Yu Itabashi

Shot Peening Effect on Low Cycle Fatigue Properties of Ti-6Al-4V and Inconel 718

This study investigates the shot peening effect on low cycle fatigue properties of two turbo engine materials, Ti-6Al-4V and Inconel 718, in view of the residual stress relaxation by the cyclic load and the thermal relaxation. Strain controlled fatigue tests for Ti-6Al-4V were carried out at room temperature. The fatigue tests for Inconel 718 were conducted at room temperature and 620C. An X-ray diffraction method was used to measure the residual stresses induced by shot peening. The compressive residual stress of Ti-6Al-4V specimen had retained about 60% after half the number of cycles to failure. It is confirmed that shot peening enhances strain range capability of Ti-6Al-4V at the life between 104 and 105 cycles region about 1.5 times higher than that of non-peened specimen. This result is attributed to the retained compressive layer even after applied cyclic loading. Shot peening enhanced the strain range capability of the Inconel 718 specimen at room temperature, by a factor of 1.3 compared to polished specimen at the life of 104 cycles region. The residual stress near the surface has been relaxed rapidly at turbine engine temperature, however, the residual stresses in the deep subsurface have been retained. The peened specimen tested at 620C tended to be slightly higher strain range than those of polished specimen at the life of 105 cycles.Copyright

Verification of Probabilistic Fracture Mechanics Analysis Code PASCAL

1. 緒言 国内において確率論的破壊力学 (PFM) の適用性向上を図るためには、破損頻度の算出に用いら れる PFM 解析コードの検証が不可欠である。著者らは、原子力機構が整備を進めている原子炉圧力容器 (RPV) に対する PFM 解析コード PASCAL に対する検証の一環として、PASCAL に導入した機能の検証を 行うとともに、米国の PFM 解析コード FAVORとのベンチマーク解析を実施した。 2. PASCALの機能整備 PASCALの適用性向上を目的として、条件付亀裂進展確率 (CPI) 及び条件付亀裂 貫通確率 (CPF) に関する低損傷確率評価機能、認識論的不確実さを考慮した信頼度評価機能等を PASCAL に導入した。例えば、信頼度評価機能の整備では、これまで不確実さの分類を考慮せずにモンテカルロ法 により CPI 及び CPFを算出していたのに対し、二重ループ化する改良を行った。具体的には、内側のルー プで偶然的不確実さのみを考慮して求めた CPIと CPFを、外側のループで認識論的不確実さに基づき統計 処理することにより、前報で示したとおり破損頻度の信頼度を評価できるようにした。また、整備した全 ての機能について検証を実施し、適切に動作していることを確認した。 3.ベンチマーク解析 PASCAL の信頼性向上を図る ため、FAVOR との比較計算を実施した。PASCALと FAVOR の機能の相違を確認した上で、単一亀裂に対 する CPI及び CPF の比較、RPV 炉心領域に対する亀 裂進展頻度 (FCI) 及び亀裂貫通頻度 (TWCF) の比 較等を実施した。PASCAL と FAVOR とでは開発機 関が異なるため、個々の機能には違いがあるが、本 比較計算を通じて、RPV 寸法や化学成分、応力拡大 係数等の条件を合わせた場合にはほぼ同等の結果が 得られることを確認した (図1)。 4. 結論 PASCAL に整備した個々の機能検証、及び PASCAL と FAVOR のベンチマーク解析を通じて、 PASCALの信頼性を検証し、実用性の向上を図った。 5. 謝辞 本報告は、原子力規制庁からの受託事業「平成28年度高経年化技術評価高度化事業(原子炉一 次系機器の健全性評価手法の高度化)」で得られた成果である。関係各位に謝意を表する。

Evaluation of SCC Crack Growth Rate in BWR Water Based on Elastic-Plastic Fracture Mechanics Parameters

