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• 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]

• 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]

• 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]