Papers by Author: Woo Sung Choi

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Authors: Woo Sung Choi, Eric Fleury, Gee Wook Song, Jung Seob Hyun
Abstract: An important characteristic of a steam power plant is its ability to maintain reliability and safety of the plant against frequent start-ups and load changes. Transient regimes arising during start-ups, shut-downs and load changes give rise to unsteady temperature distribution with time in steam turbine rotor(HP/IP), which results in non-uniform strain and stress distribution. The rapid increase of temperature and rotational speed during starts-ups, especially, makes conditions more severe and causes main components’ damage and reduction of life span for steam turbine. Thus accurate knowledge of thermal and centrifugal stresses are required for the integrity and lifetime assessment for the turbine rotor. So far, only elastic calculations are currently performed for simplicity. However, it is well known that the materials of steam turbine rotor deform inelastically at high temperature. Existing models proposed to describe the viscoplastic(rate-dependent) behavior are rather elaborate and difficult to incorporate with computer simulations in the case of complex structures. In this paper, the life assessment for steam turbine rotor was established by combining the inelastic behavior and the finite element method. The inelastic analysis was particularly focused on viscoplastic behavior that is simple enough to be used effectively in computer simulation and matches the essential features of the time-dependent inelastic behavior of materials reasonably well for cyclic loading under non-isothermal conditions. Using this study, life consumption of steam turbine rotor can be obtained.
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Authors: Woo Sung Choi, S.T. Choi, Sang Uk Son, Seung Seob Lee, S.Y. Yang, Y.Y. Earmme
Abstract: In order to measure the mechanical properties of gold films on silicon substrate, two types of specimens, i.e., bridged films and circular membranes, are manufactured. Using a wedge tip, the bridged gold films are indented so that the films are pushed off, which is called as V-peel test. The load-deflection curves obtained by the V-peel test are analyzed with the concept of geometrically nonlinear beam by using the minimum potential energy theory together with Ritz method. Thus, Young’s modulus and residual stress of the bridged gold films are obtained. Blister test is also conducted to measure the Young’s modulus and residual stress of a circular gold membrane, of which deformation is measured by Twyman-Green interferometer. By gradually increasing the external pressure applied on the membrane, the interfacial fracture toughness between the gold membrane and silicon substrate is measured based on the concepts of interfacial fracture mechanics.
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Authors: Bum Shin Kim, Jung Soo Ha, Gee Wook Song, Jung Seob Hyun, Woo Sung Choi
Abstract: As a number of aged fossil power plants recently increased, the precise life assessment of critical equipments gets to be important more than ever. Despite of infrequent likelihood of failure, the equipments in high pressure and temperature operation condition have traditionally been considered as critical because of huge consequence of the equipments and hence life assessment of fossil power plant has been focused on all of the severe operated equipments for past decades. Nowadays, with Risk-Based Inspection technology being developed rapidly, most of the power plant utilities get a chance to reduce the scope of the inspection and test and to extend the interval for the life assessment. This paper provides methodology based on Risk-Based Inspection technology to optimize the life assessment work scope and interval and also demonstrates the enhanced life assessment procedure including risk assessment of equipments.
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