Section 6.2.1. Slow Crack Growth Structure
The first example is a heavy-section spar cap (Figure 6.2.1a).
The spar cap will be treated as Slow Crack Growth structure. The initial flaw has to be assumed at the
most critical location. Assume that
this is location A (Figure 6.2.1a). Due to assembly drilling the skin is assumed
to be flawed also. If there is load
transfer from the cracked spar cap to the skin, it should be taken into consideration.
The damage development for the spar cap is shown schematically in Figure 6.2.1b, the change of the
stress-intensity factor is shown schematically in Figure
Figure 6.2.1. Damage Development in Slow Crack Growth Structure
Crack 1 starts as a 0.05 inch crack. It grows until the remaining ligament fails at K = KIc. The continuing damage is a 0.005 inch flaw
at the other side of the hole (point B).
Its prior growth need not be considered, since the primary damage
terminated by ligament failure (JSSG-2006 paragraph A3.12.1e). Hence, it may be assumed to have been
stationary thus far.
At ligament failure, crack 2 is suddenly introduced and the
stress-intensity factor is determined by the total damage size, consisting of
the failed ligament, the hole, and a 0.005 inch crack. This damage grows to failure by the growth
of crack 2.
Now consider the case that B is the critical location (Figure 6.2.1a).
In that case, crack 1 would be absent (ligament intact), but crack 2
would start at 0.05 inch (dashed lines in Figure 6.2.1b,c). Due to the
absence of crack 1 it will grow slower.
Now assume that C is the most critical location. This case is depicted in Figure 6.2.1d, e. Crack 3 will start as a 0.05 inch crack, and
terminates in the next hole. Continuing
damage is a 0.005-inch crack at the other side of the hole, plus its prior
growth, Da, must be assumed
(JSSG-2006 paragraph A3.12.1e).
Contrary to the previous case, the 0.005 inch crack was growing
previously. Its independent growth, Da, has to be calculated. Due to this previous growth there is an increase of K.
When crack 3 terminated in the next hole the stress-intensity factor of
crack 4 jumps, because crack 3 and 4 together now constitute the total
damage. Therefore, the growth of crack
4 will be much faster than before.