THERMO-MECHANICAL CONSTITUTIVE MODEL OF SHAPE MEMORY POLYMER – NUMERICAL MODELING, EXPERIMENTAL VALIDATION AND ITS APPLICATION TO AERO-MORPHING STRUCTURES
Balogun, Olaniyi A
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In this dissertation a one-dimensional thermo-mechanical constitutive model was developed for thermally actuated shape memory polymers (SMPs) in which a single variable, called a binding factor, was defined and used to simplify the model compared to other existing models that typically require many variables. The binding factor is an approximation that captures the behavior of the storage strain and considers the polymer's molecular architecture and morphology in the glass transition temperature region. Next, the one-dimensional viscoelastic model developed was further expanded into a three-dimensional model. The three-dimensional translation was initiated by transforming the Young's modulus into bulk and shear modulus. Also, the compliance and stiffness matrix for correlating elements were defined and applied into the model. Developing a three-dimensional model allows for the prediction of the transverse and shear direction of the material under different loading conditions. Finally, in order to make use of the model in finite elements tools, the three-dimensional model was rewritten into a numerical constitutive model. A numerical algorithm of this model was developed using the UMAT subroutine capabilities of the finite element software ABAQUS. Using experimental data available in literature, validation of the model was conducted which shows that the model has good response to shape fixity, stress relaxation and strain release.A novel concept to apply SMPs as the primary material for flapping wing micro air vehicle (FW-MAV) was proposed. Using SMPs will allow the vehicle to morph its wings, a trait common in nature for flying creatures at that scale. Comprehensive analysis was carried out using the new numerical constitutive model to investigate the possibility of self-induced morphing of the SMP wing. The numerical simulation results demonstrated that morphing of FW-MAV with the new numerical model is promising. The results reported in this dissertation also provide credible confidence that the new model can be used to advance development and simulation of morphing structures.
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