BS 7910:2019 Annex F — leak-before-break length margin (2c_L detectable vs 2c_c limiting)
LBB answers: once the pressure boundary cracks, does it leak detectably first or break suddenly? The detectable-leakage route (F.2.2) compares the limiting through-wall length 2c_c (critical length, via the Clause 7 fracture assessment — reused from the E3 engine) against the detectable-leakage length 2c_L (from your leak-detection capability). LBB is demonstrated when 2c_L < 2c_c with a sufficient margin.
2c_L is user-supplied (the full two-phase leak-rate model of F.2.4 is a separate fluid-mechanics module, deferred). The crack-opening area (Eq F.1, with bulging factor a(\lambda) for shells) is reported to confirm the flaw opens. Margins are not prescribed by the standard (F.2.2.4) — set your own.
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