• Proceedings of the KWS Conference
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• 2005.11a
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• pp.17-19
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• 2005

To achieve high productivity of assembly hull blocks, it is important to predict welding deformations accurately and to apply these data to the production planning. In the deformation analysis of hull block, simplified methods (elastic analysis) such as inherent method, equivalent loading method and local & global approach are usually used instead of thermal-elastic-plastic analysis because of calculating time and cost. To be much more practical, these simplified methods should consider gravity effect of plate and contact condition between the plate and the positioning jig. In this research, using finite element method, practical predicting method for the welding deformation of the curved hull blocks with considering welding sequence, gravity effect and contact condition is proposed.

• Journal of Welding and Joining
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• v.26 no.3
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• pp.61-66
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• 2008

As ships become to be larger than ever, the thicker plate and the higher tensile steel plate are used in naval shipyard. Though special chemical composition is needed for high-tensile steels, recent high-tensile steels are made by the TMCP(Thermo-Mechanical control process) skill. The increase of yield stress and tensile stress of TMCP steels is induced from bainite phase which is transformed from austenite, but that increased yield stress can be vanished by another additional thermal cycle like welding and heating. As thermal deformations are deeply related by yield stress of material, the study for prediction of plate deformation by heating should reflect principle of TMCP steels. This study developed an algorithm which can calculate inherent strain. In this algorithm, not only the mechanical principles of thermal deformations, but also the predicting of the portion of initial bainite is considered when calculating inherent strain. The simulations of plate deformation by these values showed good agreements with experimental results of normalizing steels and TMCP steels in welding and heating. Finally we made an inherent strain database of steels used in Class rule.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.626-631
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• 2002

This paper describes the weldability and mechanical behavior of JIS S45C medium carbon steel (corresponding to KS SM45C and SAE 1045) for machine structures in CW Nd:YAG laser welding. ill general, medium carbon steels have a limited application to the industrial fields in spite of good mechanical characteristics. This is due to welding difficulty because of the high carbon contents and impurities in this material. Therefore, in this study the laser weldability of medium carbon steel with adjusted contents of S and P has been investigated in order to extend the application to medium carbon steels. Several experiments and numerical simulations have been conducted to determine the characteristics of mechanical behavior in CW Nd:YAG laser welds. The results of the simulations concur with the experiment results. From the result of this study, the application possibility of CW Nd:YAG laser welding to medium carbon steel has been confirmed. Also, the appropriateness of mechanical behavior simulation has been verified to analyze and predict the welding phenomena.

• Proceedings of the KWS Conference
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• 2003.11a
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• pp.216-218
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• 2003

Most of ferrous b.c.c weld materials may experience martensitic transformation during rapid cooling after welding. And it is well known that volume expansion due to phase transformation could influence in the case of welding of high tensile strength steels on the relaxation of welding residual stress. To apply this effect practically, it is a prerequisite to establish a numerical model which is able to estimate the effect of phase transformation on residual stress relaxation quantitatively. In this study, we investigated the effect of phase transformation on the relaxation of welding residual stress through experiment. And three-dimensional thermal elastic-plastic FEM analysis is conducted to compare the effect of phase transformation on the relaxation of welding residual stress in high strength steels(POSTEN60, POSTEN80) with analytical results which is not considering the effect of phase transformation on residual stress relaxation. According to the results, the extents of welding residual stress relaxation due to phase transformation in the case of welding of POSTEN60, POSTEN80 are 0.85 $\sigma$/$\sigma$$\sub$Y0/, 0.75$\sigma$/$\sigma$$\sub$Y0/, respectively.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.87-91
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• 2009

Welding is one of the most important joining processes and the effect of welding residual stresses in the structure has a great deal of influence on its quality. In this paper, recent development in computational welding mechanics, particularly calculation of welding residual stresses, is introduced. The hypoelastic formulation of finite element analysis for thermoelastic-plastic deformation is applied to welding processes to find residual deformations and stresses. Leblond's phase evolution equation coupled with the energy equation is employed to calculate the phase volume fraction; this plays an important role as a kinetics parameter affecting phase fraction effects in the mechanical constitutive equation of welded materials. Furthermore, transformation plasticity is taken into account for an accurate evaluation of stress. The influence of the phase transformation and the transformation plasticity on residual stress is investigated by means of numerical analyses using metallurgical parameters in Leblond's phase evolution equation that are adjusted with respect to various cooling rates in a CCT-diagram. Coding implementation is conducted by way of the ABAQUS user subroutines, UMAT.

