Date of Award

Spring 2018

Project Type

Thesis

Program or Major

Mechanical Engineering

Degree Name

Master of Science

First Advisor

Igor Tsukrov

Second Advisor

Marko Knezevic

Third Advisor

Judson DeCew

Abstract

Fatigue performance of structural materials depend on their microstructure and manufacturing processes. In this thesis, two different materials are investigated: additively manufactured Inconel 718 and a fiberglass reinforced vinylester composite.

The microstructural features of a direct metal laser sintered Inconel 718 are compared to a traditionally manufactured wrought Inconel 718. The effects of 3D printing direction and heat treatment are studied. Low cycle fatigue specimens manufactured by Turbocam International are tested at strain amplitudes from 0.6% to 1.4% engineering strain. These data are correlated to the microstructural features seen in the electron backscatter diffraction. High cycle fatigue specimens are tested at stress amplitudes ranging from 200 MPa to 1200 MPa. Images of the fracture surfaces are utilized to find trends in the ductile and brittle fracture areas on the surface to understand the material performance in fatigue. Strain-life and stress-life curves are generated for the low cycle and high cycle fatigue data, respectively.

The low cycle fatigue performance of fiberglass reinforced vinylester is also investigated. Specimens manufactured by HALO Maritime Defense Systems are subjected to tension-tension fatigue loading with maximum stresses ranging from 250 MPa to 350 MPa. A stress-life curve is generated and the results are compared to data from literature. It is observed that these composite materials have high uncertainty in fatigue performance so it is recommended to use a lower bound S-N curve for design purposes.

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