In this paper, a nonlinear constitutive relation of foam plastics is formulated. First, based on the "Eshelby equivalent inclusion method" and the "differential scheme" , effective moduli of a porous material with linear elastic matrix are derived. Then, a lower bound expression for the macroscopic strain potential of foam plastics is obtained. The polymeric matrix of this foam plastics is assumed to obey the Ramberg-Osgood relation. Finally, a method to give a more accurate expression for the macroscopic strain potential is developed"and a system of nonlinear ordinary differential equations is established. Solutions of these differential equations may be used to provide a more realistic prediction for nonlinear constitutive relation of foam plastics.
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