In life data analysis and accelerated life testing data analysis, the objective is to obtain a life distribution that describes the times-to-failure of a component, subassembly, assembly or system. (For more details, see ReliaSoft's Life Data Analysis Reference and ReliaSoft's Accelerated Life Testing Reference.) The analysis to determine the life distribution is based on the time of successful operation or time-to-failure data of the item, either under use conditions or from accelerated life tests.
For any life data analysis, the analyst chooses a point at which no more detailed information about the object of analysis is known or needs to be considered. At that point, the analyst treats the object of analysis as a "black box." The selection of this level (e.g. component, subassembly, assembly or system) determines the detail of the subsequent analysis.
The main objective of system reliability is the construction of a model (life distribution) that represents the times-to-failure of the entire system based on the life distributions of the components, subassemblies and/or assemblies ("black boxes") from which it is composed, as illustrated in Figure 2.1.
Figure 2.1: Obtaining a system pdf from the pdfs of the components.
To accomplish this, the relationship between components are considered and decisions as to the choice of components can be made to improve or optimize the overall system reliability, maintainability and/or availability. There are many specific reasons for looking at component data to estimate the overall system reliability. One of the most important is that in many situations it is easier and less expensive to test components/subsystems rather than entire systems. (Note: In this reference, the terms component, assembly and subsystem will be used interchangeably since a block can represent any level in the system configuration. Furthermore, we will use the terms block and component interchangeably to describe the item being analyzed.) Many other benefits of the system reliability analysis approach also exist, and they will be presented throughout this reference.
This chapter includes the following sections:
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