Thermally activated iron-based shape memory alloy for strengthening metallic girders. (August 2019)

• Record Type: Journal Article
• Authors: Izadi, Mohammadreza; Hosseini, Ardalan; Michels, Julien; Motavalli, Masoud; Ghafoori, Elyas
• Is Part Of: Thin-walled structures. Volume 141(2019)
• Page Start: 389
• Page End: 401
• Publication Date: 2019-08
• Subjects: Steel structures -- Prestressed strengthening -- Iron-based shape memory alloy (Fe-SMA) -- Activation temperature -- High-cycle fatigue (HCF); Thin-walled structures -- Periodicals; 690.1
• DOI: 10.1016/j.tws.2019.04.036
• Languages: English
• ISSNs: 0263-8231

Abstract

Abstract: The study presents a new retrofit solution for strengthening metallic I-girders. The retrofit system involves two iron-based shape memory alloy (Fe-SMA, 'memory-steel') strips (each with a width and thickness of 50 and 1.5 mm, respectively) that are mechanically anchored (using friction clamps) to the girders. The shape memory effect (SME) of the Fe-SMA material has been used to activate/prestress the strips by heating to a predefined temperature. The main advantage of the proposed SMA-retrofit system is that, unlike conventional systems, it can prestress itself without a need for heavy hydraulic jacks, which then results in a significant reduction of the required time, labor works and cost of prestressing process. In order to evaluate the efficiency of the proposed retrofit system, in this study, a series of static and fatigue four-point bending tests were performed on a 6.4-m SMA-retrofitted beam. Five static tests were performed on a steel beam with different SMA prestressing levels and included a reference un-strengthened test. The test results indicated that the achieved prestressing levels (i.e., recovery stresses) in the Fe-SMAs for activation temperatures of 100, 160, and 260 °C were approximately 160, 330, and 430 MPa, respectively. The induced compressive stresses in the bottom flange were in the range of 10–30 MPa. It was demonstrated that the Fe-SMAs could be re-activated for multiple times even up to higher temperatures (than the initial activation temperature), which would then result in higher prestressing levels. These features make the proposed SMA-based system a versatile and adaptable retrofit solution. Furthermore, the SMA-strengthened beam with the maximum prestressing level (activation temperature of 260 °C) was subjected to 2 million load cycles with a load ratio of R = 0.2 and a loading frequency of 4.35 Hz. The results of the high-cycle fatigue (HCF) tests showed no slippage in the anchorage system and a stable prestressing in the Fe-SMA members during the tests, which demonstrates the reliability of the proposed system under HCF loading regime. Highlights: Prestressed iron-based shape memory alloy system for retrofitting steel beams. Utilization of an un-bonded anchorage system for SMA-to-steel joints. Activation of Fe-SMA strips by electrical heating up to 100, 160, and 260 °C. High cycle fatigue (HCF) behavior of SMA-strengthened steel beam.