| NiTi shape memory alloy has been extensively used in aerospace and bio-medical fields due to its super-elasticity, shape memory effect and excellent bio-compatibility. The components and devices made by the NiTi SMA are often subjected to a cyclic loading in service. Thus, the damage accumulation and fatigue failure of the NiTi shape memory alloy are key issues that should be investigated. In recent years, experimental observations have been conducted on the fatigue behavior of NiTi shape memory alloys, and life-prediction models are also constructed. However, most of the experiments are performed under the strain-controlled cyclic loading conditions, and at room temperature. Moreover, these experiments only focused on the fatigue failure of the NiTi alloy in the uniaxial and bending load cases, the fatigue failure in the non-proportional multiaxial cases have not been reported yet. Meanwhile, the observations to the evolution of whole-life transformation ratchetting are still insufficient, and the life-prediction models are also so limited, especially for the fatigue failure of super-elastic NiTi micro-tubes which are used in the endovascular stents. Such limitation extremely hinders the design of NiTi medical devices. Thus, it is necessary to perform comprehensive stress-controlled fatigue experiments under the axial, torsional and multi-axial loading conditions, and analyze the whole-life transformation ratchetting and fatigue lives of NiTi shape memory alloy and their evolutions. Moreover, the morphology of fracture surface and residual martensite phase on the surface are also observed for establishing the reasonable damage evolution and fatigue failure models, in order to improve the ability of the model in predicting the damage evolution and fatigue life of NiTi shape memory alloy.Therefore, to perform systematic research on the damage evolution and fatigue behavior of NiTi shape memory alloys in different loading cases, following studies have been carried out:1. At human-body temperature (310K), systematic stress-controlled fatigue experiments were conducted on NiTi micro-tubes in axial, torsional, as well as multiaxial cases. The evolution of transformation ratchetting in different loading cases were summarized; the effects of stress level, loading path and P/V stress holds on the transformation ratchetting were discussed as well. Meanwhile, the effect of transformation extent and the re-orientation induced plasticity on the evolution of transformation ratchetting are also analyzed. The studies provide an experimental basis for the establishment of damage evolution model for NiTi shape memory alloys.2. The fatigue failure of super-elastic NiTi shape memory alloys in uniaxial, torsional and multi-axial loading cases is analyzed, and the relationship between fatigue life and stress level/transformation extent is also summarized. Meanwhile, the different fatigue failure features in uniaxial and multiaxial loading cases are compared, and the effect of re-orientation induced plasticity on fatigue life is also discussed. Experimental observations are also performed on the fracture surface and residual martensite phase on the surface in different loading cases. Some significant conclusions useful to understand the fatigue failure of NiTi shape memory alloys are obtained. The studies play important roles in the construction of fatigue failure models for NiTi shape memory alloy.3. Based on the obtained fatigue data and the analysis of fracture morphology, a new damage evolution model for NiTi shape memory alloy is constructed to describe the damage accumulation during the cyclic loading, the model considers the effects of micro-cracks initiation and propagation, and the transformation induced damage. Meanwhile, for the multiaxial loading cases, the re-orientation induced plasticity is also taken into account. The fatigue failure models are also constructed based on the damage evolution, reasonable life-predictions can be obtained. |
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