Pattern of Failures

The failure rates or rate of occurrence of failures (ROCOF) of repairable systems can also vary with time and an important implication can be derived from these trends.

A constant failure rate (CFR) is indicative of externally induced failures, as in the constant hazard rate situation for non-repairable systems. A constant failure rate (CFR) is also typical of complex system if subjected to repair and overhaul where different parts exhibit different patterns of failures with the time and parts have different ages since repair or replacement. Repairable system can show a decreasing failure rate (DFR) when reliability is improved by progressive repair, as defective parts which fails relatively early are replaced by good parts. An increasing failure rate (IFR) occurs in repairable systems when wear out failure mode parts begins to predominate.

The impact of designed reliability is greatly influenced by variations in operating environment which can be controlled through monitoring the trend. Also, to estimate the reliability it is essential to know the failure patterns and the behaviour of the machine to determine the distribution from which the sample has come from. Experiences say that the simple way to predict most of the distributions is the trend analysis.