Shape Memory Alloy Actuators

Design, Fabrication, and Experimental Evaluation

Mohammad H. Elahinia

• Publisher: Wiley
• Year: 2015
• ISBN:
  9781118426944
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ciencias de los materiales

This book provides a systematic approach to realizing NiTi shape memory alloy actuation, and is aimed at science and engineering students who would like to develop a better understanding of the behaviors of SMAs, and learn to design, simulate, control, and fabricate these actuators in a systematic approach.

Several innovative biomedical applications of SMAs are discussed. These include orthopedic, rehabilitation, assistive, cardiovascular, and surgery devices and tools. To this end unique actuation mechanisms are discussed. These include antagonistic bi-stable shape memory-superelastic actuation, shape memory spring actuation, and multi axial tension-torsion actuation. These actuation mechanisms open new possibilities for creating adaptive structures and biomedical devices by using SMAs.

1
Introduction

Christoph Haberland, Mahmoud Kadkhodaei and Mohammad H. Elahinia

This chapter is on introductory materials on shape memory alloys (SMA) behavior. Shape memory effect, and superelasticity will be covered. In this context, the benefits of SMAs in actuation will be highlighted. Phase transformation as the underlying phenomenon for the unique properties of these alloys will be presented and discussed. Different actuation mechanisms and designs will be presented and compared. Example of aerospace, automotive, industrial, and biomedical applications of SMA actuation will be used to discuss the benefits and limitations of actuations using these alloys. Particular attention will be on rotary SMA actuators. This type of actuators will be used as a continuous example throughout the book.

1.1 Shape memory alloys

SMAs are distinguished from conventional metallic materials by their ability to restore their shape after large deformations, which can significantly exceed the actual elastic deformability of the material. This is referred to as Shape memory effect (SME) characteristic and was first observed in 1932 in a gold–cadmium¹ alloy following a thermally induced change in the crystal structure [1, 2]. Nearly 20 years later, Chang and Read [3] identified the fundamental mechanisms in the crystal lattice and attributed this phenomenon to a thermoelastic behavior of the martensitic phase. In the following years, the SME was observed in other alloys, more than 25 binary, ternary, or quaternary alloys and alloy systems are now known to show shape memory properties [4]. In contrast to nickel–titanium (NiTi), majority of these systems however have only been considered in principle and as such have not yet achieved any practical technological importance [5]. In NiTi, the SME was observed for the first time by Buehler et al. at the US Naval Ordnance Laboratory (NOL, White Oak, Maryland) in the 1960s [6, 7]. Because of the place of discovery, besides NiTi or TiNi, the term nitinol is also commonly used for this alloy. The application of SMAs spans a wide range of length scales, and these alloys are now used in multiscale devices ranging from nanoactuators used in nanoelectromechanical systems to very large devices used in civil engineering applications. SMA devices range from simple parts like cell phone antennas or eyeglass frames to complicated devices in mechanical [8–10], biomechanical [11–13], aerospace [14], and civil engineering [15].

Today, more than 90% of all commercial shape memory applications are based on binary NiTi or ternary NiTi-Cu and NiTi-Nb alloys [5]. This is despite the relatively high world market prices for high-purity nickel and especially for high-purity titanium. It should be noted that the price of Fe- or Cu-based SMAs is lower. Additionally, as explained in Chapter 6, the manufacturing processes of NiTi are complex and challenging, which adds to the production costs. The main reason for the dominance of NiTi-based SMAs is due to their excellent structural and functional properties. The SME in NiTi allows for relatively large reversible deformations of up to 8%, characterized by good functional stability [5, 16–18]. In addition, NiTi has good wear and corrosion resistance and biocompatible properties, making it an attractive candidate for various medical applications such as surgical tools, stents, or orthodontic wires [19–22]. Furthermore, the low stiffness of NiTi attracts interest for use in bone implant applications and in regenerative medicine [23]. For actuation and motion control applications, this alloy can be easily heated by passing an electrical current while offering several advantages for system miniaturization such as high power-to-mass ratio, maintainability, reliability, and clean and silent actuation. Due to its outstanding predominant role amongst other SMAs, in this book we mainly focus on NiTi.

The fundamental reason for the unique behavior of these alloys is due to the martensitic phase transformation. Originally, this term referred to the crystallographic phase transformation, which results in rapid cooling to a specific crystallographic phase in the Fe–C structure. This is also the basic mechanism in the hardening of steels. With increasing scientific understanding of t...