Long-term behaviour of binary Ti-49.7Ni (at.%) SMA actuators-the fatigue lives and evolution of strains on thermal cycling
Long-term behaviour of binary Ti-49.7Ni (at.%) SMA actuators-the fatigue lives and evolution of strains on thermal cycling
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DOI：
10.1088/0964-1726/19/11/115019
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发表时间：
2010-11-01
影响因子：
4.1
通讯作者：
Lindroos, Tomi
中科院分区：
文献类型：
--
作者：
Karhu, Marjaana;Lindroos, Tomi
SMA驱动器的长期行为和疲劳耐久性是其应用的关键问题，但在这一领域还需要进行系统的研究工作。本研究主要研究了三个主要设计参数对二元Ti-49.7Ni基致动器长期性能的影响：热循环温度间隔的影响、应力水平的影响和所用线材的热处理状态的影响。在定制的疲劳试验台中，在恒定应力水平下对钢丝进行热循环，研究了钢丝的长期行为。记录了试件的疲劳寿命以及热循环过程中相变应变和塑性应变的演化过程。在疲劳试验之前，对钢丝进行了一系列热处理，以产生最佳的致动器性能。研究的主要结论之一是，热循环的温度间隔对疲劳耐力有很大的影响：减少热循环的温度间隔可以显着提高疲劳寿命。当转变完成时，最终温度升高20℃，所研究的Ti-49.7Ni线材的疲劳寿命至多减少一半。对于部分转变，效果更加明显：即使最终温度升高5摄氏度，疲劳寿命也会减少一半。应力水平和热处理状态对钢丝的致动器性能有显著影响，但对疲劳寿命的影响较小。疲劳试验结果表明，由于Ti-49.7Ni二元致动器的性能对焊丝的热处理状态高度敏感，因此设计和控制其长期行为是非常具有挑战性的。即使延长5min的热处理时间，也最多只能产生两倍的塑性应变值和低30%的稳定相变应变值。塑性应变量可以表示为二元Ti-49.7Ni的失效判据之一，但每种热处理状态和加载水平都应分开处理和表示，并建立各自的设计关系。
Long-term behaviour and fatigue endurance are the key issues in the utilization of SMA actuators, but systematic research work is still needed in this field. This study concentrates on the effects of three major design parameters on the long-term behaviour of binary Ti-49.7Ni-based actuators: the effect of the temperature interval used in thermal cycling, the effect of the stress level used and the effect of the heat-treatment state of the wire used. The long-term behaviour of the wires was studied in a custom-built fatigue test frame in which the wires were thermally cycled under a constant stress level. The fatigue lives of tested specimens and the evolution of transformation and plastic strains on thermal cycling were recorded. Before the fatigue testing, a series of heat treatments was carried out to generate optimal actuator properties for the wires. One of the major conclusions of the study is that the temperature interval used for thermal cycling has a major effect on fatigue endurance: decreasing the temperature interval used for thermal cycling increased the fatigue life markedly. When the transformation is complete, a 20 degrees C increase of the final temperature reduced the fatigue lives at the most by half for the studied Ti-49.7Ni wires. With partial transformations the effect is more distinct: even the 5 degrees C increase in the final temperature reduced the fatigue life by half. The stress level and heat-treatment state used had a marked effect on the actuator properties of the wires, but the effects on fatigue endurance were minor. The fatigue test results reveal that designing and controlling long-term behaviour of binary Ti-49.7Ni actuators is very challenging because the properties are highly sensitive to the heat-treatment state of the wires. Even 5 min longer heat-treatment time could generate, at the most, double plastic strain values and 30% lower stabilized transformation strain values. The amount of plastic strain can be stated as one of the failure criteria for binary Ti-49.7Ni, but that every heat-treatment state and loading level should be treated and presented separately with its own design relations.