System state monitoring and lifetime scales—I

This paper is devoted to failure monitoring of critical systems. The monitoring is carried out by measuring a systemʹs generalized age in a specially selected time-scale termed the best monitoring scale (BMS). The BMS is a linear combination of several observable ‘principal’ time-scales, like the operational time, the number of cycles, the calendar time. This time-scale has the property that the coefficient of variation of the lifetime in the time-scale is mininal. We discuss some theoretical aspects of finding the BMS and present a method for calculating it on the basis of failure data observed in two principal time-scales. The construction of BMS and its applications are illustrated using fatigue test data. We demonstrate the applications of the BMS to the nomination of system safe operational period and to the accelerated life testing. We describe the use of so-called damage counters (DCs) for system-state monitoring and explain that their efficiency depends on the properties of the time-scales created by the DCs.