PSI - Issue 64

Sizhe Wang et al. / Procedia Structural Integrity 64 (2024) 2075–2082 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Lifetime extension of civil structures is essential for sustainability as it reduces the demand for raw materials for new construction. Fatigue cracking jeopardizes the long-term structural integrity and lifespan of steel structures. Many techniques have been developed to address the fatigue cracking issue for extended lifetime, for example, drilling stop holes and using bonded CFRP patches (Liu et al. (2009), Yu et al. (2013), Chen et al. (2018), Yu and Wu (2018)). Furthermore, prestressed repair solutions are desirable for fatigue cracks as they apply a compressive stress to the crack resulting in a reduced stress intensity factor and thus slowing down fatigue crack growth or, in some cases, a complete crack arrest. Recently, shape memory alloy (SMA) materials have been exploited for a prestressed repair. By utilizing the shape memory effect, the prestressing can be realized through an activation process (i.e., a heating and cooling process), which is easier than a mechanical tensioning process. A patching system consisting of nickel titanium-based shape memory alloy (NiTi-SMA) wires and CFRP sheets was developed by some researchers, and experimental tests indicated that the solution was effective (Zheng and Dawood (2017), Zheng et al. (2018), Abdy et al. (2018), Li et al. (2020)). In addition to NiTi-SMAs, memory-steel (also known as iron-based shape memory alloy, Fe-SMA) is attractive for civil engineering applications owing to its excellent mechanical properties, high recovery stresses, and relatively low manufacturing cost (Cladera et al. (2014)). Experimental studies on cracked steel plates repaired using mechanically anchored or adhesively bonded Fe-SMA strips have demonstrated that the fatigue life of repaired specimen was substantially extended and even complete arrest of fatigue cracks could be achieved (Izadi et al. (2018), Izadi et al. (2019), Wang et al. (2021), Wang et al. (2024c)). In these studies, activation techniques such as electrical resistive heating (Izadi et al. (2018), Izadi et al. (2019)), heat gun (Wang et al. (2021)), hot bonder, and gas torch (Wang et al. (2024c)) were involved. Strengthening solutions to improve the fatigue performance of large-scale steel girders using adhesively bonded Fe-SMA strips were developed where handheld torches was employed for activation (Wang et al. (2023a), Wang et al. (2024b)). In addition, experimental studies were conducted to establish a comprehensive understanding of the mechanical behavior and durability of Fe-SMA bonded joints (Li et al. (2023a), Li et al. (2023b), Pichler et al. (2023), Pichler et al. (2024)). The previous studies have showcased the feasibility and effectiveness of the adhesively bonded Fe-SMA strips for fatigue crack repair. The critical role for the success of the repair solution is the activation of the Fe-SMA and the generation of prestress. Therefore, this study focuses specifically on activation tests of Fe-SMA patches bonded on cracked steel plates to investigate the recovery stress behavior. Particularly, the effect of patch length on the prestress development is investigated. 2. Experimental study 2.1. Specimens and test matrix The experimental program was designed to investigate the recovery stress behavior of adhesively bonded Fe-SMA patches, studying the effect of patch length on the recovery stress development and the final prestress level after activation. Fig. 1 illustrates the specimen configurations. As shown in Fig. 1a, steel plates with a central notch and precracks were employed. Fe-SMA strips were symmetrically bonded over the notch and precracks on both sides of the steel plates using a ductile nonlinear adhesive SikaPower-1277, respectively (Fig. 1b and c). The material properties of the Fe-SMA strips, adhesive, and steel plates can refer to previous studies (Wang et al. (2023a), Li et al. (2023b), Gu et al. (2021)).