要約のみTohoku University三浦英生課

SHIM CHOONGMOO

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Tohoku University博士（工学）博士学位論文 (Thesis(doctor))要約のみthesi

SHIM, CHOONGMOO

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Acceleration of Fatigue Crack Growth of 316Stainless Steel in Simulated PWR Primary Waterin Very Low Strain Rate Region著者 SHIM CHOONGMOO学位授与機関 Tohoku University学位授与番号 11301甲第17731号URL http://hdl.handle.net/10097/00122147          シム チュンム 氏 名          沈 忠茂 (Shim ChoongMoo) 研究科，専攻の名称 東北大学大学院工学研究科（博士課程）機械システムデザイン工学専           攻 学 位 論 文 題 目          Acceleration of Fatigue Crack Growth of 316 Stainless Steel in Simulated PWR Primary Water in Very Low Strain Rate Region （模擬加圧水型炉水中の超低ひずみ速度領域における316ステン レス鋼の疲労き裂進展の加速現象の解明） 論 文 審 査 委 員           主査 東北大学教授 三浦 英生 東北大学教授 小川 和洋 東北大学教授 坂  真澄 東北大学教授 渡邉 豊 東北大学准教授 竹田 陽一                                論文内容要約 World energy consumption has been increased in recent years and the demand of total world energy was predicted to grow by about 48% to 2040. Therefore, nuclear energy has drawn attention to satisfy the energy demand because it has satisfied the stable and carbon-free environmentally-friend energy supply. Nuclear power plants (NPPs) have been used to supply stable and enough electricity with the reasonable cost as a based-load power plant. However, a number of NPPs already have exceeded their original design life, which was 30 years. Extension of the lifetime has been considered to operate them until 60 years. Therefore, assuring the reliability of the further long-term operation (LTO) of NPPs is particularly important for the lifetime extension because NPPs have been operated for the initially designed lifetime and all the materials used in the plants have been aged. There are various degradation issues that have to be concerned to renew the license of the aged NPPs. In addition to the creep damage during a constant operating condition, the damage caused by environmentally-assisted fatigue (EAF) should be considered because shut-downs and start-ups are repeated in annual service inspection. The American Society of Mechanical Engineers fatigue design curve were established by numerous fatigue data in ambient air. The environmental effect was, however, not included in the curve. It was found that the fatigue life of austenitic stainless steels tended to significantly decrease in the laboratory test using simulated pressurized water reactor (PWR) primary water. Therefore, the environmental effect on fatigue damage of these materials should be investigated more in detail in order to quantitatively discuss the possibility of the extension of the lifetime of the aged NPPs. Environmental correction factor (Fen) is an effective parameter for the quantitative evaluation of the environmental effect on the fatigue life of materials. This parameter is the ratio of the fatigue lives obtained in ambient air and PWR environment. There were many investigations regarding Fen to estimate environmental effect as functions of various factors such as strain rate, strain amplitude, temperature, hold time, and surface finish, etc. In particular, the strain rate significantly influences the fatigue life of austenitic stainless steels in simulated PWR primary water. It was reported that the environmental effect increased with the decrease of strain rate. Nevertheless, it has been regarded that the strain rate effect was saturated below the critical strain rate (0.0004%/s) although data are not enough to determine the saturation of the environmental effect. Thus, it is indispensable for evaluating the fatigue behavior in the low strain region in simulated PWR primary water. Meanwhile, the loading rate caused by shut-downs and start-ups of NPPs should be very slow. Moreover, the loading rate of EAF gradually approaches to that of slow strain rate tensile test (SSRT)–stress corrosion cracking (SCC) when the strain rate becomes very low strain rate. The behavior of EAF and SCC should be similar at least under tensile loading mode. Thus, both behaviors must be carefully taken into consideration for precisely predicting the remaining life of the core component materials. Therefore, the objectives of this study are to propose a modified time domain analysis (TDA) based on short crack growth behavior at each given life fraction in order to evaluate the crack growth rate (CGR) in the conventional saturation region, to clarify the environmental acceleration in low strain rate region, and to validate the continuity of EAF and SCC in very low strain rate region. In order to achieve these objectives, EAF tests for the crack initiation stage and crack propagation tests for the crack propagation stage were conducted under various strain and loading rates including very low conditions compared to previous other studies. The TDA was used to clarify the disappearance of the saturation of the environmental acceleration in very low strain rate region. The continuity from EAF to SCC was evaluated on the TDA map by comparing EAF data with SSRT–SCC data. Furthermore, the overall environmental acceleration of crack growth behavior, which indicated early crack and long crack propagation, were evaluated by means of the TDA.    In Chapter 2, the relationship between EAF and SCC with respect to the environmental acceleration in very low strain rate region was evaluated by EAF tests with 316 stainless steel in simulated PWR primary water and in ambient air for the crack initiation stage. After the tests, a number of cracks were initiated on four cross sections cut from a pipe-shape specimen in the early stage and were more than those in ambient air. Therefore, the environmental effect on crack initiation was clearly validated. Those examined cracks were used to establish the maximum CGR obtained from the difference of two maximum cracks and lives at each given life for modified TDA. It was difficult to evaluate environmental acceleration in very low strain rate region using conventional TDA because EAF test took very long time for environmental acceleration in very low strain rate region. On the other hand, it was easy to evaluate the environmental acceleration at lower strain rate than conventional one by using the modified TDA. This was because the special emphasis was made on the crack depth at each given life fraction. The validation of the maximum CGR on the TDA was confirmed by comparing with the overall maximum CGR. It was found that those CGRs were on a single straight line on the modified TDA and both of them showed a similar environmental acceleration behavior on the TDA. Therefore, the maximum CGR data set on the TDA for stainless steel was also applicable to the evaluation of the environmental effect on the CGR. As a result, the environmental acceleration of shorter crack was successfully analyzed in lower da/dt|air region. The CGR data for EAF at low strain rates, which showed relatively slower CGR, exhibited that its environmental acceleration exceeded 100. It was found that there was no tendency of the saturation of the environmental acceleration with the decrease of CGR and environmental effect was rather accelerated, suggesting there was no saturation of Fen. Furthermore, SSRT–SCC data obtained from a reference was also plotted on the TDA with EAF data to understand the relationship between EAF and SCC. Environmental acceleration for SSRT–SCC data were well addressed on the TDA with EAF data. As a result, it was found that environmental acceleration of EAF approached that of SSRT data in very low strain rate region on the TDA. This result clearly indicates the continuity from EAF to SCC based on the trend of environmental acceleration in very low strain rate region.  