TY - JOUR

T1 - Thermally activated iron-based shape memory alloy for strengthening metallic girders

AU - Izadi, Mohammadreza

AU - Hosseini, Ardalan

AU - Michels, Julien

AU - Motavalli, Masoud

AU - Ghafoori, Elyas

N1 - Funding information: The first author thanks the Swiss Federal Commission for Scholarships for Foreign Students (FCS) for providing a Swiss Government Excellence Scholarship for the academic year 2016–2017 (ESKAS No. 2016.1007 ) to support the project. The mobility grant for bilateral research collaboration with the South Asia and Iran 2017–2019 funded by ZHAW (Zürcher Hochschule für Angewandte Wissenschaften , Winterthur, Switzerland), commissioned by State Secretariat for Education, Research and Innovation (SERI) is also acknowledged. The authors thank the technicians of the Structural Engineering Research Laboratory of Empa, Switzerland, for their exceptional support in performing,the experiments. Furthermore the authors acknowledge support from Von Roll Deutschland GmbH for providing GFRP laminates.

PY - 2019/8

Y1 - 2019/8

N2 - 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.

AB - 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.

KW - Activation temperature

KW - High-cycle fatigue (HCF)

KW - Iron-based shape memory alloy (Fe-SMA)

KW - Prestressed strengthening

KW - Steel structures

UR - http://www.scopus.com/inward/record.url?scp=85064896767&partnerID=8YFLogxK

U2 - 10.1016/j.tws.2019.04.036

DO - 10.1016/j.tws.2019.04.036

M3 - Article

AN - SCOPUS:85064896767

VL - 141

SP - 389

EP - 401

JO - Thin-walled structures

JF - Thin-walled structures

SN - 0263-8231

ER -