• Transactions of the Korean Society of Automotive Engineers
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• v.23 no.6
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• pp.642-649
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• 2015

Resistance Spot Welding (RSW) is the method for joining two overlapped base materials when high pressure and current is applied from electrodes. Due to the safety problem such high pressure and voltage, automation should be early adopted. In this paper, the spot welding is developed as a computational model of wheel house from GM Korea and the welding condition such as weld sequence is considered. The computational analysis is preceded as a static and elasto-plastic procedure and used thermal expansion coefficient represents a dependency of spot volume between two panels. In case of welding sequence, the efficiency which depends on the distance between current spot point and the other is calculated in several cases.

• Journal of Welding and Joining
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• v.21 no.7
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• pp.49-58
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• 2003

It has been well known that ductile fracture of steel is accelerated by triaxiality stresses. The characteristics of ductile crack initiation in steels are evaluate quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. Recently, the characteristics of critical crack initiation of steels are quantitatively estimated using the two-parameter, that is, equivalent plastic strain and stress triaxiality, criterion. This study is paid to the fundamental clarification of the effect of geometrical heterogeneity and strength mismatching, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on critical condition to initiate ductile crack using two-parameter. Then, the crack initiation testing were conducted under static and dynamic loading. To evaluate the stress/strain state in the specimens especially under dynamic loading, thermal elastic-plastic dynamic FE-analysis considering the temperature rise was used. The result showed that the critical global strain to initiate ductile fracture in specimens with strength mismatch under various loading rate cu be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mismatch and dynamic loading effects on stress/strain behavior.

• Journal of the Korean Society for Precision Engineering
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• v.10 no.3
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• pp.47-56
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• 1993

Typical plastic strains in the machined surface are very difficult to measure, since they are located within a very short distance from the surface and they change very rapidly. There is an alternative way to determine the residual strain in plastically deformed materials by measuring the grain size after a subsequent recrystallization precess. Although, this technique has been successfully applied by several researchers to find the plastic zone around notches and cracks in various materials and welding beads, few works have been reported using the recrystallization method to determine the residual strains in machined surface. Therefore, the purpose of this investigation is to explore the effectiveness of the recrystallization technique in machining applications and in particular, to find the effect of cutting parameters, i.e., depth of cut, rake angle, on the plastic strains. As the result, the recrystallization technique was succesfully applid to determine the plastic strain in machined surface.

• The Journal of Korean Academy of Prosthodontics
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• v.40 no.2
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• pp.156-171
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• 2002

The purpose of this study was to measure and compare the strains produced by screw-tightening implant frameworks fabricated by aye different fabrication methods; (1) one-piece cast using plastic sleeve, (2) one-piece cast using gold cylinder, (3) laser welding, (4) soldering, and (5) electrical discharge machining, and also to measure and compare the strains produced when the order of screw tightening was changed A research model incorporating eighteen strain gages was made to measure the fit of implant frameworks in three dimensions. Three implants aligned in an arc were fixed on the top ends of the L-shape aluminum bars of the research model, and standard abutments were joined to the implants with abutment screws. Five types of implant framework were placed on the abutments and screwed by a torque wrench using 10 Ncm. Under the conditions of this study, the following conclusions were drawn: 1. The electrical discharge machining group showed the smallest magnitude of strain, followed by the soldering group, the laser welding group, the one-piece cast group using gold cylinder, and the one-piece cast group using plastic sleeve. However, among the magnitude of strain for the remaining groups except the electrical discharge machining group, there were not significant differences. 2. When the order of screw tightening was changed, there were not significant differences in the magnitude of strain. 3. In comparison with the electrical discharge machining group, the laser welding group and the one-piece cast groups showed greater horizontal distortion and the soldering group showed greater horizontal and vertical distortion.

• Journal of Welding and Joining
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• v.15 no.4
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• pp.90-98
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• 1997

A copper-tungsten sintered alloy(Cu-W) has been friction welded to a tough pitch copper in order to investigate the effect of friction pressure on bonding strength and a charicteristic of fracture. The tensile strength of the friction welded joint was increased up to 90% of the Cu base metal under the condition of friction time 1.2 sec, friction pressure 4.5kgf/$\textrm{mm}^2$ and upset pressure 10kgf/$\textrm{mm}^2$. From the results of fracture surface analysis, the increase of friction pressure could remarkably decrease the force and the time to be normally acted on weld interface. The W particles which were included in the plastic zone of Cu side could induce fracture adjacent to the weld interface because their existance in Cu induces a decrease in available section area and an increase in notch effect. Therefore, the tensile strength was decreased at high friction pressure (6kgf/$\textrm{mm}^2$) because the destruction of W was increased by an increase in mechanical force and crack was formed at weld interface.