조 및 임의 주기 하중에 대한 비탄성 거동과 국부좌굴의 효과를 고려할 수 있다. 모델의 대조와 검증은 동반논문에 제시되어있다.

• Coupled systems mechanics
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• 제3권2호
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• pp.195-211
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• 2014

When strong seismic forces act on reinforced concrete structures, their beam-column connections are very susceptible to damage during the earthquake event. The aim of this numerical work is to evaluate the influence of the internal steel reinforcement array on the nonlinear response of a RC beam-column connection when it is subjected to strong cyclic loading -as a seismic load. For this, two specimens (extracted from an experimental test of 12 RC beam-column connections reported in literature) were modeled in the Finite Element code FEAP considering different stirrup's arrays. In order to evaluate the nonlinear response of the RC beam-column connection, the 2D model takes into account the nonlinear thermodynamic behavior of each component: for concrete, a damage model is used; for steel reinforcement, it is adopted a classical plasticity model; in the case of the steel-concrete bonding, this one is considered perfect without degradation. At the end, we show a comparison between the experimental test's responses and the numerical results, which includes the distribution of shear stresses and damage inside the concrete core of the beam-column connection; in the other hand, the effects on the connection of a low and high state of confinement are analyzed for all cases.

• Computers and Concrete
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• 제21권3호
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• pp.299-310
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• 2018

When reinforced concrete structures are subjected to strong seismic forces, their beam-column connections are very susceptible to be damaged during the earthquake event. Consequently, structural designers try to fit an important quantity of steel reinforcement inside the connection, complicating its construction without a clear justification for this. The aim of this work is to evaluate -and demonstrate- numerically how the quantity and the array of the internal steel reinforcement influences on the nonlinear response of the RC beam-column connection. For this, two specimens (extracted from an experimental test of 12 RC beam-column connections reported in literature) were modeled in the Finite Element code FEAP considering different stirrup's arrays. The nonlinear response of the RC beam-column connection is evaluated taking into account the nonlinear thermodynamic behavior of each component: a damage model is used for concrete; a classical plasticity model is adopted for steel reinforcement; the steel-concrete bonding is considered perfect without degradation. At the end, the experimental responses obtained in the tests are compared to the numerical results, as well as the distribution of shear stresses and damage inside the concrete core of the beam-column connection, which are analyzed for a low and high state of confinement.

• 대한조선학회논문집
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• 제30권4호
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• pp.136-152
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• 1993

정확하고 효율적인 해석적 박판보 모델을 개발하여 반복적인 과부하를 받고 있는 SWATH선의 전체적인 거동을 해석하고자 한다. SWATH선을 단순한 채널형 박판보로 이상화하고, 박판보 모델은 기하학적으로 완전한 비선형 보를 응력성분으로 정식화하여 유한요소법에 의해 수치해석적으로 처리한다. 또한 복잡한 하중상태하의 재료의 비선형 거동을 잘 나타낼 수 있는 효율적인 주기적 소성모델도 고려한다. 이 박판보 모델은 단면에서의 응력분포를 국부적으로 매우 정확하게 나타내고, 재료가 항복되어 가는 과정을 쉽게 추적할 수 있게 한다. 그리고 전단변형의 영향을 국부적으로 취급함으로서 변형과 전단응력의 분포를 보다 더 정확하게 나타낼 수 있다. 이와 같은 해석적 박판보 모델에 의한 초기 설계단계에서의 해석은 바람직하다고 생각되며, 이 연구방법을 좀 더 진전시켜 보다 현실적인 박판보 모델을 개발할 필요성이 있다.

• 한국압력기기공학회 논문집
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• 제14권2호
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• pp.69-76
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• 2018

The paper presents preliminary investigation results for the effect of the baseline correction in the acceleration excitation method on finite element seismic analysis results (such as accumulated equivalent plastic strain, equivalent plastic strain considering cyclic plasticity, von Mises effective stress, etc) of nuclear safety Class I components. For investigation, finite element elastic-plastic time-history seismic analysis is performed for a surge line including a pressurizer lower head, a pressurizer surge nozzle, a surge piping, and a hot leg surge nozzle using the Chaboche hardening model. Analysis is performed for various seismic loading methods such as acceleration excitation methods with and without the baseline correction, and a displacement excitation method. Comparing finite element analysis results, the effect of the baseline correction is investigated. As a result of the investigation, it is identified that finite element analysis results using the three methods do not show significant difference.

• Earthquakes and Structures
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• 제25권1호
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• pp.27-41
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• 2023

This study focused on nonlinear effective stress site response analysis using two coupled constitutive models, that is, the DM model (Dafalias and Manzari 2004), which incorporated a simple plasticity sand model accounting for fabric change effects, and the PMDY03 model (Khosravifar et al. 2018), that is, a 3D model for earthquake-induced liquefaction triggering and postliquefaction response. A detailed parametric study was conducted to validate the effectiveness of nonlinear site response analysis and porewater pressure (PWP) generation through a true coupled formulation for assessing the initiation of liquefaction at ground level. The coupled models demonstrated accurate prediction of liquefaction triggering, which was in line with established empirical liquefaction triggering relations in published databases. Several limitations were identified in the evaluation of liquefaction using the cyclic stress method, despite its widespread implementation for calculating liquefaction triggering. Variations in shear stiffness, represented by changes in shear wave velocity (Vs1), exerted the most significant influence on site response. The study further indicated that substantial differences in response spectra between nonlinear total stress and nonlinear effective stress analyses primarily occurred when liquefaction was triggered or on the verge of being triggered, as shown by excess PWP ratios approaching unity. These differences diminished when liquefaction occurred towards the later stages of intense shaking. The soil response was predominantly influenced by the higher stiffness values present prior to liquefaction. A key contribution of this study was to validate the criteria used to assess the triggering of level-ground liquefaction using true coupled effective-stress constitutive models, while also confirming the reliability of numerical approximations including the PDMY03 and DM models. These models effectively captured the principal characteristics of liquefaction observed in field tests and laboratory experiments.