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 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.
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