• Structural Engineering and Mechanics
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• 제58권4호
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• pp.719-729
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• 2016

The main objective of the present paper is to study the effect of the yield criterion on the magnitude of the strain rate and plastic work rate intensity factors in axisymmetric flow of isotropic incompressible rigid perfectly plastic material by means of a problem permitting a closed-form solution. The boundary value problem consisting of the axisymmetric deformation of a plastic tube is solved. The outer surface of the tube contracts. The radius of the inner surface does not change. The material of the tube obeys quite a general yield criterion and its associated flow rule. The maximum friction law is assumed at the inner surface of the tube. Therefore, the velocity field is singular near this surface. In particular, the strain rate and plastic work rate intensity factors are derived from the solution. It is shown that the strain rate intensity factor does not depend on the yield criterion but the plastic work rate intensity factor does.

• 한국전산구조공학회논문집
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• 제24권3호
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• pp.275-281
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• 2011

일반적으로 인장 시험에 주로 사용되는 납작한 형태의 금속 재료에 천천히 인장력을 가하고 그 힘을 증가시키면 어느 순간에 루더스 밴드(Luders band)라고 불리는 소성 변형의 띠가 갑자기 발생한다. 이 띠들은 일정 각도를 가지고 평행하게 발생하며, 여러 연구자들에 의해 특정 경계조건에 대하여 이 띠가 발생하는 조건과 그 각도에 대한 많은 연구가 선행되어 왔다. 본 연구에서는 평면 응력 조건에서 이루어진 Thomas(1961)의 연구를 기반으로 하고 $J_2$ 소성 변형 조건, 힘의 평형방정식, 그리고 구성방정식을 이용하여 평면 변형 조건에서 발생하는 밴드의 각도를 해석적으로 유도하였다. 이 결과는 음향텐서를 이용한 기존의 연구 결과와 일치함을 확인하였다.

• 한국산학기술학회논문지
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• 제7권3호
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• pp.265-270
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• 2006

복합재로 된 판재에 대하여 동적 균열이 접합면을 따라서 진행되어 가면서 찢어져 나갈 때 그 계면 주위에서의 응력 상태 및 변형, 견인응력 등을 조사하였다. 최대의 등가응력 및 소성 변형율은 접촉이 바로 떨어지는 부분에서 그리고 판 끝부분의 휘어지는 부근에서 최대의 변형량이 되었다. 견인력은 0.015mm정도로 그 면이 떨어졌을 때 최대치 75 MPa로서 되었다가 서서히 낮아지면서 0.13mm이상 떨어졌을 때는 0으로 일정하게 되었다. 이러한 연구 자료로서 정확한 파괴강도 해석과 안전설계 및 새로운 고급재료의 개발에 필요한 자료를 제공할 수 있는 기초적인 연구를 수행함이 본 연구의 목적이다.

• Structural Engineering and Mechanics
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• 제62권5호
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• pp.551-565
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• 2017

Based on the strain gradient theory (SGT), vibration analysis of an embedded micro cylindrical shell reinforced with agglomerated carbon nanotubes (CNTs) is investigated. The elastic medium is simulated by the orthotropic Pasternak foundation. The structure is subjected to magnetic field in the axial direction. For obtaining the equivalent material properties of structure and considering agglomeration effects, the Mori-Tanaka model is applied. The motion equations are derived on the basis of Mindlin cylindrical shell theory, energy method and Hamilton's principal. Differential quadrature method (DQM) is proposed to evaluate the frequency of system for different boundary conditions. The effects of different parameters such as CNTs volume percent, agglomeration of CNTs, elastic medium, magnetic field, boundary conditions, length to radius ratio and small scale parameter are shown on the frequency of the structure. The results indicate that the effect of CNTs agglomeration plays an important role in the frequency of system so that considering agglomeration leads to lower frequency. Furthermore, the frequency of structure increases with enhancing the small scale parameter.

• 전산구조공학
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• 제10권3호
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• pp.185-193
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• 1997

탄성 선형 경화 재료로 구성된 복합 구조물의 자유 경계면에서 나타나는 응력 특이도를 평면 변형률 상태에서 계산하였다. 자유 표면력 경계조건과 계면 연속조건을 만족해야하는 지배 탄성 방정식은 2점 경계치문제로 정의되며, 일반 고유치 문제의 해인 고유치가 응력 특이도가 될 것이다. 자유경계면 근처에서 응력 성분을 r/sup s-1/에 비례한다고 가정하여 특정한 s(고유치)를 구하는 고유치 문제를 뉴톤향상법과 사격법을 사용하여 수치적으로 해를 구하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.815-818
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• 2002

The elliptic relaxation model(ERM) with the inhomogeneous correction intermediate between near wall with and far from the wall. The source of the ERM usually was appled quasi-homogeneous pressure-strain correlation in homogeneous situations. This formulation was easily applied to the linear model or non-linear pressure-strain model. It is observed that the boundary conditions of the relaxation operator dominate the homogeneous pressure-strain model in the near wall region. While looking at high-Reynolds number flows, it was found necessary to modify the effect of the relaxation operator throughout the log region by accounting for gradients of the flatness variable and turbulent length scales. These effects are kinematic blocking of the wall normal velocity fluctuation and pressure reflections from the surface. This model is wall distances and unit vectors which make the model applicable to flows boundary by a complex geometry. Inhomogeneous correction model is computed inertial and non-inertial channel flow These are compared DNS(Kim et at., Kristofffrsen & Andersson) for channel flow. The present model could be predicted well for rotating flows.

