Papers by Author: Kyung Ho Lee

Abstract: A reliable fabrication method for providing close spacing between the emitter mesa and the base contact metal of InP-based heterojunction bipolar transistor is disclosed. The silicon nitride sidewall was formed on the emitter electrode and mesa periphery. It was used as a mask for emitter mesa etching and also as an overhang to self-align the base contact with respect to the emitter mesa. The self-aligned device fabricated by this technique exhibited better high-frequency performances with fT of 138 GHz and fmax of 143 GHz, respectively, superior to the re-aligned one on the same epitaxy wafer.

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: 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).

Abstract: Low cycle fatigue cracks are mainly detected at discontinuous welded locations with high stresses under repeated cyclic static loads due to cargo loading and unloading. Theoretical and analytical methods have been used to estimate the local stress and strain, which affect the prediction of fatigue life, but these methods have difficulties considering stress concentration at notched locations and complicated material behavior of welded joints or heat affected zones. Electronic speckle pattern interferometry(ESPI) system is a nondestructive and non-contact measurement system, which can provide relatively accurate full field strain at critical positions such as welded zones and structurally discontinuous locations. In this study, local strain was measured by ESPI system at the welded cruciform joint, and then low cycle fatigue test was performed. Effect of local strain on low cycle fatigue life was examined by using the strain value measured by ESPI. In order to verify the relations between local strains and fatigue lives, after theoretical local strains and stresses were calculated by using Neuber’s rule, the measured local strains corresponding the experimental fatigue lives were compared with the results of Neuber’s rule and established codes of the British standard and DNV curve.