Headed coupling behavior of large diameter Cu-Al-Mn shape memory alloy bars: Mechanical testing and microstructural analyses

Abstract

Cu-Al-Mn (CAM) superelastic alloy bars (SEAs) have received increasing attention in bridge applications. To take full advantage of their strain recovery and energy dissipation capacity, CAM SEAs are usually only applied in the plastic hinge regions of columns as part of the longitudinal reinforcement and coupled with steel rebar through mechanical splices. In this study, the feasibility of using headed coupling to connect large diameter CAM SEAs with steel rebar was investigated. Five large diameter CAM SEAs with headed ends were prepared: one 30 mm diameter and four 20 mm diameter samples. First, mechanical tests were performed on the five headed samples where each sample was coupled with one steel rebar at each end. Monotonic, incremental and constant strain cyclic loading was applied to simulate earthquake loading. The key mechanical properties were extracted and discussed. Second, microstructural analyses including electron backscatter diffraction (EBSD), metallographic imaging, Vickers hardness testing and fractographic evaluation were performed. The crystal orientation, phase composition and fracture surfaces were investigated to understand the effect of heading process on the stress induced martensitic transformation (SIMT), phase composition and failure of the headed CAM SEAs. It was found that the heading process only affects the near end portion of the specimen and it has no influence in the middle portion of the bars. Therefore, the strain recovery capacity of the CAM SEAs after heading was not reduced. Heading process led to high density of bainite phase precipitation at the end portions, which strengthened the sample. In order to uniformly strengthen the headed end of large diameter CAM SEAs, the key is to ensure a consistent cooling rate in the central and peripheral regions of the headed end after the heading process.