Due to the internal reversible martensitic transformation, Shape Memory Alloys (SMAs) can recover large (up to 10-12%) inelastic strains upon stress removal (referred to as the superelastic effect, SE) or with external heat stimuli (referred to as the shape memory effect, SME). The SE are particularly advantageous for dissipating seismic energy and protecting bridges in earthquake prone regions by substantially reducing permanents under near-fault earthquakes. Previous research on superelastic alloys (SEAs) mainly focused on the nickel-titanium (NiTi) composition, which showed stable behavior at or near room temperature. NiTi SEAs have been shown to have the necessary characteristics (strength, ductility, and energy dissipation capacity, among others) to be used as plastic hinge reinforcement in bridge columns. The first successful implementation of the NiTi SEAs in the SR99 Alaskan Viaduct Bridge in Seattle has been completed in 2017. However, certain properties of the NiTi based SEAs such as the difficulty in machining, potential loss of superelasticity at low temperature, and the high cost still drive the search for alternate materials. As an alternative material to NiTi, this research investigated the use of Cu-Al-Mn (CAM) SEAs, which are known to be cheaper and easier to machine. Considering both the manufacturing and machining, the total cost of CAM SEAs is expected to be one-half to one-quarter of that of NiTi based ones. More importantly, the CAM SEAs show comparable or even better superelasticity and a wider temperature application range than NiTi ones. In the low-cycle fatigue tests, it was found that the single crystal CAM SEAs show excellent superelasticity and fatigue resistance at all test temperatures: -40℃, room temperature, 25℃, and 50℃. The fatigue life of single crystal CAM SEAs was found to be as high as 50,000 cycles under 5% strain, and almost no deterioration was observed in the superelastic properties of single crystal CAM SEAs in the initial 100 cycles. In the long-term corrosion and electrochemical tests, it was found that the mass loss and corrosion rate of CAM SEAs is around 1/3 of mild steel. After around three years of natural corrosion, the CAM SEAs still showed excellent superelasticity: its strain recovery and energy dissipation capacity showed negligible degradation. A cost estimation study indicated that columns reinforced with CAM SEAs show economic advantage over the NiTi SEA reinforced columns particularly if machining is used to connect the SEA bars with the steel rebar. The additional cost associated with using CAM SEAs in the column was only about 1/4 of that of NiTi SEAs, indicating the cost effectiveness of CAM SEA resulting from its excellent machinability. This is confirmed by previous research by the PIs, which showed that incorporating CAM SEAs in the column plastic hinges of earthquake-prone bridges increased the overall initial cost of the bridge by only a few percent, a cost that is more than offset by not needing to conduct major bridge repair or replacement after strong earthquakes. The only major impediment for real life implementation of CAM SEAs in bridges is the mechanical splicing, which is recommended for future research.
The final report is available.