Internal Crack Initiation and Growth Starting from Artificially Generated Defects in Additively Manufactured Ti6Al4V Specimen in the VHCF Regime

The aim of the present work was to investigate the 'fine granular area' (FGA) formation based on artificially generated internal defects in additively manufactured Ti6Al4V specimens in the early stage of fatigue crack growth in the 'very high cycle fatigue' (VHCF) regime. Fatigue tests were performed with constant amplitude at pure tension-compression loading (R = -1) using an ultrasonic fatigue testing setup. Failed specimens were investigated using optical microscopy, high-resolution 'scanning electron microscopy' (SEM), and 'focused ion beam' (FIB) techniques. Further, the paper introduces alternative proposals to identify the FGA layer beneath the fracture surfaces in terms of the 'cross section polishing' (CSP) technique and metallic grindings with special attention paid to the crack origin, the surrounding microstructure, and the expansion of the nanograin layer beneath the fracture surface. Different existing fracture mechanical approaches were applied to evaluate if an FGA formation is possible. Moreover, the results were discussed in comparison to the experimental findings.