DEFINITION OF RELIABILITY

Reliability is the probability of a device performing its purpose adequately for the period of time intended under the operating condition encountered.

A failure-rate curve of the type shown in Fig. 1 has been called " bath tub " curve.

The failure early period OA is sometimes called the infant-mortality or the burn-in period. Failure occurrence can be discerned by burn in or aging of the components.

The failure rate will decrease to a lower value ( Point C ) which remain fairly constant and define the beginning of normal operation period. During this period, failure takes place at random. The important objective of the manufacturer is to minimize this failure rate as low as possible.

In general, if X represents a constant probability of failure per hour, the probability of an item surviving for H hours is (1-X )^H. For small X one can approximate:

R(t)=exp (-HX ) where R(t) is the probability of survival after H hours.
and 1/X =MTBF ( Mean Time Between Failure )