In applying JSSG-2006 paragraph 3.12 to a built-up structure, it is noted that cyclic growth behavior of primary damage may be influenced by the geometry of the structure or the arrangement of the elements.  In order to provide an orderly and progressive path for the crack that eventually causes the structure to fail, the continuing damage assumptions were incorporated.  There are three cases where the continuing damage assumptions are made in order to keep the crack moving; these cases are described with examples.

Figure 1.3.3 describes a skin-stringer construction where equivalent initial (primary) damage is assumed to exist in both elements of the hole marked A.  According to JSSG-2006 paragraph A3.12.1, all other holes are secondary cracking sites (marked B) and contain the small imperfections equivalent to the 0.005 inch radius corner flaw.  As the primary damage progresses in both the skin and stringer, eventually the radial crack in the stringer will extend to the edge of the stringer, shown in Figure 1.3.3 - cracking sequence (ii).  At this time, a new crack, equivalent to the 0.005 inch radial crack flaw plus the growth prior to the primary element failure, is assumed to exist on the diametrically opposite side of the failed hole, as shown in Figure 1.3.3 - cracking sequence (iii).  This continues the growth process until the complete stringer fails, shown in Figure 1.3.3 - cracking sequence (iv).

Figure 1.3.3.  Example of Continuing Damage Growth Terminated at Free Edge and Terminated by Failure of Member

Under the condition that the primary damage terminates due to a member or element failure, such as the stringer illustrated in Figure 1.3.3, the designer is required to assume that continuing damage is present.  The continuing damage is assumed to be present at the most critical location in the remaining element or structure.  The continuing damage is either a corner crack that starts from an initial small imperfection of 0.005 inch or a surface flaw with length of 0.02 inch and depth of 0.01 inch, plus the amount of growth which occurs prior to element failure.

Figure 1.3.4 illustrates several choices for potential critical locations where continuing damage might be assumed subsequent to the failure of the stringers.  Secondary Site 1 is assumed to be an adjacent hole, and the crack growth is in the skin and opposite in direction to the primary skin crack.  Such a situation would eventually result in a stepwise shift in the crack growth path.  Most logically, this type of damage could be assumed to exist at the primary damage site in the skin on the diametrically opposite side of the hole once the stiffener fails.  Secondary Site 2 is located in the skin and would provide a path for link-up with the primary crack.  Secondary Site 3 is located in a parallel stringer-skin hole and would also allow for possible link-up with the primary crack.

The type of continuing damage assumption that the designer must make when the assumed primary damage enters into and terminates at a fastener hole is described in Figure 1.3.5.  The continuing damage in this case is a crack on the opposite side of the hole entered by the primary crack.  The continuing damage crack is taken as the crack that has grown from an initial small imperfection of a 0.005 inch radial corner crack through the time period that it takes the primary damage to terminate at the hole.

Figure 1.3.4.  Example of Continuing Damage Types and Locations Assumed When Primary Damage Terminates Due to Element Failure

Figure 1.3.5.  Continuing Crack Assumed at Opposite Side of Hole When Primary Crack Terminates at a Hole