FRACTURE-TOUGHNESS OF REENTRANT FOAM MATERIALS WITH A NEGATIVE POISSONS RATIO - EXPERIMENT AND ANALYSIS

Fracture toughness of re-entrant foam materials with a negative Poisso n's ratio is explored experimentally as a function of permanent volume tric compression ratio, a processing variable. J(IC) values of toughne ss of negative Poisson's ratio open cell copper foams are enhanced by 80 percent, 130 percent, and 160 percent for permanent volumetric comp ression ratio values of 2.0, 2.5, and 3.0, respectively, compared to t he J(IC) value of the conventional foam (with a positive Poisson's rat io). Analytical study based on idealized polyhedral cell structures, a pproximating the shape of the conventional and re-entrant cells, discl ose for re-entrant foam, toughness increasing as Poisson's ratio becom es more negative. The increase in toughness is accompanied by an incre ase in compliance, a combination not seen in conventional foam, and wh ich may be useful in some applications such as sponges.