Nitinol one-way shape memory springs: Thermomechanical characterization and actuator design

One-way shape memory effect (OWSME) in NiTi springs has been investigated in this work. The main goal is the definition of a guide-line for the design of a linear actuator for high cycles duty. Some SMA and steel springs with various geometrical features have been produced from wires with different diameters. SMA spring's behavior has been analyzed measuring the maximum length (austenitic condition, T>A(F) - austenite finish) under different applied loads. The measurement of this length has been performed at successive thermomechanical complete working cycles (150,5350,43,000 and 600,000) under a constant applied load necessary to full recovery in the martensitic phase. It has been found that the higher the thermomechanical cycles the lower the reached maximum length. In particular the length loss is negligible at higher cycles. Starting from these considerations and the knowledge of the mechanical characteristics of the springs, a linear actuator (SMA spring-harmonic steel spring) for high-cycle duty can be designed. The right working conditions have been verified too. (C) 2009 Elsevier B.V. All rights reserved.