Part 9: Flaw Characterization Alpha · 2026-06-19

API 579-1 (2021) Part 9 §9.3.6 — Equivalent flaw dimensions for oblique and buried flaws

API 579-1 Part 9 §9.3.6 — Flaw Characterization

Before performing a fracture assessment (FAD), NDE-detected cracks must be converted from as-found irregular shapes into idealized crack geometry (semi-elliptical surface / through-wall / embedded). This calculator implements the three characterization sub-procedures of API 579-1/ASME FFS-1 (2021) Part 9, paragraph 9.3.6, Eq. 9.1–9.10.

• Mode: Equivalent Length (§9.3.6.2) — Projects an oblique flaw (angle α to the principal stress plane, biaxiality B) to obtain an equivalent Mode-I half-length c.
• Mode: Equivalent Depth (§9.3.6.3) — Uses a 7th-order polynomial W-factor (Eq. 9.7) to convert a non-perpendicular flaw depth a_m to the through-thickness-normal equivalent a.
• Mode: Recategorization (§9.3.6.6) — Converts a buried flaw with small ligament d to a surface flaw (Eq. 9.8/9.9), or a deep surface flaw (a_s/t>0.8) to a through-wall flaw (Eq. 9.10).

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