• Journal of the Computational Structural Engineering Institute of Korea
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• v.28 no.1
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• pp.1-8
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• 2015

In this paper, computational evaluation method for fatigue crack growth rate(FCGR) based on material viscoplastic-damage model is proposed. Viscoplastic-damage model expressing material constitutive behavior of 7% nickel steel is introduced and is implemented into commercial finite element analysis(FEA) code, ABAQUS, as a user defined material subroutine(UMAT) for application in the FEA environments. Verification of developed UMAT and material parameters of material model are carried out by uniaxial tensile test simulations of 7% nickel steel. Moreover, jump-in-cycles procedure and rearrangement of critical damage are employed and also implemented to the ABAQUS UMAT for fatigue damage analysis. Typical FCGR test results such as relationship between crack length and number of cycles and relationship between da/dN and ${\Delta}K$ could be obtained from FCGR test simulation using developed UMAT and these results are compared with experimental results in order to verify of proposed computational method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.4
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• pp.353-358
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• 2011

In this paper, we describe a process for optimally designing a ring-type permanent magnet thrust bearing. The bearing consists of two sets of permanent magnet rings. One set is located inside the other set. An axial offset between the two sets creates axial force, which results in a thrust bearing function. In order to realize an optimal design of the bearing where the required load capacity of the bearing is achieved with the least magnet volume, we derived analytical design equations by adopting the equivalent current sheet (ECS) method. We considered the following two types of magnet arrays: axial arrays and Halbach arrays. These two types of arrays are optimized using the analytical design equations. The results of the optimization are verified using three dimensional (3D) finite element analyses (FEA). The results show that the Halbach array can achieve the required load capacity with less amount of permanent magnet than the axial array does. The efficacy of the ECS method is also verified by using 3D FEA. It is found that the accuracy of ECS method is more sensitive to the underlying assumptions for the Halbach array than for the axial array.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.9
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• pp.1137-1143
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• 2010

This paper presents a fatigue design method for plug- and ring-type gas-welded joints, which takes into account the effects of welding residual stress. To develop this method, we simulated the gas-welding process by performing nonlinear finite element analysis (FEA) To validate the FEA results, numerically calculated residual stresses in the gas welds were then compared with experimental results obtained by the hole-drilling method. To evaluate the fatigue strength of plug- and ring-type gas-welded joints influenced by welding residual stresses, the use of stress amplitude $(\sigma_a)_R$, which includes the welding residual stress in gas welds, is proposed $(\sigma_a)_R$ on the basis of a modified Goodman equation that includes the residual stress effects. Using the stress amplitude $(\sigma_a)_R$ at the hot spot point of gas weld, the relations obtained as the fatigue test results for plug and ring type gas welded joints having various dimensions and shapes were systematically rearranged to obtain the $(\sigma_a)_R-N_f$ relationship. It was found that more systematic and accurate evaluation of the fatigue strength of plug- and ring-type gas-welded joints can be achieved by using $(\sigma_a)_R$.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.27 no.2
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• pp.95-102
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• 2014

In this paper, a coupling scheme for applying finite element analysis(FEA) programs, such as, LS-DYNA and MIDAS/Civil, to a nonlinear soil structure interaction analysis by the boundary reaction method(BRM) is presented. With the FEA programs, the structure and soil media are discretized by linear or nonlinear finite elements. To absorb the outgoing elastic waves to unbounded soil region as much as possible, the PML elements and viscous-spring elements are used at the outer FE boundary, in the LS-DYNA model and in MIDAS/Civil model, respectively. It is also assumed that all the nonlinear elements in the problem are limited to structural region. In this study, the boundary reaction forces for the use in the BRM are calculated using the KIESSI-3D program by solving soil-foundation interaction problem subjected to incident seismic waves. The effectiveness of the proposed approach is demonstrated with a linear SSI seismic analysis problem by comparing the BRM solution with the conventional SSI solution. Numerical comparison indicates that the BRM can effectively be applied to a nonlinear soil-structure analysis if motions at the foundation obtained by the BRM for a linear SSI problem excluding the nonlinear structure is conservative.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.6
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• pp.528-538
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• 2011

