Thermo-Mechanical Properties of Shape Memory Alloy. Tan Wee Choon, Saifulnizan Jamian and Mohd. Imran Ghazali

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1 Thermo-Mechanical Properties of Shape Memory Alloy Tan Wee Choon, Saifulnizan Jamian and Mohd. Imran Ghazali Faculty of Mechnical and Manufacturing, Universiti Tun Hussien Onn Malaysia Abstract The Young s modulus and the Coefficient of Stress Influence are two important parameters of thermo-mechanical properties of shape memory alloys. These parameters are important in the design but there are no technical data on it. A novel method was used to determine the Young s modulus of shape memory alloys during martensite phase and austenite phase, and the Coefficient of Stress Influence during austenite-martensite and martensite-austenite by using Universal Testing Machine. The load applied on to the Flexinol wire for one cycle was at a very low speed of testing. Throughout the test, the Flexinol wire was heated up to a certain temperature level and maintained it constant by using chamber. The Young s modulus at martensite phase and austenite phase were GPa and GPa while the Coefficient of Stress Influence during austenite-martensite and martensite-austenite were MPa/ C and 9.82 MPa/ C. Keywords Young s modulus, Coefficient of stress influence, Shape memory alloy, Universal Testing Machine III. INTRODUCTION Shape memory alloys are widely used in the engineering applications especially in the robotic, aerospace and vibration control area. To design and optimise the applications of the shape memory alloys, a clear understanding of its behavior and characteristics is required. The thermo-mechanical properties for the shape memory alloys such as the Young s modulus at martensite phase and austenite phase and the Coefficient of Stress Influence during austenite-martensite and martensite-austenite were the properties that should be determined before designing the system. Unfortunately the properties provided by the manufacture normally were not complete. So further investigation or testing need to be done. Differential Scanning Calorimetry (DSC) is one of the famous method that done to determine the phase transformation temperatures which is one of the important thermomechanical properties for the shape memory alloys. The past researchers [1,2,3,4,5], had suggested that DSC as the standard method to determine the phase transformation temperatures for their research purposes but from the analysis that was done, the DSC method is only suitable for the powder form product and not suitable for finished form product such as wire. Some other researchers proposed other methods to determine the properties of the shape memory alloys. Mohammad H Elahinia and Mehdi Ahmadian, [6] suggested to use a linear variable differential transformer (LVDT) consists of a stack of masses that was actuated vertically by an SMA wire. The stress applied to the SMA wire can be adjusted by adding mass to or removing mass from the stack. A personal computer

2 records these measurements and generates the voltage signals, which were then amplified before applied to the SMA wire. A J Żak et. al, [7] had design an experimental rig which contained force sensors displacement measurement and thermocouple to determine the phase transformation temperatures. The Shape memory alloy wire was heated up with the direct current and the testing were done with different value of dead-weight load to the wire. The experiment which designed by A. J Żak et. al. not only used to determine the phase transformation temperature but also the thermo-mechanical properties of the shape memory alloy wire. While Tan Wee Choon et. al, [8] had used Univerisal Testing Machine to determine the phase transformation temperature for the shape memory alloy. Furthermore in this paperwork, the Young s modulus at martensite phase and austenite phase and the Coefficient of Stress Influence during austenite-martensite and martensite-austenite will be determined from the experiment which was similar done by W.C., Tan et. al,. Further discussion on the specimen will be done in section 2. Section 3 introduces our enhanced data and analysis, and finally the paper is concluded in section 4. IV. EXPERIMENTAL The shape memory alloy in this paper was the Flexinol wire from Dynalloy, Inc and the detail properties are as shown in Table I. The diameter of the Flexinol wire was.2 inch which has the maximum pull force of N. The clearance length of the Flexinol wire was mm and both end of the wire have been clamped with the barrel crimp as shown in Fig. 1. This Flexinol wire was chosen because of the ready-to-use characteristic compared to the ordinary nitinol wire which required special training before it can be used and furthermore the Flexinol can be easily heated by the direct current. TABLE I PROPERTIES OF FLEXINOL WIRE [9] Parameters Value Density 6.45 g/cm 3 Specific heat 6-8 cal (mol. C) Melting Point 12 C Heat of transformation 1.4 BTU/lb Thermal conductivity.5 cal (cm- C-sec) Thermal expansion coefficient Martensite 6.6 x 1-6 / C Austenite 11. x 1-6 / C Electrical resistivity Martensite 421 Ohm/Cir Mil Ft Austenite 511 Ohm/Cir Mil Ft Linear resistance.12 Ohm/inch Density 6.45 g/cm 3 Specific heat 6-8 cal (mol. C) Melting Point 12 C

