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Handbook for Damage Tolerant Design

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Section 9.2.0. Usage Characterization for Simple Repairs

As explained in Section 9.1, there are two reasons for characterizing the usage for a damage tolerance analysis: (a) to establish the residual strength requirement and (b) to grow the crack in the sub-critical crack growth analysis.  The maximum loading conditions and their frequency determine the loading (residual strength) requirement that the structural element must withstand without failure.  Typically the more frequently occurring loading conditions are those responsible for growing a crack from its initial size to failure.

For a residual strength analysis of a structural repair, one would want to characterize the maximum loading condition that the structure might experience in its anticipated service life.  A simple choice might be based on returning the load carrying capability of the cracked structure to the original ultimate load carrying capability of the structural member without a crack.  Section 9.7 describes in more detail the methods for conducting a residual strength analysis.

For a sub-critical crack growth analysis, one is more typically interested in characterizing the average per flight loading conditions that will be experienced by the cracked or repaired structure.  It is the relatively large, frequently occurring load excursions that drive the crack growth process.  From a repair analysis standpoint, it is important that the analyst know what are the sources of large (and frequently occurring) stress excursions and have some indication of the maximum to minimum stress ratios as well as frequency of these excursions on a per flight (or per flight hour) basis.

The more critical the repair, the more important is the definition of the specifics of per flight average loading conditions for a life analysis.  If one can identify those loading conditions that affect the rate at which cracks grow in a given structural member, then simple calculations can be made to obtain first order estimates of this member's structural life.  While first order estimates can be questioned from an absolute sense, such estimates, when used in a relative sense, can provide the necessary guidance for designing a repair, or releasing an individual aircraft for flight.

In the following subsections, a sub-critical crack growth analysis approach, which was introduced in Section 5.2.5, is further described and justified for its application for repair analysis.  In Section 9.3, an example analysis of three transport wing stress histories is utilized to illustrate how a generic stress history for a given structural member could be employed to estimate the life at any given location in that member.