• Transactions of Materials Processing
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• v.13 no.7
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• pp.614-622
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• 2004

A new approach for the automatic elimination of free edges in the finite element model for the analysis of sheet metal forming processes is presented. In general, the raw finite element model constructed from an automatic mesh generator is not well suited for the direct use in the downstream forming analysis due to the many free edges which requires tedious time consuming interactive graphic operations of the users. In the present study, a general method for the automatic elimination of free edges is proposed by introducing a CAD/CAE hybrid method. In the method a trimmed parametric surface is generated to fill the holes which are orginated from the free edges by using the one step elastic finite element analysis. In addition, mesh generation algorithm is suggested which can be used in the general trimmed surface. In order to verify the validity of the proposed method, various examples including actual automobile sheet metal parts are given and discussed.

• Transactions of Materials Processing
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• v.15 no.6 s.87
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• pp.445-452
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• 2006

In most of automobile body panel manufacturing, trimming process is generally performed before flanging. To find feasible trimming line is crucial in obtaining accurate edge profile after flanging. Section-based method develops blank along manually chosen section planes and find trimming line by generating loop of end points. This method suffers from inaccurate results of edge profile. On the other hand, simulation-based method can produce more accurate trimming line by iterative strategy. In this study, new fast simulation-based method to find feasible trimming line is proposed. Finite element inverse method is used to analyze the flanging process because final shape after flanging can be explicitly defined and most of strain paths are simple in flanging. In utilizing finite element inverse method, the main obstacle is the initial guess generation for general mesh. Robust initial guess generation method is developed to handle genera] mesh with very different size and undercut. The new method develops final triangular mesh incrementally onto the drawing tool surface. Also in order to remedy mesh distortion during development, energy minimization technique is utilized. Trimming line is extracted from the outer boundary after finite element inverse method simulation. This method has many advantages since trimming line can be obtained in the early design stage. The developed method is verified by shrink/stretch flange forming and successfully applied to the complex industrial applications such as door outer flanging process.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.05a
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• pp.209-212
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• 2001

It is known that the mim forming processes show somewhat different phenomena compared with the conventional metal forming processes, namely, the size effect, enhanced friction effect and etc. Such typical phenomena, however, are not predicted by the conventional finite element analysis, which has been an efficient numerical tool to predict the metal forming processes. It is due to the fact that the constitutive relations used does not describe the microstructural characteristics of the materials. In the present investigation, the finite element formulation using the rate-dependent rigid plastic crystal plasticity model of the face-centered cubic materials is conducted to predict the micro mechanical behaviors during the mim forming processes. The finite element analysis, however, provides mesh-dependent solutions for the intragranular deformations. Therefore, the couple stress energy is additionally introduced into the variational principle and formulated within the framework of the rigid plastic finite element method to obtain mesh-independent solutions. Micro deformations of single crystal and bicrystal with various orientations are calculated to show the potential of the developed formulation.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2002.05a
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• pp.125-128
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• 2002

The automotive industry have made an effort to reduce the weight of vehicle structures with increased safety, while initial model of the final product does not contain any prehistoric effects in a design stave. It takes lots of time to calculate forming effects that have great influences on the energy absorption of structures. In this paper, finite element inverse analysis is adopted to calculate forming effects, such as thickness variation and effective plastic strain as well as an initial blank shape with small amount of computation time. Crash analysis can be directly performed after inverse analysis of the forming process without remeshing scheme. The direct mesh mapping method is used to calculate an initial guess from the sliding constraint surface that is extracted from the die and punch set. Analysis results show that energy absorption of structures is increased with consideration of forming effects and finite element inverse analysis is usefully applicable to calculate forming erects of vehicle structures for the crash analysis.

• Korean Journal of Computational Design and Engineering
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• v.18 no.6
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• pp.461-469
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• 2013

Computer simulation with FEM is very useful to analyze hypervelocity impact phenomena that are tremendously expensive or otherwise too impractical to analyze experimentally. Shock physics can be efficiently handled by mesh adaptation which allows finite element mesh to be locally optimized to resolve moving shock wave in explosion. In this paper, an adaptive meshing technique based upon quadtree data structure was applied to resolve ballistic impact phenomena. The technique can adaptively refine a mesh in the neighborhood of a shock and coarsen the mesh for the smooth flow behind the shock according to a criterion. The criterion for refinement and coarsening is based upon the standard deviation of the gradient of shock pressure on the associated field. Shock simulation starts with the rough mesh of the pressure field and mesh density is increased locally under the criterion at each time step. The results show that the mesh adaptation enables to minimize the global computation error of FEM and to increase storage and computational saving compared to the fixed resolution of the conventional static mesh approach.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.4 s.142
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• pp.388-395
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• 2005

