• Journal of the Computational Structural Engineering Institute of Korea
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• v.24 no.3
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• pp.275-281
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• 2011

When a flat bar type metal specimen for general tension test is subject to incremental uniaxial tension, a narrow plastic shear band, so called luders band, is generated at some instance. This band typically has an angle to the axis of specimen and many early researches have been done to investigate the condition and angle of this localized deformation phenomenon by many researchers. This study follows the procedure of Thomas(1961) under plane stress boundary condition. $J_2$ plasticity theory, balance of linear momentum, and constitutive equations are used to derive the angle of luders band under plain strain boundary condition. The result was confirmed by other angle based on acoustic tensor theory.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.48 no.10
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• pp.773-782
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• 2020

The development of joint FE models for deep learning neural network (DLNN)-based hybrid FEA is presented. Material models of bolts and bearings in the front axle of tractor, showing complex behavior induced by various tightening conditions, were replaced with DLNN models. Bolts are modeled as one-dimensional Timoshenko beam elements with six degrees of freedom, and bearings as three-dimensional solid elements. Stress-strain data were extracted from all elements after finite element analysis subjected to various load conditions, and DLNN for bolts and bearing were trained with Tensorflow. The DLNN-based joint models were implemented in the ABAQUS user subroutines where stresses from the next increment are updated and the algorithmic tangent stiffness matrix is calculated. Generalization of the trained DLNN in the FE model was verified by subjecting it to a new loading condition. Finally, the DLNN-based FEA for the front axle of the tractor was conducted and the feasibility was verified by comparing with results of a static structural experiment of the actual tractor.

• Journal of the Korean Society of Safety
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• v.11 no.4
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• pp.143-150
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• 1996

This is an explicit-Implicit, finite element analysis for linear as well as nonlinear hygrothermal stress problems. Additional features, such as moisture diffusion equation, crack element and virtual crack extension(VCE ) method for evaluating J-integral are implemented in this program. The Linear Elastic Fracture Mechanics(LEFM) Theory is employed to estimate the crack driving force under the transient condition for and existing crack. Pores in materials are assumed to be saturated with moisture in the liquid form at the room temperature, which may vaporize as the temperature increases. The vaporization effects on the crack driving force are also studied. The Ideal gas equation is employed to estimate the thermodynamic pressure due to vaporization at each time step after solving basic nodal values. A set of field equations governing the time dependent response of porous media are derived from balance laws based on the mixture theory Darcy's law Is assumed for the fluid flow through the porous media. Perzyna's viscoplastic model incorporating the Von-Mises yield criterion are implemented. The Green-Naghdi stress rate is used for the invariant of stress tensor under superposed rigid body motion. Isotropic elements are used for the spatial discretization and an iterative scheme based on the full newton-Raphson method is used for solving the nonlinear governing equations.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.31 no.3
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• pp.155-163
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• 2018

One of the major disadvantages of isogeometric analysis(IGA) is that local refinement is nearly impossible in a conventional manner because of the tensor product nature in NURBS. In this research, we investigate a local refinement scheme for isogeometric analysis, named multi-resolution approach where different resolutions are employed at each subdomain, using h-refinement relation to endow displacement compatibility on an interface of subdomains. Then, we develop shape sensitivity analysis possessing same compatibility condition as in the analysis. Numerical examples are shown to demonstrate the computational efficiency of the method in analysis especially stress concentration problem and accurate sensitivity results which is also compatible on the interface.

