Practical Reliability Engineering, 5th Edition

7

Design for Reliability

7.1 Introduction

The reliability of a product is strongly influenced by decisions made during the design process. Deficiencies in design affect all items produced and are progressively more expensive to correct as development proceeds. The cost of errors and design changes increases drastically over the course of the product development cycle, Figure 7.1. It can be as high as a 10-fold increase in cost from one phase to another. It is therefore essential that design disciplines are used which minimize the possibility of failure and which allow design deficiencies to be detected and corrected as early as possible. In Chapter 5 the basic requirements for failure-free design were laid down, that is adequate safety margins, protection against extreme load events and protection against strength degradation. The design must also take account of all other factors that can affect reliability, such as production methods, use and maintenance, and failures not caused by load.

The design process must therefore be organized to ensure that failure-free design principles are used and that any deviations from the principles are detected and corrected. The designers must aim to create designs which will not fail if manufactured and used as specified.

The old design concept of ‘test-analyze-and-fix’ (TAAF), in which reliance is placed on the test programme to show up reliability problems, no longer has a place in modern design and manufacturing due to shorter design cycles, ...