Leak-Before-Break (LBB) analysis is an important fracture mechanics concept for the design and integrity evaluation of pressurized components. A LBB assessment demonstrates that leakage of fluid through a crack in the component wall would be detected prior to conditions of instability at which rapid extension occurs. Two independent aspects to the potential failure are considered which are fracture mechanics and leakage assessment.
Widely applied in the nuclear industry, two approaches are generally provided: (i) simplified or, (ii) detailed. In a simplified analysis (i), an idealized through-wall crack with straight crack sides perpendicular to the surface is typically postulated, which simplifies the analysis significantly. However, in reality, the case of a surface crack that grows through the wall thickness and breakthrough at its deepest point is more realistic (ii). It is therefore very important to determine the behavior of the crack size and shape accurately to ensure the best fitness-for-service assessment using LBB principles.
Solutions necessary for a LBB assessment available in BS 7910 and API 579-1/ASME FFS-1 for a wide range of geometry will be compared and discussed. Both part-penetrating and through wall defects will be investigated.
At break-through the flaw may have a much longer length at the inner surface, and only a small crack length at the outer surface. The crack sides are not perpendicular to the surface but are more likely to be steeply angled or curved. This crack eventually grows further to a shape which does become closer to an idealized through-wall crack. The effect of through-wall cracks with shapes yet to become ‘idealized’ will be assessed for their significance in an LBB assessment. Based on the FE results, analysis of these defects shapes will be discussed.
Experimental work will be carried out on plates with a surface notch being already introduced up to 70% of the thickness. Fatigue test will be carried out and stress/strain fields on the plate’s back surface will be recorded during the transition from surface to through-thickness crack. This will allow a better understanding of the phenomenon taking place at this critical stage.