Very high cycle fatigue (VHCF) response of an Additively manufactured Nitinol

This work investigates the Very High Cycle Fatigue (VHCF) response of an Additively Manufactured (AM) Nitinol alloy. To resemble in-service loading conditions of typical Nitinol components, an innovative experimental setup has been proposed, where the martensite/austenite transformation fraction is controlled by applying a mean static compressive load through an electrodynamic machine, and the partially transformed material is fatigued up to 109 cycles through an ultrasonic machine. Hourglass-shaped hollow specimens have been designed by means of a Finite Element (FE) iterative procedure to account for the material phase transformation with applied stress. The FE model has been optimized by considering the displacement field acquired with a high-speed camera, to ensure a reliable computation of the applied stress amplitude. With the proposed setup, the partially transformed AM Nitinol alloy has been subjected to VHCF tests. FE analyses on Representative Volume Elements (RVEs) with defect size determined through µ-CT scans have been also performed to analyze the failure driving force.