Search

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]

• 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]

Abstract Low cycle fatigue (LCF) behavior of CP-Ti along transverse direction under symmetrical strain and stress-controlled modes was compared systematically. For the strain-controlled tests, tension-compression asymmetry (TCA) has little effect on LCF behavior due to the negligible compressive mean stress. For the stress-controlled tests, TCA is detrimental to LCF resistance. The microstructure evolution during cycling deformation was characterized by EBSD and TEM to identify different deformation mechanisms. It was found that strain cycling deformation was dominated by dislocation slip and exhibited the transition from planar slip to wavy slip with the increase of strain amplitude. For the stress-controlled tests, tensile twinning together with dislocation slip accommodated the increased ratcheting deformation. The strain-life curve under stress-controlled tests is lower than that at strain-controlled tests due to the heterogeneous plastic deformation caused by ratcheting deformation. In order to obtain a unified fatigue life prediction model for different control modes, a novel effective strain range related model was proposed by introducing the TCA factor into the original Manson–Coffin relation. Furthermore, the application of this effective strain range related model was extended to the condition of asymmetrical stress cycling by correcting fatigue ductility coefficient and exponent. [ABSTRACT FROM AUTHOR]

Abstract In order to understand the effects of cyclic asymmetry and control mode on low cycle fatigue (LCF) behavior of CP-Ti along rolling direction (RD), symmetrical strain and stress-controlled fatigue tests were conducted in this paper. LCF behavior of CP-Ti along RD under different control modes was systematically compared from the aspects of cyclic response, microstructure evolution and fatigue life with the technique of electron backscatter diffraction (EBSD) and scanning electron microscope (SEM). It was found that cyclic softening resulted by dislocation slip dominates strain-controlled fatigue. On the contrary, cyclic hardening dominates stress-controlled fatigue at high stress amplitude, leading to a higher cyclic stress-strain curve. Owing to asymmetrical hysteresis loop, compressive ratcheting strain is accumulated in stress-controlled LCF test despite of the fully reversed stress cycling. EBSD results show there is a transition of deformation mechanism from dislocation slip to dislocation slip and twinning dominated cyclic deformation with the increase of compressive ratcheting strain. Meanwhile, GOS analysis and SEM observation indicate that the appearance of massive twins results in the heterogeneous plastic deformation and micro-cracks are favorable to be initiated in the twin abundant area. Besides, compressive ratcheting strain has significant effects on fatigue crack growth as the fracture surface presents the serious abrasion of cleavage facets and fatigue striations. Finally, strain-life curves under different control modes are identical, indicating that fatigue life along RD is insensitive to compressive ratcheting strain under the interaction of these factors. [ABSTRACT FROM AUTHOR]

Abstract: Based on the careful analyses of the stress controlled fatigue behavior of commercially pure titanium (CP-Ti) at room temperature, two stress regions can be evidently identified by cycle strain range, ratcheting strain, min ratcheting strain rate, ratcheting failure strain and fracture morphologies. In the high stress region, the ratcheting behavior is significant, and the failure is caused by the large ratcheting deformation. However, in the low stress region, the ratcheting behavior is weak, and the failure is the typical fatigue fracture, with fatigue crack initiation, fatigue extension, and instant rupture. In the high stress region, for both ratcheting and fatigue behaviors, the influence of max stress is greater than stress amplitude. On the contrary, in the low stress region, the influence of stress amplitude on the fatigue behavior is greater than max stress. But the ratcheting behavior in the low stress region is still max stress controlled. Based on the discussion on the fatigue life prediction model, traditional Basquin model and energy based model cannot supply precise fatigue life in the wide stress range, since the ratcheting behavior is not considered. The coupled fatigue damage and ratcheting damage model is proved to make a robust prediction. [Copyright &y& Elsevier]

• 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]

• Fatigue life of CP-Ti shows non-monotonic variation with stress ratio. • Effects of prestrain on fatigue life under different control modes are clarified. • Current strain-life models including mean stress effects are evaluated. • A new strain-life model with exponential mean stress correction term is proposed. This paper investigates symmetric strain, stress and asymmetric stress-controlled fatigue behavior of prestrained and non-prestrained CP-Ti. The effects of stress ratio, mean stress, prestrain on stress/strain life were carefully analyzed. Stress-life of CP-Ti shows non-monotonic variation with stress ratio, as compressive ratcheting resulted by tension-compression asymmetry accumulates under symmetric stress cycling. Stress-life can be improved by prestrain, regardless of loading condition, due to the decrease of responded strain amplitude. Stress fatigue resistance of the material is actually deteriorated by prestrain at the stress level identical to that of non-prestrained CP-Ti. Strain-life of prestrained sample shows little variation at high strain amplitude due to mean stress relaxation and significant reduction at low strain amplitude. By introducing asymmetry factor, it is further found that strain-life of the material is insensitive to prestrain if the applied mean stress level keeps constant. Finally, current strain-life models with different mean stress correction terms are discussed to evaluate their application ability in fatigue life prediction of CP-Ti under different loading conditions. Though two additional parameters are introduced into the mean stress correction term with power law form, there are still few data points beyond the scatter bounds of two. Therefore, a new strain-life model with exponential mean stress correction term is proposed and verified by fatigue life data of different materials, which has more advantage than existing strain-life models at a wide range of mean stress level. [ABSTRACT FROM AUTHOR]