Modeling of torsional fatigue failure and experimental investigation

In this study, an elastic–plastic damage model for fatigue life prediction is developed based on continuum damage mechanics, and the fatigue characteristics of torsion specimens of 18CrNiMo7-6 steel are investigated experimentally. A damage-coupled elastic–plastic constitutive model considering non-linear isotropic/kinematic hardening is established, and the material parameters are calibrated. A damage variable coupling elastic and plastic components is introduced, and corresponding damage parameters are obtained from torsional fatigue tests with an 80 % confidence level. A finite element model is established, and the errors of predicted and experimental lives are within the triple error band, validating the correctness of the elastic–plastic damage model. The damage evolution laws and weakening effects of damage are revealed. The results indicate that purely elastic damage under lower loads weakens local yield capacity and consequently leads to plastic damage in the following stage. Kinematic hardening achieves saturation stress of 152 MPa at an early stage, while isotropic softening requires higher plastic strain to attain saturation at 115 MPa. Cracks initiate at maximum octahedral shear stress zone, resulting in instantaneous fracture with smooth contour (at ≤70 % critical torque) or ductile tearing with 5–20 µm dimples (at ≥80 % critical torque). A life factor Z_NT ≥ 1.21 is recommended to further narrow the design range beyond that suggested by the standards.