PSI - Issue 33

Emanuele Sgambitterra et al. / Procedia Structural Integrity 33 (2021) 1073–1081 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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It is important to observe that the higher the applied current the higher the maximum temperature. According to the obtained results, to guarantee a complete transformation of the wire, even when it is constrained within the resin block, at least 1.3 A of electric current should be applied. However, almost 10 seconds for the heating process and 15 second for the cooling are required for a complete cycle. In order to speed up the cyclic experiments, preliminary tests were performed with the aim to determine the most appropriate electric current to apply to activate the wire in a small time. Three heating time were set, i.e. 0.5-0.75-1 sec and the input current was increased from 1.2 to 4 A. Obtained results are reported in Fig. 6.

Fig. 6. Maximum temperature as a function of the input current and activation time.

3.2. Pull-out tests Based on the results described in the previous section, an electric current of 3 A with an actuation time of 0.75 s was adopted for thermal activation of the SMA-polymer samples that were subjected to pull-out tests. Figure 7 reports the results of these tests in terms of shear/axial stress vs displacement curves. In particular, Fig. 7.a illustrates results obtained from pull-out tests of as manufactures samples, Fig 7.b reports the curves obtained after 1000 complete thermal activation cycles whereas Fig. 7.c illustrates the results obtained from the pull-out test carried out during thermal activation.

a) c) Fig. 7. Stress vs displacement curves obtained from pull-out tests under three different thermo-mechanical loading conditions: a) as manufactured samples, b) samples subjected to 1000 thermal activation cycles and c) samples tested during thermal activation. An almost sharp drop of the load is observed in the as manufactured samples, that is with a limited residual strength beyond the maximum load, characterized by a typical serrated trend attributed to stick-slip phenomena at the SMA-polymer interface. A maximum normal stress around 900 MPa was recorded corresponding to an average shear stress at the interface of around 3.25 MPa, see Fig. 7.a. As clearly shown in the figure, the maximum stress at the interface is well above the detwinning stress of the SMA, that is a minimum requirement for SMA-polymer b)