The basic premise in arriving at the initial damage sizes is
the assumption that the as-fabricated structure contains flaws of a size just
smaller than the maximum undetectable flaw size found with the NDI procedures
used on the production line. These flaw
size shapes which are intended to be covered by the initial flaw size
assumptions include radial tears, drilling burrs, and rifle marks at fastener
holes as well as forging defects, welding defects, heat treatment cracks,
forming cracks, and machining damage at locations other than fastener holes.
Table 1.3.2 and Figure
1.3.2 summarize the initial damage assumptions as specified in JSSG-2006
paragraph A3.12.1 and Table XXX. For
slow crack growth and fail safe primary element structure, the assumed initial
flaw at holes and cutouts is a 0.05 inch through the thickness flaw at one side
of the hole if the material thickness is equal to or less than 0.05 inch. For thicker materials (> 0.05 inch), the
assumed initial flaw is a 0.05 inch radius corner flaw.
At locations other than holes, the assumed initial flaw is a
semi-circular surface flaw with a length of 0.25 inch and depth of 0.125 inch,
or, for material thickness less than 0.125 inch, a through thickness flaw of
0.25 inch length.
These assumptions - relative to the size, shape and location -
were based on a review of existing NDI data.
The crack length values given in Figure 1.3.2
and Table 1.3.2 were selected as most appropriate for
the types of cracks considered and for the two design categories.
1.3.2. Initial Flaw Assumptions
for Metallic Structure, JSSG-2006 Appendix Table XXX
Initial Flaw Assumption*
Slow crack growth and
Fail Safe primary element
For t £
0.05”, 0.05” through thickness flaw
For t > 0.05”, 0.05” radial
For t £
0.125”, 0.25” through thickness flaw
For t > 0.125”, 0.125” deep x
0.25” long surface flaw
Welds, embedded defects
* - Flaw is oritented in the most critical direction
Figure 1.3.2. Summary of Initial-Flaw Assumption for Intact Structure
The Slow Crack Growth initial damage sizes are based on NDI
probability of detection (POD) data (90 percent probability of detection with
95 percent confidence). The 0.050 inch
crack size for holes and cutouts is based on POD data obtained in the lab using
eddy current inspection with fastener removed.
The surface flaw size, 0.250 inch long by 0.125 inch deep, was obtained
from Air Force sponsored inspection reliability programs where several
techniques were used including ultrasonic, dye penetrant and magnetic
particle. In these programs, most
techniques were found to be sensitive to both surface length and flaw depth and
thus the NDI capability must be judged in terms of the flaw shape rather than
simply surface length or crack depth.