• The Journal of Engineering Geology
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• v.7 no.2
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• pp.91-99
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• 1997

In general, there are various approaches available in literature to model discontinuous rock masses and engineers are often confused which one to use for designing structures in rock masses. Modelling rock masses can be classified mainly into two approaches. One is discrete modelling of intact rock and discontinuities and the other is the equivalent continuum modelling. In this study five models are selected ;(1) Crack tensor model, (2) Equivalent volume defect model, (3) Damage model, (4) Micro - structure model (Parallel model and Series model), and (5) Homogenization model. Most of these models are mainly concerned with how to define additional strain due to discontinuities over the representative elementary volume (REV) and how to relate the stress field of discontinuities to that acting on the REV. The characteristics of these models are clarified by comparing with results of some laboratory tests.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.26 no.1
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• pp.79-87
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• 2013

An isogeometric topological shape optimization method is developed using the level sets and Heaviside enrichments. In the level set method, the initial domain is kept fixed and its boundary is represented by an implicit moving boundary embedded in the level set functions, which facilitates to handle complicated topological shape changes. The Heaviside enrichment improves the isogeometric analysis by adding some enrichment functions to model the internal boundaries. The proposed topological shape optimization method has several benefits: exact geometric models can be obtained using the isogeometric approach and the limitation of tensor-product patches can be overcome using the Heaviside enrichments to represent the internal voids. Even in a single patch, discontinuous displacement fields as well as smooth stress field can be obtained. Since the level sets offer the implicit moving boundary inside the domain, it is easy to represent the topological shape variations in the isogeometric analysis using Heaviside enrichments.

• Magazine of the Korea Concrete Institute
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• v.7 no.5
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• pp.172-178
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• 1995

Concrete contains numerous microcracks at initially poured. The growth and propagation of nicrockacsk are believed tc finally incur the faiure of concrete. These processings are understood as a damage. Damage IS represented as a second-order tensor and crack is treated as a con tinuum phenomenon. In this paper, damage is characterized through the effective stress concept together with the hypothesis of elastic energy equivalence, and damage evolution law and constitutive equation of a damage model are derived by using the Helmholtz frte eriergy and the dissipation potential by means of the thermodynamic principles. The constitutive equation of the model includes the effects of elasticity, anisotropic damage and plasticity of concrete. There are two effective tangent stiffness tensors in this model : one is for elastic-darnage and the other for plastic damage. For the verification of the model, finite element analysis was performed for the analysis of concrete subjec:t to uniaxial and biaxial loading and the results obtained were compared with test results.

• Journal of the Microelectronics and Packaging Society
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• v.30 no.4
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• pp.50-60
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• 2023

Ball Grid Array (BGA) is a widely used package type due to its high pin density and good heat dissipation. In BGA, solder balls play an important role in electrically connecting the package to the PCB. Therefore, understanding the inelastic deformation of solder balls under various mechanical loads is essential for the robust design of semiconductor packages. In this study, the geometrical effect on the inelastic deformation and fracture of solder balls were analyzed by finite element analysis. The results showed that fracture occurred in both tilted and hourglass shapes under shear loading, and no fracture occurred in all cases under compressive loading. However, when bending was applied, only the tilted shape failed. When shear and bending loads were combined with compression, the stress triaxiality was maintained at a value less than zero and failure was suppressed. Furthermore, a comparison using the Lagrangian-Green strain tensor of the critical element showed that even under the same loading conditions, there was a significant difference in deformation depending on the shape of the solder ball.

• Composites Research
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• v.36 no.1
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• pp.16-24
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• 2023

This paper presents a multi-scale progressive fatigue damage model incorporating the model for interfacial debonding between fibers and matrix. The micromechanics model for the progressive interface debonding was adopted, which defined the four different interface phases: (1) perfectly bonded fibers; (2) mild imperfect interface; (3) severe imperfect interface; and (4) completely debonded fibers. As the number of cycles increases, the progressive transition from the perfectly bonded state to the completely debonded fiber state occurs. Eshelby's tensor for each imperfect state is calculated by the linear spring model for a damaged interface, and effective elastic properties are obtained using the multi-phase homogenization method. The fatigue damage evolution formulas for fiber, matrix and interface were proposed to demonstrate the fatigue behavior of CFRP laminates under cyclic loading. The material parameters for the fiber/matrix fatigue damage were characterized using the chaotic firefly algorithm. The model was implemented into the UMAT subroutine of ABAQUS, and successfully validated with flat-bar UD laminate specimens ([0]₈,[90]₈, [30]₁₆) of AS4/3501-6 graphite/epoxy composite.