Leak-Before-Break (LBB). Engineering critical assessment. This analysis may be carried out to establish that the finished cylinder will leak in the event of a defect in the cylinder or liner growing into a through-wall crack. A leak-before-break assessment shall be performed at the cylinder side wall. If the fatigue sensitive location is outside the side wall, a leak-before-break assessment shall also be performed at that location using a Level II approach as outlined in BS PD6493. The assessment shall include the following steps:
Leak-Before-Break (LBB). F.2.1. Engineering critical assessment. This analysis may be carried out to establish that the finished cylinder will leak in the event of a defect in the cylinder or liner growing into a through-wall crack. A leak- before-break assessment shall be performed at the cylinder side wall. If the fatigue sensitive location is outside the side wall, a leak-before-break assessment shall also be performed at that location using a Level II approach as outlined in BS PD6493. The assessment shall include the following steps:
(a) Measure the maximum length (i.e. major axis) of the resultant through-wall surface crack (usually elliptical in shape) from the three cylinder cycle tested under the design qualification tests (according to paragraphs A.13 and A.14 of Appendix A) for each type of design. Use the longest crack length of the three cylinders in the analysis. Model a semi-elliptical through-wall crack with a major axis equal to twice the measured longest major axis and with a minor axis equal to 0.9 of wall thickness. The semi-elliptical crack shall be modeled at the locations specified in paragraph F.1. of Appendix F. The crack shall be oriented such that the highest tensile principal stress shall drive the crack;
(b) Stress levels in the wall/liner at 26 MPa obtained from the stress analysis as outlined in paragraph 6.6 of Annex 3 shall be used for the assessment. Appropriate crack driving forces shall be calculated using either Section 9.2 or 9.3 of BS PD6493;
(c) Fracture toughness of the finished cylinder or the liner from a finished cylinder, as determined at room temperature for aluminium and at -40 C for steel, shall be established using a standardized testing technique (either ISO/DIS 12737 or ASTM 813-89 or BS 7448) in accordance with Sections 8.4 and 8.5 of BS PD6493;
(d) Plastic collapse ratio shall be calculated in accordance with Section 9.4 of BS PD6493-91;
(e) The modeled flaw shall be acceptable in accordance with Section 11.2 of BS PD6493-91.
F.2.2. LBB by flawed cylinder burst A fracture test shall be performed by the cylinder side wall. If the fatigue sensitive locations as determined in paragraph F.1. (Appendix F) is outside the side wall, the fracture test shall also be performed at that location. The test procedure is as follows:
(a) Determination of leak-before-break flaw length The length of the LBB flaw at the fatigue sensitive site shall be twice the length of the maximum length measured of the resultant through-wall surface crack from...
