SELF-HEALING OF IMPACT DAMAGE IN VASCULAR FIBER-REINFORCED COMPOSITES BY KEVIN RICHARD HART DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Aerospace Engineering in the Graduate College of the University of Illinois at Urbana-Champaign, 2016 Urbana, Illinois Doctoral Committee: Professor Scott White, Chair Professor Nancy Sottos Professor Philippe Geubelle Professor John Lambros ABSTRACT Vascular fiber-reinforced composites mimic biological systems to allow pluripotent multifunctional behavior in synthetic engineering materials. In this dissertation, methods are explored for recovering mechanical performance of a composite material after an out-of-plane impact event using vascular self-healing technologies. To date, the critical damage modes which occur during out-of-plane impact that most significantly contribute to reductions in post- impact performance have not been identified and methods for delivering healing agents to those critical regions using internal vascular networks has not been explored. In this dissertation, out-of-plane impact damage is quantified and correlated with reductions in post- impact mechanical performance. Additionally, healing of impact-induced damage is demonstrated using vascular delivery of epoxy and amine based agents. An alternate healing agent chemistry for potential use in vascular healing schemes is also discussed. This work is the first to detail methods for healing impact-induced damage in vascular composites using segregated healing agent components and paves the way for the adoption of self-healing vascular materials in commercial applications. ii ACKNOWLEDGEMENTS I’m extremely fortunate in my life to be surrounded with amazing people. Adversity presents itself in many ways throughout the course of a graduate career and without the presence of great people in my life, I would not have completed this journey. Some were there to help me as I questioned my decision to go to and stay in graduate school; some were there as I navigated difficult coursework; some were there for research insights and discussions; some were there for tedious sample fabrication; and some were there when I wanted to get away from my work and focus on drinks, laughter, and stress relief. For that, I thank each one of you. “If I have seen further it is by standing on the shoulders of Giants” - Isaac Newton It’s impossible to list the names of everyone whose shoulders I’ve stood upon, as each interaction – ‘big’ or ‘small’ – has shaped my path and to me weighs equally in the sum of my collective experience. I simply wish to provide my utmost gratitude for each and every one of those experiences as I would not be the person I am today without them. I have truly enjoyed my time at the University of Illinois. I have the people around me to thank for that and I will cherish the memories I have made here with all of you as I move forward in my life. Thank you. iii Table of Contents CHAPTER 1: INTRODUCTION .......................................................................................................... 1 CHAPTER 2: IMPACT OF 2D AND 3D WOVEN COMPOSITES, PART I: DAMAGE MECHANISMS AND CHARACTERIZATION...................................................................................................................... 19 CHAPTER 3: IMPACT OF 2D AND 3D WOVEN COMPOSITES, PART II: POST-IMPACT MECHANICAL RESPONSE............................................................................................................... 47 CHAPTER 4: REPEATABLE HEALING OF DELAMINATION DAMAGE IN VASCULAR COMPOSITES BY PRESSURIZED DELIVERY OF HEALING AGENTS ............................................................................. 65 CHAPTER 5: SELF-HEALING OF IMPACT DAMAGE IN VASCULAR FIBER-REINFORCED COMPOSITES ................................................................................................................................. 89 CHAPTER 6: REPEATABLE SELF-HEALING OF AN EPOXY MATRIX USING IMIDAZOLE INITIATED POLYMERIZATION ....................................................................................................................... 107 CHAPTER 7: CONCLUSIONS AND FUTURE DIRECTIONS ............................................................. 135 APPENDIX A: GROWTH OF A HIERARCHICAL NETWORK IN EPOXY VIA ELECTRICAL TREEING .. 143 APPENDIX B: VASCULARIZATION OF METALLIC MATRICES USING SACRIFICIAL PRECURSORS . 163 APPENDIX C: METALLIC COATING OF MICROCAPSULES ............................................................ 175 APPENDIX D: VACUUM ASSISTED RESIN TRANSFER MOLDING OF AN IMIDAZOLE LADEN GLASS/EPOXY COMPOSITE ......................................................................................................... 