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.,
Figure 10.3.2. Selecting Design Stress Level to Meet
Residual Strength Crack Growth and Inspectability Requirements [Walker, et al.,
Redesign is done as necessary until the required life is
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 , Goranson, et al. , and
Watson . 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.