Impedance based Fatigue Monitoring of Shape Memory Alloys in elastocaloric Applications

When using superelastic shape memory alloys (SMA) in elastocaloric applications where the crystallographic phase transformation is triggered by cyclic tensile loading and unloading the fatigue life of the SMA plays an important role. Over the lifetime of the material inclusions on its surface lead to microcracks which grow the more tensile cycles are applied. At the end of the SMA lifetime the cracks lead to fracture over the complete cross section area causing a failure. The growing microcracks are expected to show a significant influence on the surface of the material over its lifetime, which could be measured by using impedance analysis.

This assumption is based on the skin effect which describes the frequency dependent skin depth of the measurement current used for impedance analysis. At high measurement frequencies above 100 MHz and NiTi as SMA material the skin depth decreases significantly to few micrometers, so that the influence of the microcracks on the impedance signal increases. To measure this influence, an impedance analyzer is added to our elastocaloric test setup so that the impedance can be determined in every cycle of the lifetime experiment.

During the experiment evaluation, a data driven approach identifies the correlation of features calculated from the impedance signals to the lifetime of the material. Based on the best correlating features a linear discriminant analysis is used to separate the cycles in groups of 10% lifetime so that information about the material condition can be derived which is presented in this contribution.