Development of 3D-Printed, Controlled Porosity Deltoid Inserts for Composite T-Joint Structures Fabricated with Resin Infusion Processing

Date of Award

2023

Degree Name

Ph.D. in Chemical and Materials Engineering.

Department

Department of Chemical and Materials Engineering

Advisor/Chair

Donald A. Klosterman

Abstract

Liquid composite molding (LCM) processes are among the most commonly used techniques to manufacture advanced composite structures such as aircraft components, boat hulls, and wind turbine blades. These processes include resin transfer molding (RTM) for smaller parts and vacuum-assisted resin transfer molding (VARTM) for larger parts. These types of processes have several advantages, such as reduced cost, faster cycle time, and efficient part fabrication, compared to other available techniques for making composite material parts, most notably autoclave curing. RTM is increasingly used to produce composite finished parts for several applications, especially in the aerospace industry. It offers mass production of composite finished parts with simple or complex geometry and with small to medium sizes. This process involves the injection of liquid resin into a closed mold that contains a compacted layup of dry fabric. The fluid flow of resin tends to be unpredictable especially for complex geometry parts, and therefore some predictive method for resin flow inside the mold is desirable. In the current research study, additive manufacturing (AM) was used to develop manufacturing processes and tooling designs for the production of composite T-joints. The focus of the study was on the "deltoid" region of the T-joint where the web and flange meet, which is known to be a region of mechanical weakness for various reasons. The objective of this research is to develop a novel approach to solve the issue of filling the deltoid area and avoid any possible racetracking of resin. AM was used to develop custom deltoid inserts with controlled porosity using different infill patterns such as zig-zag and gyroid. These inserts were fabricated from polylactic acid (PLA) or carbon fiber reinforced PLA using a low cost fused filament fabrication process. The permeability of these 3D-printed controlled porosity structures was measured with a permeability apparatus involving the flow of vegetable oil under both dynamic and steady state conditions. Furthermore, AM was used to produce the RTM molds for small-lot production of the composite T-joints. This combined effort provided a comprehensive capability to rapidly and inexpensively test various concepts and iterate designs. Additional T-joints were made containing traditional strategies for filling the deltoid region, including inserting a piece of rolled up fabric ("noodle") and inserting no solid reinforcement so that it will fill completely with resin. A mechanical test fixture was developed to allow for mechanical testing in a universal load frame apparatus. This was used to measure the finished composite part's maximum pull-off strength and to recommend the best option for porosity and design of the 3D printed insert. Optical photographs were obtained of the deltoid region throughout the test to help illuminate the sequence of damage initiation and failure. Based on this study, the best infill density result came from 60% infill zigzag pattern using PLA only inserted material. This part demonstrated a failure load capacity at 27% higher than a deltoid filled with resin only.

Keywords

Aerospace Engineering, Materials Science, Composition

Rights Statement

Copyright © 2023, Author

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