Experimental analysis of rate dependence of NiTi alloy under shear loading

Shape memory alloys (SMAs) as an optimal design solution to many engineering problems, due to their ability to exhibit fully recoverable deformations under substantial loads. While many uniaxial tension experiments of shape memory alloys have been published in the literature, relatively rare experimental studies address their behaviour in shear test. In this study, the simple shear test of NiTi alloy under quasi-static loading (7.7×10 mm/s – 7.7 mm/s) and dynamic loading (0.8 m/s) were conducted. In all the experiments, the isothermal, global, mechanical responses were measured, meanwhile the digital image correlation (DIC) was used to measure the full field quantitative strain maps and observe the evolution of the phase transformation. The results show that the deformation behaviour of NiTi alloy under quasi-static and dynamic loading are quite different. 22 Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 Mots clefs : NiTi alloy, Martensitic transformation, Simple shear test, Strain rate dependence, Digital image correlation 1 MATERIAL AND EXPERIMENTAL SETUP 1.1 NiTi specimen The material studied was from NiTi polycrystalline cold-rolling sheets, straight annealed with dark oxide surface (Johnson Matthey Inc., USA). The metallurgical composition is 50.7 wt. % Ni and 49.3 wt. % Ti. The experiment was performed at room temperature, clearly above the characteristic temperature Af, so that for stress-free condition the specimen is in a full austenite state. The overall size of the specimen was 62mm in length, 30mm in width, and 0.5mm in thickness. The length of shear zone was 20mm, and the width was 3mm, as shown in Fig. 1. In order to reduce the influence of free edges effects, the ratio of length and width should larger than 10. Figure 1. The geometry of NiTi specimen for double shear loading 1.2 Double shearing grip Double shearing test can be realized which might prevent from geometrical instabilities. Divers versions of this test had been developed in the past decades. A double shearing device to better fit with the 60mm-diameter Split Hopkinson Pressure Bar was developed, as shown in Fig. 2 left. In this study, a modified shear grip was designed for DIC observation, as shown in Fig. 2 right. 22 Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 2 SHEAR TEST PROCEDURE 2.1 Quasi-static test The NiTi shear test under different strain rates (10 /s − 10 /s) were performed on modified hydraulic MTS 810 testing machine. Experiments were conducted at five different prescribed velocities: 7.7×10mm/s (2.6×10 /s), 7.7×10mm/s (2.6×10 /s), 7.7×10mm/s (2.6×10 /s), 7.7×10mm/s (2.6×10 /s) and 7.7mm/s (2.6 /s). The temperature was 25°C and the humidity was 45%. 2.2 Dynamic test The test at moderate impact velocity around 0.8m/s was performed on the advanced Split Hopkinson Pressure Bar, the shear grip with specimen placed between the input and output pressure bars, as shown in Fig. 3. A labview program was used to record the signals from the strain gauges on the input bar (trigger signal) and output bar, meanwhile, sent a TTL signal (5 volts) to trigger the high-speed camera. Figure 3. Schematic drawing of Hopkinson bar and clamping device Figure 2. The geometry of Double shear grip 22 Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 The high-speed camera Photron Fastcam SA5 equipped with telecentrique G1 (Edmund Optics, field of view 2/3 Sensor 8.8 mm, working distance 98-123mm) was used to observe the deforming process of NiTi specimen. The frame rate was 54250 images/second. The resolution was 768*160pixels. Shutter speed was 1/54250 s. Moreover, two cold spotlights were used for lightening, which were enough for the high-speed camera and not heating the specimen. The room temperature was 24.4°C and the humidity was 34%. 3 TEST RESULTS 3.1 Shear stress-strain curve The strain gauges attached on the input bar and output bar were used to measure shear stress, and shear strain was calculated from the mean of shear strain in the shear zone. The shear stress-strain curve is shown in Fig. 4, which is compared with the quasi-static shear loading. Figure 4. Shear stress-strain curve 3.2 Shear strain maps A first analysis of DIC results allows give quantitative information on the phenomena during the test. The dynamic shear strain maps are as shown in Fig. 5, which is compared with the shear strain maps of quasi-static test. 22 Congrès Français de Mécanique Lyon, 24 au 28 Août 2015 (a) Shear strain maps under quasi-­‐static shear loading (He et al, 2013) 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 ENO12 Bar 10px −0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 (b) Shear strain maps under dynamic shear loading Figure 5. Shear strain maps