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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.