In the analysis of the intact structure, the critical damage limit is failure of the skin panel ­.  The mode of failure was slow growth of either depot level inspection type damage or initial manufacturing damage (Figure 1.3.18 and 1.3.19, respectively).  In each case, the damage is assumed to grow in a stable manner until the critical damage size in the skin panel is reached.  The critical damage size for this case would be that size at P_xx = P_DM or P_xx = P_LT where P_xx is bounded by

            P_limit £ P_xx £ 1.2P_{one lifetime}

For a balance fail safe design, the remaining structure must be capable of withstanding the effects of the major load path failing, including the redistribution of load to adjacent members at the time of load path failure.  This is the basis for the requirement that the remaining structure must support the P_yy residual strength load.  The load P_yy is dependent upon the design allowable for the first panel (Panel ­ in this case). 

Assume for example that the P_xx allowable for first panel failure is exactly P_limit.  The remaining structure must be capable of supporting P_limit with adjacent panels carrying the increment or that portion originally carried by panel ­ at P_limit.  This is illustrated in Figure 1.3.21 where the amount of load in panel ­ at the limit design condition, i.e. P₂ is redistributed after it is multiplied by 1.15 to account for dynamic effects (DP₁+DP₂+ DP₃+ DP₄+ DP₅).  The total redistribution increment then is

The residual strength capability of the remaining structure is then checked against this condition; the P_yy requirement for panel ® is .

Figure 1.3.21.  Illustration of Redistributed Panel Load P₂ to Adjacent Structure