182 APPENDIX E: HEALING OF IMPACT DAMAGE USING SEGREGATED TWO-PART DELIVERY OF HEALING AGENTS ........................................................................................................................ 186 REFERENCES ................................................................................................................................ 197 iv CHAPTER 1: INTRODUCTION 1.1 Global Adoption of Fiber-Reinforced Polymer Composites Fiber-reinforced polymer (FRP) composite material use is on the rise. In 2013, the global carbon fiber reinforced plastic (CFRP) market was valued at approximately $1.85B and is expected to reach approximately $3.60B by 2019, representing a compound annual growth rate of approximately 11.9% [1]. As the demand for renewable sources of energy increases and the cost of non-renewable fuels rises, government regulations and financial viability have promoted the use of light-weight composites. Consequently, the superior mechanistic performance per weight of FRPs has overtaken traditional engineering materials (metals and ceramics) in many commercial applications. Penetration of FRPs has been most prevalent in the aerospace, defense, wind energy, automotive, electronics, sports equipment, marine, and civil engineering sectors of the economy [1]. Most notably, the commercial deployment of the Boeing 787-8 Dreamliner in October of 2011 demonstrated the economic viability of FRP materials and represented a landmark progression in the adoption of FRPs on a global level. Since the deployment of the Dreamliner, globalization of composites continues into the present as companies like BMW (automotive) [2], Kenway Corp. (marine) [3], and GE (wind energy) [4] have recently reported significant allocations of funding to composite material research and development. 1 1.2 Impact Damage of Fiber-Reinforced Polymer Composites With the global adoption of composites comes a greater understanding of their limitations and drawbacks. In particular, the continuous matrix of composite materials typically consists of a brittle thermosetting resin which leaves the structure susceptible to crack initiation and growth. As a result, composite materials are particularly prone to out-of-plane impact events. Out-of-plane impact events initiate elastic waves from the site of impact. Material response and damage formation during wave propagation is influenced by not only the magnitude of the energy delivered, but also by the velocity of the impingent projectile [5]. Graphically, this is represented in Figure 1.1. For very short impact times (on the order of the wave speed through the thickness) the response is dominated by the propagation of dilatational p-waves (Fig. 1.1a) [5]. High velocity (1000 m/s or more) ballistic impact represents this type of impact damage [6]. For moderate velocity impact events (ca. 10-100 m/s) the response is dominated by shear (flexural) waves (Figure 1.1b) [5]. Hail strikes represent this type of damage. Finally, for impact speeds of ca. 1 m/s or less, the speed of the impactor is typically much faster than the wave speed through the material and quasi-static responses dominate. This regime is colloquially referred to as “low-velocity” impact and often occurs when the mass of the impactor is larger than the mass of the target. Tool drops on composite structures are representative of this type of damage and it is this type of damage which we focus on in this dissertation. In the case of high velocity ballistic impact, damage is often easily detectable and quickly remedied. However, with lower velocity impact strikes the damage is often sub-surface making it more difficult to detect and imperceptibly dangerous. 2 Six major damage types are observed in composites subject to low velocity out-of-plane impact: transverse shear cracks, matrix tensile cracks, interply delamination, fiber pull-out, fiber rupture, and penetration. The cross-section of an impacted composite depicting some of these damage types is provided in Figure 1.2. In this dissertation, we restrict impact energies and impact velocities to relatively low values to minimize fiber rupture and impactor penetration. This allows a greater focus on the recovery of sub-surface matrix damage. 1.3 Effect of Composite Architecture on Impact and Post-Impact Response 1.3.1 Effect of fabric architecture on impact response Fiber architecture plays a significant role on damage formation and post-impact mechanical performance of impacted composite plates and beams. For example, composites with stacking sequences in which the angle variation from ply to ply is large show greater resistance to transverse and inter-ply crack propagation as a result of increased fiber bridging and crack deflection [7–13]. Composites made with woven fabrics have also shown increased