Availability and maintenance modeling for systems subject to dependent hard and soft failures

TLDR

Hard failures halt the system immediately, soft failures only reduce performance, and each soft failure increases the hard‑failure hazard rate. The paper develops availability and maintenance models for single‑unit systems experiencing dependent hard and soft failures. The authors derive recursive equations for reliability and availability, perform periodic inspections to detect both failure types, and optimize the inspection policy by minimizing expected cost per unit time. The models are validated through a case study on an electrical distribution system.

Abstract

This paper develops availability and maintenance models for single‐unit systems subject to dependent hard and soft failures. A hard failure stops the system immediately, whereas a soft failure only reduces the performance capacity of the system. Dependence between these 2 types of failures is reflected in the fact that each soft failure directly increases the hazard rate of the hard failure. On the basis of such interaction, we derive recursive equations for the system reliability and availability functions. To detect both types of failures, inspections are executed periodically. Furthermore, we investigate the optimal inspection policy via the minimization of the expected cost per unit time. The applicability of the developed availability and maintenance models is validated by a case study on an electrical distribution system.