3 Fig. 1 Samples of Flexinol wire. The Experimental layout that will be conducted by the Universal Testing Machine is shown in the Fig. 2. The Flexinol wire was heated up to certain temperature and maintained it constant by using the chamber. The force and displacement reading of the Flexinol wire were then measured by the force transducer and the linear transducer. The Flexinol wire was initially heated up to 95 C inside the chamber for few minute and then let it cool down to the room temperature before testing. The force transducer and the linear transducer were calibrated before the test. It was then held by the clamper of the Universal Testing Machine and the wires were connected to all equipments as shown in the Fig. 2. Both load cell and the linear transducer were set to zero. The Flexinol wire was heated up to the desired temperature and placed inside the chamber for about 3 minutes. Before the test, the Flexinol wire need to be tighten up and the difference values of the linear transducer will be used to calculate the actual initial length of the Flexinol wire. The Flexinol wire was then applied with a pretension of approximately 1 N. This was done manually at constant low speed to avoid any extreme pretension to the wire. After the pretension of the Flexinol has been done, the displacement was then set to zero. The actual initial length was determined from the original length plus the changes of the length due to setup. The Flexinol wire was then ready to be tested.

4 Fig. 2 Layout of experiment setup. The experiment is divided into two major parts; the loading process and the unloading process. For the loading process, the Flexinol wire was pull to a strain level of.6 mm/mm at a speed of.25 mm/minute. When the Flexinol wire reached the maximum strain, then the unloading process will be taking place where the Flexinol wire was pushed back to it original position with the same test speed of.25 mm/minute. The load cell reading and the linear transducer reading were taken continuously at every 15 seconds. The loading and the unloading process were repeated for other sets where the value of the temperature applied to the Flexinol wire was set to 75 C, 8 C, 83 C, 85 C, 87 C, and 9 C. The Young s modulus during austenite and martensite phase can be determined with few modification. The test was done at below the martensite finish, 26 C for the loading test to determine Young s modulus during martensite phase and above austenite finish, 93 C for the unloading test to determine Young s modulus during austenite phase. Both test speed were at.2 mm/minute and the load cell reading and the linear transducer reading were taken continuously at every 1 seconds. III. RESULTS AND DISCUSSION The data from the experiment was then manually keyed into the MS Excel for analysis. Each set of the data of temperature 75 C, 8 C, 83 C, 85 C, 87 C, and 9 C were plotted as stress graph of the Flexinol wire versus the strain for the loading and unloading process as shown in Fig. 3, 4, 5, 6, 7 and 8.

5 75 celcius loading 75 unloading Fig. 3 Set data for the set experiment with 75 C temperature applied to the Flexinol wire. 8 celcius loading 8 unloading Fig. 4 Set data for the set experiment with 8 C temperature applied to the Flexinol wire celcius loading 83 unloading Fig. 5 Set data for the set experiment with 83 C temperature applied to the Flexinol wire.

6 85 celcius loading 85 unloading Fig. 6 Set data for the set experiment with 85 C temperature applied to the Flexinol wire. 87 celcius loading 87 unloading Fig. 7 Set data for the set experiment with 87 C temperature applied to the Flexinol wire. 9 celcius loading 9 unloading Fig. 8 Set data for the set experiment with 9 C temperature applied to the Flexinol wire.

7 83 celcius loading 83 unloading Fig. 9 Steps to determine the Coefficient of Stress Influence during austenite-martensite and martensite-austenite of Flexinol wire. All experiments result celcius 8 celcius 83 celcius 85 celcius 87 celcius 9 celcius Fig. 1 Thermo-mechanical characteristic for Flexinol wire from 75 celcius to 9 celcius. From each set of the experiments, the Coefficient of Stress Influence during austenite-martensite and martensite-austenite were determined as shown in the Fig. 9. The Coefficient of Stress Influence during austenite-martensite will be obtained from the loading process curve while the Coefficient of Stress Influence during martensiteaustenite from the unloading process curve which represented as red line and blue lines. Fig. 1 shows the combination of Fig. 3, 4, 5, 6, 7 and 8. From Fig. 1, it can be observed that the stresses during phase transformation increased with the increment of operating temperature, but the stress remain within the Flexinol wire will be decreased with the increment of temperature. The slope during the loading and unloading process were almost of the same gradient from the observation. Table II shows the Stress during austenite-martensite and martensite-austenite for all set of experiments for the Flexinol wire. Graphs of stress versus temperature will be plotted from the results shown in Table II as shown in Fig. 11. The graphs were linear

