: Application of Shape Memory Alloys

In the ambit of smart materials, shape memory alloys (SMA) have emerged as a remarkable class of materials capable of undergoing significant shape changes when stimulated by external factors such as mechanical, magnetic, thermal, or electrical forces [...]


Introduction
In the ambit of smart materials, shape memory alloys (SMA) have emerged as a remarkable class of materials capable of undergoing significant shape changes when stimulated by external factors such as mechanical, magnetic, thermal, or electrical forces. These unique characteristics have propelled SMAs into the forefront of scientific exploration, revolutionizing fields that demand adaptable and responsive materials. While renowned for their biocompatibility and high force-to-weight ratio in miniaturized actuators and sensors, the application of SMAs faces certain challenges. Concerns such as low energy efficiency, hysteresis, complex control mechanisms, structural fatigue, and overheating have posed significant hurdles in fully harnessing their potential. However, with their wide range of applications spanning from medical devices [1][2][3] to consumer electronics, automotive to avionics, the stage is set for groundbreaking advancements in SMA utilization.

Contents of the Special Issue
The five studies in this Special Issue explore different aspects of SMAs and their applications, showcasing current advancements and future potential. From magnetic properties and thermo-mechanical behavior to plasticity modeling and intelligent actuator design, these studies reveal the diverse nature of SMAs and their impact in various fields.
Tsai and colleagues [4] investigate the magnetic properties of a new Fe 41 Ni 28 Co 17 Al 11.5 (Ti + Nb) 2.5 SMA system. Thermo-magnetization and vibrating sample magnetometer (VSM) techniques were employed to characterize the magnetic properties. The focus of the study was on the magnetization, martensitic transformation temperatures, and microstructure of the alloy during the aging process at 600 • C. X-ray diffraction (XRD) analysis revealed the emergence of a new peak, γ , during aging, with its intensity increasing over time, while the intensity of the FCC (111) austenite peak decreased. Transmission electron microscope (TEM) observations indicated that the size of the precipitate increased with longer aging times. Thermo-magnetization results demonstrated that phase transformation occurred after at least 24 h of aging, with transformation temperatures increasing over time and with increasing magnetic field strength. Additionally, the magnetization reached saturation after 24 h of aging. Vibrating sample magnetometer (VSM) results emphasized the significant impact of thermal processes on the alloy's magnetic properties, particularly on saturated magnetic magnetization and magnetic moment reversal behavior.
Efficient temperature sensing and monitoring play a vital role in supply chain management. Conventional methods involve using electronic devices powered by batteries to measure and record temperature profiles over time. Wang and colleagues [5] introduce a cost-effective and intelligent battery-free solution for capturing crucial temperature events by leveraging the inherent thermo-mechanical properties of a shape memory alloy (SMA). The proposed approach takes advantage of the SMA's temperature-induced irreversible mechanical deformation as a natural means of capturing temperature history, eliminating the need for electronic data logging. In this study, a two-way SMA system is utilized to record both high-and low-temperature peak events. Specially trained thermo-mechanical SMA arms are integrated into a dipole antenna for radio frequency (RF) readout. The fabricated antenna sensor operates at 1 GHz and demonstrates a sensitivity of 0.24 dB/ • C and −0.16 dB/ • C for recording the temperature maxima and minima, respectively. This innovative approach presents a promising avenue for cost-effective and battery-free temperature monitoring in various applications, particularly within the supply chain management sector.
Over the past century, numerous criteria have been proposed by researchers to model the initiation of plasticity in ductile metals. Surprisingly, only a small number of researchers have attempted to model stress-induced crystalline phase transformations in SMAs using yield criteria. Chen and colleagues [6] addressed the question of whether a yield criterion originally developed to describe the plastic behavior of metals is suitable for modeling the "pseudoelastic" behavior of SMAs. To investigate this, two yield criteria proposed by the authors of this work are employed to predict the initial surface of transformation onset in two different SMAs: Cu-Al-Be and Ni-Ti alloy. The predicted transformation onset surfaces of the two SMAs are compared with experimental results and existing theories from the literature, leading to significant conclusions and recommendations. By exploring the applicability of existing yield criteria to the pseudoelastic behavior of SMAs, this study contributes to a deeper understanding of the mechanical response of these materials and provides valuable insights for future research and modeling in the field.
Mitrev and colleagues [7] present a comprehensive investigation, both theoretical and experimental, of a thermo-mechanical model for an actuator comprising a shape memory alloy wire in series with a bias spring. The developed mathematical model accounts for the actuator's dynamics in the thermal and mechanical domains. To enhance modeling accuracy, a novel algorithm is proposed to incorporate the minor and sub-minor hysteresis, thus overcoming limitations of classical models. The algorithm significantly improves accuracy, particularly when utilizing pulse-width modulation control, where minor and sub-minor hysteresis are likely to occur. Experimental studies highlight the high sensitivity of the system and the presence of physical factors not accounted for in the mathematical model. It is demonstrated that setting constant duty cycle values fails to achieve stable displacement and force outputs. A comparison between the mathematical model and experimental results indicates acceptable differences. The enhanced modeling approach serves as a foundation for actuator design and facilitates the development of improved automatic feedback control systems for maintaining specific displacements, forces, or trajectory tracking.
Actuators utilizing SMAs have gained increasing economic importance due to their numerous advantages, including high energy density. Traditionally, SMAs have been employed to control end or maximum positions, resulting in actuators moving between two fixed positions. However, achieving precise and repeatable control of intermediate positions has proven challenging, often requiring external sensors to determine the length of the SMA element. Additionally, controlling SMA actuators is complex due to the material's nonlinear behavior. Schmelter and colleagues [8] present an SMA actuator capable of controlling intermediate positions with repeatable accuracy, without the need for separate control technology. The integrated control unit operates on a mechanical principle, utilizing a shaft with a circumferential groove. The groove's height profile translates the rotational motion of the SMA-generated shaft into translational movement. Consequently, the SMA wire generates a partial stroke with each activation, incrementally rotating the shaft. By accumulating multiple activation cycles, the stroke progressively adds up until reaching the maximum position. Subsequent activation of the wire resets the actuator's stroke to its initial position. Within each complete actuator cycle, each part of the stroke can be precisely and repeatedly controlled. Through the integration of a ratchet mechanism, the actuator can store energy in each stroke. This innovative approach opens new possibilities for achieving controlled and efficient motion in SMA actuators without the need for external control systems.

Conclusions
The five studies presented in this Special Issue showcase diverse aspects of SMAs and their applications, highlighting the ongoing advancements and potential for future development. From the exploration of magnetic properties and thermo-mechanical behaviour to the modelling of plasticity and the design of intelligent actuators, these studies collectively demonstrate the multidimensional nature of SMAs and their impact across various fields. Collectively, these studies underscore the growing significance of SMAs as smart materials with vast potential in various industries, including robotics, healthcare, and transportation. The advancements in understanding their properties, refining modelling techniques, and developing intelligent control mechanisms pave the way for enhanced performance, reliability, and energy efficiency in SMA-based applications. As we embrace sustainable solutions and strive for technological advancements, it is crucial to further explore and unlock the full potential of SMAs. Collaborative efforts among researchers, engineers, and industry professionals are essential to drive innovation, refine existing designs, and develop novel applications. By doing so, we can harness the unique capabilities of SMAs and usher in a new era of smart materials, revolutionizing numerous fields and improving our daily lives.

Conflicts of Interest:
The author declares no conflict of interest.