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고승기(Koh.S.K),류창훈(Ryu.C.H),이정준(Lee.z.J),나의균(Na.E.G),백태현(Baek.T.H),전경락(Jeon.K.L.) 대한기계학회 2003 대한기계학회 춘추학술대회 Vol.2003 No.11
A failure analysis of holddown spring screw was performed using fracture mechanics approach. The<br/> spring screw was designed such that it was capable of sustaining the loads imposed by the initial<br/> tensile preload and operational loads. In order to investigate the cause of failure, a stress analysis of<br/> the top nozzle spring assembly was done using finite element analysis and a life prediction of the<br/> screw was made using a fracture mechanics approach. The elastic-plastic finite element analysis<br/> showed that the local stresses at the critical regions of head-shank fillet and thread root significantly<br/> exceeded than the yield strength of the screw material, resulting in local plastic deformation. Primary<br/> water stress corrosion cracking life of the Inconel 600 screw was predicted by using integration of<br/> the Scott model and resulted in 1.42 years, which was fairly close to the actual service life of the<br/> holddown spring screw.
고승기(Koh S.K),황한영(Hwang H.Y) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.11
Strength analysis of a diaphragm made of HNBR rubber used for a damper in a vehicle self-leveling system was performed. Finite element technique was used for the stress analysis of the diaphragm with two types of SR and SS. Loading was divided into two steps of assemblage of diaphragm in the damper and internal pressure on the diaphragm. High stress and strain concentrations were observed at the lower end of both types of diaphragms. SR type of diaphragm was superior to SS type in static strength and fatigue cracking was expected at the local region of diaphragm which was in contact with the lower holder in the damper system.
고승기(Koh S.K.),류창훈(Ryu C.H.) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.11
A lifetime prediction of holddown spring screw in nuclear fuel assembly was performed using fracture mechanics approach. The spring screw was designed such that it was capable of sustaining the<br/> loads imposed by the initial tensile preload and operational loads. In order to investigate the cause of failure and to predict the stress corrosion cracking life of the screw, a stress analysis of the top<br/> nozzle spring assembly was done using finite element analysis. The elastic-plastic finite element analysis showed that the local stresses at the critical regions of head-shank fillet and thread root<br/> significantly exceeded than the yield strength of the screw material, resulting in local plastic deformation. Normalized stress intensity factors for PWSCC life prediction was proposed. Primary<br/> water stress corrosion cracking life of the Inconel 600 screw was predicted by using integration of the Scott model and resulted in 1.78 years, which was fairly close to the actual service life of the<br/> holddown spring screw.
고승기(Koh S.K),이정준(Lee J.J.) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
Fatigue lifetime of a breech system was estimated in order to ensure the structural integrity of the breech system used for a thick-walled tube subjected to pulsating high internal pressure. A stress analysis of the breech was performed to locate the critical region vulnerable to crack initiation. Low-cycle fatigue behavior of the breech material was investigated to obtain the fatigue crack initiation properties. Elastic-plastic finite element stress analysis resulted in a stress concentration at the breech ring groove root. Local strains at the groove root was experimentally measured using strain gages and resulted in similar values compared to the calculated strains. Local strain approach was employed to estimate the fatigue life of the breech system, resulting in 9,124 cycle for crack initiation at the groove root of the breech ring. Fatigue test using simulation specimens was performed and an average fatigue life of 10,497 cycle was obtained, which was fairly close to the calculated fatigue life.
변형률에너지법에 의한 1Cr-0.5Mo-0.1V강의 피로손상평가
고승기(S.K. Koh),나의균(E.G. Na),하정수(J.S. Ha) 대한기계학회 2002 대한기계학회 춘추학술대회 Vol.2002 No.3
In order to represent and evaluate the fatigue damage of 1Cr-0.5Mo-0.1V steel used for high strength pressure vessels, strain-controlled fatigue tests using uniaxial specimens were performed. Based on the fatigue test results from different strain ratios of -2, -1, 0, 0.5, 0.75, the fatigue damage of the steel was evaluated by using a strain energy density. Mean stress relaxation depended on the magnitude of the applied strain amplitude. The high pressure vessel steel exhibited the cyclic softening behavior. Total strain energy density consisting of the plastic strain energy density and the elastic tensile strain energy density described fairly well the fatigue life of the steel, taking the mean stress effects into account. Fatigue lives predicted by the strain energy density correlerated very well with the experimental fatigue lives within factors of 3 to 4, compared to other fatigue damage parameters.
고승기(S.K. Koh),김병운(B.W. Kim),원삼용(S.Y. Won),류창훈(C.H. Ryu) 한국자동차공학회 2002 한국자동차공학회 지부 학술대회 논문집 Vol.- No.-
In order to investigate the behaviors of fatigue crack propagation of SiC-particulate-reinforced Al-Si metal matrix composites, fatigue tests using single edge notched tension(SENT) specimens were performed. Composite materials were manufactured by using both permanent die casting and extrusion processes with different volume fractions of 10% and 20%. SiCp-reinforced AI-Si composites showed higher levels of threshold stress intensity factor range, ??K??, for the higher volume fractions of SiC particles, implying the higher fatigue crack propagation resistance at the threshold or low ??.K level, compared to the unreinforced Al-Si alloy. Extruded materials showed higher values of threshold L'.K than the cast materials. However, the composites showed similar or lower resistance of fatigue crack propagation at the Paris region compared to the unreinforced Al-Si alloy. Stress intensity factor range at fracture decreased. as the volume fraction of SiC particle increased due to the lower fracture toughness of composites with higher SiCp volume fraction.<br/>
고승기(Koh S.K),송원주(W.J. Seo),서광선(Seo K.S),최현선(Choi H.S) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
In order to investigate the optimum condition of the autofrettage process for the diesel engine injection pipe, different values of autofrettage pressure, pressure rising time, pressure holding time, and repetition of autofrettage process were applied. Autofrettage was preformed by applying the hydrostatic internal pressures of 603 ㎫, 535 ㎫, 500 ㎫ on the fuel injection pipe, corresponding to theoretically 50%, 30%, and 20% overstrain levels, respectively. The autofrettage residual stresses in the injection pipe were experimentally determined by using X-ray diffractometer. As the overstrain level increased, the magnitude of compressive residual stress at the bore increased. It was found that the rising time to reach the autofrettage pressure, holding time at the autofrettage pressure, and repeating application of the autofrettage pressure on the pipe had no significant influence on the residual stress distributions.
고승기(S.K. Koh),연문흠(M.H. Yeon),나의균(E.G. Na) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.5
Application of a very high internal pressure on the thick-walled cylinder induces beneficial compressive tangential residual stresses near the bore of the cylinder after unloading the internal pressure. However, a reverse yielding due to the Bauschinger effect during the unloading process causes the reduction of the compressive residual stress near the bore. In order to evaluate the autofrettage residual stress distributions of the thick-walled cylinder, the Bauschinger effects were considered. Magnitudes of the compressive residual stresses at the cylinder bore determined by considering the Bauschinger effects decreased by about 25 percent, compared to the case of linear elastic unloading, I.e., without Bauschinger effects. Measured residual stress distuibutions agreed?fairly well with the calculated distributions considering the Bauschinger effects