Doctor of Philosophy (PhD)
Syntactic foams, renowned for their low density and high mechanical properties, are enjoying continuing growth in various civilian and military sectors. However, like laminated composites, foam cored structures are vulnerable to impact damages and suffering from inabilities in repairing macro-scale cracks. A self-healing mechanism for structural damage is genuinely desired. A recent development in self-healing structural damage is a two-step close then heal (CTH) scheme proposed by Li and Nettles  and elucidated by Li and Uppu , by mimicking the self-healing process of human skin. This concept has been further demonstrated in Nji and Li’s work  that a shape memory polymer (SMP) based and thermoplastic particles incorporated syntactic foam is able to heal structural scale fracture repeatedly, efficiently, molecularly, and almost autonomously. The purpose of this study is to establish a theoretical framework for the thermomechanical behavior of the SMP based self-healing syntactic foam as well as to investigate the effects of various parameters. Firstly, a phenomenological thermoviscoelastic constitutive model for this SMP based syntactic foam was developed by Xu and Li . The foam was initially perceived as a three-phase composite with interfacial transition zone layer coated microballoons dispersed in the SMP matrix based on the experimental observation, and later simplified into a two-phase composite by generalizing the interphase and pure SMP into an equivalent SMP matrix for convenience. The model was tested against the 2nd law of thermodynamics and validated by experimental results. Parametric studies provided design guidelines for the manufacturing and optimization. Secondly, a new concept was introduced to program SMPs at temperatures well below Tg, which effectively simplified the traditional lengthy, energy costly and inconvenient shape fixing process. Thermomechanical testing confirmed the feasibility of the proposed training method. Durability of the fixed shape under various environmental attacks was experimentally validated. A thermoviscoplastic constitutive model incorporating structure-relaxation was then developed and validated experimentally. Parameters affecting the thermomechanical behavior were further investigated. Finally, this concept was extended to the SMP based syntactic foam. A damage-allowable thermoviscoplastic model was formulated and experimentally validated. Parametric studies were conducted.
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Xu, Wei, "Modeling and Simulation for Shape Memory Polymer Based Self-healing Syntactic Foam" (2010). LSU Doctoral Dissertations. 3206.