超弹性NiTi形状记忆合金微管的疲劳行为及损伤演化模型研究

发布时间：2018-08-12 11:23

【摘要】：NiTi形状记忆合金由于其卓越的超弹性,形状记忆特性和生物相容性,被广泛应用于航天航空和生物领域。超弹性NiTi合金器件常作为结构中的关键元件,并在服役中受到循环载荷的作用。因此超弹性NiTi合金的疲劳行为是亟需关注的一个重要问题。近年来,国内外的一些学者对于超弹性NiTi合金的疲劳行为开展了较为广泛的实验观察和疲劳失效模型研究。然而,绝大部分的实验均为应变控制,并且未采用温控装置以保持恒温的实验过程。另外,实验通常仅关注超弹性NiTi合金的单轴和弯曲疲劳失效行为,而对非比例多轴加载下的疲劳行为报道较少；对于疲劳加载过程中的全寿命相变棘轮的研究还不够完善,疲劳模型的建立也存在着一定的局限性。尤其是针对应用于人体血管支架的超弹性NiTi合金微管的疲劳实验和失效模型的研究均非常缺乏,这对于医用NiTi合金器件的设计存在着极大的阻碍作用。因此,有必要首先对超弹性NiTi合金微管在单轴、扭转和非比例多轴加载下进行系统的应力控制疲劳实验,对其全寿命相变棘轮和疲劳失效行为进行深入分析。进而结合不同应力水平和加载路径下疲劳断口和残余马氏体形貌的观察,建立合理的损伤演化模型和疲劳失效模型,提高对超弹性NiTi合金损伤演化和疲劳失效行为的预测能力。为了对超弹性NiTi合金微管在应力控制循环加载下的损伤演化和疲劳失效行为进行系统的研究,本论文开展了以下研究工作：1.在人体温度310K的条件下,对于超弹性NiTi合金微管开展了系统的轴向、扭转和非比例多轴应力控制疲劳实验研究。归纳了不同加载方式下NiTi合金全寿命相变棘轮演化的形式,讨论了应力水平,加载路径和峰谷值应力保持对NiTi合金单轴和多轴全寿命相变棘轮演化的影响,并分析了马氏体相变程度对相变棘轮演化的促进作用。对于多轴加载,同时讨论了马氏体重定向过程对于相变棘轮演化的影响。这部分研究工作为建立NiTi合金的损伤演化模型提供了实验基础。2.对于超弹性NiTi合金微管在轴向、扭转和非比例多轴加载下的疲劳失效行为进行了分析,总结出材料的疲劳寿命随应力水平和相变程度的演化规律,并对比了单轴和多轴加载下疲劳失效行为的差异,讨论了马氏体重定向对材料疲劳寿命的影响；同时对超弹性NiTi合金微管在不同加载方式下的疲劳断口和残余马氏体形貌进行了观察。得到了一些关于超弹性NiTi合金微管疲劳失效行为有用的结论。这部分的研究结果对超弹性NiTi合金疲劳失效模型的建立具有重要的支持作用。3.在对于宏观和微观疲劳实验结果分析的基础上,建立起了一个新的针对超弹性NiTi合金材料的疲劳损伤演化模型,模型同时考虑了微裂纹萌生、微裂纹扩展和相变过程导致的损伤累积,对于多轴加载的情况则考虑了马氏体重定向的影响。利用上述模型对单轴和多轴加载下材料内部的损伤随循环圈数逐渐累积的过程分别进行了模拟和预测。
[Abstract]:NiTi shape memory alloys are widely used in aerospace and biological fields because of their excellent superelasticity, shape memory properties and biocompatibility. Super-elastic NiTi alloy devices are often used as key components in structures and are subjected to cyclic loading in service. Therefore, the fatigue behavior of super-elastic NiTi alloys is an urgent concern. In recent years, some scholars at home and abroad have carried out extensive experimental observation and fatigue failure model research on the fatigue behavior of superelastic NiTi alloys. However, most of the experiments are strain-controlled, and no temperature control device is used to maintain the constant temperature. In addition, experiments usually focus on superelastic NiTi alloys. The uniaxial and bending fatigue failure behavior of the alloy is less reported than that of the alloy under non-proportional multiaxial loading; the research on the life-cycle phase change ratchet during fatigue loading is not perfect enough, and the establishment of fatigue model has certain limitations, especially for the superelastic NiTi alloy used in human vascular stents. The fatigue test and failure model of the tube are very scarce, which have a great impediment to the design of the medical NiTi alloy device. Therefore, it is necessary to carry out systematic stress-controlled fatigue test on the superelastic NiTi alloy Microtube under uniaxial, torsional and non-proportional multi-axial loading, and to carry out phase change ratcheting and fatigue test for its whole life. In order to predict the damage evolution and fatigue failure behavior of superelastic NiTi alloy, a reasonable damage evolution model and a fatigue failure model were established based on the observation of fatigue fracture surface and residual martensite morphology under different stress levels and loading paths. The damage evolution and fatigue failure behavior under force-controlled cyclic loading are systematically studied in this paper. The following research work is carried out: 1. The axial, torsional and non-proportional multiaxial stress-controlled fatigue experiments of hyperelastic NiTi alloy microtubules are carried out at 310 K body temperature. The effect of stress level, loading path and peak-trough stress retention on the evolution of uniaxial and multiaxial life-cycle phase change ratchets in NiTi alloy is discussed. The effect of martensitic transformation on the evolution of phase change ratchets is also analyzed. This work provides an experimental basis for establishing the damage evolution model of NiTi alloy. 2. The fatigue failure behavior of superelastic NiTi alloy microtubules under axial, torsional and non-proportional multiaxial loading is analyzed, and the evolution rules of fatigue life with stress level and phase transformation degree are summarized. The fatigue failure behavior under uniaxial and multiaxial loading is compared, and the effect of martensitic weight orientation on the fatigue life of the material is discussed. The fatigue fracture and residual martensite morphology of the superelastic NiTi alloy microtubules under different loading modes are observed. The results of this study have important support for the establishment of the fatigue failure model of hyperelastic NiTi alloys. 3. Based on the analysis of the macro and micro fatigue experimental results, a new fatigue damage evolution model for hyperelastic NiTi alloys is established. The micro cracks are considered in the model. Damage accumulation due to initiation, microcrack propagation and phase transition is considered for the case of multi-axial loading, and the effect of Martensite weight orientation is considered.
【学位授予单位】：西南交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG139.6

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