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The effect of hold time with 0, 20, and 40 s on in-phase thermomechanical fatigue (TMF) behavior and life of P92 steel is investigated in this study. TMF tests are carried out under mechanical strain control with strain amplitudes of 0.40.4, 0.60.6, and 0.8%0.8\text{\%}, and temperature range of 550–650°C which is closely relevant to the operating condition in power plant. TMF tests are performed in a mechanical strain ratio of R=−1R=-1 and cycle time of 120 s. The fatigue life variation follows the sequence of Nf0s

Multiaxial low cycle fatigue tests under non‐proportional stress (NPSS) controlled mode were performed on commercial pure titanium (CP‐Ti). Strain responses of axial and torsional channels under highly applied stress amplitudes show an initial hardening phenomenon. Non‐proportional hardening coefficient of CP‐Ti is independent of the controlled mode. The critical plane of CP‐Ti under NPSS controlled mode is aligned with the maximum principal stress plane proved by optical microscopy observation. Optimized FSM model and KBM‐PM model with mean axial and torsional strain are established. These models are further integrated into equations related to multiaxial stress ratio with high accuracy of life prediction for CP‐Ti under NPSS controlled mode. Highlights: Non‐proportional hardening coefficient of CP‐Ti is independent of the controlled mode.The critical plane of CP‐Ti is aligned with the maximum principal stress plane.Energy and optimized critical plane model with mean strain are established. [ABSTRACT FROM AUTHOR]

In‐plane biaxial fatigue tests of commercial pure titanium (CP‐Ti) were performed to investigate the effects of load ratio and phase shift on fatigue and ratcheting behavior. The results show that with the decrease of load ratio or the increase of phase shift, biaxial ratcheting strain effect is more significant, leading to the deterioration of fatigue resistance. Analysis of the crack morphology and fracture characteristic reveals that the crack propagation path becomes complex and the fracture morphology changes due to the enhanced non‐proportional hardening effect. Finally, current multiaxial life prediction models are evaluated to compare the applicability in the biaxial fatigue life prediction. Highlights: Effects of load ratio and phase shift on biaxial ratcheting behavior are discussed.Biaxial fatigue life is more sensitive to the variation of the phase shift.The crack morphology and fracture characteristics are analyzed.The applicability of different multiaxial life prediction models is evaluated. [ABSTRACT FROM AUTHOR]

• The maximum principal strain critical plane is more effective under both IP and OP loading. • FS model and KBM-P model can be applied to both IP and OP loading in a general form. • MLCF life prediction model based on newly defined α N f and α are established. Non-proportional multiaxial low-cycle fatigue (MLCF) tests were performed on CP-Ti at different multiaxial strain ratios. The optical microscopy observations and life prediction models verified that the critical plane of CP-Ti was the maximum principal strain plane and that it was effective under non-proportional loading. A non-proportional life coefficient and strain energy density coefficient are proposed. Typical MLCF life prediction models, such as the ESN, FS, KBM, and KBM-P models, are discussed under non-proportional loading. Finally, by associating the newly defined non-proportional life coefficient with the non-proportional hardening factor, two comprehensive non-proportional MLCF life prediction models are established, which have higher accuracies than those of previous models. [ABSTRACT FROM AUTHOR]

• For CP-Ti, the threshold value for ratcheting strain accumulation of the axial stress amplitude is closer to 287 MPa. • Rate Surface Ratio is proposed to represent the difference in velocity during ratcheting accumulation. • The brand-new KBM-PH model with mean stress correction is the best overall among all models. Asymmetrical multiaxial fatigue tests were performed. A new quantification parameter is proposed to represent the difference in velocity during ratcheting accumulation. Although the stress–strain response under asymmetric loading is similar to that under symmetric loading, substantial deteriorations of life are observed. The threshold value for ratcheting strain accumulation of axial stress amplitude is closer to 287 MPa. The brand-new stress parameters-based KBM-PH model with mean stress correction is the best overall among a series of models based on strain and stress parameters with or without mean strain and stress corrections with the best accuracy and ease of use. [ABSTRACT FROM AUTHOR]