In Chapter 3, the averaged CGR calculated from all observed cracks and the fatigue life in the previous chapter was applied on the TDA to confirm and support accelerated environmental effect in very low strain rate region. The averaged CGR obtained from SSRT–SCC data was also applied to the TDA in order to support the continuity from EAF to SCC in very low strain rate region and to achieve further high accuracy. The averaged CGR was also well-interpreted on the TDA with other CGRs. The data set was distributed below the data sets of the maximum and overall maximum CGRs on the TDA. However, the environmental acceleration using the averaged CGR also increased with the decrease of strain rate for both CGRs. It was clarified that the saturation did not exist, but the acceleration of the environmental effect was accelerated in very low strain rate region. Furthermore, an extrapolation drawn from the averaged CGR data set was gradually approaching to SSRT–SCC data. This result clearly indicates the continuity from EAF to SCC in the environmental acceleration of CGR at very low strain rates. In addition, intergranular (IG) and transgranular (TG) crack facets were observed on the fracture surface and cross sections of the EAF specimens tested at lower strain rates. In general, TG crack occurs in the fracture mode of EAF. However, IG crack was observed although the applied loading pattern was cyclic loading. The fraction of IG crack facet drastically increased with the decrease of strain rate. Moreover, the estimated CGR on IG crack facet was faster than that on TG obtained from those fracture surfaces. According to these results, lower strain rate leads to more IG crack facet and higher CGR although the strain rate is much lower than the conventionally assigned critical strain rate (0.0004%/s). Those results also indicate no saturation of the environmental acceleration of CGR and the continuity from EAF to SCC in the environmental acceleration of CGR in very low strain rate region (≤ 0.0004%/s). In Chapter 4, crack propagation tests using compact tension (CT) specimen were conducted at very low strain rate in order to evaluate the environmental acceleration in the crack propagation stage and to understand overall behavior of the growth of short and long cracks. The environmental acceleration and the continuity from EAF to SCC at very low loading rate were also investigated by interpreting the crack propagation data on the TDA with the averaged CGR for SSRT–SCC. The averaged CGR data set for the long cracks was also well-interpreted on the TDA and showed the similar behavior as that of the short crack with respect to the effect of strain rate on the environmental acceleration. Moreover, the continuity from EAF to SCC was also confirmed even in the long crack propagation stage. It was concluded that all the obtained CGR were interpreted on one TDA map by using CGR in both environments. Therefore, TDA is the most suitable and strongest evaluation method for analyzing the environmental acceleration. From the viewpoint of the fracture mode, IG crack facet was also observed in the crack propagation stage and fraction of IG facet also increased with the decrease of strain rate as it was observed similarly in the crack initiation stage. The value was almost close to 100% and if the reason why CGR was increased was due to only IG crack, CGR should have increased continuously and become saturated at further lower loading rate when the IG crack facet became dominant on all over the fracture surfaces. Moreover, the environmental acceleration effect of CGR became clearer than the mechanical effect when crack length became shorter. The environmental acceleration in the crack initiation stage was much larger than that in the crack propagation stage. This result clearly indicates that the environmental acceleration of CGR increased larger in the shorter crack. This is because the effect of mechanical loading such as stress concentration factor increases monotonically with the increase of crack length. Therefore, it is very important to measure the environmental acceleration at the very low strain rate precisely by using a very short crack in order to avoid the effect of the mechanical loading. In this study, it was clearly indicated that low strain or loading rate significantly reduced the fatigue life and increased CGR of type 316 stainless steel both in the crack initiation and propagation stages. It was clarified that the modified TDA is very effective and useful for evaluating the environmental effect especially in very low strain rate region based on given crack depths in ambient air and simulated PWR primary water. The modified TDA facilitated the evaluation of environmental acceleration in even conventional saturation region. Therefore, it is strongly suggested that there is no saturation region of CGR even in very slow strain rate region. It was validated that the continuous acceleration of the environmental effect in the crack initiation and propagation stages exists even when strain or loading rate is very low. In addition, the continuity from EAF to SCC was clarified by interpreting data sets of EAF and SCC in very low strain rate region on the TDA. It was confirmed that the conventional evaluation method should lead to over estimation of the structural integrity of NPPs without any consideration of the acceleration in the conventional saturation region. Therefore, not only SCC but also EAF under cyclic loading in low strain rate region must be considered for the long-term reliability of the materials used in the aged NPPs for quantitatively estimating the remained lifetime of the material. The structural integrity of LTO of NPPs, especially, in the crack initiation stage should be evaluated more in detail in very low strain rate region by using the proposed modified TDA method for assuring the security of the extension of the operation time of the aged NPPs.  

Acceleration of Fatigue Crack Growth of 316 Stainless Steel in Simulated PWR Primary Water in Very Low Strain Rate Region

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模擬加圧水型炉水中の超低ひずみ速度領域における316ステンレス鋼の疲労き裂進展の加速現象の解明

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