Nonlinear elastic plastic load deflection curves in NiTi shape memory alloy helical coils

Abstract

Reversible deflection of coil spring due to shape memory effect was examined. Coil springs were prepared by winding the wire of Nickel -50.at.% Titanium alloy followed by the annealing at 800 C for 1h to set the coil shape. The Mf and Af temperatures were 30 C and 87 C and the spring was deformed in the fully martensite state at room temperature. A tough pitched spring with the coil diameter of 6.6 mm and the element diameter of 0.6 mm showed shape memory effect after extension larger than 1,000 %. The load-deflection curves measured showed nonlinear relationship with rapid increase of load at a certain extension. It was found that the shape memory strain of coil spring is not limited by that of element wire but by the geometry of coil spring against the extension. Noted that and the deflection was accompanied with the rotation around the coil axis. Elastic plastic deflection analysis was performed, and the prediction of the theory was compared with the experimental curves.