Papers by Author: Jang Hyun Lee

Abstract: Flame bending has been widely used for forming hull plates of marine vessels, Since it is difficult to estimated the amount of residual deformation after applying the flame heat, flame bending has been carried out by skilled workers without automatic facility. A mechanical bending such as multiple-piston pressing forming has been studied as an alternative to the flame bending. However, springback is a major problem in the press forming process. Present paper presents a combination of FEM (Finite Element Method) and springback compensation algorithm to calculate the compensated stroke of pressing points. In order to calculate the springback, the process is modeled by an elastic plastic material and shell elements. Combination of global scale factor and local scale factors is suggested to adjust the amount of strokes through an iterated numerical calculation. In each iteration, shape deviation between object surface and processed shape is minimized to reach the designed shape. The shape deviations due to springback are compensated using the residual shape estimated by FEM.

Abstract: Eddy currents of electromagnetic field leads not only to the local heating of plate but also to the thermal-elasto-plastic deformation in the induction heating. It is necessary to have a simulation model to attract the possibility of induction heating equipment and to study the deformation behavior. The goal of present paper is to investigate the possibility of induction heating equipment for steel plate bending. The residual stress distribution of induction heating is investigated by an electromagnetic analysis in conjunction with thermal-mechanical analysis. A computational model based on FEA is used to study the electromagnetic field and thermalmechanical process. Comparison with the residual strain fields and deformation of both heating shows that the induction heating has good similarity with the gas heating.

Abstract: A finite element procedure to predict residual stresses on the multi-pass arc welding considering the multi-layered beads and solid–liquid phase transformation in the melted bead is implemented and experimentally validated. The nonlinearities of material properties are considered in the thermo-elasto-plastic distortion analysis. Element birth and death technique is used to simulate the weld metal added to base metal in both heat transfer and thermo-elasto-plastic analysis. Residual stresses and strains are computed for butt welding and T-joints fillet welding. The stress distributions calculated by proposed finite element analysis procedure are compared with the measured values of experiments and data available in the other studies. The efficiency of the proposed finite element analysis and experimental procedure is discussed and demonstrated.

Abstract: In this study, an experimental method has been studied to measure the residual stresses on a specimen with I-groove weldment. The specimens were extracted from I-groove weldment which was joined by SAW (Submerged Arc Welding) with CO2 shield gas. A FEA (Finite Element Analysis) model was developed for the estimation of the residual stresses for the specimen. Measurements were carried out using ESPI(Electronic Speckle Pattern Interferometry) system which can measure the strain distribution on the surface of specimen. The residual stresses were estimated by the value of strain measured by ESPI system. Strain gages were added to evaluate the accuracy of ESPI system. In addition, a three-dimensional FE model was used to estimate the residual stresses generated by the welding procedure. A thermal elasto-plastic analysis was performed by the FEA. The stresses measured by the experiments were compared with the results of FEA. Also, discussed are the difference and agreement between the stresses obtained by experiments and FEA, respectively.

Abstract: The turbine wheels of a turbocharger are operated at high revolution speed in high temperature inlet gas. Alloy 713LC blades of the turbine wheel broke in an hour the during a model test. Two failures and several cracks were found in the turbine blades. Failures in blades are suspected to occur as a result of thermal mechanical stresses or fatigue load and other cause such as creep-rupture and resonant vibration. The present study investigates the possible causes of the failure of these blades. FEM (Finite Element Method) was used to calculate the thermal centrifugal stresses and natural frequency to find the cause of failures. LCF (Low Cycle Fatigue) life of blades was roughly estimated by using the stress and strain level calculated by FEM. The investigation indicates that the failures were associated with resonant forces and HCF (High Cycle Fatigue).