ANISOTROPIC PLASTICITY MODEL FOR FOAMS AND HONEYCOMBS

A three-dimensional plasticity model is developed for foams and honeyc ombs. The model accounts for the transverse isotropy of foams and the orthotropy of honeycombs by incorporating a shift on stress that provi des a yield surface in the form of a sphere with the centroid shifted from the origin in the principal stress space. To model the extensive strains that occur at constant stress, an elastic-perfectly plastic fo rmulation is used until a critical plastic volumetric strain is reache d. Then the initiation of lockup is represented as an isotropic harden ing function of plastic volumetric strain. Both elastic and plastic co nstants are given as functions of the initial density of the cellular material. With lockup, the elasticity tensor transforms to that of the parent solid. To account for strain-rate effects, the plateau stresse s and the hardening parameter are taken as functions of volumetric str ain rate. For each material, experimental data are given for various d ensities and illustrative plots are given to show how well the model r epresents these data.