Notation	Meaning
$i$	Component number
t	Time since the component is put into use
r	Actual utilization rate of components
${\lambda }_i(t|r)$	Failure rate function of component i
$G(r)$	Cumulative probability distribution function of utilization
$T_{ki}$	kth preventive maintenance interval of component i
$\alpha$	The increasing failure rate factor
$\beta$	The decreasing age factor
$R_{{}_{\min }}^i$	Reliability threshold when performing preventive maintenance for component i
$n_i$	Number of preventive maintenance projects before component i replacement
$\Delta R$	Reliability threshold in opportunistic maintenance
$[W_B,U_B]$	Two-dimensional warranty period. $W_B$ is the warranty period of the time dimension, and $U_B$ is the warranty period of the usage dimension
$S_{fi}$	Corrective maintenance cost of component i
$S_{pi}$	Imperfect preventive maintenance cost of component i
$S_{ri}$	Preventive replacement cost of component i
$T_{fi}$	Corrective maintenance downtime of component i
$T_{pi}$	Imperfect preventive maintenance downtime of component i
$T_{ri}$	Preventive replacement downtime of component i
$C_{ai}$	Reliability utilization value of component i
$C_d$	Downtime loss per unit time