Stress Relaxation of Cellular Materials

The theoretical expressions for describing the stress relaxation rates of cellular materials are derived analytically based on a cell-wall relaxation-bending model. Theoretical results indicate that the stress relaxation rates of hexagonal honeycombs and open-cell and closed-cell foams depend on their relative density, the imposed strain, and the stress relaxation parameters of the solid material from which they are made. Cellular materials have the same activation energy and power-law stress relaxation exponent as solid cell walls; the exponent constants for the dependences on relative density and imposed strain are also theoretically obtained. Meanwhile, it is found that the dependences on the relative density of cellular materials for stress relaxation are different from those for creep as provided by Gibson and Ashby.