A numerical procedure is proposed as a mesh-size insensitive structural stress definition that gives a stress state at a weld toe with relatively large mesh size. The structural stress values obtained using different finite element types, i.e. shell element and solid element, are examined for typical weld structures. The calculation procedures are performed using the balanced nodal forces and moments obtained from finite element solutions. A consistent formulation based on work equivalent argument has been implemented to transform the balanced nodal forces and moments from shell to line force and line moments at each nodal position. The mesh-insensitivity, the effect of distance $\delta$(where the stress is calculated) and the potential limitations of the structural stress method are examined for various types of weldments. Based on the results from this study, it is expected to develop a more precise stress estimation technique for fatigue strength assessment of welded structures.

• Structural Engineering and Mechanics
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• v.28 no.2
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• pp.207-220
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• 2008

The shear lag has been studied for many years. Nevertheless, existing research gives a variety of stress concentration factors. Unlike the elementary beam theory, the application of load is not unique in reality. For example, concentrated load can be applied as point load or distributed load along the height of the web. This non-uniqueness may be a reason for the discrepancy of the stress concentration factors in the existing studies. The finite element method has been often employed for studying the effect of the shear lag. However, not many researches have taken into account the influence of the finite element mesh on the shear lag phenomenon, although stress concentration can be quite sensitive to the mesh employed in the finite element analysis. This may be another source for the discrepancy of the stress concentration factors. It also needs to be noted that much less studies seem to have been conducted for the shear lag effect on deflection while some design codes have formulas. The present study investigates the shear lag effect in a simply supported box girder by the three-dimensional finite element method using shell elements. The whole girder is modeled by shell elements, and extensive parametric study with respect to the geometry of a box girder is carried out. Not only stress concentration but also deflection is computed. The effect of the way load is applied and the dependency of finite element mesh on the shear lag are carefully treated. Based on the numerical results thus obtained, empirical formulas are proposed to compute stress concentration and deflection that includes the shear lag effect.

• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.13 no.2
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• pp.10-15
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• 1984

The aims of this study are to obtain a suitable method and a proper mesh for investigation of the temperature distribution and heat transfer. The relative errors of temperature distribution and heat transfer for each mesh are acquired in accordance with linear finite element (FEM 3), square finite element (FEM 6), cubic finite element (FEM 10), and finite difference method (FDM). It has been found that FEM 10 is the most accurate measure to obtain the temperature distribution and heat transfer. However, no significant results have been obtained successfully, because when higher order finite element methods are used the more computational efforts are necessary due to the distribution of elements. The results of this study are as follows ; 1 . In case of a=b=L, meshes for less than $1\%$ relative errors (temperature distribution) acquired in various methods to exact solution are $2\times2,\;4\times4,\;8\times8\;and\;8\tiems8$ for each FEM 10, FEM 6, FEM 3 and FDM and a=L, b=1/2L are $10\times5$ for each FEM 3 and FDM. And the tendency of results acquired of heat transfer is similar to those above. 2 . In computational efforts (a=b=L), FEM 6 has taken 21 times. and FEM 10 154times FEM 3 and FDM and FEM 3 is the sane as FDM.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.367-371
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• 2001

In the shape optimization based on the finite element method, the accuracy of finite element analysis of a given structure is important to determine the final shape. In case of a bending dominant problem, finite element solutions by the full integration scheme are not reliable because of the locking phenomenon. Furthermore, in the process of shape optimization, the mesh distortion is large due to the change of the structure outline: therefore, we cannot guarantee the accurate result unless the finite element itself is accurate. We approach to more accurate shape optimization to diminish these inaccuracies by improving the existing finite element. The shape optimization using the modified finite element is applied to a two-dimensional simple beam. Results show that the modified finite element have improved the optimization results.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.23 no.12
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• pp.48-57
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• 2009

This paper presents optimal design of direct-driven PM wind generator using MADS (Mesh Adaptive Direct Search). Optimal design of the direct-driven PM Wind Generator, combined with MADS and FEM (Finite Element Method), has been performed to maximize the Annual Energy Production (AEP) over the whole wind speed characterized by the statistical model of the wind speed distribution. In particular, the newly applied MADS contributes to reducing the computation time when compared with Genetic Algorithm (GA) implemented with the parallel computing method.