Cracking at ultrasonic frequency – Can ultrasonic fatigue testing mimic the failure of Nitinol stents in the body?

We aim to investigate superelastic Nitinol by deploying an novel in situ synchrotron ultrasonic fatigue (USF) testing technique. Cycling at 20 kHz frequency allows testing very high numbers of cycles relevant for implants within hours and will be used to investigate the progress of fatigue damage and failure. Combining USF with (XRD), we investigate the equivalence of USF loading to testing at conventional frequencies, the role of the cyclic austenite-to-martensite transformation for VHCF , and the mechanisms of crack initiation. The proposed experiment involves ultrafast, USF synchronized time resolved 2D XRD mapping and interrupted fatigue tests with XRD and computed tomography to observe damage localization and accumulation. This promises to accelerate material characterization and development, providing critical insights into the long-term reliability of Nitinol implants and addressing fundamental questions regarding fatigue mechanisms at very high numbers of cycles.