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Effect of plastic constraint and cladding on semi-elliptical shaped crack in fracture toughness evaluation for a reactor pressure vessel steel

半楕円亀裂を有する原子炉圧力容器の破壊靭性評価に及ぼす塑性拘束とクラッドの影響

下平 昌樹 ; 飛田 徹; 名越 康人*; Lu, K.; 勝山 仁哉 

Shimodaira, Masaki; Tobita, Toru; Nagoshi, Yasuto*; Lu, K.; Katsuyama, Jinya

JEAC4206-2016における原子炉圧力容器の構造健全性評価では、材料の破壊靭性が、想定欠陥であるクラッド下半楕円亀裂の先端における応力拡大係数よりも高いことが求められている。しかしながら、破壊靭性試験片と想定亀裂の亀裂深さやクラッドの有無といった違いにより、塑性拘束状態や破壊靭性評価に影響を及ぼす可能性がある。本研究では、半楕円亀裂に対する拘束効果やクラッドが破壊靭性評価に及ぼす影響を調べるため、4点曲げ破壊靭性試験及び有限要素解析を実施した。その結果、半楕円亀裂最深点における見かけの破壊靭性がマスターカーブ法に基づく5%信頼下限を上回り、現行評価手法が保守性を有することを確認した。半楕円亀裂における破壊の起点は亀裂最深点だけでなく試験体表面近傍にも観察された。有限要素解析の結果、半楕円亀裂における塑性拘束は亀裂最深点に比べて表面近傍で弱くなっていることが分かった。また、表面亀裂の場合に比べてクラッド下亀裂の場合には塑性拘束が弱められ、その弱い拘束の影響によりクラッド下亀裂の見かけの破壊靭性が表面亀裂のそれよりも高くなることがローカルアプローチによって示唆された。

In the structural integrity assessment of a reactor pressure vessel (RPV), the fracture toughness (K$$_{Jc}$$) should be higher than the stress intensity factor at the crack tip of a semi-elliptical shaped under-clad crack (UCC), which is prescribed in JEAC4206-2016. However, differences in crack depth and existence of cladding between the postulated crack and fracture toughness test specimens would be affected to the plastic constraint state and K$$_{Jc}$$ evaluation. In this study, we performed fracture toughness tests and finite element analyses to investigate the effect of plastic constraint and cladding on the semi-elliptical shaped crack in K$$_{Jc}$$ evaluation. The apparent K$$_{Jc}$$ value evaluated at the deepest point of the crack exceeded 5% fracture probability based on the Master Curve method estimated from C(T) specimens, and the conservativeness of the current integrity assessment method was confirmed. Few initiation sites were observed along the tip of semi-elliptical shaped crack other than the deepest point. The plastic constraint state was also analyzed along the crack tip, and it was found that the plastic constraint at the crack tip near the surface was lower than that for the deepest point. Moreover, it was quantitatively showed that the UCC decreased the plastic constraint. The local approach suggested higher K$$_{Jc}$$ value for the UCC than that for the surface crack, reflecting the low constraint effect for the UCC.

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