Ac total cracked area
\(l\) surface crack length
\(\text{\ l}_{\text{av}}\) average length of the existing surface cracks
 N number of cycles
\(\text{\ N}_{\text{f\ }}\) number of cycles to failure
Z number of existing surface cracks
Δmc extent of monotonic and cyclic, respectively, crack tip plastic zone
δ, Δδ maximum and cyclic, respectively, crack tip opening displacement
σ current applied stress amplitude
σo initial axial stress amplitude
σy monotonic yield stress
σcy cyclic yield stress