Section 6.2.2. Multiple Load Path, Fail Safe Structure
The second example is academic, but illustrative. It is a multiple load path dependent beam
consisting of members A, B, C (Figure 6.2.2a). Assume that crack 1 is the critical crack. (If the critical location was at the other
side of the hole, damage development would be similar as in Figure 6.2.2).
Due to assembly drilling the two members, A and B should both be assumed
flawed. The damage development is shown
in Figure 6.2.2b, c.
Figure 6.2.2. Damage Development in Multiple Load Path, Fail Safe Structure
Cracks 1 and 2 both start as 0.05 inch flaws. Crack 1 is assumed to grow faster, and when K = KIc
rapid crack propagation (instability) will occur at which point member A is
assumed failed. Remaining structure
damage has to be assumed in the adjacent member (crack in member C). It is a 0.005 inch crack plus its prior
Due to member failure the stress-intensity factors of all
cracks will show a jump. Therefore,
cracks 2 and 3 will grow much faster than before: Final failure will occur when the stress-intensity factor (K) of crack 3 reaches KIc, or when K of crack 2 reaches Kc, whichever occurs
first. (It is assumed that plane stress
prevails in the thin member B). The
period between failure of member A and final failure (indicated by t in Figure 6.2.2c) has to be adequate for one of the
options of remaining structure damage inspection. Otherwise, the structure would not qualify as Multiple Load Path,
Fail Safe structure.