Strain-controlled torsional fatigue and fracture of EQ56 high-strength steel

Monotonic torsion tests and low-cycle torsion fatigue tests under various shear strain amplitudes are conducted on EQ56 high-strength, a material widely used in offshore platforms. Under monotonic torsional loading, EQ56 high-strength steel demonstrates high ductility and strain hardening behavior. Torsional fatigue tests are performed with equivalent strain amplitudes ranging from 0.4 % to 1.0 %. At a strain amplitude of 0.4 %, the material shows immediate rapid cyclic softening, whereas at higher strain amplitudes, the material exhibits an initial cyclic hardening followed by cyclic softening within the first few cycles. The results indicate that the initial cyclic hardening ratio is strongly dependent on the strain amplitude, whereas the cyclic softening ratio remains strain-independent. Shear fatigue parameters are determined using the Manson-Coffin relationship, accurately predicting fatigue life in strain-controlled tests with strong agreement to experimental results. Fatigue life predictions are further evaluated using three equivalent criteria (Tresca, von Mises, and maximum principal strain) and two energy-based methods. Except for the Tresca criterion, all methods provided predictions within a scatter band of 2 error factor. Scanning electron microscopy (SEM) analysis of fracture surfaces reveals cracks perpendicular to the axial direction, providing insights into the failure mechanisms under torsional fatigue.