SHAPE MEMORY ALLOYS

소개

a literature review of shape memory alloys (SMA) is presented. First, the general behavior of SMAs is discussed, including a brief description of the microstructure of SMA, the shape memory effect, and the superelastic effect. Different types of SMA are discussed briefly, including NiTi SMA (also known as Nitinol). This is followed by several sections describing the properties of Nitinol, the SMA used in this investigation. Several studies of the mechanical properties of Nitinol are then presented, followed by a decription of the various applications of SMAs. The chapter ends with studies and applications of SMAs to seismic resistant design of buildings and bridges.

목차

CHAPTER III SHAPE MEMORY ALLOYS
3.1 Microstructure of Shape Memory Alloys
3.2 The Shape Memory Effect
3.3 Types of SMA
3.4 Nitinol SMA
3.5 Testing of NiTi SMAs
3.6 Applications of Shape Memory Alloys
3.7 Seismic Studies of SMAs in Buildings
3.8 Seismic Applications of SMAs to Buildings

본문내용

These characteristics include hysteretic damping, recentering capabilities, large elastic strain capacities, highly reliable energy dissipation based on a repeatable solid state phase transformation, excellent low- and high-cycle fatigue properties, and excellent corrosion resistance. The stress-strain relationship of a typical superelastic SMA is characterized by an elastic region, a long nearly horizontal plateau, followed by a significant increase in stiffness. Upon unloading, which occurs at a lower stress plateau, the material returns to the origin with little residual deformation (or permanent offset).

3.1 Microstructure of Shape Memory Alloys
To understand the phenomena of shape memory and superelasticity it is important to have a basic understanding of the microstructure and microstructural behavior of SMA. The following discussion is based primarily on Duerig et al. (1990), which presents a thorough discussion of the subject. Figure 3.1 is a simplified illustration of the different phases of SMA. The austenitic phase (Figure 3.1a) of SMAs occurs when the planar arrangement of atoms is of a square geometry, whereas the martensitic phase exists as either a twinned (Figure 3.1b) or detwinned (Figure 3.1c) rhombus geometry. Twinning in martensite occurs when the rhombus geometry switches direction along a twin boundary as shown in Figure 3.2. When twin boundaries alternate at each level, fully twinned martensite as shown in Figure 3.1b results.

참고자료

· 없음