Graduation Semester and Year

2021

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Robert Taylor

Abstract

Fused filament fabrication (FFF) show high anisotropy and reduced mechanical properties as compared to conventional manufacturing techniques. This happens due to the poor interlaminar bonding between the layers, as a result of premature halt of bond healing process. Previous research works show multiple approaches to improve the bond quality to enhance the mechanical properties of FFF parts, however these are complex and expensive techniques. In this work a novel print head assembly is presented to offer a simple and effective solution by applying a thermal field to the part as it is being printed to preheat the previously deposited layer, thus improving the interlaminar bonds and eventually the mechanical properties of the parts. This assembly is compatible with most of the commercial FFF printers with no major modification, unlike other existing technologies. An optimized print head assembly was developed to improve the Fatigue and Fracture properties of FFF parts while minimizing the mass of the block to provide maximum enhancement of the properties while reducing geometric distortions. A design of experiments approach has been used to identify the main effects and interaction effects between the two factors (Plate Thickness and Nozzle Height) with three levels each for five response variables (Increase in Number of Cycles to Failure, Increase in Fracture Toughness, Decrease in Width, Increase in Thickness and Increase in Skewness). The DOE shows that the nozzle height and plate thickness main effects are present for all response variables. A localized cooling mechanism has been provided to cool the upper sections of the print head to prevent filament softening and save from clogging and print failure. Parts printed with the optimized print head shows good correlation with the DOE analysis with major improvements in mechanical properties and less geometric distortion of the FFF parts. In conjunction, mesostructured analysis of these parts showed a transformation in the void shape from diamond to circular indicating that these voids have much lower stress concentration rather than failure initiation points, supporting the experimental data. Meso-structure analysis of fractured surface was done to understand the change occurring at the failed surfaces.

Keywords

Additive manufacturing, FFF, Fatigue, Fracture toughness, DOE

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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