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Shape Memory Waterborne Polyurethanes
Published in Ram K. Gupta, Ajay Kumar Mishra, Eco-Friendly Waterborne Polyurethanes, 2022
Arunima Reghunadhan, Jiji Abraham, Sabu Thomas
Shape memory materials show a special type of behavior, often termed pseudoelasticity, which is alternatively referred to as superelasticity. The superelasticity may be a consequence of a stress-induced alteration from austenite to martensite and back once a sample is tested between zero and a finite; however, tiny strain at a continuing close temperature on top of a set temperature is typically observed because of the austenite end temperature. The sample experiences very little or no permanent deformation in such a strain cycle, giving the impression that the fabric has undergone solely elastic deformation; consequently, the term super elastic is being used. The stresses suffered by superelastic materials in ordinary applications are minimal, but the rotations can be rather significant [7].
Biofunctionalization of NiTi Shape Memory Alloy Promoting Osseointegration by Chemical Treatment
Published in Sam Zhang, Hydroxyapatite Coatings for Biomedical Applications, 2013
Yanli Cai, Xianjin Yang, Zhenduo Cui, Minfang Chen, Kai Hu, Changyi Li
Nickel titanium (NiTi, also known as nitinol) is a metal alloy of nickel and titanium, in which the two elements are present in roughly equal atomic percentages. It undergoes a phase transformation in its crystal structure when cooled from the stronger, high-temperature form (austenite) to the weaker, low-temperature form (martensite). This thermoelastic martensitic phase transformation in the material is responsible for its extraordinary properties. These properties include shape memory effect and superelasticity (also called pseudoelasticity). Shape memory refers to the ability of NiTi to undergo deformation at one temperature and then recover its original, undeformed shape upon heating above its “transformation temperature.” The superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity.
Phase transformations
Published in Gregory N. Haidemenopoulos, Physical Metallurgy, 2018
Pseudoelasticity. In alloy systems exhibiting thermoelastic martensitic transformation, it is possible to activate the transformation above the Ms temperature by the application of mechanical stresses. This type of mechanically-induced transformation has been characterized as pseudoelasticity due to the large observed elastic strains (about 10%). Actually these strains are fully recoverable on unloading due to the operation of the reversed transformation. An example of pseudoelastic behavior is depicted in Figure 6.54 for the Cu−39.8Zn alloy at −77°C. Although there are 24 available orientations of the habit plane, the applied stress during loading imposes a preferred habit orientation that maximizes the elongation in the direction of the tensile axis. Therefore, martensite has been fully formed and is fully oriented at the upper plateau of Figure 6.54. A lower plateau is also observed during unloading, where the transformation is reversed to form the parent phase. The stress difference between the two plateaus is attributed to the lattice resistance during interface movement. This difference can be expressed as 2τf, where τf is the friction stress. Taking into account the friction stress, the thermoelastic balance of Equation (6.31) becomes (ΔGch−τγ0)+2Acr=±τfγ0 where γo is the shear component of shape deformation and τ the applied shear stress. The term τγo is the mechanical work contribution to the chemical driving force. The term τfγo is negative when the martensite plate grows during loading and positive when the plate shrinks during unloading. An important feature depicted in Figure 6.54 is that relatively large reversible strains can be obtained under a practically constant load.
Free vibration of pseudoelastic NiTi wire: finite element modeling and numerical design
Published in Mechanics of Advanced Materials and Structures, 2022
Liangdi Wang, Jun Wang, Yingjie Xu, Jihong Zhu, Weihong Zhang
Pseudoelasticity, sometimes referred to as superelasticity, is manifested by the large inelastic but recoverable deformation in NiTi alloys [9]. It is derived from the stress-induced phase transformation between martensite and austenite. Figure 1 shows a typical pseudoelastic stress-strain response of NiTi alloy. When the material is loaded at the temperature above Af, austenite transforms to martensite along the applied stress in three stages: elasticity of austenite (a b), forward phase transformation (b c), and elasticity of martensite (c d). Upon unloading, martensite transforms back to austenite in a similar way, elastic unloading of martensite (d e), reverse phase transformation (e f), and elastic unloading of austenite (f g). The two most remarkable features in pseudoelasticity are the large nonlinear recoverable strain of more than 8% and the significant hysteresis loop between the forward and reverse phase transformation.
Large thermal hysteresis NiTi Belleville washer fabricated by metal injection moulding
Published in Powder Metallurgy, 2020
Wenbo Li, Hao He, Dongyang Li, Yimin Li, Jia Lou, Zheyu He, Yongzhi Chen, Yi Xie
NiTi alloy is one of the best SMAs. Its pseudoelasticity and shape memory effect make it widely used in medical and engineering applications. The shape memory effect of NiTi alloy is attributed to thermally induced thermoelastic martensitic transformation [5]. Precise control of transformation temperature and excellent shape design can further expand the applications. The thermal hysteresis of the NiTi alloy is one of the important characteristics of the transformation temperatures. It reflects the irreversible energy consumed by the boundary migration and creation of new defects in the long-term elimination process of martensite in the phase transformation [6]. An SMA with a small thermal hysteresis width is suitable for driving parts that need to quickly realise ‘heat–force’ conversion. A large thermal hysteresis is suitable for applications that stably maintain the phase and physical properties under temperature fluctuations [7]. How to improve the thermal hysteresis width of NiTi alloy is a problem worth discussing in fastening field. Generally, the thermal hysteresis width of the binary NiTi alloy can only reach 30–50°C [7,8]. The addition of the Nb element can effectively increase the thermal hysteresis width [9–13]. However, less research has been reported on larger thermal hysteresis width of binary NiTi alloys.
Experimental investigations on the mechanical properties and buckling behavior of the filament wound composite shells embedded with shape memory alloy wires
Published in Mechanics of Advanced Materials and Structures, 2019
Tayebeh Akbari, Seyed Mohammad Reza Khalili
Smart materials have been used in different fields of mechanical engineering like vibration, buckling and postbuckling, damping, and resistance to impact and explosion because of their unique behavior in sensing and actuating capability [1]–[7]. Shape memory alloys (SMAs) are a wide branch of smart materials. These materials have two kinds of behaviors, the first is their ability to experience the large strain about 8% in some composition and recover it without any residual strain, which is called pseudoelasticity. In the second, unloading leaves material with residual strain that can be removed by heating. This is called shape memory effect (SME) property [8]. The shape control ability of SMA may be used in the field of structural stability, where the induced internal stresses due to the SMA fibers may decrease the compressive forces, which generally result in higher buckling and postbuckling strength [9].