The last example is Crack Arrest Fail Safe structure consisting
of a skin with tear straps, as shown in Figure 6.2.3a. Due to assembly drilling, skin, tear strap
and shear clip are assumed to have 0.02
inch corner flaws, giving rise to cracks 1, 2, and 3. Damage development is shown in Figure
6.2.3b. Stress-intensity factor (K) development is shown in Figure 6.2.3c. Corresponding points on the flights axes are indicated by A, B, C,
Figure 6.2.3. Damage Development in Crack Arrest Fail Safe Structure
First consider cracks 2 and 4 in the tear strap. When crack 2 terminates due to ligament
failure, the continuing damage is a 0.005 inch crack 4 without prior
growth. From point A onwards, growth of
crack 4 will be rapid until the tear strap fails.
The independent previous growth of crack 1 was slow. However, upon tear strap failure there will
be load transfer from the cracked tear strap to the skin. Consequently, there will be a sudden
increase of the stress-intensity factor of the skin crack resulting in
accelerated growth. When K of the skin crack reaches Kc, instability (rapid crack
growth) will occur, and the crack will run to the left tear strap. Due to load transfer from the skin to the
tear strap, K will drop (point C),
and the instable crack will be arrested at the tear strap.
Subsequent damage development is strongly dependent upon
remaining structure damage assumptions (which may be mutually agreed upon by
the USAF and the contractor). In this
particular example, the most logical damage would be a 0.005 inch crack at both
5 and 6 (only prior growth of 5 should be considered). At the moment of instability of crack 1, the
shear clip will most likely be failed, because it was cracked already. Hence, there will be little load transfer to the frame.
Therefore, it is most likely that crack 6 becomes unstable immediately
in conjunction with crack 1, so that the skin crack would be from the
left to the right tear strap. This case
would be as in JSSG-2006 paragraph A3.12.1d.
(A two-bay crack with the central strap failed). It is questionable whether also the frame
should be assumed cracked. Upon failure
of the shear clip, continuing damage requirements would strictly apply to the
frame, at the next fastener hole (away from the primary damage source). The complexity of these assumptions is
Further growth of the skin crack (with continuing cracks 5 and
7) will take place until Kc
is reached again. The period CD would
have to be adequate, otherwise the structure would not qualify as Crack Arrest
Fail Safe structure.