• Steel and Composite Structures
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• 제43권6호
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• pp.707-723
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• 2022

This article aims to investigate the size dependent bending behavior of perforated nanobeams incorporating the nonlocal and the microstructure effects based on the nonlocal strain gradient elasticity theory (NSGET). Shear deformation effect due to cutout process is studied by using Timoshenko beams theory. Closed formulas for the equivalent geometrical characteristics of regularly squared cutout shape are derived. The governing equations of motion considering the nonlocal and microstructure effects are derived in comprehensive procedure and nonclassical boundary conditions are presented. Analytical solution for the governing equations of motion is derived. The derived non-classical analytical solutions are verified by comparing the obtained results with the available results in the literature and good agreement is observed. Numerical results are obtained and discussed. Parametric studies are conducted to explore effects of perforation characteristics, the nonclassical material parameters, beam slenderness ratio as well as the boundary and loading conditions on the non-classical transverse bending behavior of cutout nanobeams. Results obtained are supportive for the design, analysis and manufacturing of such nanosized structural system.

• Advances in nano research
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• 제16권3호
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• pp.265-272
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• 2024

In this paper, the problem of static bending of axially functionally graded (AFG) nanobeam is formulated with the local stress(Lσ)- and strain-driven(εD) two-phase local/nonlocal integral models (TPNIMs). The novelty of the present study aims to compare the size-effects of nonlocal integral models on bending deflections of AFG Euler-Bernoulli nano-beams. The integral relation between strain and nonlocal stress components based on two types nonlocal integral models is transformed unitedly and equivalently into differential form with constitutive boundary conditions. Purely LσD- and εD-NIMs would lead to ill-posed mathematical formulation, and Purely εD- and LσD-nonlocal differential models (NDM) may result in inconsistent size-dependent bending responses. The general differential quadrature method is applied to obtain the numerical results for bending deflection and moment of AFG nanobeam subjected to different boundary and loading conditions. The influence of AFG index, nonlocal models, and nonlocal parameters on the bending deflections of AFG Euler-Bernoulli nanobeams is investigated numerically. A consistent softening effects can be obtained for both LσD- and εD-TPNIMs. The results from current work may provide useful guidelines for designing and optimizing AFG Euler-Bernoulli beam based nano instruments.

• 한국정밀공학회지
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• 제25권8호
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• pp.80-87
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• 2008

This paper investigated into the impact characteristics of the stainless sheet with thickness of 0.7 mm on the stretching boundary condition through three-dimensional finite element analysis. High speed tensile tests were carried out to obtain strain-stress relationships with the effects of the strain rate. The FE analysis was performed by the ABAQUS explicit code. In order to improve an accuracy of the FE analysis, the hyper-elastic model and the damping factor were introduced. Through the comparison of the results of the FE analyses and those of the impact tests, a proper FE model was obtained. The results of the FE analyses showed that the absorption rate of energy maintains almost 82.5-83.5% irrespective of the impact energy level and the diameter of the impact head. From the results of FE analyses, variations of stress, strain, dissipation energy, strain energy density, and local deformation characteristics in the stainless sheet during the collision and the rebound of the impact head were quantitatively examined. In addition, it was shown that the fracture of the specimen occurs when the plastic strain is 0.42 and the maximum value of the plastic dissipation energy of the specimen is nearly 1.83 J.

• Structural Engineering and Mechanics
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• 제52권6호
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• pp.1135-1156
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• 2014

The ratcheting effect greatly challenges the design of piping components. With the assistance of the quasi-three point bending apparatus, ratcheting and the ratcheting boundary of pressurized straight Z2CND18.12N stainless steel pipe under bending loading and vertical displacement control were studied experimentally. The characteristics of progressive inelastic deformation in axial and hoop directions of the Z2CND18.12N stainless steel pipes were investigated. The experiment results show that the ratcheting strain occurs mainly in the hoop direction while there is less ratcheting strain in the axial direction. The characteristics of the bending ratcheting behavior of the pressure pipes were derived and compared under load control and displacement control, respectively. The results show that the cyclic bending loading and the internal pressure affect the ratcheting behavior of the pressurized straight pipe significantly under load control. In the meantime, the ratcheting characteristics are also highly associated with the cyclic displacement and the internal pressure under displacement control. All these factors affect not only the saturation of the ratcheting strain but the ratcheting strain rate. A series of multi-step bending ratcheting experiments were conducted under both control modes. It was found that the hardening effect of Z2CND18.12N stainless steel pipe under previous cyclic loadings no matter with high or low displacement amplitudes is significant, and the prior loading histories greatly retard the ratcheting strain and its rate under subsequent loadings. Finally, the ratcheting boundaries of the pressurized straight Z2CND18.12N stainless steel pipe were determined and compared based on KTA/ASME, RCC-MR and the experimental results.