RELIABILTTY GOALS - THE EFFECT OF DEPENDENCY 630523

In complex systems there exist many elements and an associated large number of possible strength failure modes. If the overall reliability goal assigned to the strength failure modes is large and all the failure modes are assumed to be independent, then the reliability goals for each element designed on a strength basis can be very severe. It is shown that sets of strength failure modes are related through environmental considerations. This dependence leads to less severe reliability goals, and thus to lower total design weight, than that found when assuming all the strength failure modes to be independent. The dependence of the failure mode is then associated with the a posteriori behavior of systems, indicating that the strength failure modes can essentially be represented by a few dominant modes. Simple rules are then given for allocating the required overall reliability goal to the dominant strength failure modes, and in turn, to the elements designed from strength considerations.