Thermomechanical Investigation of TiNi Shape Memory Alloy ... - Core

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ScienceDirect Procedia Engineering 74 (2014) 287 – 292

XVII International Colloquium on Mechanical Fatigue of Metals (ICMFM17)

Thermomechanical investigation of TiNi shape memory alloy and PU shape memory polymer subjected to cyclic loading

ElĪbieta A. Pieczyskaa, Katarzyna Kowalczyk-Gajewskaa, Michał Maja, Maria Staszczaka and Hisaaki Tobushib a

Institute of Fundamental Technological Research PAS, Pawinskiego 5B, 02-106 Warsaw, Poland b AICHI Institute of Technology, 1247 Jachigusa, Toyota-city 470-0392, Japan

Abstract In applications to sensors, actuators, guide wires, special grips for handicapped people, a shape memory alloy (SMA) or shape memory polymer (SMP) are used as working elements that perform cyclic motions. In order to evaluate the reliability of the shape memory materials (SMM), cycling and fatigue deformation properties are investigated. Since the SMM are very sensitive to temperature, not only mechanical properties but also their related temperature changes accompanying the deformation process should be taken into account. The presented paper embraces experimental investigation of effects of thermomechanical couplings occurring in shape memory alloy and shape memory polymer subjected to various kinds of cycling loading. The deformation was carried out on MTS 858 Testing machine. The strain was measured by a mechanical extensometer, so the stress-strain characteristics were elaborated with high accuracy. Furthermore, a fast and sensitive FLIR Co Phoenix infrared (IR) measurement system was used in order to record infrared radiation from the sample surface. It enables obtaining temperature distribution of the sample as a function of the deformation parameters. For each strain cycle, an increase in temperature during the loading and the temperature decrease during the unloading processes was observed. It was found that the temperature increment recorded during the cyclic deformation depends on the strain rate, the kind of the material and the test conditions. The higher the strain rate the higher the stress and temperature changes were obtained, since the deformation process was more dynamic and has occurred in almost adiabatic conditions. It was shown that various deformation mechanisms are active during various loading stages.

Keywords: shape memory alloy, shape memory polymer, cyclic deformation, thermomechanical coupling, infrared camera

1877-7058 © 2014 Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the Politecnico di Milano, Dipartimento di Meccanica doi:10.1016/j.proeng.2014.06.264

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Elżbieta A. Pieczyska et al. / Procedia Engineering 74 (2014) 287 – 292

1. Introduction In the intelligent materials, investigation of shape memory alloys and shape memory polymers has attracted high attention due to their shape memory properties and huge potential in practical applications. In SMA, the shape memory property appears based on the reversible martensitic transformation in which the crystal structure varies depending on the variation in stress or temperature [1-3]. In SMP, the shape memory property appears based on the radical difference of its elastic modulus and yield point below and above the glass transition temperature Tg: the elastic modulus is high at the temperature below and low at temperatures above Tg. Such behavior is caused by a significant change of the molecular motion of the polymer chains below and above Tg temperature [1, 4]. Among the polymers, the polyurethane has been most often practically used due to its good mechanical and shape memory properties [4-6]. The goal of this research is an experimental investigation of mechanical properties and thermomechanical couplings in TiNi shape memory alloy and shape memory polyurethane subjected to cyclic tension. The scheme of the experimental set-up is shown in Fig. 1a, whereas an infrared image of the sample in grips of the testing machine in Fig. 1b. A rectangular area marked on the sample has denoted chosen area for calculation of the average temperature. The average sample temperature obtained in this manner presented vs. stress, strain or time is shown in diagrams (Figs 3-6) [2, 3, 9]. a)

b)

Fig. 1. a) Scheme of experimental set-up used for investigation of thermomechanical properties of shape memory alloys and polymers; b) Thermogram showing rectangular for calculation the average temperature of the sample.

