Jae Do Kwon

A Study on Fretting Fatigue Behavior of Degraded 1Cr-0.5Mo Steel

The aged degradation of material is observed when heat-resisting steel is exposed for long periods of time at high temperatures. In the present study, the degraded 1Cr-0.5Mo steel that is used for long periods of time at high temperature(about 515°C) and artificially reheat-treated materials are prepared. These materials were used to study the effect of aged degradation on fretting fatigue behavior. Through this experiment, it is found that the fretting fatigue strength of reheat-treated 1Cr-0.5Mo steel is approximately 46 percent lower than that of the plain fatigue strength of the same material. Furthermore, the fretting fatigue strength of degraded 1Cr-0.5Mo steel was less than 53 percent of the same material™s plain fatigue strength. The maximum value of fatigue strength difference is observed as 57 percent between the fretting fatigue of degraded material and plain fatigue of reheat-treated material. These results can be used as basic data in a structural integrity evaluation of heat-resisting steel considering aged degradation effects.

Multiaxial Fatigue Life Prediction of Duplex Stainless Steels with Thermal Aging at 430°C under Axial-Torsional Load

Multiaxial low cycle fatigue tests with degradation were performed on duplex stainless steels(CF8M). The fatigue properties of CF8M to predict multiaxial fatigue life are obtained from the pure axial and torsional low cycle fatigue. CF8M is used in pipes and valves in a nuclear reactor coolant system. The specimens to be used at the experiment are virgin and degraded of CF8M. The degraded material was aged at 430°C for 3600 hrs. In the present study, von Mises equivalent strain and a critical plane method proposed by Fatemi-Socie(FS) and Smith-Watson-Topper(SWT) are used to calculate fatigue damage. The predicted result by von Mises equivalent strain and FS parameter show good agreement with experimental results. In conclusion, fatigue life by material degradation is decreased and multiaxial fatigue life prediction which was used FS parameter is not conservative but the best result. Introduction To predict the fatigue life of the machinery parts and structures that are under the complicated loading, appropriate the multiaxial fatigue theories or standards are required. Classical multiaxial fatigue theories are based on the stress that extends the static yield criteria into the fatigue. The most of those fatigue theories include the maximum principal theory, the maximum shear stress theory, and the octahedral shear stress theory. The recent approaches have defined the critical plane in the initiation and growth of the fatigue cracks to physically translate the process of the fatigue damage. There are many studies made with the critical plane theory that can predict not only the fatigue life but also the orientation of the crack or the failure plane [1,2,3]. This study used the following three theories to predict the multiaxial fatigue life under the axial-torsional loading: the first, the prediction using the equivalent strain, a von Mises yield criteria [4]; the second, that using the Fatemi-Socie (FS) parameter [5], one of the critical plane approach; and the third, that using Smith-Watson-Topper (SWT) [6,7]. The purpose of this study based on three approaches as above is to provide important data necessary for the design, maintenance and repair of the equipment through the change and prediction of the multiaxial fatigue life, which occurs due to the degradation of the cast duplex stainless steels(CF8M) that is used in pipes of the primary coolant system in the nuclear power plant. Materials and Test Procedure Specimen. Table 1 shows the chemical composition of the CF8M used in this study. The study was performed on the virgin material and the degraded material produced by the thermal aging for 3600 hours at 430°C to evaluate the effect of degradation to the axial-torsional fatigue life [8,9,10]. The profile of the used specimen was a thin walled tubular specimen as shown in Fig.1. The inside of the specimen was finished with electrolytic polishing to prevent the crack from initiation Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1183-1188 doi:10.4028/www.scientific.net/KEM.270-273.1183

The Study on Effects of Thermal Aging of Dissimilar Weld Zone in a Primary Reactor Cooling System

In a primary reactor cooling system(RCS), a dissimilar weld zone exists between austenitic-ferritic duplex cast stainless steel(CF8M) in a pipe and low-alloy steel(SA508 cl.3) in a nozzle. Thermal aging is observed in CF8M as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330°C, while no effect is observed in SA508 cl.3. An investigation of the effect of thermal aging on the various mechanical properties of the dissimilar weld zone is required. The purpose of the present investigation is to find the effect of thermal aging on the dissimilar weld zone. The specimens are prepared by an artificially accelerated aging technique maintained for 100, 300, 900, 1800 and 3600 hrs at 430°C, respectively. The various mechanical tests for the dissimilar weld zone are performed for virgin and aged specimens.

Evaluation of the Fracture Characteristics with Thermal Aging on Dissimilar Welds in Reactor Coolant System

A dissimilar weld zone exists between the pipe and nozzle in a primary reactor cooling system (RCS). Thermal aging is observed in cast stainless steel, CF8M used in a pipe as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330°C. No effect is observed in low-alloy steel. SA508 cl.3 is used in a nozzle. The artificially accelerated aging specimens are prepared to maintain for a temperature of 430°C for 300, 1800, and 3600hrs, respectively. Then, various mechanical tests such as hardness, tension, impact test, are performed in virgin and aged specimens in order to determine the existence of dissimilar weld zones. The specimens for elastic-plastic fracture toughness tests are prepared for one type, where a notch is created in the heat affected zone of CF8M. From the experiments, it was found that J-integral values decrease as age increases.

Proposal of New Biaxial Fatigue Life Prediction Parameter by Using the Effective Shear Stress for CF8M Stainless Steels

A combined axial-torsional low cycle fatigue test was carried out to predict the fatigue life under in-phase and out-out-phase loading conditions for CF8M cast stainless steels. The Fatemi-Socie (FS) parameter which is based on the critical plane approach is not only one of the many methods but also the best method that can predict the fatigue life under a biaxial loading condition. But the result showed that, a biaxial fatigue life prediction by using the FS parameter with several different parameters for the CF8M cast stainless steels is not conservative enough but at the same time it was the best result so far. So in this present research, we proposed a new fatigue life prediction parameter (Park-Kwon parameter) by considering effective the shear stress instead of the FS parameter which considers the maximum normal stress acting on the maximum shear strain and its effectiveness was verified.

Mechanical and Fatigue Properties of the SiCp/Al Composites

Mechanical and fatigue properties of the SiCp/Al with three different SiC particle volume fractions: 5%, 7.5% and 10% as well as Al6061 alloy were examined with tensile test, hardness measurement and fatigue crack propagation test. Tensile strengths and hardness of the SiCp/Al increased together with SiC particle volume fraction. It was checked that tensile strengths of the SiCp/Al had a certain relation with the corresponding hardness. On the other hand, fatigue crack propagation behavior (FCPB) of the SiCp/Al was faster than that of the AA6061. Besides, difference of the FCPB among SiC particle volume fractions was not pronounced. However, The FCPB slightly increased together with SiC particle volume fraction. It was noted that fatigue properties of the SiCp/Al was more affected by local volume fraction in front of crack tip than bulk volume fraction. In addition, the presence of SiC particles delayed fatigue crack initiation effectively because the percentage of fatigue crack initiation life to the whole fatigue life was a maximum of 38%, larger than that of ordinary structural materials.