As a forming method for curved hull plates more efficient than the flame bending, mechanical bending using multi point press forming and die-less forming is discussed in this paper. the mechanical forming is a flexible manufacturing system for automatically forming of hull parts. It is especially suited to varied curved parts. This paper discusses a multiple point pressing machine composed of a pair of reconfigurable punches in order to achieve the rapid forming of curved hull plates using division forming and presents how forming information is obtained from the given design surface. Although the mechanical forming can be efficient in the metal forming, spring back after pressing is a phenomenon which must be carefully considered when quantifying the process variables. If the spring back is not accurately controlled, the fabricated shell plate cannot meet assembly tolerance. This paper describes the principles to calculate the proper stroke of each punch at the divided areas. the strokes are determined by an iterative process of sequential pressing and spring back compensation from an unfolded flat shape to its given design surface. FEA(finite element analysis) is used to simulate the spring back of the plate and the IDA(iterative displacement adjustment) method adjusts the offset of pressing punches from the deformation results and the design surface. The shape deviations of two surfaces due to spring back are compensated by integrated system using FEA and IDA method. For the practical application, It is aimed to develop an integrated system that can automatically perform the compensation process and calculate strokes of punches of the double sides' reconfigurable multiple-press machine and some experimental results obtained with mechanical bending are presented.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.13 no.5
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• pp.364-374
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• 2003

The performance of an acoustic transducer is determined by the effects of many design variables. and mostly the influences of these design variables are not linearly independent of each other To achieve the optimal performance of an acoustic transducer, we must consider the cross-coupled effects of the design variables. In this study with the FEM. we analyzed the variation of the resonance frequency and sound pressure of a flextensional transducer in relation to Its design variables. Through statistical multiple regression analysis of the results, we derived functional forms of the resonance frequency and sound pressure in terms of the design variables, and with which we determined the optimal structure of the transducer by means of a constrained optimization technique, SQP-PD. The proposed method can reflect all the cross-coupled effects of multiple design variables, and can be utilized to the design of general acoustic transducers.

• Journal of Korean Association for Spatial Structures
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• v.7 no.3 s.25
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• pp.87-95
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• 2007

The recent completions of free-form spatial structures provide us a very attractive form. To realize such an extraordinary shape, it is absolutely necessary that the connector systems have to be investigated the characteristics of the systems and analyzed with a practicable method. In this context, this research consists of not only literature research but also numerical analysis with selected connector systems, which was adopted in real free-form spatial structures. For numerical analysis, especially, finite element analysis (FEA) is performed with a various test parameter using a commercial program ANSYS. Consequently, the general characteristics of node connectors the moment-rotation-curves are presented by considering a large deformation effect as well as a multi-linear material properties.

• Proceedings of the KSR Conference
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• 2011.10a
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• pp.412-419
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• 2011

With an increase in rail traffic, developing activities around structures of railway have been expanded. Inevitably, the changes to cross though sub-structures of railway have been getting increased. However, this situation affects on the safe operation of trains. Generated wheel load makes on the result in settlement on roadbed and damages on track materials. Therefore, via the numerical analysis were carried out for the box structure and subground using FEM analysis program called. Visual FEA/Geo 4.19. Parametric studies were performed by changing soil depth above box structure constructed in railway embankment. A standard live load was applied to simulate loads from train. Through this study, a minimum required soil depth above subground box structure was recommended based on deformation and stresses in concrete railway system.

• Journal of Electrical Engineering and Technology
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• v.12 no.1
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• pp.256-261
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• 2017

This paper deals with the experimental evaluation on power loss of a double-side permanent magnet synchronous motor/generator (DPMSM/G) applied to a flywheel energy storage system (FESS). Power loss is one of the most important problems in the FESS, which supplies the electrical energy from the mechanical rotation energy, because the power loss decreases the efficiency of energy storage and conversion of capability FESS. In this paper, the power losses of coreless DPMSM/G are separated by the mechanical and rotor eddy current losses in each operating mode. Moreover, the rotor eddy current loss is calculated by the 3-D finite element analysis (FEA) method. The analysis result is validated by separating the power loss as electromagnetic loss and mechanical loss by a spin up/down test.

• Proceedings of the Korea Institute of Applied Superconductivity and Cryogenics Conference
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• 2002.02a
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• pp.312-314
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• 2002

Magnetic circuit design of HTS (High Temperature Superconducting) motor is important to achieve the power at a given load condition, and it is essential to the thermal design for HTS motor rotors. To determine the result of thermal design, the magnetic field distribution has to be known exactly. On the basis of the 2 dimensional magnetic field analysis, the magnetic field distributions due to several cases are calculated by using Biot-Savart equation and magnetic image method. And the I$_{c}$ of HTS field coil was calculated by using I$_{c}$-B(equation omitted) curve and 3D FEA(Finite Element Analysis).is).