A structural‑integrity concept and design criterion asserting that a through‑thickness crack or defect in a pressure‑boundary will produce a detectable leak before it propagates catastrophically to fracture, provided material, geometry, loading and detection conditions permit stable crack growth and leak detection.

Principle

If crack growth is stable and the leakage rate through a growing through‑thickness defect reaches a detectable threshold prior to unstable fracture, operators can identify and mitigate the defect; the principle couples fracture mechanics, leak-rate physics and detection capability.

Demonstration

Illustrative scenario → A pressurized piping circuit develops a fatigue crack that grows through the wall. Recognition → As the crack breaches the wall, fluid begins to leak at a rate above the sensing threshold. Action → Leak detection alarms prompt inspection and repair before unstable fracture occurs. Consequence → Allows planned intervention and can justify reduced redundancy or inspection intervals when the LBB condition has been validated.

Misapplication

Assuming LBB holds regardless of material toughness, loading rate, environment (e.g., hydrogen embrittlement), crack morphology, or detectability limits. The error is treating LBB as universally applicable rather than contingent on validated stable crack growth and reliable leak detection.

Consequence

When demonstrated for a given component, LBB supports risk‑informed inspection, maintenance planning and design options that rely on detectable leakage as early warning; if incorrectly assumed, it can leave systems vulnerable to sudden, undetected catastrophic fracture.

Reversal

LBB is not valid when fracture is unstable (brittle behavior, high strain‑rate loading), leaks are undetectable due to very low leak rates or inaccessible locations, or when environmental mechanisms (e.g., embrittlement) produce rapid crack propagation without significant through‑wall leakage.

Boundary

Applies to pressure‑boundary components and geometries where fracture mechanics predict ductile or stable crack growth and where leak‑rate physics and detection systems can reliably signal through‑wall defects. Excludes cases dominated by unstable brittle fracture, non‑detectable leaks, or failure modes not involving through‑thickness penetration.

Semantic Tension

Balancing reduced structural redundancy and operational flexibility against the need for demonstrable detectability and conservative assumptions about material and environmental behavior.

Synthesis

Leak‑Before‑Break is a conditional integrity philosophy: it replaces assumed sudden failure with a requirement for demonstrable, detectable stable leakage behavior under the actual material, geometry, loading and detection constraints; it is a tool for managed risk, not a universal guarantee.