Equivalent initial damage size based structural risk analysis considering fatigue cracks under fastener head

Aging aircraft must be inspected to detect fatigue cracks that nucleate at unexpected sites. Several sites cannot be accessed through disassembly because of the presence of permanent wiring and tubing or lack of fixtures. Thus, inspections without disassembly must be performed, although the corresponding post-inspection crack states involve higher uncertainties than those pertaining to inspections with disassembly. In estimating the initial crack size distribution for structural risk analyses (SRA), the additional uncertainties in the crack states must be considered. This paper proposes a method to estimate the post-inspection crack size distribution under the fastener head and corresponding initial crack size distribution using the equivalent initial damage size method. The likelihood function, which is used as a filter to pass likely cracks, is constructed using the reliabilities of nondestructive inspection performed by the manufacturer and operator. The proposed method is applied to realize the SRA of splice fitting of F-15K aircraft.