On the imperfection of replacement

Abstract In this paper the replacement process for a N modules parallel structure is characterised by an imperfect nature. Intermediate series states are added to the state transition diagram such that a second procedure is now required to fully attain a complete regenerative process. This is quantified in term of the steady state availability. As an extention of the analysis, the effect of hard core components is presented. This has been proved that under certain circumstances it is not a necessary condition of degradation.