Actuation Fatigue and Fracture of Shape Memory Alloys
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Shape memory alloys (SMAs) are promising candidates for solid state actuators since they have the highest energy density among all active materials. However, before SMA solid state actuators can be used in practical applications, especially where their failure can cause casualties, fatigue and fracture properties of these functional materials under actuation thermal cycling conditions must be thoroughly understood. There are two general approaches for evaluating fatigue response of materials. First one is life based or total life approach, where materials are cycled until failure either under stress/load control or strain/displacement control. Second approach is called damage tolerant approach, in which the materials with known crack lengths are subjected to cyclic loading and crack’s growth is monitored. Even though it is limited, there are few studies published in literature on the life based approach for the actuation fatigue of SMAs, where the samples are thermally cycled under a mechanical load. However, to the best of author’s knowledge, there is no systematical work on the damage tolerant approach for the actuation fatigue of SMAs. The main goal of this dissertation is to systematically study the actuation fatigue behavior of SMAs with both life based and damage tolerant fatigue approaches. Moreover the effects of microstructure on the actuation fatigue performance of SMAs are investigated. Both conventional low temperature SMAs, NiTi based, and recently discovered stable high temperature shape memory alloys (HTSMAs), NiTiHf based, are studied. Two compositions of binary NiTi namely Ni₄₉.₅Ti₅₀.₅ and Ni₅₀.₃Ti₄₉.₇ and one composition of ternary Ni₅₀.₃Ti₁₉.₇Hf₂₀ are used and their microstructures are modified by precipitation heat treatments. The microstructure is evaluated using transmission electron microscopy and scanning electron microscopy. Mechanical and thermal properties are determined using differential scanning calorimetry, thermomechanical tests such as monotonic tension, incremental strain superelasticity, and isobaric heating cooling tests. It is found that microstructure has a significant effect on the actuating fatigue of SMAs. In NiTiHf, the peak aged condition outperforms overaged conditions in terms of fatigue performance. It is also found that surface finish has a strong effect on fatigue life of tested material. Counter intuitively, the samples with electric discharge machining recast layer have significantly longer fatigue lives than mechanically or electro-chemically polished samples. Damage tolerant approach on the actuation fatigue results shows stable crack growth in NiTi SMAs and the rate of crack growth per thermal cycle is reported. It is found that under comparable average stress intensity factors the crack growth rate during thermal cycling is significantly larger than those under the mechanical cycling. Improving understanding of actuation fatigue of SMAs not only addresses a missing link in scientific literature but also helps accelerate the development of solid state actuators. With better tools and data, design against fatigue can be performed more accurately and efficiently. Development of damage tolerant approach is expected to better assess remaining fatigue life of components with existing cracks. Other shape memory compositions and microstructures that are not studied in this work can be investigated in future using the methods described in this work.
Hayrettin, Ceylan (2017). Actuation Fatigue and Fracture of Shape Memory Alloys. Doctoral dissertation, Texas A & M University. Available electronically from