ABSTRACT
FOR AN ENERGY conservative solid, the stress and strain relation can be described by astrain energy potential function U, such as σij = ∂U
For a linear elastic solid, the constant linearity between the stress and the strain is called the elasticity of stiffness, which can be written as
Cijkl = ∂σij ∂kl
= ∂ 2U
The strain energy in composite materials can also be used to obtain the effective elasticity of the composites.