Dissertation
SHOCK INDUCED MARTENSITIC TRANSFORMATION OF NITINOL SHAPE MEMORY ALLOY
Washington State University
Doctor of Philosophy (PhD), Washington State University
01/2021
DOI:
https://doi.org/10.7273/000006440
Handle:
https://hdl.handle.net/2376/119038
Abstract
Due to its many important practical applications and intriguing phase transformation behavior between austenite and martensite, shape memory alloy has been studied extensively. However, nearly all of the stress-induced transformation studies have been focused on uniaxial stress loadings. The overall objective of this study was to gain insights on the effects of stress states, specifically, uniaxial stress vs. uniaxial strain loading, on the transformation stress and transformation kinetics of a Ni-Ti shape memory alloy (SMA), commercially known as Nitinol. To achieve the objective, both uniaxial stress and uniaxial strain experiments were conducted. The former was done with the Split Hopkinson Pressure Bar (SHPB) technique and the latter with the plate impact experiments. Three different configurations were designed for the plate impact experiments, namely transmission, ramp wave, and front surface impact experiments. All the three types of experiments led to a consistent result that martensitic transformation took place at ~1.9 GPa under uniaxial strain loading, in contrast to 0.5 GPa under uniaxial stress loading. There appeared to be a critical strain rate around which the transformation completed rapidly. Below this critical rate, the transformation exhibited a smooth and continuous transition from austenite to martensite, i.e., spanned over a finite range of stress and time. Above the critical rate, the material was driven to a meta-stable austenite state. For uniaxial stress loading, this critical rate was ~2000-3000/s and for uniaxial strain loading, it was ~2×106/s. In addition to transformation stress and transformation behavior, the Hugoniot for both stable and meta-stable austenite phase, a continuous stress-strain relation under moderate strain rates, ~1×105/s - 2×105/s, and the possible loading paths from the meta-stable to the martensite phase were also established from the experiments. To examine the effects of stress states on transformation, both uniaxial stress and uniaxial strain data were expressed in terms of effective stress and strain and compared in a consistent manner. Preliminary evidence indicates that the transformation stress is more sensitive to strain rate than mean stress under uniaxial stress loading. However, their relative roles are unclear under uniaxial strain loading. Further studies are needed to resolve this issue.
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Details
- Title
- SHOCK INDUCED MARTENSITIC TRANSFORMATION OF NITINOL SHAPE MEMORY ALLOY
- Creators
- Jialong Ning
- Contributors
- Jow-Lian Ding (Advisor)Amit Bandyopadhyay (Committee Member)David P Field (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Mechanical and Materials Engineering, School of
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 162
- Identifiers
- 99900591957701842
- Language
- English
- Resource Type
- Dissertation