PSI - Issue 64

Ali Jafarabadi et al. / Procedia Structural Integrity 64 (2024) 2059–2066 A. Jafarabadi et al./ Structural Integrity Procedia 00 (2019) 000 – 000

2064

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compensate the thermal response, steel tubes were initially heated in absence of a mechanical trigger, i.e., the Fe-SMA tube. Consequently, thermal compensation was applied to the steel tube, resulting in the strain-gauges solely capturing the resultant strain induced by the Shape Memory Effect (SME) of the Fe-SMA tube. The principal strains in both axial and circumferential directions were measured during activation and are depicted in Fig. 2, with SG_A and SG_B representing strain-gauges in axial and circumferential directions, respectively. Utilizing equation (1), the contact pressure evolution is calculated during activation and presented in Fig. 2. Since the strain-gauges are self compensated, almost no excitation is recorded prior to the onset of contact, while the resultant strain starts to develop only after the onset of contact. That is, the contact pressure develops in steel tube only after overcoming the initial gap between the tubes through free recovery of the Fe-SMA. The onset of contact can be observed in the strain measurement where the two grids diverge into tensile and compressive strains as shown in Fig. 2. This point can be spotted on the contact pressure evolution as the starting point for the development of contact pressure. Ultimately, through the cooling process further pressure is applied on the steel tube as a result of thermal contraction. It should be noted that during the cooling process both tubes are in contact, however, due to a higher thermal contraction of the Fe-SMA tube (16ppm m/m.K) in relation to the steel tube (12ppm m/m.K) (Hosseini et al., 2018), the Fe-SMA tube exhibits a relatively higher contraction. Therefore, additional contact pressure develops at the interface through the cooling process. The finished contact pressure, which occurs at the end of the activation process, as highlighted in Fig. 2, demonstrates the maximum developed contact pressure. Moreover, the finished contact pressure at the end of the activation, i.e., back at room temperature (25°C), is free of possible sensitivity to the thermal loadings. These values are linked to the delivered joint strength, and hence the basis for gripping capacity evaluation. Furthermore, to study the effect of heat-treatment on the free recovery behavior two samples with similar conditions as stated in Table 1 were subjected to the activation process.

Figure 3. Comparison of the finished interface contact pressures and the [free] recovery ratio for as-received and heat-treated Fe-SMA joints.

3.1. Heat-treatment effect Yang et al. demonstrated that employing a two-step aging heat-treatment enhances the Shape Memory Effect (SME) in Fe-SMA (Yang et al., 2023). This study investigates this phenomenon specifically in Fe-SMA tubes, focusing on both restrained and free recovery. The samples IID26_T2_AR and IID26_T2_HT share identical geometries and pre-straining levels, differing only in their post-processing conditions, as stated in Table 1. As depicted in Fig. 2, the heat-treatment induces greater deformations in the steel tube, indicating an enhanced interface contact pressure development compared to the as-received sample. Moreover, finished contact pressures of 17.4 and 23.4 MPa for the as-received and heat-treated sample, respectively, demonstrate interface contact pressure enhancement of