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

Carsten Wickmann,Manuela Sander,Kerstin Witte-Bodnar,Jan Schäfer,Horst Heyer,Christopher Benz

doi:10.3390/ma14185315

Carsten Wickmann, Manuela Sander + Show 4 more

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https://doi.org/10.3390/ma14185315

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Abstract

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.

Highlights

Crack initiation in the very high cycle fatigue regime for type II materials is frequently observed at non-metallic inclusions of different shapes and chemical compositions with a typical ‘fish-eye’ (FiE) formation
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
The size of an optical dark area’ (ODA) or granular bright facet’ (GBF), detected in optical microscopy and ‘scanning electron microscopy’ (SEM), respectively, does not necessarily correspond to the FGA, which refers to the existence of a nanograin layer below the fracture surface

Summary

Introduction

Crack initiation in the very high cycle fatigue regime for type II materials is frequently observed at non-metallic inclusions of different shapes and chemical compositions with a typical ‘fish-eye’ (FiE) formation. A characteristic area is formed around the crack initiating defect or structure during the fatigue loading. This area refers to ‘optical dark area’ (ODA) [3,4], ‘granular bright facet’ (GBF) [5], ‘rough surface area’ (RSA) or ‘fine granular area’ (FGA) [6] based on the identified characteristics or the microscopy method used. The size of an ODA or GBF, detected in optical microscopy and ‘scanning electron microscopy’ (SEM), respectively, does not necessarily correspond to the FGA, which refers to the existence of a nanograin layer below the fracture surface. According to the literature [10,11,12], at least a large part of the total lifetime (90% to 99%) during fatigue loading is assigned to the FGA formation, even though it is not clear whether the FGA is formed during crack growth [12,13,14,15] or crack initiation occurs at the size of the FGA [16,17,18], followed by crack growth in the fish-eye [10]

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