2. Investigation of thermomechanical couplings in TiNi Shape Memory Alloy subjected to cyclic tension loading Cyclic tension loading of the shape memory alloy was carried out on TiNi belts of the size 160x10x0,4 mm at room temperature, above the alloy Af (austenite finish) temperature, so in terms of the SMA pseudoelastic behavior [1]. Stress-strain curves obtained for 10 loading-unloading cycles with strain rate 10-2s-1 are presented in Fig. 2. One can notice that at the strain equal to approximately 1.5%, a material “yielding” caused by the stress-induced martensitic transformation (SIMT) is observed, starting with waving part of the stress-strain curve recorded on the stress level of 540 MPa. The observed waving of the curve is related to the nucleation and development of the localized martensitic transformation [2, 3]. At larger strains, a much smoother stress-strain curve is observed; however significantly inclined, which was caused by the increase of the sample temperature due to the exothermic martensitic forward transformation and the SMA loading with such a high strain rate. In the second loading cycles, the stress level at which the transformation starts decreases; however the values of the stress decrements are getting smaller in the subsequent loading cycles. It means that thermodynamic conditions of the SIMT are stabilizing. On the other hand, the residual strains increases with each cycle of the SMA loading, which is caused by accumulation of the micro-structural defects and increasing amount of the residual martensite in the subsequent loading-unloading cycles (Fig. 2).


800

.

TiNi-4

600

True stress (MPa)

-2 -1

ε = 10 s

1

10

400

200

0 0.00

0.02

0.04

0.06

0.08

0.10

True strain Fig. 2. Stress-strain curves obtained for 10 loading-unloading tension cycles of TiNi SMA at strain rate 10-2s-1.

For such a high strain rate and almost adiabatic experimental conditions a significant temperature increase is observed during the TiNi SMA loading due to the exothermic character of the martensitic forward transformation followed by the temperature decrease due to the endothermic character of the reverse one (Fig. 3). 30

TiNi-4

Temperature variation (K)

.

1

-2 -1

ε = 10 s

20

10

10

0

-10 0

200

400

600

800

True stress (MPa) Fig. 3. Temperature change vs. stress obtained for 10 loading-unloading tension cycles of TiNi SMA at strain rate 10-2s-1.

Maximal value of the temperature change is recorded during the first cycle of the SMA loading and equals to 28K. During the unloading the temperature decreases due to the endothermic reverse transformation. After the unloading is completed, the sample temperature decreases below its initial temperature (Figs 3, 4).

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The temperature changes of the TiNi SMA sample obtained during the 10 tension loading-unloading cycles are presented in Fig. 4. In the subsequent loading cycles the maximal temperature increase is smaller, which is caused by the drops in temperature during the SMA unloading and the reverse endothermic transformation.

30

TiNi-4

.

-2 -1

ε = 10 s

True stress (MPa)

ΔΤ

600

20

400

10

200

0

Temperature variation (K)

800

σ

0

-10 0

50

100

150

200

Time (s) Fig. 4. Stress (black curve) and average temperature changes (red curve) of TiNi SMA sample obtained during 10 loading-unloading cycles.

Namely, during the unloading, the sample temperature decreases below its initial temperature, so the thermodynamic conditions of the each subsequent loading cycle differ from the first and the former. The drops in temperature accompanying the SMA unloading are higher in the subsequent cycles; however the temperature and the stress changes stabilize at higher number of the loading cycles (Fig. 4). 3. Investigation of thermomechanical couplings of PU shape memory polymer subjected to cyclic tension loading In order to find the new PU-SMP fundamental properties, a dynamic mechanical analysis (DMA) was performed. The DMA was carried out in tension with frequency of force oscillation 1 Hz and heating rate 2°C/min. The obtained results are shown in Table 1. A high glass elastic modulus Eg’ (1500 MPa), proper value of the rubber modulus Er’ (15 MPa) and a high ratio of Eg’/Er’ (100) suggest that the PU-SMP material fulfils some preliminary demands to function as shape memory polymer [5]. The Tg value taken as the midpoint of glass transition region is 25°C. Table 1. Results of DMA obtained for polyurethane shape memory polymer denoted by MM 2520. Sample

PU-SMP

E’g

Tg

Er’

[MPa]

[oC]

[MPa]

1500

25

15

E’g/E’r

100

In shape memory polymers, the stress-strain curves under cyclic loading are expected in order to show the mechanical deformation properties clearly. Stress and average temperature changes of the PU-SMP obtained during 10 tension loading-unloading cycles with strain rate 2x10-1s-1 are shown in Fig. 5, while obtained for 10 times higher


strain rate - 2x100s-1 in Fig. 6, respectively.

