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Modeling and Characterization of Interfacial Stress Transfer in Mechanoluminescent Modeling and Characterization of Interfacial Stress Transfer in Mechanoluminescent Smart Particulate Polymer Composites Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Srivatsava Krishnan Graduate Program in Mechanical Engineering The Ohio State University 2020 Dissertation Committee Prof. Vishnu Baba Sundaresan, Advisor Prof. Ahmet Kahraman Prof. Ryan Harne Copyrighted by Srivatsava Krishnan 2020 ii Abstract This dissertation focuses on the development of mathematical models for stress distribution and transfer in smart particulate polymer composites and its application for structural prognosis and health monitoring. Contemporary mathematical models for polymer composites, especially particulate composites, are mainly focused on predicting macroscopic behavior and fall short of estimating of stress transferred to particulate phase. The mechanical interactions between particulate and matrix phases are extremely significant for the composite if the particulate phase exhibits smart functional behavior. To address this knowledge gap, interfacial mechanics is modeled by a cohesive zone model (CZM) and its parameters are estimated through direct experimental methods. This mathematical framework is applied to predict stress transferred to particles and model modulus relaxation in elastomeric mechanoluminescent composites. Consistent experimental evidences presented in this dissertation suggest that light emission from elastomeric mechanoluminescent composites may correlate with the structural health of the composite. It is hypothesized that health degradation in particulate composites occurs through damage accruing at the interface between the particles and the matrix. It is also hypothesized that interfacial damage leads to reduced stress transfer to the particulate which in the case of EML particles results in reduction of emission intensity. Validation of i these hypotheses will prove that EML emission reflects on the health state of the elastomer composites. Towards this objective, the interface is modeled by a CZM which defines it as a softening 푛푡 spring with an initial undamaged stiffness (푘휎 ). Damage softens the interface by ̃ 푛푡 degrading its stiffness at a rate (푘휎 ) defined by the CZM model. The interface CZM 푛푡 ̃ 푛푡 parameters 푘휎 and 푘휎 are obtained by a combination of theoretical and experimental approaches and validated by numerical methods. Simulations of macroscopic composite behavior under low strain-rate monotonic loading and high strain-rate cyclic loading prove that the CZM model accurately captures modulus relaxation, hence proving interfacial damage is the mechanism of fatigue in particulate composites. Interfacial damage also reduces stress transfer to the EML particulate phase establishing that EML emission reflects on the health of the composites. The SHM capability of EML particulates can be extended to carbon-fiber composites and adhesive bonded joints since the mechanism of interfacial damage is similar. ii Dedication To Amma and Appa iii Acknowledgments I would like to acknowledge Honda R&D Americas Inc., Toyota Research Institute and NSF I/UCRC Smart Vehicle Concept Center for supporting this research. I would particularly like to thank Nichole Verwys, Katy Richardson, Anthony Smith and Duane Detwiler of Honda R&D and Shailesh Joshi and Umesh Gandhi of Toyota Research Institute for providing valuable technical and non-technical inputs and feedback. I would like to thank my advisor Dr. Vishnu Baba Sundaresan for all the guidance, encouragement and mentorship these past four years. I am thankful to him for letting me learn by making mistakes and for his acceptance of my odd work hours. I am truly grateful for all that I’ve learnt working with him. I thank Dr. Ahmet Kahraman and Dr. Ryan Harne for serving on my examination committee. I’d like to thank Dr. Ezekiel Johnston-Halperin for access to his film fabrication facilities and to Dr. Justin Young for training me on them. I am immensely grateful to Dr. Noriko Katsube for her help and guidance with my research. I cannot thank her enough for her dedicated involvement, technical suggestions and helpful discussions. It has been a great pleasure working with my fellow grad students in the lab, Vinithra Venugopal, Prasant Vijayaraghavan, Travis Hery, Paul Gilmore, Vijay Venkatesh, Parker iv Evans and Sujasha Gupta. I thank them for all the helpful distractions and discussions. Over the years, I’ve come to see them as family and I will cherish their friendship. I thank my friends Ayush, Ramprakash, Asvin, Sripad, Mukil, Vinay and many others for making Columbus home. I can’t thank Aparna enough for being the pillar of my support structure. Lastly, I’d like to thank my parents for letting me choose my path and supporting me unconditionally, this would not be possible without them. v Vita 2013................................................................B.E. Aeronautical Engineering, Anna University 2015................................................................M.S. Mechanical Engineering, The Ohio State University, Columbus 2015 to present ..............................................Graduate Fellow, The Ohio State University Publications Krishnan, S., Van der Walt, H., Venkatesh, V., & Sundaresan, V. B. (2017). Dynamic characterization of elastico-mechanoluminescence towards structural health monitoring. Journal of Intelligent Material Systems and Structures, 28(17), 2458-2464. Fields of Study Major Field: Mechanical Engineering vi Table of Contents Abstract ................................................................................................................................ i Dedication .......................................................................................................................... iii Acknowledgments.............................................................................................................. iv Vita ..................................................................................................................................... vi Table of Contents .............................................................................................................. vii List of Tables ..................................................................................................................... xi List of Figures ................................................................................................................... xii Nomenclature ................................................................................................................. xxiii Chapter 1 – Introduction and Motivation............................................................................ 1 1.1. Light Emission Mechanisms ............................................................................. 1 1.2. Introduction to Mechanoluminescence ............................................................. 2 1.3. Elastico-mechanoluminescent Materials .......................................................... 4 1.3.1. Zinc sulfide – Optical Properties ......................................................... 5 1.3.2. EML Mechanism ................................................................................. 8 1.4. EML Architectures.......................................................................................... 11 1.4.1. EML Thin Films ................................................................................ 11 1.4.2. EML Particulate Composites ............................................................. 15 1.5. Structural Health Monitoring .......................................................................... 19 1.5.1. Structural Health Monitoring of Composites – Need ........................ 20 1.6. Hypothesis....................................................................................................... 23 1.6.1. Testing the Hypothesis – Interface Modeling ................................... 25 1.7. Motivation ....................................................................................................... 26 1.8. Organization of this Dissertation .................................................................... 27 Chapter 2 – EML Elastomeric Composites ...................................................................... 30 2.1. Introduction ..................................................................................................... 30 2.2. Background ..................................................................................................... 30 2.2.1. Materials and Methods ...................................................................... 31 vii 2.2.2. Material Characterization .................................................................. 31 2.2.3. Functional Characterization – Time-Averaged Luminance .............. 32 2.2.4. Importance of Interfacial Adhesion ................................................... 35 2.3. Composition Optimization .............................................................................. 37 2.4. Dynamic Emission Characteristics ................................................................. 38 2.4.1. Empirical model for EML emission .................................................. 42 2.5. Structural Health Monitoring Capability ........................................................ 46 2.6. Failure Prediction using Measured Mechanoluminescence ............................ 51 2.7. Conclusion
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