Introduction

MICROMECHANICS is a branch of continuummechanics. It studies the effectivemechan-ical behavior of heterogeneous materials based on their microstructures of material phases. Continuummechanics deals with materials under the assumption that the material distribution is uniform, so stress and strain satisfy the constitutive law uniformly. Obviously, at a lower scale the material distribution is not uniform, so that local stress and strain are not uniform, even if we apply a uniform loading at the boundary. However, when the material phases are distributed homogeneously in a statistical sense, at a larger length scale, the composite can be treated as a statistically uniform material, and the stress and strain at a material point can be evaluated by the averages of stress and strain on a representative volume element (RVE) [1-4]. Here an RVE in a continuum body is a material volume that statistically represents the neighborhood of a material point. From the relation between averaged stress and strain, we can derive an effectivemechanical constitutive law of the composite. Inevitably, the overall mechanical properties of the composite are affected by factors such as the volume fraction of the particles, the elastic moduli of the matrix and particles, and the microstructure of the composites. Micromechanics has become a powerful tool in design and characterization of composite materials.