S’N is the theoretical fatigue strength in a fully reversed test(Sm= 0, and R = Smin/Smax= −1) at N number of cycles; and K and b are the fatigue strength coefficient and exponent, respectively. S’N can be theoretically calculated for a specific cycle life of 103 cycles (S_1000) and 106 cycles (S’e) in a fully reversible test(R = Smin/Smax= −1), using these expressions: