System Reliability Analyses
An Overview of Basic Concepts and Directory of Other Resources

Overview

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.) This analysis is based on the time of successful operation or time-to-failure data of the item (component), 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.   

In System Reliability one constructs a "System" model from these component models. In other words in system reliability analysis we are concerned with 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. 

A system is a collection of components, subsystems and/or assemblies arranged to a specific design in order to achieve desired functions with acceptable performance and reliability. The types of components, their quantities, their qualities and the manner in which they are arranged within the system have a direct effect on the system's reliability. To accomplish this, and in addition to the reliability of the components, the relationship between these components is also  considered and decisions as to the choice of components can be made to improve or optimize the overall system reliability, maintainability and/or availability.  This reliability relationship is usually expressed using logic diagrams, such as Reliability Block Diagrams (RBD) and/or Fault Trees (See also Fault Trees and Reliability Block Diagrams).

Reliability Block Diagrams (RBDs)

Block diagrams are widely used in engineering and science and exist in many different forms. They can also be used to describe the interrelation between the components and to define the system. When used in this fashion, the block diagram is then referred to as a reliability block diagram (RBD). A reliability block diagram is a graphical representation of the components of the system and how they are reliability-wise related (connected). (Note: One can also think of an RBD as a logic diagram for the system based on its characteristics.) It should be noted that this may differ from how the components are physically connected.

After defining the properties of each block in a system, the blocks can then be connected in a reliability-wise manner to create a reliability block diagram for the system. The RBD provides a visual representation of the way the blocks are reliability-wise arranged. This means that a diagram will be created that represents the functioning state (i.e. success or failure) of the system in terms of the functioning states of its components. In other words, this diagram demonstrates the effect of the success or failure of a component on the success or failure of the system. For example, if all components in a system must succeed in order for the system to succeed, the components will be arranged reliability-wise in series. If one of two components must succeed in order for the system to succeed, those two components will be arranged reliability-wise in parallel. See RBDs and diagramming methods.

This reliability-wise arrangement of components is directly related to the derived mathematical description of the system. The mathematical description of the system is the key to the determination of the reliability of the system. In fact, the system's reliability function is that mathematical description (obtained using probabilistic methods) and it defines the system reliability in terms of the component reliabilities. The result is an analytical expression that describes the reliability of the system as a function of time based on the reliability functions of its components. This is discussed further in the Statistical Background, RBDs and Analytical System Reliability and Time-Dependent System Reliability (Analytical) sections on-line references on this site.

Learn more:   See System Analysis Reference (eTextbook) on this site.