The development of the fracture critical parts list begins with the first design studies.  This list is then maintained throughout the life of the aircraft.  It identifies those parts that would cause loss of the aircraft or endanger personnel and cargo if they failed as a result of flaw propagation.  The logic pattern and analysis necessary to identify critical parts is outlined in Figure 10.3.1 [Ehert, 1979]. 

Figure 10.3.1.  Illustration of Selection Logic for Fracture Critical Parts [Ehert, 1979]

Initially, the static analysis is used to identify the highly stressed areas of safety of flight items.  A crack growth analysis using the best estimate of an initial flaw at the time of the analysis and the design load spectrum is run until either the required life has been exceeded without a predicted failure or until a failure is predicted in the part.  Failure is usually related to a critical crack size and the required residual strength load.  This analysis is usually conducted as a part of the design trade studies used to select materials, select stress loads and to size the part.  The factors affecting the selection of design stress levels are illustrated in Figure 10.3.2 [Walker, et al., 1979].

Figure 10.3.2.  Selecting Design Stress Level to Meet Residual Strength Crack Growth and Inspectability Requirements [Walker, et al., 1979]

Redesign is done as necessary until the required life is attained.  Figure 10.3.2 shows a decision point at four lifetimes.  Actual life requirements will vary depending on the part; however, the logic is similar for all parts.  The accurate determination of the component stress field for identification of critical areas is important.  The best results can be achieved with fine grid finite element models. 

Each part finally identified as a fracture critical part is then added to the list and identified for controlled handling during the manufacturing process.  Establishment of this procedure early results in little disruption of standard procedures and makes the handling of fracture critical parts an integral part of the design and manufacturing process.

When the design load spectrum is developed to its final form, which should also be relatively early in the design process, the initial analysis of the most critical items should be repeated to determine if there are any changes in results.  Any differences must be evaluated and redesign accomplished as indicated.

The selection of the manufacturing processes for the critical part should be made with care.  Such things  as surface finish, edge finish, location of parting planes, location of identification marks, and amount of metal removal per part must be considered during the design.  Considerations of these and other items are presented in publications such as Lunde [1976], Goranson, et al. [1981], and Watson [1979].  It is not considered appropriate to present a large number of details in this handbook, but a catalogue of acceptable and unacceptable design and machining details should be developed by the manufacturer as a guide to design and fabrication.