Theoretical modelling and experimental characterization of flexible composites under finite deformation

This dissertation presents a theoretical and experimental study of the constitutive relations of flexible composites. The major focuses of this research effort are: (A) the conceptual development of flexible composites with wavy fibers; (B) the theoretical modeling development for predicting and optimizing the nonlinear elastic finite deformation behavior of flexible composites; and (C) process and test method development for making and characterizing such composites.; In a series of recent publications, Chou, Luo and Takahashi defined "flexible composites" as those composites based upon elastomeric polymers with the usable deformation range much larger than that of the conventional rigid polymer based composites. Flexible composites provide the engineers with a new degree of freedom in design, by controlling the geometric configurations of the fibers and selecting the proper matrix materials. The range of deformation and mechanical properties achieved in flexible composites were not realized in rigid composites.; In order to fully understand the ability of flexible composites to sustain large deformation, elastomeric matrices reinforced with wavy fibers have been suggested and analyzed by using the following three approaches: (1) The Lagrangian description has been used to derive the constitutive equations. Following Rivlin, the strain-energy density is assumed to be a function of the Lagrangian strain components referring to the initial principal material coordinates; and a fourth-order polynomial form is adopted. (2) The Eulerian description has been used to develop the constitutive model. The material nonlinear stress-strain relation including the stretching-shear coupling is derived by using the stress energy density referring to the deformed volume in a moving coordinate. The geometric nonlinear behavior is predicted by using an iterative calculation method. (3) A step-wise incremental analysis is used where the constitutive relations are predicted based upon a superposition of the infinitesimal deformations of the classical lamination theory.; The fabrication techniques for flexible composites based on unidirectional straight fibers and wavy fibers have been developed. Experiments have been conducted to study the elastic behavior of flexible composites under finite deformation. Comparisons between theories and experiments have been made. Finally, the applicability and limitations of these three analytical methods have been assessed.