Papers by Author: Gee Wook Song

Abstract: The vehicle suspension system is directly influenced to ride and handling. Therefore, the major components of the vehicle suspension system should have enough fatigue strength during its lifetime to protect passenger from the traffic accident. Spring is one of the major suspension part of vehicle. Thus, in this paper, a fatigue design method for leaf spring was proposed. At first, numerical stress analysis for leaf spring assembly was performed. On the base of the analysis results, fatigue strength of leaf spring was assessed. And next, after studying numerically on geometrical parameters of leaf spring assembly, an economical prediction method of fatigue design criterion for leaf spring assembly using the theory of artificial neural network was developed and certified its usefulness. Without performing a lot of additional fatigue test for a long time, fatigue design criterion for a new leaf spring assembly having different geometry can be predicted on the base of the already obtained fatigue data.

Abstract: An experimental program has been carried out to address the thermo-mechanical fatigue life of the uncoated IN738LC nickel-base superalloy. High temperature isothermal Fatigue and out-of-phase(OP), in-phase(IP) TMF experiments in strain control were performed on superalloy materials. Temperature interval of 450-850
was applied to thermo-mechanical fatigue tests. The stress-strain response and the life cycle of the material were measured during the test. The plastic strain energy based life pediction models were applied to the stress-strain history effect on the thermo-mechanical fatigue lives.

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.

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.

Abstract: The operation mode of thermal power plants has been changed from the base load to duty cycle. From the changeover, fossil power plants cannot avoid frequent thermal transient state, for example, start up and stop, which results in thermal fatigue damage at the heavy section components. The rotor is the highest capital cost component in steam turbine and requires long outage for replacing the new one. For optimized power plant operation life and inspection management of rotor is necessary. It is known as general that start-up and shutdown operation greatly affect on steam turbine life. The start-up operation condition is especially severe because of the rapid temperature and rotational speed increase, which causes damage and reduction of main components life of steam turbine. The start-up stress of rotor which is directly related life is composed of thermal and rotational stresses. The thermal stress is due to the variation of steam flow temperature and rotational stress is due to the rotation speed of itself. In this paper, the analysis method of start-up stress of rotor which considers simultaneously temperature and rotation speed transition is proposed, which includes a case study regarding 500MW fossil power plant steam turbine rotor. Also, the method of damage quantitative estimation of fatigue damage to operation condition is described. The method can be applied to find weak points to the fatigue damage. Using the method, total life consumption can be obtained, and can be also used for determining future operation mode and the life extension of old fossil power units.

Abstract: A more accurate life prediction for gas turbine blade takes into account the material behavior under the complex thermo-mechanical fatigue (TMF) cycles normally encountered in turbine operation. An experimental program has been carried out to address the thermo-mechanical fatigue life of the IN738LC nickel-base superalloy. High temperature out-of-phase and in-phase TMF experiments in strain control were performed on superalloy materials. Temperature interval of 450-850
was applied to thermo-mechanical fatigue tests. The stress-strain response and the life cycle of the material were measured during the test. The mechanisms of TMF damage is discussed based on the microstructural evolution during TMF. The plastic strain energy based life pediction models were applied to the stress-strain history effect on the thermo-mechanical fatigue lives.

Abstract: Most domestic fossil power plants have exceeded 100,000hours of operation with the severe operating condition. Also, cyclic operation of the plant subjects the piping system to mechanical and thermal fatigue mechanisms and poor of defective support assemblies can impose massive loads onto the piping system and induce unstable piping displacement. In order to prevent the serious damage and failure of the piping system in fossil power plants, 3-dimensional displacement measurement system was developed for the on-line monitoring. Displacement measurement system was developed with LVDT and rotary encoder type sensors. This system was installed and operated on a real power plant successfully.

Abstract: Acoustic emission (AE) technique was used to investigate fatigue crack growth on compact tension specimens of aging materials at room temperature. Test materials have been sampled steam pipe serviced the actual operation conditions for a long time in fossil power plant. The compact tension test specimens were subjected to load stress ratios of 0.33, 0.5, and 0.66. All the fatigue tests were performed at a frequency of 1Hz. The test results indicate that acoustic emission counts show reasonable correlation with crack propagation rates for applied stress ratios. When the crack growth rates increase, AE’s counts and energies show increment. Also, the higher stress ratios, the faster crack propagation rates. Based on these relationships it may be possible to predict the remaining service life of fatigue-damaged steam pipes.

Abstract: For prevention of failure and extension of maintenance interval, many Asian power companies have continuously tried to apply Condition Based Maintenance system to their plants. However, only few companies succeed to change their maintenance system from fixed time based to equipment condition based, although there are many brand new ITs, such as high speed computing system, huge storage device, and advanced development software tools. This paper describes the reasons why they failed to change and proposes the solutions to construct maintenance management system based on equipment condition. The solutions were also applied practically to power plant in Korea.

Abstract: Degradation of the gas turbine hot-gas-path components, the 1st stage blades and vanes, serviced for a period was evaluated by measuring the mechanical properties. For this, tensile and impact tests on these gas turbine parts were performed. Microstructure of the substrate and coating layers were also observed. The mechanical properties of the serviced blades were degraded by about 30% comparing with those of unused ones. In terms of the microstructure, the dissolution of the secondary g’ phase and subsequent coarsening of precipitates were observed in the substrate. And the interdiffusion zone near the coating layer was disappeared.