API 579-1/ASME FFS-1[1] Part 5 (2007 edition) contains procedures for assessing local metal loss based on failure by plastic collapse. Equation (5.10) defines “acceptable” tip radii for grooves to ensure a plastic collapse failure mode. Grooves failing the radius check must be treated as a crack-like flaws. The validity of Equation (5.10) is questionable, and it may be excessively conservative. This paper presents new rules for groove assessment based on brittle and ductile failure models. Computation of the Weibull stress using finite element analysis (FEA) was employed to determine the minimum groove radius required to eliminate the possibility of cleavage fracture. The Bao-Wierzbicki ductile failure model was used with FEA to evaluate burst pressure and to determine a new groove radius criterion defining the plastic collapse regime, allowing categorization as metal loss. Groove-like flaws categorized as neither sharp cracks nor as metal loss are evaluated using an effective toughness concept. This concept quantifies the difference in fracture response between a sharp crack and a notch with a finite tip radius. The upcoming 2016 API 579-1/ASME FFS-1 rules remove the excessive conservatism found in Part 5 of the 2007 edition and avoid the abrupt transition between crack and metal loss assessment types based on groove radius.

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