**1. Introduction**

34 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

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shape memory alloys. Smart Materials and Structures 18:085021.

Shape Memory Alloys (SMAs) are functional materials, characterized by some attractive features, such as pseudo-elasticity (PE) and shape memory effect (SME) [1]. The latter consist in the capability of the material to recover high deformation values by heating and can be considered very suitable for actuation applications.

In the actuation field, the most common shape memory alloy is quasi-equiatomic NiTi system, commercially known as Nitinol®. Ti-rich NiTi compounds show characteristic transformation temperatures higher than the room one and they can recover high values of deformation. Moreover, these intermetallic compounds are widely used since they exhibit high thermal and mechanical cycling stability. Other NiTi-based alloys are also employed for this kind of applications. In particular, NiTiCu, with Cu substituting Ni in the 3-10at% range, or NiTiCo system are used respectively when the application requires narrow thermal hysteresis or high stiffness.

In Figure 1, the comparison between differential scanning calorimetry (DSC) data derived from NiTi wire (200 μm in diameter) in fully annealed condition and after aging (500ºC for 10 minutes) are reported. As it can be seen, the fully annealed NiTi alloy goes into a one-step transformation both during cooling and heating. In this case, shape memory effect occurs by a martensitic transformation (MT) between a low-temperature monoclinic structure, B19', called martensite, and a high-temperature body-centered cubic parent phase, B2, called austenite. When the material is aged at specific temperatures, this transition may occur in two steps in association with a trigonal phase, called R-phase [1].

During heating, the phase transformation, named inverse MT, is defined by austenitic start (As) and finish (Af) temperatures. Similarly, during cooling direct MT is defined by martensitic start (Ms) and finish (Mf) temperatures, respectively.

© 2012 Nespoli et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** DSC scan of 200μm NiTi wire in the fully annealed state and after aging (500°C for 10 minutes).

Then, the deformation induced by a state of stress on a SMA element can be recovered heating the material over the austenitic characteristic temperatures: the shape recovery starts at As and finishes at Af.

The transition temperatures are strongly influenced by the applied load, since it stabilizes the martensitic structure causing an increase of all transition temperatures. This behavior of SMA is well described by the Clausius-Clapeyron law [1-5].

Furthermore, SMA actuators generally need a training to stabilize their fatigue properties in terms of transformation temperatures and strain recovery. For example, a possible procedure consists in the thermal cycling under a constant load. Figure 2 depicts the evolution of the temperature-stroke relationship during the first thermo-mechanical cycles. It is seen that the stabilization of the material behavior can be reached after few thermal loops.

**Figure 2.** Example of thermo-mechanical hysteresis typical of a SMA material.

Since SMA can generate mechanical work, they are employed as actuators. The functional properties of SMA element used in actuators strictly depend on several factors, such as the chemical composition and the thermo-mechanical history[1,4,5].

Nowadays, the tendency is to produce ever smaller actuators because of the continued miniaturization of all the industrial products. In general, a mini-actuator may be composed by only a SMA element or by a mini-modular device, in which the SMA element is embedded and acts as the core and active part.

In this chapter, the fundamental operating parameters, which affect the performance of shape memory thin wires, are reviewed. Then, the influence of electrical heating conditions performed by different waveforms on functional fatigue of NiTi micro-wires is also reported.

When the SMA element is embedded into a mini-modular mechanical device, its shape could be a serious problem as spaces are very restricted. To solve this drawbacks, a new SMA conformation suitable for the mini and micro scales is presented: it consists of a planar wavy formed NiTi wire, called snake-like arrangement. Currently, this configuration of the SMA wire is principally exploited in the textile and medical domains [6-8]. n the micro scale, original works are reported by Mineta et al. [7-8], who produced different SMA snake elements by means of electrochemical etching for obtaining bending motion of active catheters. Khol et al. [9] used the micro snake geometry to activate a microgripper system. Moreover, Leester-Schadel et al. [10] adopted laser technology to produce micro SMA snake actuators and then used the batch fabrication process to obtain more articulated samples. In this chapter the main mechanical performance of this unusual geometry and its exploitation in a mini-modular mechanical device are presented.

Another topic of this work is related to a current research on fabrication and characterization of SMA micro-snakes, by means of laser technology and following polishing processes. The reason why this non-traditional production technology has been chosen for this purpose is due to its flexibility in the machining of small features, high productivity and repeatability [11-12]. Considering the direction of miniaturization of the products during the recent years [13], in the last part of the chapter the rescaling of SMA mini-snakes down to micro-scale is presented. The attention is also focused on the evaluation of the capability of laser microcutting industrial process in the fabrication of micro-snakes from NiTi SMA sheets [14-17]. The evaluation of the thermo-mechanical properties of the produced actuator in correspondence of the different fabrications steps is then presented and discussed in order to describe the behavior of the micro-snake-like element for actuation.
