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Fatigue crack growth calculations for pipes considering subsurface to surface flaw proximity rules

内部欠陥の表面き裂への置き換えルールに関する疲労き裂進展計算

勝又 源七郎; Li, Y.; 長谷川 邦夫; Lacroix, V.*

Katsumata, Genshichiro; Li, Y.; Hasegawa, Kunio; Lacroix, V.*

日本及び米国の維持規格では、機器の自由表面近傍にある内部欠陥について表面欠陥に置き換える接近性ルールがある。本論文では管に内部欠陥を仮定し、従来の規格にある接近性ルールと、新たに提案した接近性ルールを用いて、疲労による余寿命を求めた。その結果、き裂形状によっては現行規格に含まれている接近性ルールは極めて非保守的であることが分かった。

If a subsurface flaw is located near a component surface, the subsurface flaw is transformed to a surface flaw in accordance with a subsurface-to-surface flaw proximity rule. The re-characterization process from subsurface to surface flaw is adopted in all fitness-for-service (FFS) codes. However, the specific criteria of the re-characterizations are different among the FFS codes. Recently, the authors have proposed a new subsurface-to-surface flaw proximity rule based on experimental data and equivalent fatigue crack growth rates. In this study, fatigue crack growth calculations were carried out for pipes with subsurface flaws, using the proposed subsurface-to-surface flaw proximity rule and the current proximity rule provided in the current JSME and ASME Section XI. Different pipe sizes, flaw aspect ratios and ligament distances from subsurface flaws to inner surface of pipes were taken into account. As the results, the current proximity rule gives less conservative fatigue lives, when the aspect ratios of the subsurface flaws are small.

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