Design of Reliability

To a large degree, reliability is an inherent attribute of a system, component or product. As such, it is a significant consideration in the engineering design process. While the life-cycle costs of a system are being analysed, reliability plays an important role as a major driver of these costs and has considerable influence on system performance. As it is seen, decisions made during conceptual and preliminary design will affect total life-cycle costs. The objective of this chapter is to describe a reliability design process that will establish and then achieve realistic reliability goals.

Reliability design is an iterative process that begins with the specification of reliability goals consistent with cost and performance objectives. This requires consideration of the life-cycle costs of the system and the effect that reliability has on overall costs and system effectiveness.

Once the reliability goals have been established, these goals must be translated into individual component, subcomponent, and part specifications. This is not necessarily an easy task, and it generally requires a reliability block analysis. After individual component and part requirements have been determined, various design methods can be applied in order to meet the goals. Following completion of preliminary and detailed design and along with initial development and prototyping a failure analysis may be performed to determine whether specifications are being met and to provide a systematic approach for identifying, ranking and eliminating failure modes. This requires the use of reliability testing, including, perhaps, a formalised reliability growth-testing program.

Once reliability goals have been achieved, verification that safety margins are also being met must be made. If either the reliability or safety goals are not met, the design process must continue. This may require reallocating reliability goals among the components if it is not possible to achieve desired component reliability. More often the entire system must be redesigned. The effect in design changes should then be verified through continued use of failure analysis and reliability testing.