Section 126.96.36.199. Risk Analysis
A number of structural risk assessments have been performed in
which damage in the structural detail is modeled in terms of the distribution
of cracks or equivalent cracks.
Examples of such risk analyses can be found in Lincoln , Berens,
et al. , Alford, et al. 
and Lincoln . If the risk
analysis calculations start with a virgin structure the crack sizes are
equivalent initial cracks. If the risk
analysis is being performed for in-service or aging aircraft, the crack size distribution is usually obtained
either from the sizes of the cracks discovered during fleet inspections
or from tear down inspections of structures removed from the fleet. The cracks detected during fleet inspections
would have experienced different total service times and would have to be
translated to a common service age to obtain a representative crack size
distribution for the population of details.
The cracks from tear down inspections may be from one or many
airframes. In either case, the crack
sizes usually need to be translated to a common or different service age. Typically, to locate the crack sizes at a
common number of flight hours, the crack sizes are translated using a fracture
mechanics based crack size versus flight hour curve for expected or observed
usage. This process is illustrated in Figure 3.2.11.
After all cracks have been translated to a common service age, a crack
size distribution can be established for use in calculating probability of
failure as a function of flight hours.
Figure 3.2.11. Schematic Demonstrating the Translation of
Crack Sizes to a Common Size Using Predicted a versus T