Multiaxial fatigue behavior of EQ56 marine high-strength steel under different strain ratios

Multiaxial low-cycle fatigue tests were conducted on EQ56 high-strength steel for offshore platforms, investigating the effects of multiaxial strain ratios (λ = $\sqrt{3}/2$, $\sqrt{3}$, $1.5\sqrt{3}$, $2\sqrt{3}$) and strain amplitudes (∆ε/2 = ±0.29 %, ±0.35 %, ±0.42 %, ±0.57 %, ±0.75 %) on cyclic behavior. The material exhibits pronounced cyclic hardening at high strain ratios and amplitudes, while at low strain ratios and amplitudes, the hardening behavior is not observed. Throughout the fatigue life, the material predominantly exhibits cyclic softening. Fractographic analysis using 3D depth-of-field imaging reveals a transition from tensile to shear failure modes with increasing strain ratio, indicating a mixed failure mode. Scanning electron microscopy (SEM) observations further confirm this transition through tire-like traces and increasingly blurred cleavage features, attributed to the influence of shear components. A comparative analysis of four critical plane multiaxial fatigue life prediction models was conducted. The Wu-Hu-Song (WHS) model, which accounts for mixed failure modes, achieved 78 % prediction accuracy with a mean relative error (MRE) of 22.8 %. The modified Kandil-Brown-Miller (KBM) model and WHS model (i.e., KBM-M, WHS-M), which incorporate the effect of multiaxial strain ratios and show improved accuracy and reduced prediction dispersion, with MRE values of 12.6 % and 15 %, respectively.