8 and the slope for each curve will be the Coefficient of Stress Influence during austenitemartensite and martensite-austenite for Flexinol wire. TABLE II STRESS DURING PHASE TRANSFORMATION FOR FLEXINOL WIRE WITH DIFFERENT TEMPERATURE CONDITIONS Stress during Stress during Temperature austenite-martensite martensite-austenite ( C) ( MPa ) ( MPa ) Stress influence coefficients Slope = Stress (MPa) 1 Slope = Temperature (Celcius) Stress AM Stress MA Fig. 11 : Coefficient of Stress Influence during austenite-martensite and martensiteaustenite of Flexinol wire From the Fig. 3, 4, 5, 6, 7 and 8, the slope of the loading and unloading process can be determined but the values obtained from these slope were not the real Young s modulus for austenite phase and martensite phase of the Flexinol wire. This is due to at these conditions the Flexinol wire consists of both austenite phase and martensite phase. So another test need to be done where the testing done on below the martensite finish for the loading test to determine Young s modulus during austenite phase and above austenite finish for the unloading test to determine Young s modulus during martensite phase.

9 The reading for both displacement and force were then converted to strain and stress and plotted in the Fig. 12. The slope for the loading process is the Young s modulus during austenite phase while the Young s modulus during martensite phase is given from the slope of unloading process graph. Young's moduli 3 Slope = GPa 3 2 Stress (MPa) 1 Slope = GPa Strain (mm/mm) martensite austenite Fig. 12 : Young s modulus during austenite phase and martensite phase of Flexinol wire From Fig. 11 and Fig. 12, the Young s modulus at martensite phase and austenite phase and the Coefficient of Stress Influence during austenite-martensite and martensiteaustenite of the Flexinol wire are as shown in the Table III and Table IV. TABLE III YOUNG S MODULUS FOR FLEXINOL WIRE Young s modulus during austenite (GPa) Young s modulus during martensite (GPa) TABLE IV COEFFICIENT OF STRESS INFLUENCE FOR FLEXINOL WIRE COEFFICIENT OF STRESS INFLUENCE DURING AUSTENITE-MARTENSITE (MPA/ C) MARTENSITE-AUSTENITE (MPA/ C) COEFFICIENT OF STRESS INFLUENCE DURING IV. CONCLUSION This method can be used to determine the Young s modulus at martensite phase and austenite phase and the Coefficient of Stress Influence during austenite-martensite and martensite-austenite for the shape memory alloy.

10 The test conducted with the Universal Testing Machine with chamber is more convenient and constant temperature was maintained throughout the testing area and this help a lots in term of temperature controlling and monitoring. Although this method can be used but there is still some shortage where the reading capture rate must be carefully selected. If the capture rate is too often then it may a waste but if the capture rate is too rare then it may not capture the changes during the starting and ending the phase transformation occurs within the Flexinol wire. Beside this, the space of chamber also constrain the length of the specimen to be tested inside chamber if compared to the testing using DC to heated up the Flexinol wire. The Young s modulus at martensite phase and austenite phase were GPa and GPa while the Coefficient of Stress Influence during austenite-martensite and martensite-austenite were MPa/ C and 9.82 MPa/ C. REFERENCES [1] Kiyohide Wada and Yong Liu, Shape recovery of NiTi shape memory alloy under various pre-strain and constraint conditions, Smart Mater. Struct. 14 (5) S273 S286 [2] Antonio Vitiello, Giuseppe Giorleo and Renata Erica Morace, Analysis of thermomechanical behaviour of Nitinol wires with high strain rates, Smart Mater. Struct. 14 (5) [3] Z.G. Wan, X.T. Zu, X.D. Feng, L.B. Lin, S. Zhu, L.P. You and, L.M. Wang, Design of TiNi alloy two-way shape memory coil extension spring, Science and Technology of Advanced Materials 6 (5) [4] Z.G. Wang, X.T. Zu, X.D. Feng, S. Zhu, J.W. Bao and, L.M. Wang, Characteristics of two-way shape memory TiNi springs driven by electrical current, Materials and Design 25 (4) [5] J. Rena and, K.M. Liewa, Meshfree modelling and characterisation of thermomechanical behaviour of NiTi alloys, Engineering Analysis with Boundary Elements 29 (5) [6] Mohammad H Elahinia and Mehdi Ahmadian, An enhanced SMA phenomenological model: II. The experimental study, Smart Mater. Struct. 14 (5) [7] A J Żak, M P Cartmell, W M Ostachowicz and M Wiercigroch, One-dimensional shape memory alloy models for use with reinforced composite structures, Smart Mater. Struct. 12 (3) [8] Tan Wee Choon, Abdul Saad Salleh, Saifulnizan Jamian and MOhd. Imran Ghazali, Phase transformation temperatures for shape memory alloy wire, Proceeding of XIX International Conference On Computer, Information, and System Science and Engineering January 29-31, 7 Bangkok, Thailand, [9] Technical characteristics of Flexinol actuator wires, Dynalloy, inc.