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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]

• Influence of micronotch size on fatigue life is quantified by a new model. • Average crack arrest cycle decreases with the equivalent micronotch size. • Post-arrest behavior is characterized by crack coalescence and the alternation of slip system. • A high crack propagation rate is correlated with the dominate slip system of prismatic . • Micronotch size has little effect on the long crack propagation rate. The influence of the micronotch size on the small fatigue-crack behavior of CP-Ti was investigated in this study. With an increase in the equivalent micronotch size, the fatigue life and average crack-arrest cycle decreased. Microstructural analysis revealed that crack arrest results from high-angle grain boundaries, and the post-arrest behavior is mainly characterized by the connection of main and secondary cracks, or the alternation of the slip system. A high crack propagation rate is correlated with prismatic < a > slip. The activation of other slip modes may decrease the crack propagation rate. The micronotch size affected the distribution of the crack growth regions but had little influence on the long crack propagation rate. [ABSTRACT FROM AUTHOR]

Low cycle fatigue (LCF) behavior of commercially pure titanium (CP-Ti) was investigated under stress cycling and strain cycling along the rolling direction (RD), 30° to the rolling direction (RD-30°), 60° to the rolling direction (RD-60°) and the transverse direction (TD), respectively. The loading direction brings about eminent differences in the fatigue properties, which is summarized as follows; Cyclic strain hardening exponent: RD＞RD-30°＞RD-60°＞TD Cyclic stress amplitude: TD＞RD-60°＞RD-30°＞RD Ratcheting effect: TD＞RD-60°＞RD-30°＞RD Fatigue life: RD＞RD-30°＞RD-60°＞TD Based on the comprehensive analysis of Schmid factor, texture evolution, twinning activities, in-grain misorientation axis and geometrically necessary dislocation density, it is concluded that the anisotropic slip behavior characterizes the planar anisotropy of LCF behavior. The promotion of dislocation activities of non-prismatic slip with higher activation stress from the RD to the TD leads to the higher cyclic stress amplitude, lower cyclic strain hardening and more significant ratcheting effect, though prismatic slip always acts as the primary slip mode. The observation of fracture surfaces also confirms the critical role of dislocation activities on fatigue crack initiation. Introducing yield ratio parameter into the asymmetry coefficient related model, the fatigue lives in both the fatigue were accurately predicted. [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]

The role of thermal cycling on the deformation behaviour and microscopic damage mechanisms of 316L stainless steel are clarified through isothermal fatigue (IF) and thermomechanical fatigue (TMF) tests under various temperature ranges. Corresponding microscopic damage mechanism is revealed based on the analysis of dislocation structure, plastic deformation, and crack initiation mode. Results show that the introduction of thermal cycling, as well as an increase in average temperature and temperature difference, result in the saturation of cyclic peak stresses, a decrease in fatigue life, and an intensification of dynamic strain ageing (DSA). Notably, under TMF loading at the same average temperature, there is a significant recovery process compared to IF. High-density dislocations transform into low-energy dislocation structures through cross-slip, indicating a pronounced recovery phenomenon. At large temperature differences and high average temperatures, the accumulation of dislocation tangles at grain boundaries and the formation of dislocation cell structures lead to severe incoherence in plastic deformation and the accumulation of damage. Furthermore, the introduction of thermal cycling and an increase in temperature differences also induce severe creep damage. It is observed that the dominant crack initiation mode changes from fatigue-dominated transgranular cracking to oxidation-induced intergranular cracking as the average temperature increases. Finally, a modified life prediction model is developed by incorporating thermal plastic strain energy to reflect the effect of thermal cycling on TMF behaviour. [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]

High-temperature components utilised in practical service are often subjected to multiaxial non-proportional loading due to their complex geometry. In the present study, a set of strain-controlled fatigue tests were conducted on P92 steel at 600 °C, including uniaxial, multiaxial proportional, and multiaxial non-proportional loading conditions. Results reveal that the axial and shear stresses exhibit varied responses to the different non-proportionalities, with proportional loading resulting in the shortest fatigue life when the strain ratio is small. The non-proportional loading induces accelerated cyclic softening at axial direction while the shear stress is enhanced. Electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) analysis indicate that the increased role of shear stress promotes the dislocation motion from the planar slip to cross-slip under non-proportional loading, which in turn facilitates the microstructure evolution. It is also important to note that the evident development of microstructure and the combined effect of more crack initiations and more secondary cracks contribute to the lowest fatigue life under proportional loading. • The non-proportional loading accelerates axial cyclic softening while the shear stress is enhanced. • The lowest fatigue life of P92 steel appears under proportional loading when the strain ratio is small. • The increased role of shear stress promotes the dislocation motion from the planar slip to cross-slip. [ABSTRACT FROM AUTHOR]

• A new cyclic softening model was established to describe the cyclic softening behavior of P92 steel. • The effect of different temperature range on fatigue life of P92 steel was analyzed. • It was considered that the mean stress and dynamic strain aging are two important factors of the difference between the fatigue life of thermal-mechanical fatigue and that of isothermal fatigue. P92 steel is a typical steel used in the main steam pipeline of an ultra-supercritical power plant. In most cases, the main load consists of the thermal-mechanical fatigue (TMF) load due to the fluctuation of temperature and pressure during operation. TMF tests at different temperature ranges and isothermal fatigue (IF) tests at different temperatures were performed in this study. The effects of different temperature ranges on the fatigue life of P92 steel were analyzed. It was found that the TMF life decreased first and then increased, as a result of an increase in the temperature range and decrease in the mean temperature. The cyclic softening of the TMF and IF was also investigated. In addition, it was found that the typical characteristics of dynamic strain aging (DSA) occurred in the TMF and IF process. This is considered to be an important reason for the decrease in the TMF life of P92 steel. A new cyclic softening model was established to describe the cyclic softening behavior of P92 steel, and the effect of the load conditions on the cyclic softening was reflected in this model. [ABSTRACT FROM AUTHOR]