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Session: Airworthiness considerations
Session starts: Wednesday 28 June, 10:50
Presentation starts: 11:10

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Jianhong Lin (Aircraft Strength Research Institute (ASRI), Chinese Aeronautical Establishment)

Abstract:
To determine the inspection thresholds and intervals is the majority task in aircraft structural designs with damage tolerance (DT) philosophy. This article conducts a briefly review on various influence factors on fatigue failures, both for durability and crack propagations. It is well known that the fatigue lives for crack growing from the detectable crack length to critical crack length have significantly less scatters than the ones for the crack from initiation growing to the detectable crack length. From this point view, different scatter factors should be applied to the determine the inspection thresholds and intervals. In other hands, the fatigue lives for open hole, lug, fastener joints and other structural features are not only linked to the materials, but also linked to the machining and assembly processes for these structural features. Such kinds of differences lead to ASIP’s requirements to verify the designs on various levels in the test pyramid, i.e., from materials, to coupons, elements, components, up to the whole aircraft. The existing aircraft development experiences also demonstrated that the fatigue quality of aircraft structures can be improved by increasing the number of levels to do the testing verifications. Based on the reviews in this article, the author proposes that the inspection thresholds should be determined by the durability analysis, with the design values determined by the coupon specimens at different levels of the test pyramid, because doing this way can reduce the uncertainties of fatigue durability for the structural features. The scatter factors to determine the inspection threshold can be obtained from the statistical analysis for enough testing results. In other hand, the inspection intervals should be determined by the crack growth analyses for the cracks growing from detectable crack length to the critical crack length, and applying a scatter factor of two.