Session: 04-07: Shape Memory Alloy Enabled Mechanisms II

Paper Number: 90335

90335 - Characteristic Value-Based Design System for Shape Memory Springs 

Shape memory alloys (SMA) represent thermosensitive materials that change their geometry in dependence upon reaching defined temperatures. Due to these characteristics, actuator springs made of SMA are of particular importance, as they can realize large forces and strokes at precise switching temperatures. In particular, the energy-self-sufficient mode of operation, like the activation by the ambient medium, predestines SMA springs for self-regulating applications, for example in the field of thermal management. However, the development of these actuators represents a complex iterative process, since there is neither a suitable design system nor standardized methods for determining the required material characteristics for SMA springs. The resulting costs and development times hinder the widespread usage in suitable industrial applications. 

In order to enable and facilitate the access to this technology for small and medium-sized enterprises, the project aims to develop an industrial-suited design systematology. In this poster, we present measurement methods for the characterization of semi-finished SMA material parameters such as the elastic modulus, Poisson's ratio and the activation temperatures. The experimental results form the basis for creating a material model for the finite element method (FEM) simulation of SMA properties. Unfortunately, the interdependency of the geometry of a linear formed SMA element and the material parameters and the actuator performance cannot be transferred without further ado to SMA springs. Due to the implemented internal stressed resulting from the winding the material parameter such as the transition temperatures change which in turn directly affects the actuator performance. 

Therefore, in the next step based on the validated material characteristics, the model of a SMA coil spring with constant parameters such as outer diameter and slope is implemented into the simulation. For the validation of the model validation, a set of coil springs made of previously analysed semi finished SMA is manufactured, whereby the characteristic values of the springs originate from current industrial applications. An outlook on future works shows the procedure for alternative spring geometries such as wave springs and torsion springs. The experimentally validated geometry models are used to determine cause-effect relationships between the geometry of the SMA-spring (outer diameter and slope) and material parameters (transition temperature) and the actuator performance (realizable force and stroke) depending on the lifecycle via design of experiments. The identified relationships enable the development of a design tool that allows users without specific expertise to take advantage of SMA and to design SMA springs as easily as conventional steel springs. 

Presenting Author: Kenny Pagel Fraunhofer IWU