Appendix 3 Methods for inspection threshold determination

Different approaches have been used to calculate inspection thresholds, although these are essentially variants of one of two methods, being:

(a)the fatigue (stress-life or strain-life) method, which uses fatigue endurance data collected under constant stress or constant strain conditions, and a linear damage accumulation model (Palmgren-Miner rule);

(b)the crack growth method, which uses crack propagation and residual strength data to calculate the growth from an assumed initial crack size to a critical crack length, according to fracture mechanics principles. CS 25.571(a)(4) requires certain types of structure to have thresholds based upon crack growth analyses or test assuming the maximum probable flaw due to manufacturing or service-induced damage. This approach applies to:

(a)single load path structure; and

(b)multiple load path ‘fail-safe’ structure and crack arrest ‘fail-safe’ structure, where it cannot be demonstrated that the resulting load path failure or partial failure (including arrested cracks) will be safely detected and repaired during normal maintenance, inspection, or operation of an aeroplane prior to failure of the remaining structure. Paragraph 8(c) of this AMC provides further details on identifying this structure. In lieu of other data, an acceptable threshold for inspection for cracks emanating from the maximum probable manufacturing flaw at a fastener hole may be obtained for aluminium alloy airframe structure if an initial corner crack of radius 0.05’’ (1.27 mm) is assumed and the total crack growth life is divided by 2. Whether this approach is also sufficient to conservatively address all probable forms of manufacturing and service-induced damage needs careful consideration and is highly design dependent. Where specific test or service data for service damage exists that can be used to reliably establish an appropriate threshold for all likely types of service damage then crack growth analysis may only need to consider the manufacturing flaw. For structure susceptible to WFD specific methods for setting inspection thresholds are applicable when agreed to be practical; see Section 11 and Appendix 2 to this AMC. Regardless of the approach used, the calculated thresholds should be supported with appropriate fatigue test evidence. The best sources of fatigue test evidence are from service experience and large component or full-scale fatigue tests. Large component and full-scale fatigue test specimens are generally constructed using the same manufacturing processes as on the actual aircraft. The results of such tests should provide sufficient information to reliably establish the typical manufacturing quality and possibly its lower bound, especially when those results are combined with service experience. Conversely, simple test specimens used to generate fatigue endurance and crack growth data, which are typically assembled under laboratory or workshop conditions, may not be representative of the actual range of manufacturing quality in the structure under consideration. Therefore, in the absence of information from the full-scale fatigue tests and service experience, consideration should be given to generating fatigue endurance and crack growth data on simple test specimens which include artificial damages that are introduced at the beginning of the test, and are representative of the lower bound of manufacturing quality.

[Amdt 25/19]