Year

2020

Season

Summer

Paper Type

Master's Thesis

College

College of Computing, Engineering & Construction

Degree Name

Master of Science in Mechanical Engineering (MSME)

Department

Engineering

NACO controlled Corporate Body

University of North Florida. School of Engineering

First Advisor

Dr. Jutima Simsiriwong

Second Advisor

Dr. Paul Eason

Third Advisor

Dr. Alexandra Schonning

Department Chair

Dr. Osama Jadaan

College Dean

Dr. William Klostermeyer

Abstract

Additive Manufacturing (AM) techniques have recently gained popularity for fabrication of parts used in aerospace applications. Some of these parts may be subjected to cyclic loading at very high frequencies, leading to service life requirements exceeding ten-million cycles (>107 cycles). Therefore, understanding the very high-cycle fatigue (VHCF) behavior of these AM parts is an important step in their design and qualification processes. In this thesis, both high-cycle fatigue (HCF) and VHCF behaviors of Inconel 718, a Ni-base superalloy, manufactured via a Laser Beam-Powder Bed Fusion (LB-PBF) process, are investigated. Uniaxial, fully reversed force-controlled fatigue tests were conducted utilizing a ultrasonic fatigue test system operating at 20 kHz. Specimen fracture surfaces were analyzed using a scanning electron microscope (SEM). Fatigue test specimens were fabricated in two different build orientations, including vertical and 45 degrees (i.e., diagonal) with respect to the build plate, and subjected to a post-process solution annealing and aging heat treatment. In addition, specimens were testing in both the as-built and post-process machined condition. Fatigue response of LB-PBF Inconel 718 was compared to that of wrought Inconel 718.

Comparison of the stress-life (S-N) response of LB-PBF Inconel 718 fabricated in the vertical and diagonal build orientations showed almost no discrepancy in fatigue life for the as-built condition. For machined LB-PBF specimens, the effects of build layer orientation were more apparent: fatigue resistance of vertically oriented specimens was superior to diagonally oriented specimens. An increase in fatigue resistance of LB-PBF Inconel 718 was observed for specimens of the machined surface condition due to the removal of surface defects from the as-built surface. AM process induced defects significantly influenced fatigue crack initiation for as-built specimens: all fatigue cracks in as-built specimens originated from either surface micro-notches or sub-surface lack of fusion. Fatigue cracks in all specimens, regardless of build orientation and surface quality, were found to initiate from both specimen surfaces and sub-surface anomalies in the microstructure. Fatigue limit estimation was performed using Murakami’s approach, which accounts for AM process induced defects. Additionally, S-N curve fitting was performed using the Basquin equation. Despite the presence of defects seen on VHCF fractures of as-built vertical specimens, the Murakami model proved to be insufficient for the specimens used in this study.

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