Long-term Durability of Iron-based Shape Memory Alloys (Fe-SMAs) and their Performance in Structural Strengthening Applications

Abstract

A wide range of repair and strengthening techniques have been developed to address vulnerabilities in aging civil infrastructure. Nevertheless, most conventional approaches rely on passive mechanisms that contribute resistance only when external demands are applied. Active retrofitting strategies, by contrast, introduce permanent restorative forces, such as prestressing, that proactively enhance structural performance, reducing damage accumulation and extending service life. Shape memory alloys (SMAs) offer a unique advantage in this context due to their heat-activated shape memory effect, which enables prestressing without bulky equipment or invasive procedures. Among them, iron-based shape memory alloys (Fe-SMAs) have emerged as a cost-effective and promising solution for structural applications. Despite their growing implementation, the long-term durability and sustained mechanical performance of activated Fe-SMAs remain insufficiently understood, limiting their reliable implementation under environmental exposure and repeated loading. This thesis addresses this critical gap through an integrated experimental–numerical investigation of the durability, fatigue behaviour, and seismic retrofit applications of activated Fe-SMA systems. The experimental program quantifies the evolution of mechanical and functional properties under corrosion and repeated loading. Accelerated durability tests were conducted on activated Fe-SMA dogbone specimens exposed to a sodium chloride solution for varying durations to characterize corrosion induced degradation. Likewise, fatigue tests were performed on pre-cracked reinforced concrete (RC) beams strengthened with Fe-SMA strips to evaluate structural-level performance and service-life implications under cyclic loading. Complementing the experimental work, the numerical investigation evaluates an innovative SMA– based seismic retrofitting strategy. Advanced nonlinear static and dynamic time-history analyses were utilized to quantify global seismic response, identify governing SMA material parameters for retrofit design, and determine critical structural demand parameters influencing performance. Additionally, a resilience-based assessment framework incorporating post-earthquake recovery time was implemented to extend evaluation beyond conventional seismic demand metrics toward functional performance. Experimental results demonstrate that activated Fe-SMAs experience progressive reductions in recovery stress, ultimate tensile strength, and deformation capacity as corrosion develops, while retaining reactivation capability at reduced prestress levels. At the structural scale, RC beams strengthened with Fe-SMA exhibit enhanced fatigue resistance, characterized by reduced crack growth, lower deflections, and decreased of steel reinforcement and concrete strain accumulation. Numerical simulations indicate that the proposed retrofit strategy significantly reduces lateral displacements and residual deformations under seismic loading, although potential increases in floor accelerations highlight important design considerations for non-structural components and functional recovery. Overall, this research establishes a multi-scale understanding of activated Fe-SMA performance by explicitly linking material degradation, structural fatigue behaviour, and system-level seismic response. The findings provide new experimental evidence on long-term durability, performance, structural validation under repeated loading, and application-oriented SMA-based retrofit solutions evaluated through resilience metrics. Together, these contributions support the development of more reliable, durable, and resilient active retrofitting strategies for strengthening deficient and seismically vulnerable infrastructure.