Durability of Nitinol for Structural Applications

Abstract

Nitinol is a shape memory alloy that has recently been studied for its suitability as a material in energy dissipation devices in structures. The alloy has excellent structural capabilities in terms of strength and elasticity, but it has not been evaluated in terms of durability issues such as corrosion resistance. This thesis investigates the corrosion-related durability of nitinol in environments that are commonly corrosive to civil structures. Furthermore, this research compares the corrosion performance of nitinol to that of A992 and A588 steel, since steel is the metal most abundantly used in structures and is in many ways similar to nitinol. The ultimate objective of this research is to determine whether nitinol is more or less resistant to corrosion than structural steel. If nitinol is shown to have better corrosion resistance than structural steel, it can safely be used in the same environments in which structural steel is currently used. Since laboratory corrosion test results cannot be easily correlated to real-life performance, this research studies steel and nitinol in identical testing conditions and compares the performance of the two. The procedure for corroding the specimens is based on ASTM International standards. After the specimens are corroded, their mechanical performance is evaluated by tension testing. Additionally, the solutions used to corrode the specimens are analyzed as another means of comparing corrosion performance. The combination of results is used to determine whether or not nitinol is suitable as a structural engineering material, based on strength degradation due to corrosion. Testing performed for this research indicates that nitinol resists corrosion much better than structural steel of grades A992 and A588. Therefore, nitinol can be used in the same environments in which these grades of steel are currently used, without undue risk of failure due to corrosion. Based on the results of this research, it can be concluded that nitinol is a safe material for structural engineering in terms of corrosion resistance.