Environmental Engineering Reference
In-Depth Information
Units of operating time
-1
Operating time
t
, s
1. 7
Reliability of objects at different operating times to failure.
operating time
S
have the dimension [units of operating time], and the
dispersion
D
- [units of square operating time].
1.2.4 Mathematical models of reliability theory.
Statistical processing of test results
To solve the problems of estimating the reliability and its prediction, it is
necessary to develop a mathematical model that is represented by analytical
expressions of one of the parameters
P
(
t
) or
f
(
t
) or λ(
t
). The main procedure
to construct such a model is based on testing, calculation of the statistical
estimates and approximations of these estimates by analytic functions.
The models used in reliability theory will be investigated.
Let us explain how the failure-free operation of objects changes during
service so that the models can be classified and modalities for their
application can be identified.
Experience has shown that changes of FP λ(
t
) of the vast majority of
the objects can be described by the U-shaped curve (Fig. 1.8).
The curve can be divided into three characteristic regions:
first - burn-in period;
second - normal operating period;
third - ageing period.
The burn-in period of the object has a higher failure rate caused
by burn-in failures due to defects in manufacturing, assembly and
adjustment. Sometimes the end of this period is also the end of the warranty
period for the object during which failuresare put right by the manufacturer.
During the normal operating period the failure rate decreases and remains
almost constant, while failures are random and appear suddenly, primarily
due to violation of the service conditions, random load changes, adverse
external factors, etc. This period is consistent with the main operating time
of the object.
Failure rate increases mainly in the ageing period of the object and is
caused by an increase in the number of failures from wear, ageing and other
factors associated with prolonged operation.
The type of analytic function describing the change of reliability
Search WWH ::
Custom Search