A novel shock-dependent preventive maintenance policy for degraded systems subject to dynamic environments and N-critical shocks

As a protective device for vehicles, shock absorbers are designed to maintain the safety and stability of vehicles. However, the constantly changing road conditions along with various dependent failure mechanisms pose challenges to their performance stability and maintenance strategy formulation. This paper investigates a linearly degraded system subject to both a dynamically changing operating environment and a series of external shocks, where an N-critical shock model is utilized to describe the limited shock resistance of shock absorbers and a continuous-time Markov process is employed to characterize the random evolution of environments. Explicit formulas for computing system reliability functions are proposed, along with a simulation algorithm as the alternative method. Furthermore, a novel shock-dependent preventive maintenance policy is developed to fully take advantage of the information on shock arrivals, which is observable and measurable, for degraded systems operating in a dynamic environment. Valuable conclusions are drawn that increasing the critical or shock failure threshold can enhance system performance while reducing maintenance costs; an increase in corrective maintenance costs indicates a need for earlier replacement, while an increase in preventive maintenance costs leads to postponed replacement.