PSI - Issue 64

E. Choi et al. / Procedia Structural Integrity 64 (2024) 2028–2035 Eunsoo Choi / Structural Integrity Procedia 00 (2019) 000 – 000

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Plastic hinge zone The RC-SMA specimens were designed to investigate the effect of using NiTi SMA in the plastic hinge region at the lowest cost. The as-received NiTi SMA rods, 400 mm long and 25 mm in diameter, were fabricated into 200 mm bars with a diameter of 20 mm at the middle part, and 30 mm long of both ends were threaded. The 9.9 mm transition part from the middle part was cut, and the remaining 90.1 mm length was uncut. The shape of the NiTi SMA rods prevented the threaded ends from fracturing. Figures 4(a) and (b) show an fabricated SMA bar and an SMA bar with couplers at the both ends.

(a) Shape of the fabricated NiTi SMA rods

(b) NiTi SMA rod connected to couplers

Figure 4. Details and photographs of NiTi SMA rods

Test setup and procedure The test setup was established by combining a constant axial force of 614 kN and lateral loading using column footing assemblages. A hydraulic actuator was mounted on the head of the column to apply cyclic loading, and the loading point was placed 1400 mm from the footing surface. As shown in Figure 5, the front was opposite to the actuator, and the rear was on the side of the actuator. Both sides were left and right facing front. Thus, force was initially applied from rear to front, which is called ‘pushing,’ and the reversed loading is called ‘pulling.’ Four linear voltage displacement transducers (LVDTs) were installed 400, 800, 1100, and 1400 mm from the footing surface. Draw-wire displacement transducers were used, and the wire ends were attached to the column surface in the circles, as shown in Figure 5. Draw-wire displacement sensors are more stable against rotational movement than stick-type displacement sensors, and their measurement range is from several meters to 50 m. Two strain gauges were also attached to the middle surfaces of the two SMA bars from the neutral axis. Each column was subjected to lateral cyclic loading, which was applied in terms of a drift ratio starting from ±0.25%; the drift ratio was obtained by dividing the lateral displacement by the column height (1400 mm). This was then increased by 0.25% increments until failure. Cycles were applied once for each drift ratio to reduce experimental time. Loading was applied using a displacement control at a speed of 0.6 mm/s. Heating specimen An external heating jacket was used to activate the SMA rods inside concrete. The jacket was 400 mm high, which was sufficient to heat the plastic hinge zone. The jacket was installed to heat the SMA wire jackets around the concrete surface, successfully inducing the shape memory effect on the SMA wire. However, in the case, the SMA rods were embedded 40 mm deep; thus, the temperature of the SMA rod increased up to 65 °C during heating for 24 hours, whereas the concrete surface temperature reached 200 °C. The heating jacket effectively increased the temperature of the SMA wires on the concrete surface (Choi E et al.(2008)). However, it did not work well for the SMA bar inside the concrete. Thus, the temperature of 65 °C did not reach the temperature As = 75 °C and failed to activate the phase transformation in the SMA bars. The SMA used in this study was a two-way type, and the temperature was maintained higher than that of the phase transformation to activate the recovery stress (Yoo YI and Lee JJ (2011)). Thus, the jacket was placed in the plastic hinge zone during the test.