Time-dependent behavior of Fe-based Shape Memory Alloys: A Comprehensive Analysis of Creep, Stress Relaxation, and Corrosion Behavior for Concrete Reinforcement Applications

Shape memory alloys (SMAs) exhibit unique characteristics, enabling them to recover their original shape either upon unloading (pseudoelasticity) or through heating (shape memory effect, SME). Due to their exceptional mechanical properties, the SME, and their ease of application, iron-based SMAs (Fe-SMAs) have gained significant attention as prestressed reinforcement for structural strengthening and retrofitting in concrete applications. The typical life span of a concrete structure is in the range of 50-100 years. Therefore, time-sensitive properties such as creep and stress relaxation of reinforcement play a crucial role in determining the service life of prestressed concrete structures. However, the understanding of these time-dependent characteristics of Fe-SMA reinforcement remains limited, highlighting a significant knowledge gap.

This doctoral project aims to investigate the underlying mechanism of the time-dependent properties of Fe-SMA Fe-17Mn-5Si-10Cr-4Ni-1(V,C) and parameters influencing it. The time-dependent properties investigated in this study include creep, stress relaxation, and corrosion performance of Fe-SMA. These properties were investigated by in-situ synchrotron X-ray diffraction to understand the mechanism, ex-situ mechanical tests for long-term behavior, and electrochemical experiments to elucidate environmental corrosion.

The creep and stress relaxation were evaluated at different stress levels, temperatures, and heat treatment conditions. Stacking fault probability and phase volume fraction quantification provided an understanding of the dominating mechanism at different stress levels. The prominent austenite peak evolution of solutionized Fe-SMA at different temperatures hinted towards accelerated transformation at -45°C, resulting in higher creep. The integrated intensities of the martensite peaks over time showed limited transformation for aged Fe-SMA, resulting in higher creep and stress relaxation resistance as compared to solutionized and as-received Fe-SMA. The austenite integrated intensity reduction during stress relaxation was lower than creep at the same stress levels, suggesting limited transformation. The effect of stress on the long-term creep strain followed a trend similar to in-situ experiments.

The corrosion behavior of heat treated Fe-SMA was evaluated in a Ca(OH)2 environment with and without chloride to simulate the conditions during its application as a reinforcement. Fe-SMAs behaved passively in solution without chlorine. In contrast, Fe-SMA shows pitting corrosion and localized attack depending on the heat treatment in solution with chlorine.

The outcomes of this doctoral study provide a comprehensive fundamental understanding of the creep, stress relaxation, and corrosion mechanism of Fe-SMA. This knowledge is crucial for advancing the understanding of the long-term performance of concrete structures reinforced with Fe-SMA bars. In particular, the understanding gained from this study could