Fig. 5. Stress (black curve) and average temperature changes (red curve) of the SMP obtained during 10 tension loading-unloading cycles with strain rate 2x10-1s-1.

Fig. 6. Stress (black curve) and average temperature changes (red curve) of the SMP obtained during 10 tension loading-unloading cycles with strain rate 2x100s-1.

The stress and average temperature changes presented in Figs 4 and 5 manifests how significant is the impact of strain rate in case of SMP loading and how huge influence of thermomechanical couplings on the SMP mechanical behavior can be observed.

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The constitutive model of SMP valid in finite strain regime has been developed. In the frame of the model, shape memory polyurethane is described as a two-phase material composed of a soft hyperelastic rubbery phase and a hard elastic-viscoplastic glassy phase [7-9]. The volume content of phases is specified by the SMP current temperature, taking into account the DMA results. The comparison of experimental and modeling results obtained for both mechanical characteristics, as well as temperature changes accompanying the SMP loading until the sample rupture has been presented in [9]. The research on mathematical description of the PU-SMP cyclic deformation is in progress. 4. Conclusions Cyclic deformation of TiNi shape memory alloy and PU shape memory polymer taking into account the effects of thermomechanical couplings have been carried out. For each strain cycle, an increase in temperature during the loading and the temperature decrease during the unloading processes were observed. It was confirmed that the temperature increment recorded during the cyclic deformation depends on the strain rate, the kind of the material and the test conditions. The higher the strain rate, the higher the stress and temperature changes were obtained, since the deformation process was more dynamic and has occurred in almost adiabatic conditions. The obtained results emphasize high sensitivity and usefulness of the infrared camera to study the effects of thermomechanical couplings in shape memory alloys and shape memory polymers. According to the model an increase of temperature causes an increase of the volume fraction of the soft rubbery phase. The research on mathematical description of the PU-SMP cyclic deformation is in progress. Acknowledgements The research has been carried out with support of the Polish National Center of Science under Grant No. 2011/01/M/ST8/07754. Authors are grateful to Dr. Mariana Cristea for elaborating the PU-SMP dynamic mechanical analysis (DMA) and valuable comments and to Leszek UrbaĔski for obtaining mechanical data. References [1] H. Tobushi, R. Matsui, K. Takeda, and E. Pieczyska, Mechanical Properties of Shape Memory Materials. Materials Science and Technologies, Mechanical Engineering Theory and Applications; NOVA Publishers, New York, 2013. [2] E. Pieczyska, H. Tobushi, K. KulasiĔski, Development of transformation bands in TiNi SMA for various stress and strain rates studied by a fast and sensitive infrared camera, Smart Materials & Structures, 22 (2013) 035007-1-8. [3] E. Pieczyska, Activity of stress-induced martensite transformation in TiNi shape memory alloy studied by infrared technique, Journal of Modern Optics, 57 (2010) 1700-1707. [4] S. Hayashi, Properties and Applications of Polyurethane-series Shape Memory Polymer, Int. Progress in Urethanes, 6 (1993) 90-115. [5] W.M. Huang, B. Young, and Y.Q. Fu, Polyurethane Shape Memory Polymers, Taylor & Francis Group, 2012. [6] E.A. Pieczyska, W.K. Nowacki, H. Tobushi H, S. Hayashi, Thermomechanical properties of shape memory polymer subjected to tension in various conditions, QIRT Journal, 6 (2010) 189 - 205. [7] T.D. Nguyen, C.M Yakacki, P.D Brahmbhatt. and M.L Chambers, Modeling the relaxation mechanisms of amorphous shape memory polymers. Advance Materials, 22 (2010) 3411–3423. [8] H.J Qi, T.D. Nguyen, F. Castro, C.M. Yakacki and R. Shandas, Finite deformation thermomechanical behavior of thermally induced shape memory polymers. J. of Mechanics and Physics of Solids 56 (2008) 1730-1751 [9] E.A. Pieczyska, M. Maj, K. Kowalczyk-Gajewska, M. Staszczak, L. UrbaĔski, H.Tobushi, S. Hayashi, M. Cristea, Mechanical and Infrared Thermography Analysis of Shape Memory Polymer, Journal of Materials Engineering and Performance, ((2014), in press, DOI: 10.1007/s11665-014-0963-2.