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8.
Assessing Product Reliability
8.1. Introduction 8.1.2. What are the basic terms and models used for reliability evaluation?
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| Repair Rate models are based on counting the cumulative number of failures over time | A different approach is used
for modeling the rate of occurrence of failure incidences for a repairable
system. In this chapter, these rates are called repair rates (not
to be confused with the length of time for a repair, which is not discussed
in this chapter). Time is measured by system power-on-hours from initial
turn-on at time zero, to the end of system life. Failures occur at given
system ages and the system is repaired to a state that may be the same
as new, or better, or worse. The frequency of repairs may be increasing,
decreasing, or staying at a roughly constant rate.
Let N(t) be a counting function that keeps track of the cumulative number of failures a given system has had from time zero to time t. N(t) is a step function that jumps up one every time a failure occurs and stays at the new level until the next failure. Every system will have its own observed N(t) function over time. If we observed the N(t) curves for a large number of similar systems and "averaged" these curves, we would have an estimate of M(t) = the expected number (average number) of cumulative failures by time t for these systems. |
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| The Repair Rate (or ROCOF) is the mean rate of failures per unit time | The derivative of M(t), denoted m(t),
is defined to be the
Repair Rate or the Rate Of Occurrence Of
Failures at Time t or ROCOF.
Models for N(t), M(t) and m(t) will be described in the section on Repair Rate Models. |
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