PSI - Issue 64

Mao Ye et al. / Procedia Structural Integrity 64 (2024) 1824–1831 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1825

2

1. Introduction Shape memory alloys (SMAs) are functional metallic materials known for their distinctive properties such as shape memory effect (SME). After deformation, the SME enables the SMAs to partially recover to their original predefined shape through activation (typically achieved with heat). This characteristic has garnered significant attention across various fields, including civil engineering, due to its potential application in prestressed strengthening. In contrast to traditional prestressed strengthening methods, which rely on hydraulic jacks and require adequate operational space, the use of SMAs offers a less demanding alternative. When deformed SMAs are attached to structures and thermally activated, they can generate considerable recovery stress, providing active prestressed strengthening to the structures. Many studies have demonstrated that SMA prestressed strengthening significantly improves the fatigue behavior of steel components and structures (Izadi et al. (2019), Vůjtěch et al. (2021) and Chen et al. (2023)). Considering the efficiency of prestressed strengthening with SMAs, two aspects are of great concern: One important precondition is whether the SMAs can generate sufficient recovery stress (at room temperature) through thermal activation. Among the most commonly used SMAs are the iron-based SMAs (Fe-SMAs) and nickel titanium based SMAs (NiTi-SMAs). Recently developed Fe-SMAs (Lee et al. (2013) and Gu et al. (2021)) showed many advantages in the high generation of recovery stress and popular price. Incorporating Nb into NiTi forms a modified ternary alloy NiTiNb (Jiang et al. (2016)). Existing research on the NiTiNb-SMA wires (Suhail et al. (2020) and Yang et al. (2023)) has confirmed their competitive recovery properties. However, despite its significance, research on the NiTiNb-SMA plates remains rather limited. The other issue pertains to the connection between SMAs and the strengthened structure. In the case of steel structure, possible solutions were proposed as mechanical anchor (Izadi et al. (2018) and Izadi et al. (2019)) and nail anchor (Fritsch et al. (2019)). However, all these solutions may potentially introduce damage to the parent structure. Recently, an emerging non-destructive bonded prestressed strengthening solution utilizing Fe-SMAs has been proposed and proven promising (Wang et al. (2021), Li et al. (2023a), Wang et al. (2023) and Li et al. (2023b)). Since the adhesive properties are temperature dependent, elevated temperatures may cause potential degradation of the adhesive, therefore a relatively lower activation temperature is more beneficial for maintaining the bond strength. In conclusion, based on pertinent research, the SMAs capable of generating sufficient recovery stress at a moderate activation temperature should exhibit promising potential for bonded prestressed strengthening of structures. In this study, experiments were carried out to investigate the mechanical behavior and stress recovery behavior of a NiTiNb-SMA plate. Tests include: tensile failure test, prestraining, activation, and re-activation. The influence of key parameters such as prestrain level and activation temperature on the stress recovery behavior of this NiTiNb-SMA plate were evaluated. The influence of re-activation was also discussed. This study also aimed to find an optimal thermal activation strategy for this NiTiNb-SMA plate to exert the best recovery behavior.

2. Experimental program 2.1. Material and specimen

The NiTiNb-SMA plates tested in this study were provided by the supplier (Beijing Gee SMA Technology Co., Ltd., China). According to the datasheet from the supplier, the plates consisted of 47%Ni, 44%Ti, and 9%Nb (at.%). Dog-bone specimens with a gauge length of 72 mm and a cross-section of 10  1.5 mm 2 were manufactured according to Chinese Code GB/T 228.1-2021, as shown in Fig. 1. The two holes at both ends of each specimen were designed to fit the test set-up of activation tests (detailed in section 2.3). All specimens were produced from the same batch, cold-rolled into plates (thickness 1.5 mm), and then cut to the designed geometry. The main manufacturing process included: smelting, casting, cold-rolling, electrical discharge machining, stretching, and grinding (to avoid the notch effect). The effect of cutting position was not considered during the manufacturing process. The as-received specimens had good flatness and surface smoothness.