| dc.description.abstract | There is an increasing demand for compact and lightweight actuators in aerospace and automotive industries. Shape memory alloys (SMAs) are energy-dense active materials with the potential to replace current actuators through the reduction of weight, volume, and design complexity due to a solid-to-solid phase transformation. However, very little is known about the fatigue and fracture of SMAs, especially in thermal cycles. NiTiHf is a promising ternary NiTi-based SMA due to the combination of its low cost, high work output, and dimensional stability, and Ni50.3Ti29.7Hf20 has been chosen as the material in this study.
In this work, the damage tolerant approach to fatigue is used, measuring the crack growth rate of a defect of known size. To use this approach to measure the crack growth rate, material baselines must be established. Using a modified elastic plastic fracture mechanics approach, the J-integral can capture the fracture toughness of NiTiHf through a range of temperatures. The stress-induced transformation appears to be a toughening mechanism resulting in stable crack growth when the temperature is close to the martensite start temperature.
With this baseline, it is possible to characterize SMAs for fatigue and fracture in both mechanical and actuation cycles. Because running actuation fatigue tests can be time consuming, a unified methodology is proposed to capture the driving forces in both mechanical and actuation fatigue. It is seen that, for both precipitate-free and nanoprecipitated microstructures, ΔJ as a driving force can relate mechanical and actuation fatigue crack growth rates.
Lastly, it is also important to understand underlying microstructural mechanisms behind crack growth. To this end, electron backscatter diffraction (EBSD) and an in-situ tensile stage were used together to observe grain orientations, martensitic traces, and crack growth in a notched tension specimen of a Ni50.6Ti49.4 (at%) shape memory alloy. Cracks on the {100} family of planes appear to grow preferentially to those on the {110} family of planes.
The present work seeks to better understand fatigue crack growth characteristics in NiTiHf, focusing on identifying a proper driving force, relating actuation and mechanical fatigue crack growth, and understanding underlying microstructural mechanisms behind crack nucleation and growth. | |
