A TREATISE ON SYSTEM RELIABILITY AND DESIGN COMPLEXITY

The quality improvement process of reliability assessment and improvement is often a post design exercise that is traditionally practiced on the component level of products as a post design validation exercise. This article defines system reliability from a function delivery perspective that lays the foundation for reliability improvement in the concept design stage. Design vulnerability is not intentionally designed into a design but it is the result of poor conceptual design processes that result in the selection of sub-optimal designs. By sub-optimal designs we mean designs that are not entirely capable of providing designed functions with certainty. The uncertainty of the provision of functions is a measure of the design vulnerability. Of interest to design science is a method for evaluating designs from a vulnerability point of view. This article clarifies the relationship between system reliability, cost of failures and design vulnerability. The mathematical formulation of system design vulnerability from an Axiomatic Design Principles and a Multi-Level Hierarchical (MLH) perspective is stated in the form of a function failure cost model. The MLH cost model as a measure of design vulnerability is discussed. The MLH cost model adapts itself well to design evaluation because it captures the complexity of a design from a function reliability perspective with respect to the design independence and design uncertainty dimensions and also quantifies the impact of the uncertainty to deliver the design functions. A framework for applying the system design vulnerability criteria in the evaluation of reliability of alternative concepts is explained with an application. The MLH analytical system model is explained by applying the methodology to an overhead projector design.