Surface Treatment and Debonding Failure Mechanisms in Iron-based SMA Connections

• Applicant: Professor Dr. Elyas Ghafoori
• Subject Area: Structural Engineering, Building Informatics and Construction Operation ; Construction Material Sciences, Chemistry, Building Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 546183955

Repair and strengthening of old steel structures including steel support structures of wind turbines, cranes, rail and roadway bridges, which are all subjected to high-cycle fatigue (HCF), have been identified as one of the effective solutions to improve the sustainability in construction (toward net zero 2050). Therefore, there is a need for development of new repair materials and reliable strengthening solutions. Iron-based shape memory alloy (Fe-SMA) is a new class of smart steel, which have recently attracted the research attention in construction sector. This advanced steel shows a shape memory effect (SME), which can be utilized to develop prestressing via an activation process. The activation process of iron-based SMAs includes different steps of pre-straining, heating and cooling to the room temperature, which ultimately results in great prestressing levels of up to 350-400 MPa (when heated to 150-170 °C). Prestressed SMA member applies a favorable compressive force to the parent structure. The iron-based SMA offers superior mechanical properties (e.g., high Young´s modulus and strength), low production cost (e.g., in contrast to NiTi SMAs) and ease of prestressing (without the need for hydraulic jacks in conventional prestressing techniques), which make them attractive for construction sector. Iron-based SMAs have been used for repair and strengthening of aging steel structures subjected to HCF loading regime to improve their strength and prolong the lifetime. Nevertheless, the majority of the available systems use mechanical fasteners (e.g., bolts, clamps and nails) to connect iron-based SMA to existing steel members, which may ultimately weaken the steel substrate by creating stress concentrations. Recently, a series of tests were conducted to demonstrate the feasibility of using adhesively-bonded SMA connections. Despite promising results in the preliminary tests, several new questions related to the fracture failure mechanisms in the bonded interface between steel and SMA have been raised. Therefore, there is a need for more research to fill this knowledge gap and to guarantee the soundness of such bonded connections. This project aims to conduct a systematic study to ensure the integrity of the bonded iron-based SMA under quasi-static loading (although the ultimate goal is fatigue repair). To reach this goal, firstly, the most appropriate surface preparation techniques for the iron-based SMA are identified. Secondly, the failure envelops of the bonded iron-based SMA connections are determined and a precise mode partitioning is proposed. Thirdly, a comprehensive numerical analysis methodology using cohesive zone modelling (CZM) methods to analyze the debonding failure in the SMA bonded connections. The insights gained from the results of the test campaign and the established analysis methodology will be then used to ensure the secure design of SMA strengthening systems.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Mastaneh Moattari