Improving residual stress and heat transfer in shape memory alloy actuation
Open Access
- Author:
- Bradley, Lawrence
- Graduate Program:
- Mechanical Engineering (MS)
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 14, 2023
- Committee Members:
- Robert Francis Kunz, Program Head/Chair
Md Amanul Haque, Thesis Advisor/Co-Advisor
Jing Du, Committee Member - Keywords:
- shape memory alloy
residual stress
actuation
recoverable strain
NiTi - Abstract:
- Shape memory alloys (SMA) are a class of functional materials that are best known for their very high strength to weight ratio as actuators. This is due to temperature dependent phase transformation, which allows 4-6% strain. When heated, they contract in length – thereby producing very large actuation force. Despite their unique properties, the applications of shape memory alloys as actuators are limited by (a) slow actuation rate that is critically influenced by cooling time and (b) residual stress in the material that reduce the amount of the recoverable strain. In the first part of this research, we propose a new actuation mode where we integrate conduction mode heat transfer in the SMA wire, without any additional element that increases size of weight of the actuators. This is motivated by the observation that the it is not possible to improve natural convective heat transfer for a given specimen cross-section and that conduction mode is faster than convection. We propose a ‘segmented actuation’ where instead of heating the entire length of the wire, we piecewise segment it and perform controlled heating. This allows the unheated segment (neighboring two heated ones) act as a temporary heatsink that receives heat by conduction. Using a controller along with segmented heating of the alloy, we demonstrate that the actuation rate can be improved three-fold, without any change in the shape memory properties. However, the segmented approach essentially costs some recoverable strain. In the second part of this research, we focus on the residual stress in the SMA wire, which is difficult to avoid because the wires are manufacturing by drawing process. We propose a unique annealing process that works at significantly lower temperature (<200℃) compared to conventional annealing processes. In this process, we apply very high current pulses (100A) with small pulse width (4 micro-seconds) and low frequency (2 Hz). This unique power application does not allow large accumulation of heat (to raise the temperature), but the electron wind force is very strong to mobilize and eliminate the defects and precipitates. We demonstrate our proposed Electropulse Aging Treatment (EAT) to improve the recoverable strain by 30%, mitigating some of the loss from segmented heating. Through higher current electropulsing, the phase transformation temperatures can be altered, in addition to improving the recoverable strain, signifying another mechanism in which the actuation rate could be improved. Additional work was done on non-destructive testing methods, where a new method of mapping residual stresses in welded materials was developed. By using the cooling rates at each pixel in the thermal image data of a sample during cooling, defects could be determined through changes in those cooling rates due to changes in heat transfer properties from defect concentrations.