Reliability allocation and apportionment: addressing redundancy and life-cycle cost

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Virginia Tech


Two reliability analysis techniques, allocation and apportionment, have the potential to influence a system's design (a distinction is made here between allocation and apportionment). Algorithms that account for the ever increasing design complexities are constructed here for both. As designs of aircraft, railway systems, automobiles and space systems continue to push the envelope in terms of their capabilities, the importance of performance criteria such as reliability and associated life-cycle cost (LCC) consequences become even more important. These interrelated criteria are the foundation for the reliability allocation and apportionment algorithms derived in this thesis.

Reliability allocation is the process of assigning reliability targets to lower-level assemblies to ensure the top-level assembly's goal is achieved. Reliability apportionment involves the analysis of an existing design configuration to determine the most cost-effective means of adding redundancy. In the apportionment problem, acquisition cost is the traditional cost-effectiveness measure. The apportionment algorithm defined herein expands the definition of cost-effectiveness to include downstream costs, thereby addressing LCC.

A well-behaved, allocation routine is derived to account for any combination of serial, parallel and partially redundant configurations. In addition, a closed-form analytic solution provides the framework for economically adding redundancy to a system's structure in order to achieve a system-level reliability goal. An Apportionment Criterion Ratio (ACR), which contrasts the incremental reliability benefits of adding redundant components with the corresponding incremental LCC, is used.

The Rate of Occurrence of Failure (ROCOF) is the reliability metric used in both the allocation and the apportionment routines. The formulation of the LCC model carefully distinguishes between failures and an allied measurement, demands.