Search

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]

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]

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]