Annex J: Fracture Toughness

BS 7910:2019+A1:2020 Annex J — $K_{mat}$ from Charpy CVN

Annex J: Fracture Toughness from Charpy CVN

When direct fracture toughness test data is not available, the assessment toughness K_{mat} can be estimated from Charpy V-notch (CVN) impact energy via BS 7910:2019 Annex J. This is a common substitute because Charpy data is far cheaper and more widely available than full K, J or CTOD tests. The result feeds the Clause 7 fracture assessment.

Two paths are offered:

• J.2.1 lower-shelf formula — applies when C_v \le 27 J and the fracture appearance is at least 80% crystalline (brittle).
• J.2.2 Master Curve — the more general method: it converts the 27 J transition temperature to the reference temperature T_0 = T_{27J} - 18°C (or you can enter a measured T_0 from ASTM E1921 directly), then evaluates K_{mat} at the assessment temperature for a chosen failure probability P_f.

A size effect (weakest-link scaling) lowers K_{mat} for thickness above 25 mm; the recommended P_f is 0.05 (a 5% lower bound).

An optional API 579 comparison runs the API 579-1:2021 §9F.4.3.2 T_0 conversion in parallel, so you can see the BS 7910 and API 579 toughness estimates side by side. That path needs the yield strength and an upper-shelf energy (defaulted by sulphur grade if left blank).

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