Annex F: Leak-before-Break Calculator (E4)

BS 7910:2019 Annex F — leak-before-break length margin ($2c_L$ detectable vs $2c_c$ limiting)

Leak-before-Break (Annex F)

Leak-before-break (LBB) answers a safety question for pressure boundaries: once a crack grows through the wall, does the component leak detectably first, giving warning and time to shut down, or does it break suddenly? Demonstrating LBB lets you rely on leak detection as a safeguard, per BS 7910:2019 Annex F. The calculator assumes a through-wall flaw is present and supports flat plates and cylinders / spheres.

The detectable-leakage route (F.2.2) compares two lengths:

• the limiting through-wall length 2c_c — the critical length at which the through-wall flaw would cause fracture or collapse, found by reverse-solving the Clause 7 fracture assessment (via E3 → E1);
• the detectable-leakage length 2c_L — the through-wall length whose leak rate is just large enough for your leak-detection system to detect; you enter it from your detection capability (F.2.2.3).

LBB is shown when 2c_L < 2c_c with enough margin, i.e. the crack becomes detectable by leakage before it reaches the length that would cause failure.

The crack opening area (COA) and crack opening displacement, which control the leak rate, are computed from the Annex F equations (F.2.3): the COA uses a plane-strain modulus E'=E/(1-\nu^2) and a stress ratio S=P_m/\sigma_f with a flow strength \sigma_f you select (mean of yield and tensile, yield, or 1.2\,\sigma_Y). For pressurised cylinders the bulging factor a(\lambda) > 1 enlarges the COA. The standard does not set a required margin (F.2.2.4) — set it per your safety case (for example NUREG-1061). The two-phase leak-rate model (F.2.4) is deferred in this version, so you supply 2c_L directly.

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