Reliability Growth Enhancing Defense System Reliability (2015) / Chapter Skim
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3 Reliability Metrics
3 Reliability Metrics
Pages 39-46
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From page 39... ...
Tracking a reliability metric over time, as a system design is modified and improved, leads to the topic of reliability growth models, which are the subject of the next chapter. CONTINUOUSLY OPERATING REPAIRABLE SYSTEMS In developmental and operational testing, continuously operating systems that are repairable perform their functions as required until interrupted by a system failure that warrants repair or replacement (ordinarily at the subsystem or component level)
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From page 40... ...
-- or as the probability of successfully completing a prescribed operational mission of a given time duration without experiencing a major failure.1,2 Standard DoD reliability analyses normally entail three analytical assumptions: 1. Restoration activities return a failed test article to a state that is "as good as new." That is, the time to first failure (from the begin ning of the test3)
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From page 41... ...
An additional issue relates to the interpretability of models that portray nonconstant failure intensities: In particular, what sort of a summary estimate for a completed developmental or operational test event should be reported for comparison to a simply specified mean time between failure prediction or requirement (that did not contemplate time-variant intensities)
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From page 42... ...
Implicit in assumption 3 above is that the environment and operating conditions remain constant for the test article each time it is repaired and returned to service. Unless statistical extrapolation methods are applied, reliability estimates generated from a single test's observed failure data should be interpreted as representative solely of the circumstances of that test.5 The possible effects of influential factors (e.g., characterizing the execution of the testing or description of the past usage or maintenance and storage profiles)
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From page 43... ...
In addition, analytical models can be developed to relate expected remaining life to concomitant data, such as recorded information on past environmental or usage history, measures of accumulated damage, or other predictive indicators obtained from sensors on the systems. Nonrepairable systems are common in many commercial settings, but they are rare in DoD acqusition category I testing.
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From page 44... ...
Observed system failures logically could be examined from the perspective of mean time between failures, facilitating the construction of estimates of in-flight reliability that correspond to distinct operational mission scenarios that span a wide spectrum of launch-to-target ranges. To obtain measures of overall reliability, these estimates could be augmented by results from separate one-shot tests, in the same developmental or operational testing event, that focus on the probabilities of successful performance for other non-in-flight elements of cruise missile performance (e.g., launch, target recognition, warhead activation, and warhead detonation)
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From page 45... ...
For a given developmental or operational test event, knowledge of the nature of the reliability data and of the particulars of the testing circumstances, including composition and characteristics of test articles, are available. This information can support the development of alternative models and associated specific forms of reliability metrics that are potentially useful for describing reliability performance demonstrated in testing and projected to operational missions.
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From page 46... ...
Again, as appropriate, distinctions should be drawn between estimates of system reliability and estimates of system operational reliability.
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