This paper presents an applicability of elastic-plastic fracture mechanics parameters for evaluating a crack growth rate of stress corrosion cracking (SCC). Currently linear fracture mechanical approaches have been applied for the SCC crack growth evaluation, even though some cracks due to SCC are found in plastic deformation zones near welding where linear fracture mechanics is no longer applicable. In this paper, the authors have proposed an elastic-plastic parameter “equivalent stress intensity factor KJ” for evaluating the SCC crack growth rate based on the J-integral value, which is valid in both elastic and plastic stress fields. In order to verify the applicability of the evaluation by KJ, SCC crack growth tests were carried out in a simulated boiling water reactor (BWR) water. When the SCC crack growth rate was evaluated by the stress intensity factor K, no linear relationship between the K values and the crack growth rates was observed in the high K-value region, where a small-scale yielding condition was not met. The crack growth rates increased exponentially according to increasing the stress intensity factor to exceed the linear relationship. On the other hand, when the crack growth rate was evaluated by the elastic-plastic parameter KJ, a linear correlation between the KJ values and the crack growth rates was confirmed regardless the specimen size and the stress condition. This result suggests that by applying the elastic-plastic parameter KJ, the SCC crack growth rates in a wider range could be estimated easily with using a smaller specimen.Copyright

Study on Validity of Stress Intensity Factor in CT Specimen for Materials With Low Yield Stress

This paper discusses the validity of a stress intensity factor K for compact tension (CT) specimens of stainless and low-alloy steels by computing J-integral values. In fracture mechanics testing, specimen configurations and loading conditions are strictly regulated to meet the small-scale yielding (SSY) condition and obtain valid data. For some materials, particularly those without an obvious yield point, it is hard to meet the SSY condition. The regulations specified in fracture mechanics test standards are based on experimental results. The theoretical reason for the SSY condition is not necessarily clear and the condition seems conservative.This study focuses on a correlation with the stress intensity factors K and the J-integral values under the SSY condition. J-integral values of CT specimens were analyzed by elasto-plastic finite element analysis. Subsequently, the validity of the stress intensity factor K was checked by comparing K and J–integral values to determine whether CT specimens were under the SSY condition or not. To simplify the comparison, J-integral values were converted to K values, equivalent stress intensity factor KJ values.When K values were low enough to meet the SYY condition of the specimen, they equaled KJ values. Meanwhile, KJ values exceed K values in a high K region, which means the specimen is no longer under a SSY condition. It is possible to determine whether the specimen is under the SSY condition or not by comparing the values of KJ and K. This paper evaluated the validity of K values using the correlative relationship and showed that they were valid to some extent even if loading conditions and specimen configurations were outside the scope of the specification by the test standards. The validity of K values with 0.2% offset strength or flow strength instead of yield stress is also confirmed by this method.Copyright

Stress Corrosion Cracking Behavior Around Interface Between Nickel Base Alloy (Alloy182) and Low-Alloy Steel

A modified middle scaled creviced bent beam (CBB) test was conducted in order to investigate stress corrosion cracking (SCC) behavior around fusion boundary between nickel base alloy (Alloy82) and low-alloy steel (LAS) under high stress conditions in light water reactor (LWR) coolant environment.An Alloy182/LAS clad plate specimen was placed between the CBB jigs so that the tensile stress occurs in Alloy182 and the SCC cracks propagate from the Alloy182 surface to the fusion boundary. In pure water, only oxides and cracks propagating along the fusion boundary were observed. Meanwhile in Na2SO4 injected water with electrical conductivity 0.3μS/cm, deep cracks propagated into the LAS. The higher conductivity enhanced the driving force of the SCC cracks.A notched specimen was also tested to investigate SCC propagating behavior in more severe stress conditions than those in a smooth surface specimen. When a notched specimen was used, even in pure water condition, some cracks penetrated the fusion boundary and oxides were observed. In the notched specimen, crack nucleation points are limited in the notch root and the stress relaxation hardly occurs due to multiple crack nucleations. Such severe stress conditions also contribute the cracks to propagate into the LAS.To discuss the crack propagation behavior in the vicinity of the fusion boundary, the cross section was observed with an optical microscope. The observation indicates that the crack propagating depth into the LAS and the diameter of oxides increase with increasing the Alloy182 thickness. Although an effect of applied strain and welding direction of specimen to SCC behavior was investigated, there was no significant difference in the morphology by them.Since 1% strain was applied to CBB specimen surface and much greater strain was observed at a notched specimen, elasto-plasticity should be considered to stress condition of a SCC crack tip. An ‘equivalent stress intensity factor, KJ’ was, therefore, proposed to describe a stress condition in elasto-plastic area. A good relationship was observed between depth of SCC crack in LAS and KJ value. It was found that the proposed parameter KJ was effective to describe the stress state at the crack tip from the elastic region to the plastic region up to approximately 1% strain.Copyright