Jtam-A4.dvi JOURNAL OF THEORETICAL AND APPLIED MECHANICS 56, 2, pp. 447-456, Warsaw 2018 DOI: 10.15632/jtam-pl.56.2.447 MECHANICAL PROPERTIES AND ADVANCED SUBJECTS IN SHAPE MEMORY ALLOYS AND POLYMERS Ryosuke Matsui, Kohei Takeda, HISAAKI TOBUSHI Department of Mechanical Engineering, Aichi Institute of Technology, Toyota, Japan e-mail: r matsui@aitech.ac.jp; k-takeda@aitech.ac.jp; tobushi@aitech.ac.jp Elzbieta A. Pieczyska Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw, Poland e-mail: epiecz@ippt.pan.pl Advanced subjects in mechanical properties of shape memory alloys and polymers are di- scussed. In the subloop loading under a stress-controlled condition of the shape memory alloy, the transformation-induced stress relaxation appears due to variation in temperature. The enhancement of corrosionand corrosion fatigue life of the shapememory alloy is discus- sed. The development of a functionally-graded shapememory alloy and polymer is expected to obtain better performance. Three-way motion appears in the shape memory composite with the shape memory alloy and polymer. Keywords: shapememory alloy, shapememory polymer, functionally-graded shapememory material, shapememory composite 1. Introduction The development of shape memory alloys (SMA) has attracted high attention because of the uniquepropertiesof the shapememoryeffect (SME)andsuperelasticity (SE)appearance (Duerig et al., 1990; Funakubo, 1987; Lagoudas, 2008; Lexcellent, 2013; Otsuka andWayman, 1998; Sun et al., 2017; Tobushi et al., 2013; Ziolkowski, 2015). If we use the SME and SE in practical applications, not only large recovery strain but also high recovery stress, energy storage and energy dissipation can be obtained. The main features of the SME and SE are induced due to martensitic transformation (MT). Since the deformation properties due to the MT depend on temperature, stress and thermomechanical hysteresis, they are, therefore, complex. They also depend on the loading rate. In the case of subloop loading, the deformation behaviors are quite different between strain- and stress-controlled loading conditions. The transformation-induced creep and stress relaxation appear in the subloop loading under the stress-controlled condition. The corrosion and corrosion fatigue properties are important in practical application of SMA elements. The shape memory polymer (SMP) has also been developed (Hayashi, 1993; Huang et al., 2012; Tandon et al., 2016; Yahia, 2015). Themain features of SMPappear due to the glass trans- ition. Elasticmodulus differs at temperatures above and below the glass transition temperature, and the rigidity of SMP elements, therefore, varies depending on the temperature change. Based on this property, the shape fixity and shape recovery can be used. Although elastic modulus and yield stress are high at high temperatures and low at low temperatures in SMAs, they are high at low temperatures and low at high temperatures in SMPs. The dependence of rigidity on temperature is, therefore, quite different between the SMA and SMP. If composite materials with SMA and SMP are developed, new characteristics of the shapememorymaterials can be obtained. 448 R.Matsui et al. In order to obtain a better performance, the development of functionally-graded SMAs and SMPs is expected. The 3D-printing of SMPs is requested as a simple method to manufacture complex SMP elements. In the present paper, advanced subjects in mechanical properties of SMA such as the de- formation behavior subjected to the stress-controlled subloop loading and the corrosion fatigue properties of SMA are discussed. Next, the functionally-graded SMA and SMP are discussed. Following these subjects, the mechanical properties of the shapememory composite with SMA and SMP such as the characteristics of the three-way bending properties and the 3D printing of SMPwill be discussed. 2. Deformation and fatigue properties of SMAs 2.1. Stress relaxation in subloop loading SMA elements are subjected to variation in stress, strain and temperature with various ranges accompanying the MT in practical applications. The analysis in the subloop loading is therefore important. Although the return-point memory appears under a low strain rate in the subloop loading, it does not appear under the stress-controlled condition. In the case of the sub- loop loading under the stress-controlled condition, the transformation-induced creep and creep recovery appear under a constant stress, and the transformation-induced relaxation and stress recovery under a constant strain. It should benoticed that the stress-strain curve depends on the loading rate (Ikeda, 2015; Pieczyska et al., 2006; Yin et al., 2014). The transformation-induced stress relaxation in the subloop loading of the TiNi SMA under various loading conditions will be discussed in this Section. Fig. 1. Stress-strain curves of the TiNi SMA in the stress relaxation test with various holding strains and in the low strain rate dε/dt=2.5 ·10−5s−1 The stress-strain curves obtained in a stress relaxation test with various holding strains are shown in Fig. 1. The loading conditions of the stress relaxation tests in Fig. 1 were as follows. The load was applied at a stress rate dσ/dt=5MPa/s until a pointH2 (orH3,H4,H5 andH6) at a strain εh =2% (or 3%, 4%, 5% and 6%) followed by holding the strain εh constant till the decrease in stress finishedandthereafter unloadedat a stress ratedσ/dt=−5MPa/s.The stress- -strain curve shown by a black line in Fig. 1 was obtained at a strain rate dε/dt=2.5 ·10−5 s−1 during loading andunloadingwith themaximumstrain 8%.As canbe seen inFig. 