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Title:Dual scale porosity effects on crack-defect interactions in additively manufactured Ti-6Al-4V
Author(s):Muro-Barrios, Raymundo
Advisor(s):Chew, Huck Beng; Lambros, John
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Additive manufacturing
Numerical methods
Dual-scale porosity
Modified boundary layer
Void defects
Additive manufacturing defects
Gurson yield criterion
Fracture process Zone
Fracture mechanics
Ductile fracture
Abstract:Microstructural defects and unpredictable fracture behavior have limited the widespread use of additively manufactured (AM) alloys in load bearing components. In addition to background pores (2-12 μm) nucleated from particle inclusions responsible for ductile fracture in conventional metals, larger defects (20-50 μm) can be introduced during the additive manufacturing process resulting in a dual-scale porosity failure process in AM alloys. The effect of these AM defects on the fracture behavior of AM Direct Metal Laser Melted Ti-6Al-4V has been previously observed in Scanning Electron Microscopy and Digital Image Correlation analyses, which suggest AM defects lead to the premature failure in fracture and fatigue of AM metals. However, the specific failure mechanisms associated with the AM defects have not been identified. In this thesis, a numerical approach is undertaken to quantitatively elucidate the role of the dual-scale porosity and resulting crack-defect interactions in AM Ti-6Al-4V alloys. A small-scale yielding, modified boundary layer model with imposed monotonically increasing K_I (Stress Intensity Factor) remote displacement loading was used to study crack propagation through a local distribution of dual size-scale voids. The Gurson yield function was implemented to model the background porosity while the larger AM defects were explicitly represented. Micrographs were taken of physical AM Ti-6Al-4V specimen cross-sections to determine the expected size and frequency of AM defects. Fracture resistance curves were generated for random AM void distributions with increasing levels of AM defects. Over and underperforming material samples with off-nominal fracture resistance were analyzed in more detail through observation of 3D void interactions in cross-sectional model images. It is shown that AM defects activate isolated and clustered damage zones ahead of the crack tip, blunt the crack tip, promote crack tortuosity, and at times appear to increase the local material toughness over a conventional alloy. Conversely, planar clusters of AM defects can form preferential crack planes that may be responsible for the premature failure of AM components. Inclusion of the AM defects also generates more opportunities for localized dissipation of plastic work, which suggests the potential for achieving “fracture-by-design” through strategic void placements. Preliminary materials design concepts resulting in significantly improved apparent fracture toughness over conventional alloys are discussed.
Issue Date:2021-04-30
Rights Information:Copyright 2021 Raymundo Muro-Barrios
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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