$a_{PR}:b_{PR}$	Material parameters for Papuga–Růžička criterion
$a_{SCF},b_{SCF}$	Material parameters for Stulen–Cummings–Findley criterion
$k_C,k_D,k_{CS},k_M,k_P$	Material parameters for the Crossland, Dang Van, Carpinteri–Spagnoli, Matake, and Papadopoulos criteria, respectively
$N_f$	Number of cycles for failure
$r_{\sigma }\left(N_f\right)$	Ratio of fatigue strength in fully reversed torsion to axial stress (R = −1) at a given number of cycles to failure $N_f$
$r_{\sigma 0}\left(N_f\right)$	Ratio of fatigue strength in fully reversed torsion to zero-pulsating axial stress (R = 0) at a given number of cycles to failure $N_f$
$\Delta r_{\sigma }$	Range of $r_{\sigma }$ observed over the span of experimental fatigue lives
${\sigma }_f\left(N_f\right)$	Fatigue strength in fully reversed axial loading at a given number of cycles to failure $N_f$
${\sigma }_{f0}\left(N_f\right)$	Fatigue strength in zero-pulsating axial loading at a given number of cycles to failure $N_f$
${\sigma }_{n,max}, {\sigma }_{H,max}$	Maximum value of normal and hydrostatic stresses, respectively
${\sigma }_{n,a}, {\tau }_{ns,a}$	Normal and shear stress amplitudes on critical plane, respectively
${\sigma }_f^{’}$	Axial fatigue strength coefficient
${\tau }_f\left(N_f\right)$	Fatigue strength in fully reversed torsion loading at given number of cycles to failure $N_f$
R	Stress ratio, $R={\sigma }_{min}/{\sigma }_{max}$
$T\left(0.95\right)$	Fatigue scatter band for a 95% confidence level
$T_a$	Average resolved shear stress amplitude for the Papadopoulos et al. criterion
$\sqrt{J_{2,a}}$	Amplitude of second invariant of stress deviator tensor