"Characterization of NiTi Shape Memory Damping Elements Designed for Automotive Safety Systems"

Actuator elements of NiTi shape memory material are more and more known in industry because of their unique properties. Due to the martensitic phase change they can revert to their original shape by heating when they have got an appropriate treatment. This thermal shape memory effect can show a significant shape change combined with a considerable force. Therefore such elements can be used to solve many technical tasks in the field of actuating elements and mechatronics and will play an increasing role in the next years, especially within the automotive technology, energy management, power and mechanical engineering as well as medical technology. Beside this thermal shape memory effect these materials also show a mechanical shape memory effect, characterized by a superelastic plateau with reversible elongations in the range of 8%. This behaviour is based on the building of stress-induced martensite of loaded austenite material at constant temperature and facilitates a lot of applications especially in the medical field.

Both shape memory effects are attended by energy dissipation during the martensitic phase change. This paper describes the first results obtained on different actuator and superelastic NiTi wires concerning their use as damping elements in automotive safety systems. In a first step the damping behaviour of small NiTi wires up to 0,5 mm diameter was examined at testing speeds varying between 0,1 mm/s and 33 mm/s upon an adapted tensile testing machine. In order to realize higher testing speeds a drop impact testing machine was designed that allows testing speeds up to 6000 mm/s. After introducing this new type of testing machine first results of vertical-shock tests of superelastic and electrically activated actuator wires are presented. The characterization of these high dynamic phase change parameters represents the basis for new applications for shape memory damping elements especially in automotive safety systems.