Investigation of Long-Time Aged Ni-Ti Shape Memory Actuator Wires for Future Use in Automotive Safety Systems

The use of shape memory alloys (SMA) as quick-changing actuators can simplify existing automotive systems performing the same function through new mechanisms with reduced size, weight and costs. One drawback for their use especially in automotive safety systems is the lack of material knowledge concerning the durability of the switching function. In order to investigate the long-time stability of SMA in these applications, extensive tests were carried out with Ni-Ti material intended to be used in a bonnet lifting system for pedestrian protection. Ni-Ti wires were exposed within the temperature range up to 140 degrees C for up to 1.560 days under tension and tested regarding their phase change temperatures, switching times and strokes. Furthermore, metallurgical investigations were carried out with the objective of studying the mechanism for the changes in the microstructure caused through the ageing. By means of this investigation it can be shown that e.g. the shape memory effect (SME) still exists even after the longest exposure period at the highest temperature and that the phase change temperatures were shifted to higher temperatures throughout the ageing. Concerning the contraction time of exposed wires no changes were measured in the fast activation of the specimens in the functional testing plant. By means of structural investigations and micro hardness measurements at different exposed material no differences could be detected.