Microscale strain accumulation during fatigue and fracture of additively manufactured Ti-6Al-4V

The goal of this thesis is to relate the different microstructural features in additively manufactured and conventionally manufactured Ti-6Al-4V alloys to the observed fatigue and fracture behaviors. Additively manufactured Direct Metal Laser Melted (DMLM) Ti-6Al-4V samples with two different build orientations and build layer thicknesses were compared to conventionally manufactured annealed Ti-6Al-4V samples. Optical images of the microstructure were recorded after these samples had been heat tinted in a laboratory oven. Using a modified single edge notch tension geometry, the samples were fatigue precracked then subjected to monotonic loading, followed by unloading and reloading. The Digital Image Correlation (DIC) measured plastic strain from fatigue precracking (i.e. plastic wake) was compared to the optical microstructural images. While the plastic wake was observed to be relatively uniform and symmetric about the crack line in the conventional samples, the prior β grain boundaries were seen to influence the size and shape of the plastic wake in the additively manufactured samples. Unlike the relatively straight precrack in the conventional Ti-6Al-4V, the fatigue crack in the additively manufactured samples was observed to periodically propagate parallel to the α’ laths and deflect at prior β grain boundaries. The fatigue crack also propagated towards voids observed on the surface of additively manufactured samples. Voids and other defects observed within the additively manufactured alloy could be responsible for the premature failure of the samples in several experiments. Needle-like regions of high strain that correlated to the α’ laths were observed in the additively manufactured samples, suggesting significant influence of the microstructure on the plastic zone. In one experiment, cracks nucleated at these regions of localized strain after subsequent reloading in additively manufactured Ti-6Al-4V.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Aerospace Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2020

Author dc:creator

• VanSickle, Raeann

Contributors dc:contributor

• Lambros, John
• Chew, Huck Beng

• Fracture
• Fatigue
• Additive Manufacturing
• Direct Metal Laser Melting
• Titanium
• Digital Image Correlation

Statement dc:rights

• Copyright 2019 Raeann VanSickle

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/106305

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/106305