ABSTRACT Investigation of the Impact Resistance of Carbon Nanotube Forests within Carbon Fiber Laminated Composites through Experimental Confirmation and Finite Element Substantiation John E. Christoph II, M.S.M.E. Mentor: David A. Jack, Ph.D. Carbon fiber laminated thermoset composites have become the industry standard for applications dictating a high strength-to-weight ratio. However, their brittle nature limits energy dissipation characteristics, often leading to catastrophic failure under low energy impact loadings. In an effort to increase the energy dissipation of these structures, this research examines the potential effects of including in-house synthesized vertically aligned multi-walled carbon nanotube forests within a layered laminate structure using the VARTM technique. Drop tower tests similar to ASTM D7136 are performed on fabricated laminates, including those with alternate insert materials and different resin cures for an added level of evaluation. A linear elastic finite element model with orthotropic properties from micromechanics theory is developed in COMSOL Multiphysics to compare with the experimental tests. Though the nanotube inserts provide some improvement from the base carbon fiber structure, it is anticipated these structures will be more useful as an impact sensor. Investigation of the Impact Resistance of Carbon Nanotube Forests within Carbon Fiber Laminated Composites through Experimental Confirmation and Finite Element Substantiation by John E. Christoph II, B.S. A Thesis Approved by the Department of Mechanical Engineering William Jordan, Ph.D., Chairperson Submitted to the Graduate Faculty of Baylor University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Mechanical Engineering Approved by the Thesis Committee David A. Jack, Ph.D., Chairperson Jordan R. Raney, Ph.D. Douglas E. Smith, Ph.D. Ian A. Gravagne, Ph.D. Accepted by the Graduate School August 2015 J. Larry Lyon, Ph.D., Dean Page bearing signatures is kept on file in the Graduate School. Copyright © 2015 by John E. Christoph II All rights reserved TABLE OF CONTENTS LIST OF FIGURES ........................................................................................................... vi LIST OF TABLES ............................................................................................................. xi ACKNOWLEDGMENTS ................................................................................................ xii CHAPTER ONE Introduction ............................................................................................ 1 1.1 Motivation and Vision .......................................................................................... 2 1.2 Thesis Statement and Overview ............................................................................ 4 CHAPTER TWO Literature Review .................................................................................. 9 2.1 Purpose .................................................................................................................. 9 2.2 Ballistic Testing Studies ..................................................................................... 13 2.3 Kevlar® Impact Studies ...................................................................................... 16 2.4 Carbon Fiber Impact Studies .............................................................................. 20 2.5 Carbon Nanotube Studies ................................................................................... 24 2.6 Results of Literature Review............................................................................... 42 CHAPTER THREE Material Synthesis, Laminate Fabrication and Materials Testing ... 44 3.1 Carbon Nanotube Manufacturing ....................................................................... 44 3.2 Experimental Results: Carbon Nanotube Forests ............................................... 63 3.3 Laminate Manufacturing ..................................................................................... 76 3.4 Drop Tower Testing ............................................................................................ 93 CHAPTER FOUR Finite Element Model....................................................................... 115 4.1 Introduction ....................................................................................................... 115 iv 4.2 COMSOL Impact Model Development ............................................................ 117 4.3 Drop Tower Test Simulation ............................................................................ 120 4.4 Final Model Results .......................................................................................... 129 4.5 Comparison of Modeling Results with Experimental Test Results .................. 143 4.6 Reexamination of Impact Model through 1-D Spring Mass Model ................. 143 CHAPTER FIVE Conclusions and Future Work ........................................................... 147 5.1 Conclusions ....................................................................................................... 147 5.2 Future Work ...................................................................................................... 152 BIBLIOGRAPHY ........................................................................................................... 163 v LIST OF FIGURES Figure 1.1 - (a) Sandwich Composite and (b) Armorcore® Panel Following Ballistic Testing......................................................................................................................... 3 Figure 1.2 – Illustration of Proposed Carbon Fiber/VACNT Composite Structure ........... 4 Figure 1.3 – Carbon Fiber and VACNT Laminate (a) and (b) Compared with a Carbon Fiber Laminate (c) and (d) Following Ballistic Test. ................................................. 6 Figure 2.1 – X-ray Image of Dragon Skin® Armor [9] .................................................... 10 Figure 3.1 – (a) Scanning Electron Microscope (SEM) Images of Nanotubes and (b) Vertically Aligned Nanotubes Synthesized On Substrate ........................................ 44 Figure 3.2 – Synthesis Diagram ........................................................................................ 45 Figure 3.3 – Ferrocene and Toluene Solution after Mixing. (Note the Small Amount of Ferrocene Solid Indicated by the Arrow along the Bottom of the Beaker) .............. 46 Figure 3.4 – (a) Carbolite Horizontal Split Tube (HST) 12/200 furnace and (b) the NE- 4000 Programmable 2 Channel Syringe Pump Utilized in the Research ................. 47 Figure 3.5 – Luer Locks That Connect Syringes to Tubing ............................................. 47 Figure 3.6 – Custom Quartz Tubes ................................................................................... 48 Figure 3.7 – Substrate breakdown: (a) Part One & (b) Part Two ..................................... 50 Figure 3.8 – Post Synthesis inside Furnace Tube Viewed From Exit of Tube ................. 50 Figure 3.9 – Complete Synthesis Setup underneath Fume Hood ..................................... 51 Figure 3.10 – SEM Imaging of Nanotube Samples .......................................................... 51 Figure 3.11 – Stainless Steel Connectors Linking the Syringes and Mass Flow Controller with Slight Angle for Continuous Downward Flow ................................................. 59 Figure 3.12 – Old (a) and New (b) Furnace Tube End Comparison................................. 60 Figure 3.13 – Substrate Removal Using Knife from (a) Side View and (b) Top View Showing Downward Direction Resulting in the (c) Final Sample ........................... 62 vi Figure 3.14 – HF Sterolithography Process Adapted From Chai et al. [47] .................... 63 Figure 3.15 – CNT Sample with Area Measured using Dino-Lite Desktop Microscope . 67 Figure 3.16 – Example CNT Compression Stress vs. Strain Plot of CNT Compression . 69 Figure 3.17 – Instron System with Blowup of Compressive Test Setup .......................... 69 Figure 3.18 – Compressed Sample Desktop Microscope Image at 18x ........................... 71 Figure 3.19 – CNT Compressive Stress vs. Strain of Sample in Figure 3.18 ................... 72 Figure 3.20 – Loading Stress versus Strain of CNT Samples........................................... 73 Figure 3.21 – Stress versus Strain Comparison Between Sample from Current Study to a Curve from Raney et al. [46] .................................................................................... 76 Figure 3.22 – First Attempt at VARTM with Insert ......................................................... 79 Figure 3.23 – (a) Microscopy of CNT/Carbon Fiber Structure with Following Attempted Resin Infusion and (b) Microscopy of Different CNT/Carbon Fiber Structure ........ 81 Figure 3.24 – Placement of the Insert Relative to Ply Stack ............................................ 81 Figure 3.25 – Spiral Tubing Placement Prior To Peel-Ply ............................................... 84 Figure 3.26 – Placement of Peel-Ply and Mesh ................................................................ 84 Figure 3.27 – Sealed Tool Prior to Vacuum Test ............................................................. 85 Figure 3.28 – Sealed Tool with Vacuum Applied ............................................................ 86 Figure 3.29 – Example Laminate (a) Top and (b) Bottom Following Post-Cure ............. 90 Figure 3.30 – Laminate (a) Top and (b) Bottom