Investigation on recovery stress and stability of hot-drawn Ni₄₇Ti₄₄Nb₉ SMA

Abstract

Ni47Ti44Nb9 shape memory alloys (SMA) were applied to bridge pier column to resist earthquake loads by the force of recovery stress over the past decades. However, the application of hot-drawn SMA on pier column reinforcement is hardly researched despite the advantages of cost savings and simplifying service process compared to annealed SMA. Aiming at exploring the usability of hot-drawn SMA on the reinforcement, this paper focused on certain behaviors about pre-strain, thermal activation and partial recovery of recoverable strain that influence the capacity of constraint SMA to gain recovery stress, and then researched recovery stress stability and tensile properties of constraint SMA under cyclic loads both at room temperature and low temperature. Test pieces adopted in this study were 2 mm diameter hot-drawn NiTiNb SMA wires. All setups involved in the tests were self-developed. The states of recovery stress and its stability were mainly evaluated by three tests: uniaxial tensile test, thermal activation test and cyclic loading test. The results marked that hot-drawn SMA could obtain sufficient recovery stress when deformed at room temperature. Moreover, the stability of recovery stress under cyclic loading and low temperature could satisfy the demands of pier column reinforcement for practical use.