The configuration evolution and macroscopic elasticity of fluid-filled closed cell composites : micromechanics and multiscale homogenization modelling

For fluid-filled closed cell composites widely distributed in nature, the configuration evolution and effective elastic properties are investigated using a micromechanical model and a multiscale homogenization theory, in which the effect of initial fluid pressure is considered. Based on the configuration evolution of the composite, we present a novel micromechanics model to examine the interactions between the initial fluid pressure and the macroscopic elasticity of the material. In this model, the initial fluid pressure of the closed cells and the corresponding configuration can be produced by applying an eigenstrain at the introduced fictitious stress-free configuration, and the pressure-induced initial microscopic strain is derived. Through a configuration analysis, we find the initial fluid pressure has a prominent effect on the effective elastic properties of freestanding materials containing pressurized fluid pores, and a new explicit expression of effective moduli is then given in terms of the initial fluid pressure. Meanwhile, the classical multiscale homogenization theory for calculating the effective moduli of a periodical heterogeneous material is generalized to include the pressurized fluid "inclusion" effect. Considering the coupling between matrix deformation and fluid pressure in closed cells, the multiscale homogenization method is utilized to numerically determine the macroscopic elastic properties of such composites at the unit cell level with specific boundary conditions. The present micromechanical model and multiscale homogenization method are illustrated by several numerical examples for validation purposes, and good agreements are achieved. The results show that the initial pressure of the fluid phase can strengthen overall effective bulk modulus but has no contribution to the shear modulus of fluid-filled closed cell composites.