D	fatigue damage
Di	cumulative damage at the ith loading cycle
D 0	initial damage
D c	critical damage
m′	strengthening coefficient
N	loading cycles
ni	loading cycles of the ith-level load
N f	fatigue life
Ni *	equivalent fatigue life subjected to the ith-level load
Yi	cyclic ratio
${\mu }_{\tilde{D}}({\sigma }_i)$	membership function
Σ	stress
σi	stress of the ith-level load
σ a	stress amplitude
σ F	lower limit of strengthening stress
σ m	mean stress
σ max	maximum stress
σ min	minimum stress
σ 0	fatigue limit
b0, M0, α, β	material parameters
k, α′, β′, σc	membership function parameters