Ductile Fracture of Laser Powder Bed Fusion Additively Manufactured Ti-6Al-4V

Date of Award

2021

Degree Name

M.S. in Mechanical and Aerospace Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Robert L. Lowe

Abstract

Understanding the mechanical performance of additively manufactured aerospace metals is of paramount importance for the development of new structural materials and next-generation aerospace components. Of particular interest in this investigation is the titanium alloy Ti-6Al-4V produced using laser powder bed fusion (LPBF), and its structural integrity in the event of sudden aerospace-related impacts. These impacts, known as foreign/domestic object damage, can cause catastrophic damage to aircraft components and serious injuries to passengers. To enable simulation-aided design and analysis for foreign/domestic object damage survival, the overarching goal of this project is to develop a stress-state-dependent ductile fracture model for LPBF Ti-6Al-4V that is representative of the material's behavior across a broad range of loading conditions. The foundational steps presented in this thesis include the design of an experimental program to calibrate the LPBF Ti-6Al-4V fracture locus, a three-dimensional representation of the variation of equivalent plastic strain at fracture with stress triaxiality and Lode parameter, commonly utilized in continuum damage models. The proposed experimental program provides a family of mechanical tests that employ different specimen geometries (e.g., notched plane stress, plane strain, axisymmetric, and thin-walled tube specimens) and loading conditions (e.g., tension and torsion) to access a broad window of stress states (triaxiality and Lode parameter combinations). Six axisymmetric tension specimens were chosen from the aforementioned family of candidate specimen designs for subsequent mechanical testing and stress state analysis. The grain morphology and porosity of the LPBF-printed Ti-6Al-4V material was characterized using electron backscatter diffraction (EBSD) and X-ray computed tomography (XCT), respectively. Quasi-static mechanical testing was performed on a servo-hydraulic load frame, with full-field surface strains measured using three-dimensional digital image correlation (DIC). The effective plastic strain (EPS) at fracture for each test (thirty total, with five runs for each of the six specimen geometries) was obtained through direct DIC measurements and the customary plastic incompressibility assumption. Traces and weighted average values of the stress state parameters (triaxiality and Lode parameter) over the plastic deformation history were obtained from parallel numerical simulations of each test. Acceptable agreement between simulation and experiment -- as quantified by comparing force-displacement and principal strain-displacement curves -- was observed across the majority of the test series. However, a general trend was noticed across the force-displacement plots where the simulations predicted sometimes meaningfully higher yield stresses and ultimate tensile stresses than the experiments. This discrepancy is attributed to the material constitutive modeling and/or specimen metrology. Arithmetic averages of the stress state parameters and EPS at fracture were reported across the five tests performed for each specimen geometry. A trend of the EPS at fracture (ductility) decreasing with an increasingly negative (tensile) triaxiality is observed, consistent with a significant body of previous literature in the ductile fracture community, including previously published results on wrought Ti-6Al-4V. The research presented in this thesis complements the few existing studies on ductile fracture of LPBF Ti-6Al-4V, providing additional fracture data that can be used to calibrate tabulated or parameterized ductile fracture models used in predictive simulations of aerospace-related structural impacts.

Keywords

Aerospace Engineering, Aerospace Materials, Engineering, Mechanics, Mechanical Engineering

Rights Statement

Copyright © 2021, author

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