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Introduction of subsurface proximity criteria in the world and stress intensity factors for transformed surface flaws

世界各国の内部欠陥の接近性クライテリアと置き換えられた表面欠陥の応力拡大係数

長谷川 邦夫; Li, Y.; 勝又 源七郎*; Dulieu, P.*; Lacroix, V.*

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

検出された内部欠陥が、機器の表面近傍にあるとき、応力拡大係数が大きくなる。このリガメントからの破壊を防ぐために維持規格では内部欠陥の接近性のルールがある。このルールによれば、内部欠陥を、内部欠陥のままにして破壊評価を扱うか、表面欠陥に置き換えて破壊評価を行う。表面欠陥に置き換えた場合、亀裂先端の応力拡大係数は増大する。この接近性のルールの考え方は世界各国の維持規格で同じであるが、具体的なクライテリアは異なる。そこで、世界各国の接近性クライテリアの紹介をするとともに、ASME規格で用いられるクライテリアをベースにして応力拡大係数を比較する。

Net-section stress at the ligament between component free surface and subsurface flaw increases when the ligament distance is short. It can be easily expected that stress intensity factors increase when the subsurface flaw locates near the free surface. To avoid catastrophic failures caused by ligament failure, fitness-for-service (FFS) codes provide flaw-to-surface proximity rules. The proximity rules are used to determine whether the flaws should be treated as subsurface flaws as-is, or transformed to surface flaws. The stress intensity factor for the transformed surface flaw increases furthermore. The increment of the stress intensity factor before and after transformation depends on the location of the subsurface flaw. Although the concept of the proximity rules are the same, the specific criteria for the rules on transforming subsurface flaws to surface flaws differ amongst FFS codes. Particularly, the criteria are different amongst the same organizations of ASME (American Society of Mechanical Engineers). The proximity criteria of the FFS codes in the world were introduced in this paper. In addition, the stress intensity factors based on the different criteria used in the ASME Codes are compared.

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