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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1. Introduction Seismic design philosophy requires the prevention of the collapse of structures from strong earthquakes to save human lives. In reinforced concrete (RC) columns, ductile behavior is critical for achieving these two requirements. Thus, conventional RC columns generally have plastic hinge zones, where the yielding of steel reinforcements occurs to dissipate energy and permit large rotations to provide ductile behavior. Steel yielding and large rotations at plastic hinges induce severe damage and lead to large residual displacements, both of which disturb the repair operation and serviceability of columns after strong earthquakes. Recently, self-centering (or re-centering) and ductility have been required in RC columns for the serviceability of structures; self-centering is the capacity to recover the applied displacement of a column to its original location. After earthquakes, RC columns displaced from their original locations may interrupt disaster recovery operations and the transportation of vehicles. Recently, the use of smart or innovative RC columns, which can provide full ductile behavior and self-centering capacity after earthquakes, has increased. One solution for smart RC columns is to use shape memory alloys (SMAs) as reinforcements in plastic hinge zones. SMAs exhibit two unique properties: super-elasticity (SE) and shape memory effect (SME). Super- elasticity, sometimes termed as ‘pseudo - elasticity,’ occurs without any change in temperature, as long as the temperature is above the transformation temperature to austenite (Nemat-Nasser S and Guo WG (2006)). When a mechanical strain is applied, austenite is transformed into martensite, which is called ‘stress -induced martensite (SIM).When the stress is removed, the martensite reverts to austenite, and the material recovers its original shape. This effect causes the alloy to appear extremely elastic; thus, it is called as super-elasticity or pseudo-elasticity. The shape memory effect delineates the ability of an alloy to return to its original shape after deformation by heating to increase its temperature (Narech C et al. (2016)). The alloy is deformed in the martensitic phase and then reheated to the austenitic phase, recovering its original shape. Super-elastic NiTi SMAs are the most commonly used self centering elements in RC structures, such as beams, columns, and beam-column joints (Raza S et al. (2022)). The excellent flag-shaped behavior of RC columns is promising for recovering their drift and reducing their residual drift. Over the past decade, a new concept of self-centering RC columns using SMAs with the shape memory effect has not been introduced. Based on Choi’s study (Choi et al. (2022)), it is possible to apply the shape memory effect SMAs to self-center RC columns. Thus, this study tried to use martensitic SMA bars with SME for the self-centering of the conventional RC columns. 2. SMA bar specimen and coupler in tensile tests The martensitic SMA bar used in this study was composed mainly of Ni (55.74%) and Ti (44.12 %), with small amounts of mixed carbon (C), oxygen (O), and ferrous (Fe). The as-received SMA bars were 300 mm and 30 mm in length and diameter, respectively, and were then cut into dog-bond-shaped specimens for tensile testing (Figure 1). They had a total length of 197 mm and a gauge length of 110 mm in the middle. Each end with a length of 33.6 mm and a diameter of 25 mm was threaded for connection, and the middle part had a diameter of 20 mm. The phase transformation temperatures of the SMA to austenite were A s = 75 °C and A f = 101 °C, where As is the starting temperature and A f is the finishing temperature of the austenite. Conversely, the phase transformation temperatures to martensite were found to be M s = 24 °C and M f = -13 °C, with M s indicating the start of the martensite transformation and M f is the finishing temperature. The elastic limit was conjectured till 182.9 MPa of 0.53% strain, and the ultimate strength of the SMA bar was 529.1 MPa at 11.6% strain. The initial secant elastic modulus was approximately 34.5 GPa.

Figure 1. Dimensions of SMA bar specimens (unit: mm)