Fracture Toughness of Re-entrant Foam Materials with a Negative Poisson's Ratio: Experiment and Analysis

Fracture toughness of re-entrant foam materials with a negative Poisson's ratio is explored experimentally as a function of permanent volumetric compression ratio, a processing variable. JIC values of toughness of negative Poisson's ratio open cell copper foams are enhanced by 80%, 130%, and 160% for permanent volumetric compression ratio values of 2.0, 2.5, and 3.0, respectively, compared to the JIC value of the conventional foam (with a positive Poisson's ratio). Analytical study based on idealized polyhedral cell structures approximating the shape of the conventional and re-entrant cells disclose for re-entrant foam, toughness increasing as Poisson's ratio becomes more negative. The increase in toughness is accompanied by an increase in compliance, a combination not seen in conventional foam, and which may be useful in applications such as sponges.