MIL-STD-1798C
5.3.2.7.1 Damage tolerance inspections.
Inspection requirements necessary to ensure damage never reaches a size able to induce catastrophic failures are inherent in damage-tolerant designs. Initial and repeat inspections are required for both fail-safe and slow damage growth designs and are described in 5.3.2.7.3 and
5.3.2.7.4, respectively. There inspections are necessary up to the point of the onset of widespread damage. At the onset of widespread damage, inspections are not sufficient to ensure safety.
5.3.2.7.2 Damage tolerance action categories and guidance.
The most appropriate damage tolerance approach shall be selected for each component, based on its design, manufacturing method, application, and material, with the approval of the Procuring Service.
The specification design usage shall be the basis for load spectrum development to be used in the crack growth analysis and verification tests. The calculations of critical flaw sizes, residual strengths, safe crack growth periods, and inspection intervals shall be based on pertinent design handbook fracture test data and any additional crack growth rate data generated as a part of the design development test program. Fracture mechanics analyses performed for damage tolerance should use linear elastic fracture mechanics as the basis of the analysis method. Any additional methodology considerations should be supported by appropriate data. In general, these considerations would include time-dependent crack growth, effects of out-of- phase stress and temperature, load interaction (overload crack retardation and/or
under-load crack acceleration), and consideration of residual stress fields due to surface treatments (e.g., shotpeen).
5.3.2.7.3 Fail-safe.
Fail-safe analysis shall establish that the required residual strength of the remaining intact component structure will be maintained for a period of unrepaired usage through the use of multiple load paths or damage arrest features after a failure or partial failure. The period of unrepaired usage necessary to achieve fail-safety must be long enough to ensure the failure or partial failure will be detected and repaired prior to the failure of the remaining intact component structure. Initial inspections for fail-safe designs shall be established based on either: 1) fatigue analyses and tests with an appropriate scatter factor, or 2) slow damage growth analysis and tests assuming an appropriate initial flaw size. Repeat inspections shall occur at or before one- half the life from the minimum detectable flaw size (based on probability of detection) to the critical flaw size.
5.3.2.7.4 Slow damage growth.
Crack growth analysis and/or sub-element/component crack growth testing shall demonstrate that the residual strength capability is maintained for the crack growth service life requirement. The initial flaw size used is the flaw size consistent with the specified inspection process and resultant demonstrated required reliability-based probability of detection/confidence level (POD/CL) capabilities. Initial inspections for slow damage growth designs shall occur at or before one-half the life from the assumed maximum probable initial flaw size to the critical flaw size. Repeat inspections shall occur at or before one-half the life from the minimum detectable flaw size (based on probability of detection) to the critical flaw size.
5.3.2.7.5 Leak before burst.
Fracture mechanics analysis shall confirm that a through crack in a fluid container will leak fluid before burst by demonstrating tolerance of a through thickness crack two times (2X) the wall
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