Due to numerous advantages, including high energy density, in addition to the small, required volume, low weight, and high biocompatibility, NiTi shape memory actuators offer the possibility of replacing conventional actuators. Besides the few mass production examples of pseudoplastic shape memory elements in medical technology and even fewer mass produced shape memory actuators, they are currently rarely used in large series, since the cost of a durability test in the development process is very high due to the low switch-off dynamics and the associated cooling times. To perform a complete durability test cycle, several months are necessary. On the other hand, a test of electromagnets of the same service life and performance class would only take three to five days. Therefore, shape memory technology is often not considered to be applicable in actuator development despite its tremendous advantages.
A particular obstacle in the development process is structural fatigue, which cannot be reliably estimated today. The shape memory effect and its change over the lifetime of the component depend significantly on the material composition, the heat treatment as well as the operating parameters, such as the applied load, the ambient temperature or the type of activation and there is still little information available on the extent to which these parameters influence the lifetime of the shape memory elements.
Further knowledge in the field of damage mechanisms would simplify and thus accelerate the development of shape memory actuators. For this purpose, two different series of experiments are carried out for this publication, comparing whether and how damage caused by changing mechanical stresses differs compared to changing current levels for activation on a metallographic level.
The main objective of this study is to reduce development time. As a first step an improved understanding of the relationships between the critical material-dependent parameters and the resulting damage to the NiTi shape memory elements will be established.
For this purpose, a test rig was constructed which is capable of reproducible and thus comparable tests in order to subsequently examine the stressed samples metallographically and assess the microstructure. The aim of the test environment was to create standardized, uniform test conditions by developing a standardized test method including measuring equipment adapted to the technical test conditions for smart memory actuator systems.
Based on various test series, analyses were carried out using conventional light microscopy and REM methods in order to investigate the differences between various stress levels within a test series as well as differences between the various test series, i.e., the influences of the changes in various parameters on the microstructure of the shape memory elements.