1, in the strain Mechanical properties and advanced subjects in shape memory alloys and polymers 449 holding process at εh following the loading till the strain εh at the stress rate dσ/dt=5MPa/s, the stress decreases to σRF2 (or σRF3, σRF4,σRF5 and σRF6), resulting in stress relaxation. The stress σRF2 (or σRF3, σRF4, σRF5 and σRF6) at a point εh after relaxation is almost the same as the stress of theMT start σMS in the stress-strain curve at a strain rate dε/dt=2.5 ·10 −5 s−1, in which an increase in the stress is smaller than that at a stress rate dσ/dt=−5MPa/s. In the loading process at a constant stress rate dσ/dt=5MPa/s, strain rate becomes high in the upper stress plateau region, and the heat is generated due to the exothermicMT, resulting in an increase in temperature of the specimen. In the strain holding stage from the pointH2 (or H3, H4, H5 and H6) to σRF2 (or σRF3, σRF4, σRF5 and σRF6), the temperature decreases due to the heat transfer into the ambient air and the condition for the transformation to progress is satisfied, resulting in progress of theMT. As a result, stress relaxation occurs while holding the strain constant. The relationship between the stress decrease ∆σ and temperature decrease ∆T during hol- ding a constant strain in the stress relaxation tests with various conditions is shown in Fig. 2. Temperaturewasmeasured by the infrared thermography. The forced convection was performed by air flow in order to observe the influence of cooling rate on stress relaxation. As can be seen, the stress decrease∆σ is proportional to the temperature decrease∆T . Thebroken line inFig. 2 is calculated by∆σ= a∆T and shows a good overall match with the experimental results. The value of the coefficient a is 13.2MPa/K. Fig. 2. Relationship between stress decrease and temperature decrease in the stress relaxation test with various conditions 2.2. Corrosion and fatigue properties of SMA 2.2.1. Corrosion fatigue life The corrosion fatigue life is important in practical applications of SMAs.However, the report on the corrosion fatigue properties is little. The corrosion fatigue life of a TiNi SMA wire was investigated through a bending fatigue test. The relationships between the strain amplitude εa and the number of cycles to failure Nf obtained by the rotating-bending fatigue test in the air and the 10%-NaCl water solution are shown in Fig. 3 (Yamada and Matsui, 2016). The materials used in the experiment were TiNi SMAwires (Ti-49.7 at% Ni) with a diameter of 0.7mm. Thematerials were heat-treated in an electrical furnace for 1h at 673K. The materials were then allowed to cool inside the furnace. The fatigue life in 10%-NaCl water solution (i.e. the corrosion fatigue life) is shorter than that in the air as shown in Fig. 3. Accordingly, engineers have to be careful with fatigue life of SMA 450 R.Matsui et al. devices, particularly, whenused in corrosive environment (i.e. in humanbody, seawater, etc.). In order to enhance the corrosion fatigue life, we have developed a thermal treatment to generate a strong and homogeneous passive layer on the surface of TiNi SMAs. This subject will be discussed in the next Section. Fig. 3. Fatigue life curves for TiNi shape memory alloys in the air and 10%-NaCl water solution The enhancement of fatigue life can also be achieved by the surface treatment of materials through the ultrasonic shot peening (USP) and the nitrogen ion implantation (NII). The influ- ence of NII, USP and thermal treatment conditions on the corrosion fatigue life of an SME tape and an SE tape is the future subject. 2.2.2. Corrosion resistance In order to promote the application of SMAs into devices used in corrosive environment, we have developed a procedure to generate a passive layer on TiNi SMAs. The proposal process called the thermal nitridation (TN) treatment to generate thin and homogeneous passive layers contains heat treatment of amechanically-polished SMAwire at temperature of 673K for 3.6ks in a furnace filled with pure nitrogen gas. Fig. 4. Anodic polarization curves for TiNi SMA (TN), conventional TiNi SMA and pure Ti Figure 4 shows anodic polarization curves for TiNi SMAwith the passive layer generated by TNprocess, a conventional TiNi SMAwith a thick oxide layer and a pureTi in a 3%-NaClwater Mechanical properties and advanced subjects in shape memory alloys and polymers 451 solution.The results reveal that corrosion resistance of theTN-treatedTiNi SMA ismuchhigher than that of the conventional TiNi SMAs and is almost the same as that of pureTi, which is the most common material as a biomaterial, with current density of up to above 1 ·10−2mA/cm2. From the energy dispersive X-ray spectrometry analysis and other microscopic investigations, we found that a thin titanium-nitride layer with a thickness of tens of nanometers is generated on the TN-treated TiNi SMA. Since the corrosion fatigue life for theTN-treated TiNi SMA is one of the important proper- ties to design devices, we are now investing in clarifying the characteristics of corrosion fatigue life from a fatigue test in the 10%-NaCl water solution. 3. Functionally-graded shape memory alloy and shape memory polymer 3.1. Functionally-graded shape memory alloy In order to develop a more advanced actuator, such as a self-stroke controlling device de- pending on the ambient temperature, a TiNi SMA having a functionally-graded property of the transformation temperatures will be a major candidate material for the element. If the functionally-graded shape memory alloy (FGSMA) coil is employed to an actuator, the shape- -recoverable region exceeding the austenitic transformation finish temperature Af will change continuously, resulting in length of the coil extending or shortening without a bias element, depending only on its temperature. Figure 5 shows a demonstration ofmovement of anFGSMAcoil having different transforma- tion temperaturesAf;Af1 =293K,Af2 =318K andAf3 =338K.The coil subjected to tensile load along its axial direction and then it was unloaded at temperature T = 298K as shown in Figs. 5b and 5c. In this state, a part of the coil, which had the transformation temperatureAf1, recovered its original shape without heating due to superelasticity. The coil was subsequently heated up to T =328 and 348K, the shape recoverable region extended and length of the coil became shorter as shown in Figs. 5d and 5e. Fig. 5. Movement of the SMA coil having different transformation temperatures;Af1 =293K, Af2 =318K andAf3 =338K: (a) initial state, (b) loading at T =298K (Af1